Patent Publication Number: US-11398534-B2

Title: Flexible display apparatus and touch sensitive display apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation application based on U.S. patent application Ser. No. 16/216,013, filed Dec. 11, 2018 (now issued as U.S. Pat. No. 10,797,251), the disclosure of which is incorporated herein by reference in its entirety. U.S. patent application Ser. No. 16/216,013 claims priority benefit, under 35 U.S.C. § 119, of Korean Patent Application No. 10-2018-0013437, filed on Feb. 2, 2018, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference for all purposes. 
    
    
     BACKGROUND 
     1. Field 
     One or more embodiments relate to a flexible display apparatus, and more particularly, to a flexible display apparatus having a bending area. 
     2. Description of the Related Art 
     Organic light-emitting display apparatuses have a self-emission characteristic, thus no separate light source is needed, allowing a thickness and a weight thereof to be reduced. In addition, the organic light-emitting display apparatuses have high-grade characteristics such as low power consumption, high brightness, and a quick response speed. 
     An organic light-emitting display apparatus includes a substrate, a thin-film transistor on the substrate, an organic light-emitting device of which emission is controlled by the thin-film transistor, and a plurality of insulation layers arranged among electrodes forming the thin-film transistor. Recently, organic light-emitting display apparatuses including a flexible substrate and having a bending area have been developed. 
     SUMMARY 
     According to one or more embodiments, a flexible display apparatus includes: a flexible substrate including a display area and a bending area outside the display area, the bending area to be bent around a bending axis; an inorganic insulating layer on the flexible substrate; a cut unit in the inorganic insulating layer in the bending area; a stress relaxation layer filling the cut unit and extending into the display area; a wiring part on the stress relaxation layer in the bending area; a planarization layer covering the wiring part and on the stress relaxation layer; and a display on the planarization layer in the display area and electrically connected to the wiring part. 
     The inorganic insulating layer may include a plurality of inorganic films. 
     In the display area, the planarization layer may be on the stress relaxation layer. 
     The cut unit may extend in a direction parallel to the bending axis. 
     The stress relaxation layer may include an organic insulating material. 
     A thickness of the stress relaxation layer in the cut unit may be greater than a depth of the cut unit. 
     A lower surface of the stress relaxation layer in the cut unit may come in direct contact with an upper surface of the flexible substrate. 
     The flexible display apparatus may further include a thin-film transistor electrically connecting the display to the wiring part and in the display area, wherein a distance from an upper surface of the planarization layer to the flexible substrate in a region overlapping the thin-film transistor may be substantially the same as a distance from the upper surface of the planarization layer to the flexible substrate in a region overlapping the cut unit. 
     The planarization layer may include an organic insulating material. 
     The flexible display apparatus may further include a thin-film transistor electrically connecting the display to the wiring part and in the display area, wherein an upper surface of the uppermost electrode of the thin-film transistor is higher than an upper surface of the stress relaxation layer. 
     The wiring part may include the same material as the uppermost electrode of the thin-film transistor. 
     The flexible display apparatus may further include, in the display area, a thin-film transistor electrically connecting the display to the wiring part, wherein an upper surface of the uppermost electrode of the thin-film transistor is lower than or equal to an upper surface of the stress relaxation layer. 
     The wiring part may include the same material as the uppermost electrode of the thin-film transistor. 
     The stress relaxation layer may have an opening and the uppermost electrode of the thin-film transistor may be in the opening. 
     The flexible substrate may include: a first substrate including a polymer resin; a second substrate on the first substrate and including a polymer resin; and a barrier film between the first substrate and the second substrate. 
     In the cut unit, the lower surface of the stress relaxation layer may come in direct contact with an upper surface of the second substrate. 
     The flexible display apparatus may further include an encapsulation part covering the display and including at least one inorganic film and at least one organic film. 
     According to one or more embodiments, a touch-detecting display apparatus includes: a flexible substrate including a display area and a bending area outside the display area, the bending area to be bent around a bending axis; an inorganic insulating layer on the flexible substrate; a stress relaxation layer filling a cut unit in the inorganic insulating layer in the bending area, and extending into the display area; a wiring part on the stress relaxation layer in the bending area; a planarization layer covering the wiring part and on the stress relaxation layer; a display on the planarization layer in the display area and electrically connected to the wiring part; a flexible encapsulation part covering the display; a touch detection layer on the flexible encapsulation part; color filters on the touch detection layer; and a black matrix between the color filters. 
     The display may include a first electrode, an organic light-emitting layer, and a second electrode, and a tilt angle of the first electrode with a plane parallel to the flexible substrate may be smaller than 0.1°. 
     The color filters and the black matrix may share an overlapping area. 
     In the display area, the planarization layer may be on the stress relaxation layer. 
     A pad electrode electrically connected to the wiring part may be at an edge of the flexible substrate, and when the bending area is folded, the pad electrode may overlap the display area. 
     In the cut unit, a thickness of the stress relaxation layer may be greater than a depth of the cut unit. 
     The touch-detecting display apparatus further include, in the display area, a thin-film transistor electrically connecting the display to the wiring part, wherein a distance from an upper surface of the planarization layer to the flexible substrate in a region overlapping the thin-film transistor may be substantially the same as a distance from the upper surface of the planarization layer to the flexible substrate in a region overlapping the cut unit. 
     Other aspects, features, and advantages other than those described above will be clear from the detailed description, the claims, and the drawings below to carry out the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features will become apparent to those of skill in the art by describing in detail exemplary embodiments with reference to the attached drawings in which: 
         FIG. 1  illustrates a schematic perspective view of a flexible display apparatus according to a first embodiment; 
         FIG. 2  illustrates a schematic cross-sectional view of the flexible display apparatus shown in  FIG. 1 ; 
         FIG. 3  illustrates a schematic perspective view showing an unfolded state of the flexible display apparatus shown in  FIG. 1 ; 
         FIG. 4  illustrates a schematic cross-sectional view showing a display area DA and a bending area BA of the flexible display apparatus shown in  FIG. 1 ; 
         FIG. 5  illustrates a schematic cross-sectional view showing a display area DA and a bending area BA of a flexible display apparatus according to a comparative example; 
         FIG. 6  illustrates color separation degrees of external light according to tilt angles when a pixel electrode of a green light-emitting pixel is tilted from a surface parallel to a flexible substrate; 
         FIG. 7A  illustrates an image showing a phenomenon that a reflective color of external light is separated when a tilt angle of the pixel electrode exceeds 1.0°; 
         FIG. 7B  illustrates an image showing a phenomenon that a reflective color of external light is separated when a tilt angle of the pixel electrode is smaller than 1.0°; 
         FIG. 8  illustrates a schematic cross-sectional view showing a display area DA and a bending area BA of a flexible display apparatus according to a second embodiment; 
         FIG. 9  illustrates a schematic cross-sectional view showing a display area DA and a bending area BA of a flexible display apparatus according to a third embodiment; and 
         FIG. 10  illustrates a schematic cross-sectional view showing a display area DA and a bending area BA of a flexible display apparatus according to a fourth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. In this regard, the present embodiments may have different forms and should not be construed as being limited to the descriptions set forth herein. Accordingly, the embodiments are merely described below, by referring to the figures, to explain aspects of the present description. 
     When it is described through the embodiments that a certain element, such as a layer, a film, a region, or a substrate, is located “on” another element, it may be understood that the certain element may be located “on” another element directly or via another element in the middle. In addition, for convenience of description, in the accompanying drawings, sizes of components may be exaggerated or reduced. For example, the size and the thickness of each component shown in the drawings are arbitrarily shown for convenience of description, and thus the present disclosure is not necessarily limited thereto. 
     In the embodiments, an x axis, a y axis, and a z axis are not limited to three axes of a rectangular coordinate system but may be analyzed as a wide meaning including the same. For example, the x axis, the y axis, and the z axis may be orthogonal to each other or may indicate different directions that are not orthogonal to each other. 
       FIG. 1  is a schematic perspective view of a flexible display apparatus  100  according to a first embodiment.  FIG. 2  is a schematic cross-sectional view of the flexible display apparatus  100  shown in  FIG. 1 .  FIG. 3  is a schematic perspective view showing an unfolded state of the flexible display apparatus  100  shown in  FIG. 1 .  FIG. 4  is a schematic cross-sectional view showing a display area DA and a bending area BA of the flexible display apparatus  100  shown in  FIG. 1 . 
     Referring to  FIGS. 1 to 4 , the flexible display apparatus  100  according to an embodiment may include a flexible substrate  110 , a display  120  formed on the flexible substrate  110 , an inorganic insulating layer  130 , a stress relaxation layer  140 , a wiring part  150 , a planarization layer  160 , and an encapsulation part  210 . In addition, the flexible display apparatus  100  may further include a pad electrode  170 , a chip on film (COF)  180 , and a printed circuit board (PCB)  190 . 
     In the present embodiment, the flexible substrate  110  includes the display area DA in which the display  120  is formed, and includes the bending area BA extending in a first direction (+y direction) in a non-display area NDA outside the display area DA and bent around a bending axis BAX. 
     The flexible substrate  110  may include various materials having a flexible or bendable characteristic, e.g., a polymer resin such as polyethersulphone, polyacrylate, polyetherimide, polyethylene naphthalate, polyethyleneterephthalate, polyphenylene sulfide, polyarylate, polyimide, polycarbonate, cellulose acetate propionate, and the like. The flexible substrate  110  may also be variously modified such that the flexible substrate  110  may have a multi-layer structure including two layers including such a polymer resin and a barrier layer between the two layers, e.g., an inorganic material such as silicon oxide, silicon nitride, or silicon oxynitride. 
     The display  120  includes a plurality of pixels and displays an image by combining light emitted from the plurality of pixels. Each pixel may include a pixel circuit and an organic light-emitting device. The pixel circuit may include at least two thin-film transistors and at least one capacitor to control light emission of the organic light-emitting device. A detailed structure of the display  120  will be described later. 
     The bending area BA may be a portion of the non-display area NDA. A plurality of pad electrodes  170  may be formed at an edge of the flexible substrate  110  extending from the bending area BA in a second direction (+x direction). The wiring part  150  including signal lines such as scan lines and data lines electrically connected to the plurality of pixels and power lines such as driving voltage lines are electrically connected to the pad electrodes  170 . 
     The pad electrodes  170  may be connected to the COF  180 . The COF  180  may be replaced by a flexible printed circuit (FPC). The COF  180  may be connected to the PCB  190 . 
     The COF  180  may include an output wiring part  181 , a driving chip  183 , and an input wiring part  182 . The PCB  190  is connected to the input wiring part  182  of the COF  180  and inputs, to the COF  180 , a control signal for controlling the driving chip  183  of the COF  180 . The output wiring part  181  of the COF  180  is connected to the pad electrodes  170  and outputs, to the pad electrodes  170 , power and various kinds of signals for controlling display of the flexible display apparatus  100 . 
     When the non-display area NDA including the wiring part  150  and the pad electrodes  170  is located alongside the display area DA without being bent (see  FIG. 3 ), an area of a dead space outside the display  120  increases. The flexible display apparatus  100  according to the present embodiment has the bending area BA formed by bending a portion of the non-display area NDA in which the wiring part  150  extends. As a bending result, the edge of the flexible substrate  110 , on which the pad electrodes  170  are formed, overlaps the display area DA at the rear of the display area DA. Thus the dead space outside the display  120  is minimized. 
     The bending area BA is bent around the bending axis BAX. The center of curvature of the bending area BA is located on the bending axis BAX. Referring to  FIGS. 1 and 2 , the bending axis BAX is parallel to the y axis. 
     The inorganic insulating layer  130  including at least one inorganic film among a barrier film  131 , a buffer film  132 , a gate insulating film  133 , a first interlayer insulating film  134 , and a second interlayer insulating film  135  on the flexible substrate  110 . The inorganic insulating layer  130  may be between electrodes and wirings included in the display  120  to insulate the same. 
     In the present embodiment, the inorganic insulating layer  130  in the bending area BA may have a cut unit CU that extends in a direction (y-axis direction) parallel to the bending axis BAX. The cut unit CU may be filled with the stress relaxation layer  140  including an organic insulating material. The stress relaxation layer  140  extends into the display area DA. For example, as may be seen in  FIG. 4 , the stress relaxation layer  140  is on the inorganic insulating layer  130  in the display area DA, i.e., between the inorganic insulating layer  130  and the planarization layer  160 . In particular, in the display area DA, the planarization layer  160  may be on, e.g., in direct contact with, the stress relaxation layer  140 . 
     The wiring part  150  may be on the stress relaxation layer  140  in the non-display area NDA. The wiring part  150  may include a metal material. The inorganic insulating layer  130  has much less flexibility than the wiring part  150 , e.g., may be sufficiently brittle that it may be broken by an external force. Therefore, if the inorganic insulating layer  130  is in the bending area BA, cracks may occur therein by a tensile force due to bending. These cracks may be propagated to other regions of the inorganic insulating layer  130 . The cracks in the inorganic insulating layer  130  may cause the wiring part  150  to be disconnected, thereby causing abnormal display of the flexible display apparatus  100 . 
     In the present embodiment, the stress relaxation layer  140  is formed of a material having lower brittleness and higher flexibility than the inorganic insulating layer  130 . For example, the stress relaxation layer  140  may be formed of an organic insulating material, e.g., acryl, epoxy, acrylate, polyimide, benzocyclobutene (BCB), or hexamethyldisiloxane (HMDSO). The stress relaxation layer  140  may be formed by filling the cut unit CU and closely attaching to side walls of the cut unit CU such that there is no gap between the stress relaxation layer  140  and the inorganic insulating layer  130 . 
     In addition, in the present embodiment, the stress relaxation layer  140  may have a lower surface that comes in direct contact with not only the side walls of the cut unit CU but also an upper surface of the flexible substrate  110 , which is a lower surface of the cut unit CU. Thus, an adhesive force with the flexible substrate  110  is also improved. 
     Therefore, the flexible display apparatus  100  according to the present embodiment may suppress the occurrence of cracks in the inorganic insulating layer  130  due to bending stress and prevent disconnection of the wiring part  150 , by forming the cut unit CU in the inorganic insulating layer  130  in the bending area BA and forming the stress relaxation layer  140  having low brittleness in the cut unit CU. 
     In more detail, the barrier film  131  may be on the flexible substrate  110 . The barrier film  131  blocks infiltration of moisture and oxygen through the flexible substrate  110  and may be formed of a multi-layer film in which silicon oxide (SiO 2 ) and silicon nitride (SiN x ) are alternately and repetitively stacked. 
     The buffer film  132  may be on the barrier film  131 . The buffer film  132  provides a planar surface for forming a pixel circuit thereon and may include SiO 2  or SiN x . 
     A semiconductor layer  121  and a first capacitor electrode  125  are on the buffer film  132 . The semiconductor layer  121  may be a polysilicon or oxide semiconductor, and the semiconductor layer  121  may be an oxide semiconductor, which may be covered by a separate protective film. The semiconductor layer  121  may include a channel region that is undoped, and a source region and a drain region located at both sides of the channel region and doped with impurities. The first capacitor electrode  125  may include the same material as the semiconductor layer  121 . 
     The gate insulating film  133  is on the semiconductor layer  121  and the first capacitor electrode  125 . The gate insulating film  133  may be formed of a single film of SiO 2  or SiN x  or a stacked film thereof. 
     A gate electrode  122  and a second capacitor electrode  126  are formed on the gate insulating film  133 . The gate electrode  122  overlaps the channel region of the semiconductor layer  121 . The second capacitor electrode  126  may include the same material as the gate electrode  122 . 
     The first interlayer insulating film  134  is formed on the gate electrode  122  and the second capacitor electrode  126 , and a third capacitor electrode  127  may be formed on the first interlayer insulating film  134 . The third capacitor electrode  127  may include the same material as the gate electrode  122 . 
     The gate insulating film  133  forms a first dielectric film between the first capacitor electrode  125  and the second capacitor electrode  126 , and the first interlayer insulating film  134  forms a second dielectric film between the second capacitor electrode  126  and the third capacitor electrode  127 , thereby forming a storage capacitor  202 . 
     The second interlayer insulating film  135  may be on the third capacitor electrode  127 , and a source electrode  123  and a drain electrode  124  may be on the second interlayer insulating film  135 . The first and second interlayer insulating films  134  and  135  may be formed of a single film of SiO 2  or SiN x  or a stacked film thereof. 
     The source electrode  123  and the drain electrode  124  in the display area DA are respectively connected to the source region and the drain region of the semiconductor layer  121  through contact holes formed in the gate insulating film  133 , the first and second interlayer insulating films  134  and  135 , and the stress relaxation layer  140 . Thus, the source electrode  123  and the drain electrode  124  may be on, e.g., directly on, the stress relaxation layer  140 . Therefore, the source electrode  123  and the drain electrode  124  are higher than an upper surface of the stress relaxation layer  140 . 
     The wiring part  150  in the bending area BA may be formed of the same material as that of the source electrode  123  and the drain electrode  124 . The source electrode  123 , the drain electrode  124 , and the wiring part  150  may be formed of a metal multi-layer film, e.g., molybdenum (Mo)/aluminum (Al)/Mo or titanium (Ti)/Al/Ti. 
     The pixel electrode  141  is formed on the planarization layer  160  for each pixel and is connected to the drain electrode  124  of a driving thin-film transistor  201  through the via hole VH formed in the planarization layer  160 . The driving thin-film transistor includes the semiconductor layer  121 , the gate electrode  122 , the source electrode, and the drain electrode  124 . 
     In addition to the driving thin-film transistor  201  and the storage capacitor  202 , the pixel circuit may also include a switching thin-film transistor. 
     The planarization layer  160  is disposed on the driving thin-film transistor  201  and the wiring part  150 . The driving thin-film transistor  201  is connected to an organic light-emitting device  203  and drives the organic light-emitting device  203 . The planarization layer  160  may include an organic insulating material or an inorganic insulating material or may be in a composite form of the organic insulating material and the inorganic insulating material. 
     The organic light-emitting device  203  may include a pixel electrode  141 , an emission layer  142 , and a common electrode  143 . 
     The pixel electrode  141  is formed on the planarization layer  160  for each pixel and is connected to the drain electrode  124  of the driving thin-film transistor  201  through a via hole VH formed in the planarization layer  160 . 
     A pixel defining film  204  is formed on an upper part of the planarization layer  160  and an edge of the pixel electrode  141 . The pixel defining film  204  defines a pixel with an opening through which a central part of the pixel electrode  141  is exposed. In addition, the pixel defining film  204  prevents the occurrence of an arc and the like at the edge of the pixel electrode  141  by increasing a distance between the edge of the pixel electrode  141  and the common electrode  143  on an upper part of the pixel electrode  141 . The pixel defining film  204  may be formed of an organic material such as polyimide or HMDSO. 
     The emission layer  142  may be on the pixel electrode  141 , and the common electrode  143  may be all over the display area DA regardless of pixel. 
       FIG. 4  shows only the emission layer  142  on an upper part of the pixel electrode  141 , but an intermediate layer besides the emission layer  142  may be further included between the pixel electrode  141  and the common electrode  143 . The intermediate layer of the organic light-emitting device  203  may include a low or high molecular material. When the intermediate layer includes a low molecular material, the intermediate layer may have a structure in which a hole injection layer (HIL), a hole transport layer (HTL), an emission layer (EML), an electron transport layer (ETL), an electron injection layer (EIL), and the like are stacked in a single or composite structure, and may include various organic materials, e.g., including copper phthalocyanine (CuPc), N,N′-Di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB), tris-8-hydroxyquinoline aluminum (Alq3), and the like. These layers may be formed by a vacuum deposition method. 
     When the intermediate layer includes a high molecular material, the intermediate layer may mainly have a structure including an HTL and an EML. In this case, the HTL may include polyethylenedioxythiophene (PEDOT), and the EML may include a high molecular material of a poly-phenylenevinylene (PPV) group, a polyfluorene group, or the like. The intermediate layer may be formed by screen printing, inkjet printing, laser induced thermal imaging (LITI), or the like. 
     Any one of the pixel electrode  141  and the common electrode  143  injects holes into the emission layer  142 , and the other one thereof injects electrons into the emission layer  142 . The electrons and the holes are bonded in the emission layer  142  such that excitons are generated, and light is emitted by energy generated when the excitons transit from an excited state to a ground state. 
     The pixel electrode  141  may be formed of a reflective film, and the common electrode  143  may be formed of a transparent film or a translucent film. The light emitted from the emission layer  142  is reflected from the pixel electrode  141 , transmits through the common electrode  143 , and then is output to the outside. When the common electrode  143  is formed of a translucent film, a portion of the light reflected from the pixel electrode  141  is re-reflected from the common electrode  143 , and the pixel electrode  141  and the common electrode  143  may form a resonance structure, thereby increasing light extraction efficiency. 
     The organic light-emitting device  203  is covered by the encapsulation part  210 . The encapsulation part  210  may seal the organic light-emitting device  203  such that deterioration of the organic light-emitting device  203  due to moisture and oxygen included in external air is suppressed. The encapsulation part  210  may have a stacked structure of an inorganic material and an organic material and may include, e.g., a first inorganic film  211 , an organic film  212 , and a second inorganic film  213 . 
     Referring again to the stress relaxation layer  140 , as shown in  FIG. 4 , in the present embodiment, the stress relaxation layer  140  fills the cut unit CU and extends into the display area DA. Thus, a thickness T of the stress relaxation layer  140  filling the cut unit CU of the inorganic insulating layer  130  may be greater than a depth D of the cut unit CU. Therefore, the wiring part  150  may be in direct contact with the stress relaxation layer  140 , not with the inorganic insulating layer  130 . 
       FIG. 5  is a schematic cross-sectional view showing a display area DA and a bending area BA of a flexible display apparatus  10  according to a comparative example. Referring to  FIG. 5 , unlike the present embodiment, in the flexible display apparatus  10  according to the comparative example, a stress relaxation layer  140 ′ including an organic insulating material fills a cut unit CU, but the stress relaxation layer  140 ′ does not extend to the display area DA. As shown in  FIG. 5 , the planarization layer  160  and the inorganic insulating layer  130  are in direct contact with each other in the bend area BA outside the cut unit CU. Therefore, the thickness T of the stress relaxation layer  140 ′ in the cut unit CU of the inorganic insulating layer  130  cannot be greater than the depth D of the cut unit CU. 
     In the flexible display apparatus  10  according to the comparative example of  FIG. 5 , the stress relaxation layer  140 ′ filling the cut unit CU in the bending area BA does not extend to the display area DA and a planarization layer  160 ′ may be different than the planarization layer  160  of  FIG. 4 . Since the stress relaxation layer  140 ′ is not in the display area DA in  FIG. 5 , the source electrode  123  and the drain electrode  124  are respectively connected to the source region and the drain region of the semiconductor layer  121  through contact holes in the first and second interlayer insulating films  134  and  135  and the gate insulating film  133 . Otherwise, elements of  FIG. 5  are the same as those of  FIG. 4 , and description thereof will not be repeated. 
     As described above, unlike the comparative example, in the flexible display apparatus  100  according to the present embodiment, a difference between the thickness T of the stress relaxation layer  140  filling the cut unit CU and the depth D of the cut unit CU may prevent color separation of reflective light reflected from a light-emitting device of the display  120 , as described in detail below. 
     External light incident to the organic light-emitting device  203  from the outside of the flexible display apparatus  10  is emitted by being reflected from various kinds of electrodes and wirings located inside the flexible display apparatus  10 . The reflected external light may be mixed with light emitted from the emission layer  142 , thereby causing noise. Particularly, according to flatness of the pixel electrode  141  acting as a reflective electrode, a color separation phenomenon of the reflected external light may be affected. 
     The flatness of the pixel electrode  141  depends on flatness of the planarization layer  160  in direct contact with the pixel electrode  141 . The flatness of the planarization layer  160  may depend on a design of a thin-film transistor, a capacitor, and wirings arranged at a lower part of the planarization layer  160 . 
     In the flexible display apparatus  10  according to the comparative example, the planarization layer  160  is formed to make the display  120  flat. However, due to a level difference by a structure of the thin-film transistor, the capacitor, and the wirings arranged at a lower part of the planarization layer  160 , a tilt angle θ (see  FIG. 6 ) of the pixel electrode  141  with a plane parallel to the flexible substrate  110  exceeds 1.0°. 
     However, referring to  FIG. 4 , the flexible display apparatus  100  according to the present embodiment has the stress relaxation layer  140  including an organic insulating material at a lower part of the planarization layer  160  and also extends to the display area DA. Thus, a level difference by a structure of a thin-film transistor, a capacitor, and wirings arranged at a lower part of the stress relaxation layer  140  is alleviated. Since the planarization layer  160  is formed in a state in which the level difference is alleviated, the tilt angle θ of the pixel electrode  141  with a plane parallel to the flexible substrate  110  does not exceed 1.0°. That is, an upper surface of the planarization layer  160  is substantially flat. 
     Therefore, according to the present embodiment, a distance from the upper surface of the planarization layer  160  to the flexible substrate  110  in a region overlapping the driving thin-film transistor  201  may be substantially the same as a distance from the upper surface of the planarization layer  160  to the flexible substrate  110  in a region overlapping the cut unit CU. 
       FIG. 7A  illustrates an image showing a phenomenon that a reflective color of external light is separated when a tilt angle of the pixel electrode  141  exceeds 1.0°. That is, reflected external lights are not mixed to emit white light but separated to respective unique colors, thereby generating noise. 
       FIG. 7B  illustrates an image showing a phenomenon that a reflective color of external light is separated when a tilt angle of the pixel electrode  141  is smaller than 1.0°. Although color separation of reflected external lights does not fully disappear, the color separation is significantly alleviated when compared with  FIG. 7A . 
       FIG. 6  illustrates color separation degrees of external light according to tilt angles when the pixel electrode  141  of a green light-emitting pixel is tilted from a surface parallel to a flexible substrate. 
     (a) of  FIG. 6  shows relative locations of a red band R and a green band Ga of reflected external light when the tilt angle θ is 0.5°, (b) of  FIG. 6  shows relative locations of the red band R and the green band Ga of reflected external light when the tilt angle θ is t1.0°, and (c) of  FIG. 6  shows relative locations of the red band R and the green band Ga of reflected external light when the tilt angle θ is 1.9°. 
     A difference of the green band Ga based on the red band R in (a) of  FIG. 6  is Δa, a difference of the green band Ga based on the red band R in (b) of  FIG. 6  is Δb, and a difference of the green band Ga based on the red band R in (c) of  FIG. 6  is Δc. A color separation phenomenon increases from (a) of  FIG. 6  to (c) of  FIG. 6 , i.e., as the tilt angle θ increases. 
     In the present embodiment, since the stress relaxation layer  140  extends into the display area DA, roughness due to a thin-film transistor, a capacitor, and wirings arranged at a lower part of the pixel electrode  141  may be reduced. Since the planarization layer  160  is on the stress relaxation layer  140 , a tilt angle of the pixel electrode  141  may be reduced. 
     Therefore, the flexible display apparatus  100  according to the present embodiment may have the bending area BA to reduce a dead space. In addition, by forming the cut unit CU in the bending area BA and filling the stress relaxation layer  140  therein, cracks in an inorganic insulating layer and disconnection of a wiring may be prevented. In addition, by reducing a tilt angle of the pixel electrode  141  by having the stress relaxation layer  140  extending into the display area DA, color separation of external light may be prevented, thereby increasing display quality. 
     Hereinafter, various embodiments of the present disclosure will be described with reference to  FIGS. 8 to 10 . 
       FIG. 8  is a schematic cross-sectional view showing a display area DA and a bending area BA of a flexible display apparatus  200  according to a second embodiment. A description will be made based on differences when compared with  FIG. 4  showing the flexible display apparatus  100  according to the first embodiment. Like reference numerals in  FIG. 4  refer to like elements in  FIG. 4 . 
     The flexible display apparatus  200  according to the second embodiment differs from the flexible display apparatus  100  according to the first embodiment with respect to a pattern of the stress relaxation layer  140  in the display area DA. According to the present embodiment, a stress relaxation layer  140   a  filling the cut unit CU of the bending area BA and simultaneously extends into the display area DA, but an opening OP 1  is in the stress relaxation layer  140   a  to expose a location where the source electrode  123  and the drain electrode  124 . 
     The source electrode  123  and the drain electrode  124  are formed in the opening OP 1  such that upper surfaces of the source electrode  123  and the drain electrode  124  are not higher than an upper surface of the stress relaxation layer  140   a , e.g., may be lower than the upper surface of the stress relaxation layer  140   a . Thus, the source electrode  123  and the drain electrode  124  are not on the stress relaxation layer  140   a . When the source electrode  123  and the drain electrode  124  are formed, the wiring part  150  may also formed simultaneously on the stress relaxation layer  140   a.    
     The planarization layer  160  is formed on the source electrode  123 , the drain electrode  124 , and the wiring part  150 , and the drain electrode  124  is connected to the pixel electrode  141  through a via hole VH formed in the planarization layer  160 . 
     Since the source electrode  123  and the drain electrode  124  are formed in the opening OP 1 , roughness due to the source electrode  123  and the drain electrode  124  arranged at a lower part of the pixel electrode  141  is small. Thus, a tilt angle of the pixel electrode  141  may be further reduced when compared with the first embodiment. 
       FIG. 9  is a schematic cross-sectional view showing a display area DA and a bending area BA of a flexible display apparatus  300  according to a third embodiment. A description will be made based on differences when compared with  FIG. 4  showing the flexible display apparatus  100  according to the first embodiment. Like reference numerals in  FIG. 4  refer to like elements in  FIG. 4 . 
     The flexible display apparatus  300  according to the third embodiment differs from the flexible display apparatus  100  according to the first embodiment in that the flexible display apparatus  300  further includes a touch layer  220 , color filters  240 , a black matrix  230 , and a protective layer  250  on the encapsulation part  210 . 
     The touch layer  220  includes a first insulating layer  221  formed on the encapsulation part  210 , a second insulating layer  222  formed on the first insulating layer  221 , and a plurality of touch electrodes  223  formed between the first insulating layer  221  and the second insulating layer  222 . Alternatively, the touch layer  220  may include various electrode structures, e.g., a mesh electrode pattern and a transparent segment electrode. 
     The touch layer  220  may detect a touch input based on a mutual capacitance change caused by the touch input. That is, when a touch input is applied, a mutual capacitance is changed by the touch input, and a touch detector connected to the touch layer  220  may detect a location at which the mutual capacitance is changed, thereby detecting the touch input 
     The color filters  240  and the black matrix  230  are arranged on the touch layer  220 . The black matrix  230  overlaps a non-emission region of a pixel, and the color filters  240  overlap emission regions, respectively. The color filters  240  may include red color filters  242 , green color filters  241 , and blue color filters  243 . 
     The black matrix  230  includes a material capable of blocking light. For example, the black matrix  230  may include an organic material having high light absorption ratio. The black matrix  230  may include a black pigment or a black dye. The black matrix  230  includes a light-sensitive organic material, e.g., may include a coloring agent such as a pigment or a dye. The black matrix  230  may have a single- or multi-layer structure. 
     Although  FIG. 9  shows that the color filters  240  cover a portion of the black matrix  230 , according to a forming sequence of the black matrix  230  and the color filters  240 , the black matrix  230  may cover a portion of the color filters  240 , or the color filters  240  may cover a portion of the black matrix  230 . 
     The color filters  240  may not only transmit light generated by the organic light-emitting device  203  to the outside but also reduce a reflectance of light incident from the outside. When external light passes through the color filters  240 , light intensity of the external light is reduced to about one third thereof. 
     A portion of the light which has passed through the color filters  240  is extinguished, and the remaining portion of the light is reflected from components arranged under the color filters  240 , for example, a thin-film transistor, a capacitor, wirings, the encapsulation part  210 , and the like arranged at a lower part of the pixel electrode  141 . 
     The reflected light is incident to the color filters  240 , and brightness of the reflected light is reduced while passing through the color filters  240 . As a result, since only a portion of the external light is reflected and discharged from the flexible display apparatus  300 , external light reflection may be reduced. 
     In addition, according to the present embodiment, since not only external light reflection is reduced by using the color filters  240  and the black matrix  230  but also a polarizing film generally used to reduce external light reflection does not have to be used, a thickness of the flexible display apparatus  300  may be reduced. In addition, a thin display apparatus may be manufactured, and the present embodiment may be used for a foldable or bendable display apparatus. 
     As described above, compared with the flexible display apparatus  100  according to the first embodiment in  FIG. 4 , the flexible display apparatus  300  including the color filters  240  and the black matrix  230  may reduce external light reflection, but a tilt angle of the pixel electrode  141  functioning as a reflective electrode still affects external light reflection of the flexible display apparatus  300 . According to the present embodiment, since the stress relaxation layer  140  extends into the display area DA, roughness due to a thin-film transistor, a capacitor, and wirings arranged at a lower part of the pixel electrode  141  may be small, and since the planarization layer  160  is once more formed on the stress relaxation layer  140 , a tilt angle of the pixel electrode  141  may be reduced, thereby much efficiently reducing external light reflection. 
       FIG. 10  is a schematic cross-sectional view showing a display area DA and a bending area BA of a flexible display apparatus  400  according to a fourth embodiment. A description will be made based on differences when compared with  FIG. 9  showing the flexible display apparatus  300  according to the third embodiment. Like reference numerals refer to like elements. 
     The flexible display apparatus  400  according to the fourth embodiment differs from the flexible display apparatus  300  according to the third embodiment with respect to a pattern of the stress relaxation layer  140  formed in the display area DA. According to the present embodiment, the stress relaxation layer  140   a  fills the cut unit CU of the bending area BA and simultaneously extends to the display area DA, but the opening OP 1  is in the stress relaxation layer  140   a  to expose a location where the source electrode  123  and the drain electrode  124  are formed. 
     The source electrode  123  and the drain electrode  124  are formed in the opening OP 1  such that upper surfaces of the source electrode  123  and the drain electrode  124  are not higher than an upper surface of the stress relaxation layer  140   a . When the source electrode  123  and the drain electrode  124  are formed, the wiring part  150  is also formed together on the stress relaxation layer  140   a.    
     The planarization layer  160  is formed on the source electrode  123 , the drain electrode  124 , and the wiring part  150 , and the drain electrode  124  is connected to the pixel electrode  141  through a via hole VH formed in the planarization layer  160 . Since the source electrode  123  and the drain electrode  124  are formed in the opening OP 1 , roughness due to the source electrode  123  and the drain electrode  124  arranged at a lower part of the pixel electrode  141  is small, and thus a tilt angle of the pixel electrode  141  may be further reduced when compared with the third embodiment. 
     A flexible display apparatus, according to an embodiment, may have a bending area formed outside a display area, thereby reducing a dead space. A cut unit filled with a stress relaxation layer may be in the bending area. Thus, cracks in an inorganic insulating layer and disconnection of a wiring may be prevented. 
     In addition, the flexible display apparatus, according to an embodiment, may have the stress relaxation layer extending into the display area to reduce a tilt angle of a pixel electrode, such that color separation of external light is prevented, thereby increasing display quality. 
     Example embodiments have been disclosed herein, and although specific terms are employed, they are used and are to be interpreted in a generic and descriptive sense only and not for purpose of limitation. In some instances, as would be apparent to one of ordinary skill in the art as of the filing of the present application, features, characteristics, and/or elements described in connection with a particular embodiment may be used singly or in combination with features, characteristics, and/or elements described in connection with other embodiments unless otherwise specifically indicated. Accordingly, it will be understood by those of skill in the art that various changes in form and details may be made without departing from the spirit and scope of the present invention as set forth in the following claims.