Display panel, manufacturing method thereof and display device

Abstract

This invention discloses a display panel, its manufacturing method, and a display device. By setting a transition layer between the light extraction layer and the thin film encapsulation layer, the transition layer can make the first inorganic layer and the light extraction layer tightly connected. When the display panel is bent, the risk of peeling between the first inorganic layer and the light extraction layer can be reduced, thereby improving the flexible bending performance of the display panel.

Description

[0001] This application is a divisional application. The original application has the application number 202210161485.7 and the original application date is February 22, 2022. The original application is entitled "Display Panel, Method of Manufacturing Thereof and Display Device". The entire contents of the original application are incorporated herein by reference. Technical Field

[0002] This invention relates to the field of display technology, and more particularly to display panels, their manufacturing methods, and display devices. Background Technology

[0003] With the continuous development of display technology, the trend of miniaturization and thinning of electronic products has gradually emerged, and flexible displays have developed rapidly under this trend. However, OLED (Organic Light-Emitting Diode) devices in flexible displays are easily corroded by water and oxygen, resulting in a shortened device lifespan. The encapsulation layer can isolate water and oxygen and protect the OLED devices.

[0004] In existing flexible displays, the bottom flexible substrate has a large elastic modulus and is relatively thick. When bending occurs, the neutral layer (which can be understood as a film layer with zero or near-zero stress) is located on the flexible substrate, while the top encapsulation layer (inorganic layer) experiences the greatest stress and strain, making it prone to damage. This allows water and oxygen to penetrate, causing OLED aging and failure, and reducing the lifespan of the flexible display. The OLED device layer located above the flexible substrate also experiences stress, which may damage the OLED device. Summary of the Invention

[0005] The present invention provides a display panel, its manufacturing method, and a display device to improve the service life of the display panel.

[0006] This invention provides a display panel including a display area having a bent area and a non-bent area. The display panel includes: a flexible substrate, a display device located on one side of the flexible substrate, a light extraction layer located on the side of the display device away from the flexible substrate, a thin film encapsulation layer located on the side of the light extraction layer away from the flexible substrate, and a transition layer located between the light extraction layer and the thin film encapsulation layer. The thin-film encapsulation layer includes a first inorganic layer, the light extraction layer is made of an organic material, and the transition layer includes an organic-inorganic chain for connecting the first inorganic layer and the light extraction layer.

[0007] Optionally, in the display panel provided in the embodiments of the present invention, the organic elements in the organic-inorganic chain originate from the light extraction layer, and the inorganic elements in the organic-inorganic chain originate from the first inorganic layer.

[0008] Optionally, in the display panel provided in the embodiments of the present invention, the thin-film encapsulation layer further includes an organic layer located on the side of the first inorganic layer facing away from the flexible substrate; wherein, The thickness of the organic layer in the bending region is less than the thickness in the non-bending region.

[0009] Optionally, in the display panel provided in the embodiments of the present invention, the non-bending area includes: a transition area connected to the bending area, and a first area located on the side of the transition area away from the bending area; the thickness of the organic layer in the transition area is less than the thickness in the first area.

[0010] Optionally, in the display panel provided in the embodiments of the present invention, the thickness of the organic layer gradually decreases in the transition region along the non-bending region toward the bending region, and the thickness of the organic layer in the bending region is the same as the minimum thickness in the transition region.

[0011] Optionally, the display panel provided in the embodiments of the present invention further includes a leveling layer located on the side of the thin film encapsulation layer opposite to the flexible substrate, wherein the thickness of the leveling layer in the bending region is greater than the thickness in the non-bending region.

[0012] Optionally, the display panel provided in the embodiments of the present invention further includes: a polarizer located on the side of the leveling layer opposite to the flexible substrate, and a touch structure located on the side of the polarizer opposite to the flexible substrate.

[0013] Optionally, the display panel provided in the embodiments of the present invention further includes: a touch structure located on the side of the leveling layer away from the flexible substrate, a first insulating layer located on the side of the touch structure away from the flexible substrate, and a filter layer located on the side of the first insulating layer away from the flexible substrate. The touch structure includes a first metal layer, a second insulating layer, and a second metal layer stacked sequentially, wherein the first insulating layer and the second insulating layer are made of organic materials.

[0014] Optionally, the display panel provided in the embodiments of the present invention further includes: a touch structure located between the leveling layer and the thin film encapsulation layer, a first insulating layer located between the touch structure and the thin film encapsulation layer, and a filter layer located on the side of the leveling layer away from the flexible substrate; The touch structure includes a first metal layer, a second insulating layer, and a second metal layer stacked sequentially, wherein the first insulating layer and the second insulating layer are made of inorganic materials.

[0015] Optionally, in the display panel provided in the embodiments of the present invention, the filter layer is a polarizer or a color filter.

[0016] Accordingly, embodiments of the present invention also provide a display device, including the display panel described above in embodiments of the present invention.

[0017] Accordingly, embodiments of the present invention also provide a method for manufacturing the above-mentioned display panel, comprising: A flexible substrate is provided, the flexible substrate including a display area, the display area having a bent area and a non-bent area; A display device is formed on one side of the flexible substrate; A light extraction layer is formed on the side of the display device away from the flexible substrate; the material of the light extraction layer is an organic material. A transition layer is formed on the side of the light extraction layer opposite to the flexible substrate; A thin-film encapsulation layer is formed on the side of the transition layer opposite to the flexible substrate; the thin-film encapsulation layer includes a first inorganic layer; wherein... The transition layer includes an organic-inorganic chain for connecting the first inorganic layer and the light extraction layer.

[0018] Optionally, in the fabrication method provided in the embodiments of the present invention, forming the light extraction layer, the transition layer, and the first inorganic layer specifically includes: The light extraction layer is formed on the side of the display device away from the flexible substrate, and the structure with the light extraction layer is placed in the vapor deposition chamber; SiH4, O2, and N2 are introduced into the vapor deposition chamber to perform the first plasma process. SiH4, O2, and N2 are introduced into the vapor deposition chamber to perform a second plasma process, forming the first inorganic layer and a transition layer connecting the first inorganic layer and the light extraction layer. The organic elements in the organic-inorganic chain originate from the light extraction layer, and the inorganic elements in the organic-inorganic chain originate from the first inorganic layer. The gas content of the first plasma process is less than that of the second plasma process.

[0019] Optionally, in the above manufacturing method provided in the embodiments of the present invention, an organic layer for forming the thin film encapsulation layer is further included, wherein the organic layer is located on the side of the first inorganic layer away from the flexible substrate; The formation of the organic layer specifically includes: A first organic material film layer is printed in the first region of the non-bending area using a first inkjet printing process, and then flows to the transition region; The first organic material film layer is cured; A second organic material film layer is printed in the bending area using a second inkjet printing process. The second organic material film is cured, and the cured first organic material film and the cured second organic material film constitute the organic layer; wherein, the thickness of the cured first organic material film in the non-bending region is greater than the thickness of the cured second organic material film.

[0020] Optionally, in the above manufacturing method provided in the embodiments of the present invention, an organic layer for forming the thin film encapsulation layer is further included, wherein the organic layer is located on the side of the first inorganic layer away from the flexible substrate; The formation of the organic layer specifically includes: An organic material film layer is printed in the non-bending area using a single inkjet printing process and then flows to the bending area. The organic material film is cured; wherein the thickness of the cured organic material film in the non-bending region is greater than its thickness in the bending region.

[0021] The beneficial effects of the embodiments of the present invention are as follows: The display panel, its manufacturing method, and display device provided in this invention provide a transition layer between the light extraction layer and the thin film encapsulation layer. The transition layer enables a tight connection between the first inorganic layer and the light extraction layer. When the display panel is bent, the risk of peeling between the first inorganic layer and the light extraction layer can be reduced, thereby improving the flexible bending performance of the display panel. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 A plan view of a display panel provided in an embodiment of the present invention; Figure 2 for Figure 1 A schematic diagram of a cross-section along the CC' direction; Figure 3 for Figure 1 Another cross-sectional diagram along the CC' direction; Figure 4 for Figure 1 Another cross-sectional diagram along the CC' direction; Figure 5This is a simulated stress cloud diagram of a display panel during bending in related technologies; Figure 6 This is a simulated stress cloud diagram of a display panel under bending conditions provided in an embodiment of the present invention. Figure 7 for Figures 2-4 A schematic diagram of the cross-section of the organic layer in the structure shown; Figure 8 for Figures 2-4 Another cross-sectional schematic diagram of the organic layer in the structure shown; Figure 9 A plan view of another display panel provided in an embodiment of the present invention; Figure 10 for Figure 9 A schematic diagram of a cross-section along the DD' direction; Figure 11 This is a schematic diagram of the specific structure of the light extraction layer, the transition layer, and the first inorganic layer; Figure 12 for Figure 2 and Figure 10 The diagram shows the combination of the structures shown. Figure 13 for Figure 3 and Figure 10 The diagram shows the combination of the structures shown. Figure 14 for Figure 4 and Figure 10 The diagram shows the combination of the structures shown. Figure 15 for Figure 10 and Figure 2 The diagram shows the combination of the structures shown. Figure 16 for Figure 10 and Figure 3 The diagram shows the combination of the structures shown. Figure 17 for Figure 10 and Figure 4 The diagram shows the combination of the structures. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Furthermore, the embodiments and features in the embodiments of the present invention can be combined with each other without conflict. Based on the described embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning understood by one of ordinary skill in the art to which this invention pertains. The terms "first," "second," and similar terms used in this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as "comprising" or "including" mean that the element or object preceding the word encompasses the elements or objects listed following the word and their equivalents, without excluding other elements or objects. Terms such as "connected" or "linked" are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect.

[0026] It should be noted that the dimensions and shapes of the figures in the accompanying drawings do not reflect actual proportions and are intended only to illustrate the content of the invention. Furthermore, the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout.

[0027] This invention provides a display panel, such as... Figures 1-4 As shown, Figure 1 This is a plan view of the display panel. Figures 2-4 for Figure 1 Several cross-sectional schematic diagrams along the CC' direction are shown. The display panel includes a display area AA, which has a bent area A1 and a non-bent area A2. The display panel includes: a flexible substrate 1, a display device 2 located on one side of the flexible substrate 1, and a thin-film encapsulation layer 3 located on the side of the display device 2 facing away from the flexible substrate 1; the thin-film encapsulation layer 3 includes an organic layer 31; wherein, The thickness H1 of the organic layer 31 in the bending region A1 is less than the thickness H2 in the non-bending region A2.

[0028] The display panel provided in this embodiment of the invention reduces the distance between the inorganic layer (second inorganic layer) furthest from the display device and the neutral layer by setting the thickness of the organic layer 31 in the bending region to be less than that in the non-bending region. This places the second inorganic layer near the neutral layer, thereby reducing the stress on the second inorganic layer when the display panel is bent, preventing damage to the second inorganic layer due to excessive force, and also preventing water and oxygen from entering the display panel due to damage to the second inorganic layer, thus improving the service life of the display panel.

[0029] It should be noted that the display panel in the embodiments of the present invention can be a foldable screen, a display screen with curved edges, etc. Figures 1-4 This example uses a display screen with a curved display panel on all four sides.

[0030] Specifically, such as Figures 2-4 As shown, H1 can be 3~8μm, and H2 is about 10μm.

[0031] Specifically, such as Figures 2-4 As shown, the flexible substrate 1 can be made of materials such as PI (polyimide film) and PET (polyethylene terephthalate). The display device 2 (OLED) includes an anode layer, an organic light-emitting layer, and a cathode layer. When an appropriate voltage is applied to the cathode layer and the anode layer, positive electrode holes and cathode charges combine in the organic light-emitting layer to produce light. Depending on the formulation, it produces the three primary colors of red, green, and blue (RGB), which constitute the basic colors.

[0032] Specifically, such as Figures 2-4 As shown, the thin-film encapsulation layer 3 is used to encapsulate the display device 2 to block water and oxygen, preventing water and oxygen from entering the display device 2 and causing OLED device failure. The thin-film encapsulation layer 3 also includes a first inorganic layer 32 located between the organic layer 31 and the display device 2, and a second inorganic layer 33 located on the side of the organic layer 31 facing away from the flexible substrate 1. Of course, the thin-film encapsulation layer 3 can also include more inorganic and organic layers, but generally, the topmost layer of the thin-film encapsulation layer 3 must be an inorganic layer to effectively block water and oxygen. In this embodiment, the material of the first inorganic layer 32 can be SiO (silicon oxide), SiN (silicon nitride), or SiON (silicon oxynitride), and the material of the second inorganic layer 33 can be SiO (silicon oxide), SiN (silicon nitride), or SiON (silicon oxynitride). The organic layer can be an organic material suitable for inkjet printing.

[0033] In specific implementation, in the display panel provided in the embodiments of the present invention, such as Figures 2-4 As shown, the non-bending region A2 includes a transition region A21 connected to the bending region A1, and a first region A22 located on the side of the transition region A21 away from the bending region A1; the thickness of the organic layer 31 in the transition region A21 is less than the thickness in the first region A22. By providing the transition region A21 connected to the bending region A1 in the non-bending region A2, the stress on the second inorganic layer 33 near the bending region A1 in the non-bending region A2 can be buffered during bending, further preventing excessive stress on the second inorganic layer 33 during bending. Specifically, the width of the transition region A21 can be 1 / 3 of the width of the bending region A1.

[0034] In specific implementation, in the display panel provided in the embodiments of the present invention, such as Figures 2-4 As shown, from the non-bending region A2 to the bending region A1, the thickness H3 of the organic layer 31 in the transition region A21 gradually decreases, and the thickness H1 of the organic layer 31 in the bending region A1 is the same as the minimum thickness H3 in the transition region A21.

[0035] In specific implementation, in the display panel provided in the embodiments of the present invention, such as Figures 2-4As shown, the thin-film encapsulation layer 3 also includes a leveling layer 4 located on the side opposite to the flexible substrate 1. The thickness H4 of the leveling layer 4 in the bending region A1 is greater than the thickness H5 in the non-bending region A2. By thinning the organic layer 31 in the bending region A1 and thickening the leveling layer 4 in the bending region A1, the distance between the second inorganic layer 33 and the neutral layer can be effectively reduced, thereby reducing the stress on the second inorganic layer 33 during bending and reducing the risk of breakage of the thin-film encapsulation layer 3. Specifically, the thickness H4 of the leveling layer 4 in the bending region A1 is 0~10 μm thicker than the thickness H5 in the non-bending region A2.

[0036] In specific implementation, in the display panel provided in the embodiments of the present invention, such as Figure 2 As shown, it also includes: a polarizer 5 located on the side of the leveling layer 4 facing away from the flexible substrate 1; a touch screen panel (TSP) 6 located on the side of the polarizer 5 facing away from the flexible substrate 1; a protective cover 7 located on the side of the touch screen panel 6 facing away from the flexible substrate 1; and a back film 8 located on the side of the flexible substrate 1 facing away from the touch screen panel 6. The polarizer 5 and the touch screen panel 6 are external film layers. The polarizer 5 can be bonded to the leveling layer 4 by a lamination adhesive, the touch screen panel 6 can be bonded to the polarizer 5 by an optical adhesive, the protective cover 7 can be bonded to the touch screen panel 6 by an optical adhesive, and the back film 8 can be bonded to the flexible substrate 1 by a lamination adhesive.

[0037] In specific implementation, in the display panel provided in the embodiments of the present invention, such as Figure 3 As shown, it also includes: a touch structure 6 (FMLOC, Flexible Multi-Layer On Cell) located on the side of the leveling layer 4 away from the flexible substrate 1, a first insulating layer 9 located on the side of the touch structure 6 away from the flexible substrate 1, and a filter layer 10 located on the side of the first insulating layer 9 away from the flexible substrate 1. The touch structure 6 includes a first metal layer 61, a second insulating layer 62, and a second metal layer 63 stacked sequentially. The first insulating layer 61 and the second insulating layer 62 are made of organic materials. Figure 3 Because the touch structure 6 is fabricated directly on the leveling layer 4, i.e., the touch structure 6 is embedded within the film layer structure, there is no need for a separate touch panel, which helps to reduce the thickness of the display panel; and Figure 3 The first insulating layer 9 and the second insulating layer 62 in the display panel shown are made of organic materials, which can further improve bending performance.

[0038] In specific implementation, in the display panel provided in the embodiments of the present invention, such as Figure 4As shown, it also includes: a touch structure 6 (FMLOC, Flexible Multi-Layer On Cell) located between the leveling layer 4 and the thin film encapsulation layer 3, a first insulating layer 9 located between the touch structure 6 and the thin film encapsulation layer 3, and a filter layer 10 located on the side of the leveling layer 4 away from the flexible substrate 1. The touch structure 6 includes a first metal layer 61, a second insulating layer 62 and a second metal layer 63 stacked in sequence. The materials of the first insulating layer 61 and the second insulating layer 62 are inorganic materials. Figure 4 Since the touch structure 6 is fabricated directly on the thin film encapsulation layer 3, that is, the touch structure 6 is set in the film layer structure, there is no need to set up a separate touch panel, which helps to reduce the thickness of the display panel.

[0039] In specific implementation, in the display panel provided in the embodiments of the present invention, such as Figure 3 and Figure 4 As shown, it also includes: a protective cover plate 7 located on the side of the filter layer 10 away from the flexible substrate 1, and a back film 8 located on the side of the flexible substrate 1 away from the touch structure 6.

[0040] In specific implementation, in the display panel provided in the embodiments of the present invention, such as Figure 3 and Figure 4 As shown, the filter layer 10 can be a polarizer or a color filter. Specifically, polarizers are generally thicker. When the filter layer 10 uses a color filter, a color filter on encapsulation (COE) technology is used, which can reduce the thickness of the display panel.

[0041] like Figure 5 and Figure 6 As shown, Figure 5 When the thickness of the organic layer 31 in the bending region A1 and the non-bending region A2 is the same in the related technology (the thickness of the organic layer 31 in the bending region A1 and the non-bending region A2 is 12μm, and the thickness of the leveling layer 4 in the bending region A1 and the non-bending region A2 is 3μm), the bending simulation of the display panel is performed, and the maximum strain of the thin film encapsulation layer 3 is 14.1‰. Figure 6 To employ the present invention Figures 2-4 When the thickness of the organic layer 31 in the bending region A1 is less than the thickness of the organic layer 31 in the non-bending region A2 (the thickness of the leveling layer 4 in the bending region A1 is 7 μm, and the thickness of the organic layer 31 in the bending region A1 is 8 μm), the maximum strain of the thin film encapsulation layer 3 is 11.3‰ when the bending simulation of the display panel is performed; this is 19.8% lower than the simulation results in related technologies.

[0042] pass Figure 5 and Figure 6The simulation results show that by reducing the thickness of the organic layer 31 in the bending region A1, the maximum strain of the thin-film encapsulation layer 3 during bending can be reduced. Therefore, by setting the thickness of the organic layer in the bending region to be less than its thickness in the non-bending region, this embodiment of the invention can reduce the distance between the inorganic layer (the second inorganic layer) furthest from the display device and the neutral layer, placing the second inorganic layer near the neutral layer. This reduces the stress on the second inorganic layer during bending of the display panel, preventing damage to the second inorganic layer due to excessive stress and avoiding water and oxygen ingress that could cause display panel failure, thereby improving the lifespan of the display panel.

[0043] In specific implementation, such as Figure 2 As shown, Figure 2 The structure of the display panel shown can be prepared by the following method, with the steps as follows: (1) A driving circuit (TFT structure) is fabricated on a flexible substrate 1, and a display device 2 is fabricated on the driving circuit. The display device 2 includes an anode layer, an organic light-emitting layer and a cathode layer. (2) A first inorganic layer 32 is deposited on the display device 2 by means of processes such as vacuum deposition, vapor deposition or spin coating; (3) An organic layer 31 is printed on the first inorganic layer 32 by inkjet printing, such that the thickness of the organic layer 31 in the bending region A1 is less than the thickness in the non-bending region A2. (4) A second inorganic layer 33 is deposited on the organic layer 31, and the first inorganic layer 32, the organic layer 31 and the second inorganic layer 33 constitute the thin film encapsulation layer 3; (5) A leveling layer 4 is coated on the thin film encapsulation layer 3 so that the overall film thickness of the bending area A1 and the non-bending area A2 is consistent, and the thickness of the leveling layer 4 in the bending area A1 is 0~10μm greater than the thickness of the leveling layer 4 in the non-bending area A2. (6) A back film 8 is attached to the side of the flexible substrate 1 away from the thin film encapsulation layer 3, a polarizer 5 is attached to the leveling layer 4, a touch structure 6 is attached to the polarizer 5, and an adhesive layer is bonded to the touch structure 6. (7) The protective cover 7 is attached to the touch structure 6 through the adhesive layer on the touch structure 6, thus forming Figure 2 The display panel shown.

[0044] In specific implementation, such as Figure 3 As shown, Figure 3 The structure of the display panel shown can be prepared by the following method, with the steps as follows: (1) A driving circuit (TFT structure) is fabricated on a flexible substrate 1, and a display device 2 is fabricated on the driving circuit. The display device 2 includes an anode layer, an organic light-emitting layer and a cathode layer. (2) A first inorganic layer 32 is deposited on the display device 2 by means of processes such as vacuum deposition, vapor deposition or spin coating; (3) An organic layer 31 is printed on the first inorganic layer 32 by inkjet printing, such that the thickness of the organic layer 31 in the bending region A1 is less than the thickness in the non-bending region A2. (4) A second inorganic layer 33 is deposited on the organic layer 31, and the first inorganic layer 32, the organic layer 31 and the second inorganic layer 33 constitute the thin film encapsulation layer 3; (5) A leveling layer 4 is coated on the thin film encapsulation layer 3 so that the overall film thickness of the bending area A1 and the non-bending area A2 is consistent, and the thickness of the leveling layer 4 in the bending area A1 is 0~10μm greater than the thickness of the leveling layer 4 in the non-bending area A2. (6) A first metal layer 61, a second insulating layer 62 and a second metal layer 63 are sequentially stacked on the leveling layer 4. The first metal layer 61, the second insulating layer 62 and the second metal layer 63 constitute the touch structure 6; the material of the second insulating layer 62 is an organic material. (7) A first insulating layer 9 is fabricated on the touch structure 6; the material of the first insulating layer 9 is an organic material; (8) A filter layer 10 (e.g., a polarizer) is attached to the first insulating layer 9, a back film 8 is attached to the side of the flexible substrate 1 away from the thin film encapsulation layer 3, and an adhesive layer is bonded to the filter layer 10. (9) The protective cover 7 is attached to the filter layer 10 through the adhesive layer on the filter layer 10, thus forming Figure 3 The display panel shown.

[0045] In specific implementation, such as Figure 4 As shown, Figure 4 The structure of the display panel shown can be prepared by the following method, with the steps as follows: (1) A driving circuit (TFT structure) is fabricated on a flexible substrate 1, and a display device 2 is fabricated on the driving circuit. The display device 2 includes an anode layer, an organic light-emitting layer and a cathode layer. (2) A first inorganic layer 32 is deposited on the display device 2 by means of processes such as vacuum deposition, vapor deposition or spin coating; (3) An organic layer 31 is printed on the first inorganic layer 32 by inkjet printing, such that the thickness of the organic layer 31 in the bending region A1 is less than the thickness in the non-bending region A2. (4) A second inorganic layer 33 is deposited on the organic layer 31, and the first inorganic layer 32, the organic layer 31 and the second inorganic layer 33 constitute the thin film encapsulation layer 3; (5) A first insulating layer 9 is fabricated on the thin film encapsulation layer 3; the material of the first insulating layer 9 is an organic material; (6) A first metal layer 61, a second insulating layer 62 and a second metal layer 63 are sequentially stacked on the first insulating layer 9. The first metal layer 61, the second insulating layer 62 and the second metal layer 63 constitute the touch structure 6; the material of the second insulating layer 62 is an organic material. (7) A leveling layer 4 is coated on the touch structure 6 so that the overall film thickness of the bent area A1 and the non-bent area A2 is consistent, and the thickness of the leveling layer 4 in the bent area A1 is 0~10μm greater than the thickness of the leveling layer 4 in the non-bent area A2. (8) Attach a filter layer 10 (e.g., a polarizer) to the leveling layer 4, attach a back film 8 to the side of the flexible substrate 1 away from the thin film encapsulation layer 3, and attach an adhesive layer to the filter layer 10. (9) The protective cover 7 is attached to the filter layer 10 through the adhesive layer on the filter layer 10, thus forming Figure 3 The display panel shown.

[0046] Specifically, the above-mentioned production Figures 2-4 When fabricating the organic layer 31 in the structure of the display panel shown, the following steps may be included: (1') A first organic material film layer 311 is printed on the first inorganic layer 32 in the first region A22 of the non-bending region A2 using the first inkjet printing process, and flows to the transition region A21; (2') Curing the first organic material film layer 311; (3') A second organic material film layer 312 is printed in the bending area A1 using a second inkjet printing process; (4') The second organic material film layer 312 is cured, and the cured first organic material film layer 311 and the cured second organic material film layer 312 constitute the organic layer 31; wherein, the thickness of the cured first organic material film layer 311 in the non-bending region A2 is greater than the thickness of the cured second organic material film layer 312; as Figure 7 As shown, Figure 7 The structure of the organic layer 31 obtained by using the above steps (1') to (4') is as follows: Figure 7 The left side is a plan view of the first inorganic layer 32 and the organic layer 31. Figure 7 The right side shows an enlarged cross-section diagram within the dashed box on the left.

[0047] Specifically, the above-mentioned production Figures 2-4 When fabricating the organic layer 31 in the structure of the display panel shown, the following steps may also be included: (1'') An organic material film is printed on the first inorganic layer 32 in a single inkjet printing process in the non-bending area A2 and flows to the bending area A1; (2'') The organic material film is cured; wherein the thickness of the cured organic material film in the non-bending region A2 is greater than its thickness in the bending region A1; such as Figure 8 As shown, Figure 8 The structure of the organic layer 31 obtained by using the above steps (1'') and (2'') is as follows: Figure 8 The left side is a plan view of the first inorganic layer 32 and the organic layer 31. Figure 8 The right side shows an enlarged cross-section diagram within the dashed box on the left.

[0048] In summary, the embodiments of the present invention are adopted. Figures 2-4 The proposed solution addresses the issue that the second inorganic layer, located furthest from the flexible substrate in the thin-film encapsulation layer, experiences the greatest stress and strain, making it prone to breakage. This leads to the intrusion of water and oxygen, causing OLED aging and failure, and reducing the lifespan of the flexible display.

[0049] As mobile phone screens become increasingly diversified, foldable screens, as a new screen form, are attracting major manufacturers. Currently, a major bottleneck for foldable screens is their bending resistance. Improving performance after exceeding a certain limit in the number of bends or the radius of curvature will be a key focus at present.

[0050] Currently, most flexible OLED structures consist of an OLED device and a thin-film encapsulation layer (TFE). A light extraction layer (CPL) is also placed between the OLED device and the TFE. The CPL is made of organic materials, and the typical TFE structure is formed by a first inorganic layer, an organic layer, and a second inorganic layer. After repeated bending, the first inorganic layer in contact with the CPL is prone to peeling, causing product failure and affecting its lifespan.

[0051] To address the issue of peeling easily between the CPL and the first inorganic layer, this invention provides a display panel, such as... Figure 9 and Figure 10 As shown, Figure 9 This is a plan view of the display panel. Figure 10 for Figure 9 A cross-sectional schematic diagram along the DD' direction shows that the display panel includes a display area AA, which has a bent area A1 and a non-bent area A2. Figure 9 (Only the bending area A1 is shown) The display panel includes: a flexible substrate 1, a display device 2 located on one side of the flexible substrate 1, a light extraction layer 11 located on the side of the display device 2 away from the flexible substrate 1, a thin film encapsulation layer 3 located on the side of the light extraction layer 11 away from the flexible substrate 1, and a transition layer 12 located between the light extraction layer 11 and the thin film encapsulation layer 3. The thin-film encapsulation layer 3 includes a first inorganic layer 32, the light extraction layer 11 is made of an organic material, and the transition layer 12 includes an organic-inorganic chain for connecting the first inorganic layer 32 and the light extraction layer 11.

[0052] The display panel provided in this embodiment of the invention provides a transition layer 12 between the light extraction layer 11 and the thin film encapsulation layer 3. The transition layer 12 can make the first inorganic layer 32 and the light extraction layer 11 tightly connected. When the display panel is bent, the risk of peeling between the first inorganic layer 32 and the light extraction layer 11 can be reduced, thereby improving the flexible bending performance of the display panel.

[0053] It should be noted that the embodiments of the present invention Figure 10 The structure shown is Figures 2-4 The structures shown have identical membrane layers with the same structure and function; see also [link to relevant documentation]. Figures 2-4 The structural description shown is omitted here.

[0054] In practical implementation, the chemical formula of the organic material used to fabricate the light extraction layer 11 can be C x H y O z N w …The first inorganic layer 32 is generally fabricated using plasma-enhanced chemical vapor deposition (CVD). The raw materials for the plasma include silane (SiH4), N2, and O2. In this embodiment of the invention, after the photoextraction layer 11 is deposited, the structure with the photoextraction layer 11 is placed in a CVD chamber. SiH4 gas, N2, and O2 are introduced into the CVD chamber to perform the first plasma process. By adjusting the power and gas flow rate in the CVD chamber, the organic material of the photoextraction layer 11 loses some hydrogen atoms using a plasma process. The chemical bonds that have lost hydrogen atoms react with the SiH4 in the CVD chamber via plasma to form silicon-containing organic compounds. The chemical reaction process is simplified as follows: Then, C-Si chains are connected to the light extraction layer 11, such as... Figure 11As shown in step A; next, the first inorganic layer 32 is fabricated using plasma-enhanced chemical vapor deposition (CVD). Specifically, SiH4, O2, and N2 are introduced into the vapor deposition chamber for a second plasma process. By adjusting the power and gas flow rate within the CVD chamber, where the gas content of the first plasma process is lower than that of the second, the plasma process causes the silicon-containing organic material formed in the second plasma process to lose some hydrogen (H). The chemical bonds resulting from the loss of H react with the SiH4 in the CVD chamber used to fabricate the first inorganic layer 32 via plasma, as shown... Figure 11 As shown in step B, an organic-inorganic chain (i.e., transition layer 12) is formed connecting the first inorganic layer 32 and the light extraction layer 11. The organic elements in the organic-inorganic chain originate from the light extraction layer 11, and the inorganic elements in the organic-inorganic chain originate from the first inorganic layer 32.

[0055] It should be noted that, Figure 11 This is merely an illustrative description of how the light extraction layer 11 and the first inorganic layer 32 are connected by an organic-inorganic chain included in the transition layer 12. The specific organic-inorganic chain structure depends on the actual chemical reaction that occurs.

[0056] In summary, this embodiment of the invention controls the plasma gas atmosphere in the CVD chamber before fabricating the first inorganic layer 32, causing a chemical reaction between the surface of the light extraction layer and SiH4 to form an organic-inorganic silicon transition film. The chemical bonds formed simultaneously connect the light extraction layer and the inorganic silicon. This solves the problem of peeling of the first inorganic layer, which is in contact with the light extraction layer, leading to product failure, and thus improves the product's lifespan.

[0057] Specifically, the above Figures 2-4 The structure shown is designed to address the problem that the second inorganic layer 33 experiences excessive strain stress during bending, which can damage the thin-film encapsulation layer 3 and affect the lifespan of the display panel. Figure 10 The structure shown is designed to address the problem of peeling easily between the light extraction layer 11 and the first inorganic layer 32. Figures 2-4 Although the structure shown solves the problem that the second inorganic layer 33 is subjected to large strain stress during bending, which leads to damage to the thin film encapsulation layer 3 and affects the life of the display panel, the problem of peeling between the light extraction layer and the second inorganic layer 33 is also prone to occur during bending. Figure 10 Although the structure shown solves the problem of peeling easily between the light extraction layer 11 and the first inorganic layer 32, Figure 10 The second inorganic layer 33 of the structure shown has the same thickness in the bending region A1 and the non-bending region A2, therefore Figure 10There is also a problem that the second inorganic layer 33 experiences high strain stress during bending, leading to damage to the thin-film encapsulation layer 3 and affecting the lifespan of the display panel. Therefore, in order to simultaneously solve the problem of the second inorganic layer 33 experiencing high strain stress during bending, leading to damage to the thin-film encapsulation layer 3 and affecting the lifespan of the display panel, and the problem of easy peeling between the light extraction layer 11 and the first inorganic layer 32, it is possible to... Figures 2-4 Based on the structure shown, Figure 10 The scheme shown, and in Figure 10 Based on the structure shown, Figures 2-4 plan.

[0058] Specifically, such as Figures 12-14 As shown, Figures 12-14 In order to be in Figures 2-4 Based on the structure shown, a transition layer 12 is provided between the light extraction layer 11 and the first inorganic layer 32. The display panel also includes: a light extraction layer 11 located between the first inorganic layer 32 and the display device 2, and a transition layer 12 located between the light extraction layer 11 and the first inorganic layer 32; the material of the light extraction layer 11 is an organic material, and the transition layer 12 includes an organic-inorganic chain for connecting the first inorganic layer 32 and the light extraction layer 11.

[0059] In specific implementation, in the display panel provided in the embodiments of the present invention, such as Figures 12-14 As shown, the organic elements in the organic-inorganic chain originate from the light extraction layer 11, and the inorganic elements in the organic-inorganic chain originate from the first inorganic layer 32.

[0060] It should be noted that, Figures 12-14 The specific structure of the intermediate transition layer 12 and the specific connection structure between the light extraction layer 11, the transition layer 12 and the first inorganic layer 32 are described above. Figure 11 The structural description shown is redundant and will not be repeated here.

[0061] Specifically, such as Figures 15-17 As shown, Figures 15-17 They are respectively in Figure 10 Based on the structure shown, the thickness of the organic layer 31 in the bending region A1 is set to be less than its thickness in the non-bending region A2. The thin film encapsulation layer 3 also includes an organic layer 31 located on the side of the first inorganic layer 32 facing away from the flexible substrate 1, and a second inorganic layer 33 located on the side of the organic layer 31 facing away from the flexible substrate 1; wherein, The thickness H1 of the organic layer 31 in the bending region A1 is less than the thickness H2 in the non-bending region A2.

[0062] It should be noted that, with Figure 15 Other membrane structures and Figure 2 The structures shown are identical, meaning that the touch structure 6 is an external type. Figure 15 The setting method of organic layer 31 in the middle and Figure 2 The organic layer 31 shown is configured in the same way, regarding the organic layer 31 and Figure 15 Other membrane layers also included are described above. Figure 2 The structural description shown is redundant and will not be repeated here.

[0063] It should be noted that, with Figure 16 Other membrane structures and Figure 3 The structures shown are the same, that is, the touch structure 6 uses a TSP (Touch Screen Panel). Figure 16 The setting method of organic layer 31 in the middle and Figure 3 The organic layer 31 shown is configured in the same way, regarding the organic layer 31 and Figure 16 Other membrane layers also included are described above. Figure 3 The structural description shown is redundant and will not be repeated here.

[0064] It should be noted that, with Figure 17 Other membrane structures and Figure 4 The structure shown is the same, that is, the touch structure 6 also uses TSP (Touch Screen Panel). Figure 17 The setting method of organic layer 31 in the middle and Figure 4 The organic layer 31 shown is configured in the same way, regarding the organic layer 31 and Figure 17 Other membrane layers also included are described above. Figure 4 The structural description shown is redundant and will not be repeated here.

[0065] Based on the same inventive concept, embodiments of the present invention also provide a method for manufacturing Figures 2-4 The method for manufacturing the display panel shown includes: A flexible substrate is provided, the flexible substrate including a display area, the display area having a bent area and a non-bent area; A display device is formed on one side of a flexible substrate; A thin-film encapsulation layer is formed on the side of the display device away from the flexible substrate; the thin-film encapsulation layer includes an organic layer; wherein... The thickness of the organic layer in the bending region is less than that in the non-bending region.

[0066] In specific implementation, the above-described fabrication process provided in the embodiments of the present invention... Figures 2-4 The method for manufacturing the display panel shown includes, in part, the fabrication of an organic layer, which may specifically include: The first organic material film layer is printed in the first region of the non-bending area using the first inkjet printing process, and then flows to the transition region; The first organic material film layer is cured; A second organic material film layer is printed in the bending area using a second inkjet printing process. The second organic material film is cured, and the cured first organic material film and the cured second organic material film constitute an organic layer; wherein, the thickness of the cured first organic material film in the non-bending region is greater than the thickness of the cured second organic material film.

[0067] Specifically, the above-mentioned production method Figures 2-4 The specific manufacturing method of the organic layer in the display panel shown can be found in the description of the manufacturing method of the organic layer in the aforementioned display panel, and the repeated parts will not be repeated here.

[0068] In specific implementation, the above-described fabrication process provided in the embodiments of the present invention... Figures 2-4 The method for manufacturing the display panel shown may further include, in part, the fabrication of the organic layer: An organic material film layer is printed in the non-bending area using a single inkjet printing process and then flows to the bending area. The organic material film is cured; wherein the thickness of the cured organic material film in the non-bending region is greater than its thickness in the bending region.

[0069] Specifically, the other production method mentioned above Figures 2-4 The specific manufacturing method of the organic layer in the display panel shown can be found in the description of the manufacturing method of the organic layer in the aforementioned display panel, and the repeated parts will not be repeated here.

[0070] In specific implementation, the above-described fabrication process provided in the embodiments of the present invention... Figures 2-4 In the method for manufacturing the display panel shown, after forming the display device and before forming the organic layer of the thin film encapsulation layer, it may further include: forming a light extraction layer, a transition layer and a first inorganic layer that are stacked sequentially; The process involves forming a light extraction layer, a transition layer, and a first inorganic layer, specifically including: A light extraction layer is formed on the side of the display device away from the flexible substrate, and the structure with the light extraction layer is placed in a vapor deposition chamber; the material of the light extraction layer is an organic material. SiH4, O2, and N2 are introduced into the vapor deposition chamber to perform the first plasma process. SiH4, O2, and N2 are introduced into the vapor deposition chamber to perform a second plasma process, forming a first inorganic layer and a transition layer connecting the first inorganic layer and the photoextraction layer. The transition layer includes an organic-inorganic chain, in which the organic elements originate from the photoextraction layer and the inorganic elements originate from the first inorganic layer. The gas content of the first plasma process is less than that of the second plasma process.

[0071] Specifically, the above-mentioned production Figures 2-4 For the specific fabrication methods of the light extraction layer, transition layer, and first inorganic layer in the display panel shown, please refer to the aforementioned display panel. Figure 10 The description of the fabrication methods of the light extraction layer, transition layer and first inorganic layer shown is repeated here.

[0072] Based on the same inventive concept, embodiments of the present invention also provide a method for manufacturing Figure 10 The method for manufacturing the display panel shown includes: A flexible substrate is provided, the flexible substrate including a display area, the display area having a bent area and a non-bent area; A display device is formed on one side of a flexible substrate; A light extraction layer is formed on the side of the display device away from the flexible substrate; the material of the light extraction layer is an organic material. A transition layer is formed on the side of the light extraction layer that is away from the flexible substrate; A thin-film encapsulation layer is formed on the side of the transition layer away from the flexible substrate; the thin-film encapsulation layer includes a first inorganic layer; wherein... The transition layer includes an organic-inorganic chain for connecting the first inorganic layer and the light extraction layer.

[0073] In specific implementation, the above-described fabrication process provided in the embodiments of the present invention... Figure 10 The method for manufacturing the display panel shown includes forming a light extraction layer, a transition layer, and a first inorganic layer, which may specifically include: A light extraction layer is formed on the side of the display device away from the flexible substrate, and the structure with the light extraction layer is placed in a vapor deposition chamber. SiH4, O2, and N2 are introduced into the vapor deposition chamber to perform the first plasma process. SiH4, O2, and N2 are introduced into the vapor deposition chamber to perform a second plasma process, forming a first inorganic layer and a transition layer connecting the first inorganic layer and the photoextraction layer. The organic elements in the organic-inorganic chain originate from the photoextraction layer, and the inorganic elements in the organic-inorganic chain originate from the first inorganic layer. The gas content of the first plasma process is less than that of the second plasma process.

[0074] Specifically, the above-mentioned production method Figure 10 The specific manufacturing methods for forming the light extraction layer, transition layer and first inorganic layer in the display panel shown can be found in the description of the manufacturing method for forming the light extraction layer, transition layer and first inorganic layer in the aforementioned display panel. The repetitions will not be repeated here.

[0075] In specific implementation, the above-described fabrication process provided in the embodiments of the present invention... Figure 10The method for manufacturing the display panel shown also includes an organic layer forming a thin film encapsulation layer, the organic layer being located on the side of the first inorganic layer away from the flexible substrate; The formation of an organic layer specifically includes: The first organic material film layer is printed in the first region of the non-bending area using the first inkjet printing process, and then flows to the transition region; The first organic material film layer is cured; A second organic material film layer is printed in the bending area using a second inkjet printing process. The second organic material film is cured, and the cured first organic material film and the cured second organic material film constitute an organic layer; wherein, the thickness of the cured first organic material film in the non-bending region is greater than the thickness of the cured second organic material film.

[0076] Specifically, the above-mentioned production method Figure 10 For the specific manufacturing method of the organic layer in the display panel shown, please refer to the aforementioned display panel. Figures 2-4 The description of the method for making the organic layer shown is repeated here, and the parts that are repeated will not be repeated.

[0077] In specific implementation, the above-described fabrication process provided in the embodiments of the present invention... Figure 10 The method for manufacturing the display panel shown also includes an organic layer forming a thin film encapsulation layer, the organic layer being located on the side of the first inorganic layer away from the flexible substrate; The formation of an organic layer specifically includes: An organic material film layer is printed in the non-bending area using a single inkjet printing process and then flows to the bending area. The organic material film is cured; wherein the thickness of the cured organic material film in the non-bending region is greater than its thickness in the bending region.

[0078] Specifically, the other production method mentioned above Figure 10 For the specific manufacturing method of the organic layer in the display panel shown, please refer to the aforementioned display panel. Figures 2-4 The description of the method for making the organic layer shown is repeated here, and the parts that are repeated will not be repeated.

[0079] Based on the same inventive concept, embodiments of the present invention also provide a display device, including any of the display panels described above. This display device can be any product or component with display functionality, such as a mobile phone, tablet computer, television, monitor, laptop computer, digital photo frame, or navigator. Implementation of this display device can refer to the embodiments of the aforementioned display panels; repeated details will not be elaborated further.

[0080] The display panel, its manufacturing method, and display device provided in this invention reduce the distance between the inorganic layer (the second inorganic layer) furthest from the display device and the neutral layer by setting the thickness of the organic layer in the bending region to be less than that in the non-bending region. This places the second inorganic layer near the neutral layer, thereby reducing the stress on the second inorganic layer when the display panel is bent, preventing damage to the second inorganic layer due to excessive stress, and also preventing water and oxygen from entering the display panel due to damage to the second inorganic layer, thus improving the service life of the display panel.

[0081] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A display panel, characterized in that, The display panel includes a display area having a bent area and a non-bent area. The display panel includes: a flexible substrate, a display device located on one side of the flexible substrate, a light extraction layer located on the side of the display device opposite to the flexible substrate, a thin film encapsulation layer located on the side of the light extraction layer opposite to the flexible substrate, and a transition layer located between the light extraction layer and the thin film encapsulation layer. The thin-film encapsulation layer includes a first inorganic layer, the light extraction layer is made of an organic material, and the transition layer includes an organic-inorganic chain for connecting the first inorganic layer and the light extraction layer.

2. The display panel according to claim 1, characterized in that, The organic elements in the organic-inorganic chain originate from the light extraction layer, and the inorganic elements in the organic-inorganic chain originate from the first inorganic layer.

3. The display panel according to claim 1 or 2, characterized in that, The thin-film encapsulation layer further includes an organic layer located on the side of the first inorganic layer facing away from the flexible substrate; wherein, The thickness of the organic layer in the bending region is less than the thickness in the non-bending region.

4. The display panel according to claim 3, characterized in that, The non-bending region includes: a transition region connected to the bending region, and a first region located on the side of the transition region away from the bending region; the thickness of the organic layer in the transition region is less than the thickness in the first region.

5. The display panel according to claim 4, characterized in that, As the organic layer moves from the non-bending region toward the bending region, its thickness gradually decreases within the transition region, and the thickness of the organic layer in the bending region is the same as its minimum thickness within the transition region.

6. The display panel according to claim 4, characterized in that, It also includes a leveling layer located on the side of the thin film encapsulation layer opposite to the flexible substrate, wherein the thickness of the leveling layer in the bending region is greater than the thickness in the non-bending region.

7. The display panel according to claim 6, characterized in that, Also includes: A polarizer located on the side of the leveling layer opposite to the flexible substrate, and a touch structure located on the side of the polarizer opposite to the flexible substrate.

8. The display panel according to claim 6, characterized in that, Also includes: A touch structure located on the side of the leveling layer opposite to the flexible substrate, a first insulating layer located on the side of the touch structure opposite to the flexible substrate, and a filter layer located on the side of the first insulating layer opposite to the flexible substrate. The touch structure includes a first metal layer, a second insulating layer, and a second metal layer stacked sequentially, wherein the first insulating layer and the second insulating layer are made of organic materials.

9. The display panel according to claim 6, characterized in that, Also includes: A touch structure located between the leveling layer and the thin film encapsulation layer, a first insulating layer located between the touch structure and the thin film encapsulation layer, and a filter layer located on the side of the leveling layer away from the flexible substrate; The touch structure includes a first metal layer, a second insulating layer, and a second metal layer stacked sequentially, wherein the first insulating layer and the second insulating layer are made of inorganic materials.

10. The display panel according to claim 8 or 9, characterized in that, The filter layer is a polarizer or a color filter.

11. A display device, characterized in that, Includes the display panel according to any one of claims 1 to 10.

12. A method for manufacturing a display panel as described in any one of claims 1-10, characterized in that, include: A flexible substrate is provided, the flexible substrate including a display area, the display area having a bent area and a non-bent area; A display device is formed on one side of the flexible substrate; A light extraction layer is formed on the side of the display device away from the flexible substrate; the material of the light extraction layer is an organic material. A transition layer is formed on the side of the light extraction layer opposite to the flexible substrate; A thin-film encapsulation layer is formed on the side of the transition layer opposite to the flexible substrate; the thin-film encapsulation layer includes a first inorganic layer; wherein... The transition layer includes an organic-inorganic chain for connecting the first inorganic layer and the light extraction layer.

13. The manufacturing method according to claim 12, characterized in that, The formation of the light extraction layer, the transition layer, and the first inorganic layer specifically includes: The light extraction layer is formed on the side of the display device away from the flexible substrate, and the structure with the light extraction layer is placed in the vapor deposition chamber; SiH4, O2, and N2 are introduced into the vapor deposition chamber to perform the first plasma process. SiH4, O2, and N2 are introduced into the vapor deposition chamber to perform a second plasma process, forming the first inorganic layer and a transition layer connecting the first inorganic layer and the light extraction layer. The organic elements in the organic-inorganic chain originate from the light extraction layer, and the inorganic elements in the organic-inorganic chain originate from the first inorganic layer. The gas content of the first plasma process is less than that of the second plasma process.

14. The manufacturing method according to claim 12 or 13, characterized in that, It also includes an organic layer forming the thin film encapsulation layer, the organic layer being located on the side of the first inorganic layer away from the flexible substrate; The formation of the organic layer specifically includes: A first organic material film layer is printed in the first region of the non-bending area using a first inkjet printing process, and then flows to the transition region; The first organic material film layer is cured; A second organic material film layer is printed in the bending area using a second inkjet printing process. The second organic material film is cured, and the cured first organic material film and the cured second organic material film constitute the organic layer; wherein, the thickness of the cured first organic material film in the non-bending region is greater than the thickness of the cured second organic material film.

15. The manufacturing method according to claim 12 or 13, characterized in that, It also includes an organic layer forming the thin film encapsulation layer, the organic layer being located on the side of the first inorganic layer away from the flexible substrate; The formation of the organic layer specifically includes: An organic material film layer is printed in the non-bending area using a single inkjet printing process and then flows to the bending area. The organic material film is cured; wherein the thickness of the cured organic material film in the non-bending region is greater than its thickness in the bending region.

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