Patent Publication Number: US-11647648-B2

Title: Electronic devices with flexible display cover layers

Description:
This application is a division of non-provisional patent application Ser. No. 16/774,948, filed Jan. 28, 2020, which claims the benefit of provisional patent application No. 62/824,168, filed Mar. 26, 2019, which are hereby incorporated by reference herein in their entireties. 
    
    
     FIELD 
     This relates generally to electronic devices, and, more particularly, to electronic devices with displays. 
     BACKGROUND 
     Electronic devices often include displays for presenting images to a user. Displays are typically formed from rigid planar substrates. Although satisfactory in many situations, rigid displays such as these may be difficult to integrate into certain devices, such as devices with bendable housings. 
     SUMMARY 
     An electronic device may have a hinge that allows the device to be flexed about a bend axis. A display may span the bend axis. To facilitate bending about the bend axis without damage, the display may include a display cover layer with a flexible portion. The flexible portion of the display cover layer may be interposed between first and second rigid portions of the display cover layer in one example. 
     During operation of an electronic device, the display cover layer for the electronic device may be scratched or dented. To improve the aesthetics of the electronic device, it may be desirable for the presence of scratches and dents to be minimized. To help mitigate the number of dents, scratches, or other imperfections in a display cover layer, the display cover layer may include a layer of self-healing material. 
     The layer of self-healing material may be formed across the entire display cover layer or may be formed only in the flexible region of the display cover layer. The display cover layer may include a layer of elastomer in the flexible region of the display cover layer for increased flexibility. The layer of self-healing material may cover the layer of elastomer in the flexible region. 
     Self-healing may occur in the layer of self-healing material without prompting (e.g., when the self-healing coating is dented, the material of the coating may fill the dent even without external intervention). Alternatively, the self-healing may be initiated or expedited by externally applied heat, light, electric current, or other type of external stimulus. 
     When heat is used as a stimulus for the self-healing process, the display cover layer may include transparent conductors that form a heating layer in the display cover layer. The heating layer may be used to generate heat to stimulate self-healing. The heating layer may be used to generate heat in response to user input, according to a predetermined schedule, or when the electronic device is charging. 
     To promote flexibility in the display cover layer, the display cover layer may include a transparent dielectric layer with slits. The slits may be filled with an index-matching material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view of an illustrative electronic device having a display in accordance with an embodiment. 
         FIG.  2    is a schematic diagram of an illustrative electronic device having a display in accordance with an embodiment. 
         FIGS.  3  and  4    are cross-sectional side views of electronic devices with flexible displays in accordance with an embodiment. 
         FIG.  5    is a diagram of an illustrative display with an array of light-emitting pixels in accordance with an embodiment. 
         FIG.  6    is a cross-sectional side view of a display showing how a display cover layer may have a flexible portion interposed between rigid portions in accordance with an embodiment. 
         FIG.  7    is a cross-sectional side view of a display that includes a display cover layer with a layer of self-healing material in accordance with an embodiment. 
         FIG.  8    is a cross-sectional side view of an illustrative display having a display cover layer formed from multiple layers of transparent dielectric material in accordance with an embodiment. 
         FIG.  9    is a cross-sectional side view of an illustrative display having a display cover layer with a layer of elastomer formed in a flexible region of the display cover layer in accordance with an embodiment. 
         FIG.  10    is a cross-sectional side view of an illustrative display in which the display cover layer includes a self-healing coating across the display cover layer and an elastomer layer in the flexible region of the display cover layer in accordance with an embodiment. 
         FIG.  11    is a cross-sectional side view of an illustrative display in which the display cover layer includes a self-healing coating across the display cover layer and a heating layer that heats the self-healing coating in accordance with an embodiment. 
         FIG.  12    is a cross-sectional side view of an illustrative display in which the display cover layer includes index-of-refraction-matching layers between layers of the display cover layer in accordance with an embodiment. 
         FIG.  13    is a cross-sectional side view of an illustrative display in which the display cover layer includes a self-healing coating across the display cover layer and a heating layer positioned adjacent to the self-healing coating in accordance with an embodiment. 
         FIG.  14    is a cross-sectional side view of an illustrative display having a display cover layer with slits in a transparent dielectric layer in a flexible region of the display cover layer in accordance with an embodiment. 
         FIG.  15    is a cross-sectional side view of an illustrative display having a display cover layer with slits in a transparent dielectric layer in a flexible region of the display cover layer and a layer of adhesive between adjacent transparent dielectric layers in accordance with an embodiment. 
         FIG.  16    is a cross-sectional side view of an illustrative display having a display cover layer with partial slits in a transparent dielectric layer in a flexible region of the display cover layer in accordance with an embodiment. 
         FIG.  17    is a cross-sectional side view of an illustrative display having a display cover layer with three transparent dielectric layers that include slits in a flexible region of the display cover layer in accordance with an embodiment. 
         FIG.  18    is a top view of an illustrative transparent dielectric layer including slits of the type shown in  FIGS.  14 - 17    in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     An illustrative electronic device of the type that may be provided with a flexible display is shown in  FIG.  1   . Electronic device  10  may be a computing device such as a laptop computer, a computer monitor containing an embedded computer, a tablet computer, a cellular telephone, a media player, or other handheld or portable electronic device, a smaller device such as a wrist-watch device, a pendant device, a wearable or miniature device of other types, a computer display that does not contain an embedded computer, a computer display that includes an embedded computer, a gaming device, a navigation device, an embedded system such as a system in which electronic equipment with a display is mounted in a kiosk or automobile, equipment that implements the functionality of two or more of these devices, or other electronic equipment. In the illustrative configuration of  FIG.  1   , device  10  is a portable device such as a cellular telephone, media player, tablet computer, watch or other wrist device, or other portable computing device. Other configurations may be used for device  10  if desired. The example of  FIG.  1    is merely illustrative. 
     In the example of  FIG.  1   , device  10  includes a display such as display  14  mounted in housing  12 . Housing  12 , which may sometimes be referred to as an enclosure or case, may be formed of plastic, glass, ceramics, fiber composites, metal (e.g., stainless steel, aluminum, etc.), other suitable materials, or a combination of any two or more of these materials. Housing  12  may be formed using a unibody configuration in which some or all of housing  12  is machined or molded as a single structure or may be formed using multiple structures (e.g., an internal frame structure, one or more structures that form exterior housing surfaces, etc.). Housing  12  may have hinge structures such as hinge  20  to allow device  10  to bend about bend axis  22 . Housing  12  may have first and second housing portions that rotate with respect to each other as device  10  is bent (folded) about bend axis  22  using hinge  20  or other flexible structures joining the first and second housing portions. 
     Display  14  may be a touch screen display that incorporates a layer of conductive capacitive touch sensor electrodes or other touch sensor components (e.g., resistive touch sensor components, acoustic touch sensor components, force-based touch sensor components, light-based touch sensor components, etc.) or may be a display that is not touch-sensitive. Capacitive touch screen electrodes may be formed from an array of indium tin oxide pads or other transparent conductive structures. A touch sensor may be formed using electrodes or other structures on a display layer that contains a pixel array or on a separate touch panel layer that is attached to the pixel array (e.g., using adhesive). 
     Display  14  may include pixels formed from liquid crystal display (LCD) components, electrophoretic pixels, microelectromechanical (MEMs) shutter pixels, electrowetting pixels, micro-light-emitting diodes (small crystalline semiconductor die), organic light-emitting diodes (e.g., pixels in a thin-film organic light-emitting diode display), or pixels based on other display technologies. Configurations in which display  14  has an array of light-emitting pixels such as an array of organic light-emitting diode pixels may sometimes be described herein as an example. 
     Display  14  may have a portion that overlaps bend axis  22 . To facilitate bending of device  10  about axis  22 , all of display  14  may be formed using flexible structures or at least the portion of display  14  that overlaps bend axis  22  may be formed using flexible structures. A display cover layer or other layer may form the outermost surface of the display. Display layers such these (e.g., display cover layers) may be formed from glass, plastic, and/or other transparent display cover layer structures and may be flexible (at least where these layers overlap bend axis  22  of device  10 ). 
     As shown in  FIG.  1   , for example, display  14  may have three portions such as portions  14 A,  14 B, and  14 C. In portions  14 A and  14 C, display  14  may be flexible or may be rigid (e.g., the pixel array in these areas may be rigid and/or the display cover layer structures in these regions may be rigid). Flexible portion  14 B overlaps bend axis  22  and forms a strip that lies between portions  14 A and  14 C and that extends across the width of the display between opposing edges of the display. To ensure that flexible portion  14 B is sufficiently flexible to allow device  10  to bend about axis  22 , display layers such as a display cover layer for display  14  may be formed from a thin flexible glass or polymer layer that accommodates bending of display  14  about axis  22  and underlying display layers (e.g., a polymer substrate, metal traces, a polarizer layer, a touch sensor layer, adhesive layers, and other conducting and dielectric layers in an organic light-emitting diode pixel array) may also be formed from flexible materials and structures. 
     A schematic diagram of an illustrative electronic device such as device  10  of  FIG.  1    is shown in  FIG.  2   . As shown in  FIG.  2   , electronic device  10  may have control circuitry  50 . Control circuitry  50  may include storage and processing circuitry for supporting the operation of device  10 . The storage and processing circuitry may include storage such as hard disk drive storage, nonvolatile memory (e.g., flash memory or other electrically-programmable-read-only memory configured to form a solid state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Processing circuitry in control circuitry  50  may be used to control the operation of device  10  (e.g., to process sensor signals and other input and to control adjustable components such as a display, a heating element, etc.). The processing circuitry may be based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors, power management units, audio chips, application specific integrated circuits, etc. 
     Input-output circuitry in device  10  such as input-output devices  52  may be used to allow data to be supplied to device  10  and to allow data to be provided from device  10  to external devices. As shown in  FIG.  2   , input-output devices  52  may include display  14 . Display  14  may be a touch screen that incorporates a two-dimensional touch sensor or may be insensitive to touch. A two-dimensional touch sensor for display  14  may be formed from an array of capacitive touch electrodes touch sensor or other touch sensor components (e.g., force sensors, resistive touch sensors, acoustic touch sensors, optical sensors, etc.). 
     Input-output devices  52  may include sensors  56 . Sensors  56  may include a capacitive proximity sensor, a light-based proximity sensor, a magnetic sensor, a force sensor such as a force sensor that gathers user input, a touch sensor for gathering user touch input, a temperature sensor, a pressure sensor, an ambient light sensor, a microphone or other sound sensor that gathers ambient noise measurements and user input such as voice commands, sensors for gathering data on device position and motion such as inertial measurement units that include accelerometers, compasses, and/or gyroscopes, and/or other sensors. 
     Input-output devices  52  may also include other components  54  such as buttons, joysticks, scrolling wheels, touch pads, key pads, keyboards, speakers, tone generators, vibrators, cameras, light-emitting diodes and other status indicators, data ports, etc. A user can control the operation of device  10  by supplying user input commands through input-output devices  52  and may receive status information and other output from device  10  using the output resources of input-output devices  52 . 
     Control circuitry  50  may be used to run software on device  10  such as operating system code and applications. During operation of device  10 , the software running on control circuitry  50  may display images on display  14  (e.g., video, still images such as text, alphanumeric labels, photographs, icons, other graphics, etc.) using an array of pixels in display  14 . 
     As shown in  FIG.  3   , device  10  may be folded (bent by 180° or other suitable amount) about bend axis  22  so that display  14  is visible from the outside of device  10  in its folded state.  FIG.  4    shows how device  10  may be folded about bend axis  22  so that display  14  is protected within the interior of device  10 . Device  10  may have flexible structures (e.g., a hinge) to allow outward bending of the type shown in  FIG.  3   , to allow inward bending of the type shown in  FIG.  4   , or to allow bending of both the type shown in  FIG.  3    and the type shown in  FIG.  4   . Configurations in which device  10  is flexed by different amounts (e.g., more than 180° or less than 180°) may also be used. 
     Display  14  may have a rectangular shape (i.e., display  14  may have a rectangular footprint and a rectangular peripheral edge that runs around the rectangular footprint) or may have other suitable shapes. A top view of circuitry in an illustrative display with a rectangular shape is shown in  FIG.  5   . As shown in  FIG.  5   , display  14  may have an array of pixels  42  formed on substrate  36 . Substrate  36  may be formed from glass, metal, plastic, ceramic, or other substrate materials. Pixels  42  may receive data signals over signal paths such as data lines D and may receive one or more control signals over control signal paths such as horizontal control lines G (sometimes referred to as gate lines, scan lines, emission control lines, etc.). There may be any suitable number of rows and columns of pixels  42  in display  14  (e.g., tens or more, hundreds or more, or thousands or more). Each pixel  42  may have a light-emitting diode  26  that emits light  44  under the control of a pixel circuit formed from thin-film transistor circuitry such as thin-film transistors  28  and thin-film capacitors). Thin-film transistors  28  may be polysilicon thin-film transistors, semiconducting-oxide thin-film transistors such as indium gallium zinc oxide transistors, or thin-film transistors formed from other semiconductors. Pixels  42  may contain light-emitting diodes of different colors (e.g., red, green, and blue diodes for red, green, and blue pixels, respectively) to provide display  14  with the ability to display color images. 
     Display driver circuitry may be used to control the operation of pixels  42 . The display driver circuitry may be formed from integrated circuits, thin-film transistor circuits, or other suitable circuitry. Display driver circuitry  30  of  FIG.  2    may contain communications circuitry for communicating with system control circuitry such as control circuitry  50  of  FIG.  2    over path  32 . Path  32  may be formed from traces on a flexible printed circuit or other cable. During operation, the control circuitry (e.g., control circuitry  50  of  FIG.  2   ) may supply circuitry  30  with information on images to be displayed on display  14 . 
     To display the images on pixels  42 , display driver circuitry  30  may supply image data to data lines D while issuing clock signals and other control signals to supporting display driver circuitry such as gate driver circuitry  34  over path  38 . If desired, circuitry  30  may also supply clock signals and other control signals to gate driver circuitry on an opposing edge of display  14  or may use display driver circuitry with other layouts. The configuration of  FIG.  5    is illustrative. 
     Gate driver circuitry  34  (sometimes referred to as horizontal control line control circuitry) may be implemented as part of an integrated circuit and/or may be implemented using thin-film transistor circuitry. Gate lines G (sometimes referred to as horizontal control lines) in display  14  may carry gate line signals (sometimes referred to as scan line signals, emission enable control signals, etc.) for controlling the pixels of each row. There may be any suitable number of control signals per row of pixels  42  (e.g., one or more, two or more, three or more, four or more, etc.). 
     Display  14  may have an outermost layer formed from clear glass, transparent plastic, sapphire, or other transparent materials that serve as a protective layer for thin-film transistor circuitry and other display structures. The outer display layer may sometimes be referred to as a display cover layer. In some configurations for display  14 , the outermost layer of the display may serve both as a protective layer (display cover layer) and as a substrate for display structures (touch sensors electrodes, color filter elements, thin-film transistors, etc.). In other configurations, the display cover layer is free of circuitry and serves solely as a protective layer for underlying display structures (e.g., one or more underlying display panels). 
     As shown in  FIG.  6   , a display cover layer for display  14  (i.e., display cover layer  24 ) may have rigid portions such as rigid portions  24 A and flexible portions such as flexible portion  24 B. Rigid portions  24 A may, for example, be rigid planar layers. Configurations in which portions  24 A have non-planar shapes and/or are formed from flexible structures may also be used. Between rigid planar portions  24 A of display cover layer  24 , display cover layer  24  may have flexible portion  24 B. Portion  24 B may bend about bend axis  22  to allow display  14  to bend as housing  12  is bent about hinge  20 . 
     In the example of  FIG.  6   , flexible portion  24 B of display cover layer  24  aligns with flexible region  14 B of display  14  (e.g., the region of display  14  that bends about bend axis  22 ). This is, however, merely illustrative. If desired, display cover layer  24  may include flexible regions  24 B in rigid areas  14 A of display  14  (e.g., in areas of display  14  that do not bend, that are prevented from being bent, or that are otherwise less flexible than regions  14 B). 
     In some cases, display cover layer  24  may be formed from a single transparent layer formed from glass, plastic, sapphire, or another transparent material. In other arrangements, however, display cover layer  24  may include more than one layer of material. For example, in addition to a transparent dielectric layer, the display cover layer may include a layer with self-healing properties. 
     During operation of an electronic device, the display cover layer for the electronic device may be scratched or dented. To improve the aesthetics of electronic device, it may be desirable for the presence of scratches and dents to be minimized. To help mitigate the number of dents, scratches, or other imperfections in a display cover layer, the display cover layer may include a layer with self-healing properties (e.g., a layer of self-healing material). For example, a self-healing coating may be formed on a transparent dielectric layer. The self-healing coating may be dented or scratched during operation of the electronic device. However, because of the self-healing properties of the self-healing coating, the self-healing coating may return to its original shape and the dents and scratches may be removed. The self-healing may occur without prompting (e.g., when the self-healing coating is dented, the material of the coating may fill the dent even without intervention from an external influence). Alternatively, the self-healing may be initiated or expedited by externally applied heat, light, electric current, or other type of external stimulus. 
       FIG.  7    is a cross-sectional side view of a display that includes a display cover layer with a layer of self-healing material. As shown in  FIG.  7   , display cover layer  24  includes a transparent dielectric layer  62 . The display cover layer may cover underlying display layers  66  in the display (e.g., substrate  36  and other layers that form the light-emitting diodes and control circuitry of  FIG.  5   ). Transparent dielectric layer  62  may be formed from a transparent layer of plastic, glass, sapphire, or any other desired material. Display cover layer  24  also includes a layer of self-healing material  64 . The layer of self-healing material  64  may, for example, form an exterior surface of the display cover layer (meaning that the self-healing layer  64  is exposed to the exterior of the electronic device). By forming self-healing material at the exterior of the display cover glass, the self-healing material will be exposed during operation of the device. However, as previously mentioned, the self-healing material may be able to automatically repair any damage to itself. The self-healing material may automatically repair any damage to itself without external intervention or the self-healing process may be initiated by heat, light, or another external stimulus. The self-healing layer  64  may be formed from polymer or any other desired material having self-healing properties. 
     To ensure that flexible portion  24 B of display cover layer  24  has sufficient flexibility, the transparent dielectric layer  62  may have a reduced thickness in flexible portion  24 B. As shown in  FIG.  7   , transparent dielectric layer  62  has a thickness  68  in rigid portions  24 A of the display cover layer and a thickness  70  in flexible portion  24 B of the display cover layer. Thickness  70  is less than thickness  68 . Thinning the transparent dielectric layer in this way may increase flexibility in the flexible region. 
     Self-healing layer  64  may be elastic to allow for sufficient flexibility in the flexible portion of the display cover layer. Self-healing layer  64  may be more elastic than transparent dielectric layer  62 . Self-healing layer  64  is thicker in flexible region  24 B than in the rigid regions  24 A of the display cover layer. As previously mentioned, flexible region  24 B may bend about bend axis  22  to allow display  14  to bend as housing  12  is bent about hinge  20 . Self-healing layer  64  may therefore both provide a self-healing coating across the display cover layer for improved aesthetic and mechanical display cover layer performance and provide increased flexibility to allow bending in the display cover layer. 
     In  FIG.  7   , display cover layer  24  is depicted as having a single transparent dielectric layer  62  that has a reduced thickness portion in flexible region  24 B of the display cover layer. This example, however, is merely illustrative. Display cover layer  24  may instead include multiple layers of transparent dielectric material that are attached together. An example of this type is shown in  FIG.  8   . 
       FIG.  8    is a cross-sectional side view of an illustrative display having a display cover layer formed from multiple layers of transparent dielectric material. As shown in  FIG.  8   , display cover layer  24  includes transparent dielectric layers  62 - 1 ,  62 - 2 , and  62 - 3 . The display cover layer may cover underlying display layers  66  in the display (e.g., substrate  36  and other layers that form the light-emitting diodes and control circuitry of  FIG.  5   ). Transparent dielectric layers  62 - 1 ,  62 - 2 , and  62 - 3  may be formed from transparent layers of plastic, glass, sapphire, or any other desired material. 
     Display cover layer  24  also includes a layer of self-healing material  64 . The layer of self-healing material  64  may, for example, form a portion of an exterior surface of the display cover layer in flexible region  24 B of the display cover layer. To ensure that flexible portion  24 B of display cover layer  24  has sufficient flexibility, the cumulative thickness of the transparent dielectric layers is reduced in flexible portion  24 B. As shown in  FIG.  8   , transparent dielectric layers  62 - 1  and  62 - 2  are formed on first and second opposing sides of self-healing material  64 . An adhesive layer  72  attaches the transparent dielectric layers  62 - 1  and  62 - 2  to transparent dielectric layer  62 - 3 . Adhesive layer  72  may be formed from pressure sensitive adhesive or a liquid adhesive. Adhesive layer  72  may be an optically clear adhesive (OCA). 
     Transparent dielectric layer  62 - 3  may be thinner than transparent dielectric layers  62 - 1  and  62 - 2 . In this way, the shape of the single transparent dielectric layer  62  in  FIG.  7    is approximated with multiple transparent dielectric layers in  FIG.  8   . Using multiple transparent dielectric layers attached together to produce a reduced transparent dielectric layer thickness area (as in  FIG.  8   ) may result in reduced manufacturing time and costs compared to using a single transparent dielectric layer with a thinned region to produce a reduced transparent dielectric layer thickness area (as in  FIG.  7   ). 
     Self-healing layer  64  may be elastic to allow for sufficient flexibility in the flexible portion of the display cover layer. Self-healing layer  64  may be more elastic than transparent dielectric layers  62 - 1 ,  62 - 2 , and  62 - 3 . As previously mentioned, flexible region  24 B may bend about bend axis  22  to allow display  14  to bend as housing  12  is bent about hinge  20 . Self-healing layer  64  may therefore both provide a self-healing portion of the display cover layer and provide increased flexibility to allow bending in the display cover layer. 
       FIG.  9    is a cross-sectional side view of an illustrative display having a display cover layer with an elastomer formed in a flexible region of the display cover layer. As shown in  FIG.  9   , similar to as in  FIG.  8    display cover layer  24  includes transparent dielectric layers  62 - 1 ,  62 - 2 , and  62 - 3 . The display cover layer may cover underlying display layers  66  in the display (e.g., substrate  36  and other layers that form the light-emitting diodes and control circuitry of  FIG.  5   ). Transparent dielectric layers  62 - 1 ,  62 - 2 , and  62 - 3  may be formed from transparent layers of plastic, glass, sapphire, or any other desired material. The nomenclature of referring to transparent dielectric layers  62 - 1  and  62 - 2  as separate dielectric layers is merely illustrative. In some cases, these transparent dielectric layers may be referred to as a single transparent dielectric layer with an opening (e.g., that is filled with elastomer and/or self-healing material). 
     Display cover layer  24  also includes a layer of self-healing material  64 . The layer of self-healing material  64  may, for example, form a portion of an exterior surface of the display cover layer in flexible region  24 B of the display cover layer. To ensure that flexible portion  24 B of display cover layer  24  has sufficient flexibility, an elastomer layer  74  is also formed in flexible portion  24 B of the display cover layer. Elastomer layer  74  may be more elastic than self-healing layer  64 . Therefore, the presence of elastomer layer  74  may increase the flexibility of flexible region  24 B (compared to when only the self-healing layer  64  is present in flexible region  24 B as in  FIG.  8   ). Elastomer layer  74  may be formed from any desired material (e.g., a natural or synthetic polymer). Elastomer layer  74  may have a Young&#39;s modulus that is lower than the Young&#39;s modulus of self-healing layer  64  and self-healing layer  64  may have a Young&#39;s modulus that is lower than the Young&#39;s modulus of the transparent dielectric layers. 
     Adhesive layer  72  attaches the transparent dielectric layers  62 - 1  and  62 - 2  as well as elastomer layer  74  to transparent dielectric layer  62 - 3 . Adhesive layer  72  may be formed from pressure sensitive adhesive or a liquid adhesive. Adhesive layer  72  may be an optically clear adhesive (OCA). As previously mentioned, flexible region  24 B may bend about bend axis  22  to allow display  14  to bend as housing  12  is bent about hinge  20 . 
       FIG.  10    is a cross-sectional side view of an illustrative display in which the cover layer includes a self-healing coating across the display cover layer and an elastomer layer in the flexible region of the display cover layer. As shown in  FIG.  10   , display cover layer  24  includes transparent dielectric layers  62 - 1 ,  62 - 2 , and  62 - 3 . The display cover layer may cover underlying display layers  66  in the display (e.g., substrate  36  and other layers that form the light-emitting diodes and control circuitry of  FIG.  5   ). Transparent dielectric layers  62 - 1 ,  62 - 2 , and  62 - 3  may be formed from transparent layers of plastic, glass, sapphire, or any other desired material. 
     Display cover layer  24  also includes a layer of self-healing material  64 . The layer of self-healing material  64  may, for example, form an exterior surface of the display cover layer (meaning that the self-healing layer  64  is exposed to the exterior of the electronic device). The self-healing material may extend across the entire exterior surface of the display cover layer. By forming self-healing material at the exterior of the display cover glass, the self-healing material will be exposed during operation of the device. However, as previously mentioned, the self-healing material be able to automatically repair any damage to itself. The self-healing material may automatically repair any damage to itself without external intervention or the self-healing process may be initiated by heat, light, or another external stimulus. The self-healing layer  64  may be formed from polymer or any other desired material having self-healing properties. 
     To ensure that flexible portion  24 B of display cover layer  24  has sufficient flexibility, an elastomer layer  74  is also formed in flexible portion  24 B of the display cover layer. Elastomer layer  74  may be more elastic than self-healing layer  64 . Therefore, the presence of elastomer layer  74  may increase the flexibility of flexible region  24 B. Elastomer layer  74  may be formed from any desired material (e.g., a natural or synthetic polymer). Elastomer layer  74  may have a Young&#39;s modulus that is lower than the Young&#39;s modulus of self-healing layer  64  and self-healing layer  64  may have a Young&#39;s modulus that is lower than the Young&#39;s modulus of the transparent dielectric layers. 
     Adhesive layer  72  attaches the transparent dielectric layers  62 - 1  and  62 - 2  as well as elastomer layer  74  to transparent dielectric layer  62 - 3 . Adhesive layer  72  may be formed from pressure sensitive adhesive or a liquid adhesive. Adhesive layer  72  may be an optically clear adhesive (OCA). As previously mentioned, flexible region  24 B may bend about bend axis  22  to allow display  14  to bend as housing  12  is bent about hinge  20 . 
       FIG.  11    is a cross-sectional side view of an illustrative display in which the display cover layer includes a self-healing coating across the display cover layer and a heating layer that heats the self-healing coating. As shown in  FIG.  11   , display cover layer  24  includes transparent dielectric layers  62 - 1 ,  62 - 2 , and  62 - 3 . The display cover layer may cover underlying display layers  66  in the display (e.g., substrate  36  and other layers that form the light-emitting diodes and control circuitry of  FIG.  5   ). Transparent dielectric layers  62 - 1 ,  62 - 2 , and  62 - 3  may be formed from transparent layers of plastic, glass, sapphire, or any other desired material. 
     Display cover layer  24  also includes a layer of self-healing material  64 . The layer of self-healing material  64  may, for example, form an exterior surface of the display cover layer (meaning that the self-healing layer  64  is exposed to the exterior of the electronic device). By forming self-healing material at the exterior of the display cover glass, the self-healing material will be exposed during operation of the device. However, as previously mentioned, the self-healing material be able to automatically repair any damage to itself. The self-healing process of the self-healing material may be initiated or expedited by heat. Therefore, a heating element may be included in the display cover layer to heat the self-healing layer. 
     As shown in  FIG.  11   , heating layer  76  may be formed between transparent dielectric layer  62 - 3  and adhesive layer  72 . The heating layer may be a resistive heater in which the passage of electric current through a conductor produces heat. In other words, the heating layer may include conductive traces (e.g., on a surface of transparent dielectric layer  62 - 3 ) that receive an applied electric current. For example, a first end of the conductive traces may be coupled to a positive power supply terminal and a second end of the conductive traces may be coupled to a ground power supply terminal. The conductive traces of heating layer  76  may follow a meandering path (e.g., the conductive traces may have a series of bends to extend the length of the traces and accordingly increase the resistance of the conductive traces). The conductive traces of heating layer  76  may be formed from a transparent conductive material such as indium tin oxide (ITO) or silver nanowire. 
     To ensure that flexible portion  24 B of display cover layer  24  has sufficient flexibility, an elastomer layer  74  is also formed in flexible portion  24 B of the display cover layer. Elastomer layer  74  may be more elastic than self-healing layer  64 . Therefore, the presence of elastomer layer  74  may increase the flexibility of flexible region  24 B. Elastomer layer  74  may be formed from any desired material (e.g., a natural or synthetic polymer). Elastomer layer  74  may have a Young&#39;s modulus that is lower than the Young&#39;s modulus of self-healing layer  64  and self-healing layer  64  may have a Young&#39;s modulus that is lower than the Young&#39;s modulus of the transparent dielectric layers. 
     Adhesive layer  72  attaches the transparent dielectric layers  62 - 1  and  62 - 2  as well as elastomer layer  74  to heating layer  76  and transparent dielectric layer  62 - 3 . Adhesive layer  72  may be formed from pressure sensitive adhesive or a liquid adhesive. Adhesive layer  72  may be an optically clear adhesive (OCA). As previously mentioned, flexible region  24 B may bend about bend axis  22  to allow display  14  to bend as housing  12  is bent about hinge  20 . 
     The heating layer may selectively be heated in order to stimulate the self-healing of self-healing material  64 . Control circuitry within the electronic device (e.g., control circuitry  50  in  FIG.  2   ) may be used to provide signals (e.g., an electric current) to heating layer  76  to generate heat with the heating layer. The control circuitry may heat the heating layer in response to user input. For example, a user of the electronic device may provide instructions to the electronic device using one or more of the input-output devices in the electronic device (e.g., input-output devices  52  in  FIG.  2   ). The user may, for example, use the input-output devices to provide instructions for immediately generating heat with the heating layer for the self-healing process. Alternatively, the user may schedule a time or times at which the heating layer will generate heat for the self-healing process. 
     In other arrangements, control circuitry in the electronic device may determine when to use the heating layer to generate heat without user input. For example, the control circuitry may provide signals to the heating layer to generate heat after a predetermined amount of electronic device usage or according to a predetermined schedule. The control circuitry may analyze the historical device usage and provide signals to the heating layer to generate heat based on the historical device usage. In other embodiments, the control circuitry may provide signals to the heating layer to generate heat when the electronic device is being charged (e.g., when the electronic device receives wireless or wired power to provide power for operating the electronic device or to recharge the electronic device battery) or in other predetermined usage scenarios. 
     Control circuitry in the electronic device may monitor the temperature of the heating layer to ensure that the temperature of the electronic device does not exceed certain temperature thresholds. For example, the electronic device may have a maximum allowable temperature above which the internal components of the electronic device may be compromised. The electronic device may have a safety temperature above which the electronic device may not be safe for a user to touch. The control circuitry may stop generating heat with the heating layer if any desired temperature limit is exceeded. 
     In general, it may be desirable for the components of display cover layer  64  to have similar indices of refraction in order to avoid different portions of the display cover layer having different appearances. The materials used to form the display cover layer may be selected to have similar indices of refraction. For example, elastomer  74  may have a similar index of refraction as transparent dielectric layers  62 - 1  and  62 - 2  (e.g., the two indices of refraction may be within 0.2, within 0.1, within 0.05, without 0.03, within 0.01, etc.). Elastomer  74  may have a similar index of refraction as self-healing layer  64  (e.g., the two indices of refraction may be within 0.2, within 0.1, within 0.05, without 0.03, within 0.01, etc.). Transparent dielectric layers  62 - 1  and  62 - 2  may have a similar index of refraction as self-healing layer  64  (e.g., the two indices of refraction may be within 0.2, within 0.1, within 0.05, without 0.03, within 0.01, etc.). Elastomer  74  may have a similar index of refraction as adhesive layer  72  (e.g., the two indices of refraction may be within 0.2, within 0.1, within 0.05, without 0.03, within 0.01, etc.). 
     These examples are merely illustrative. In general, all of the materials within the display cover layer may be selected to have similar indices of refraction when possible. In some cases, however, there may be differences between the indices of refraction of materials within the display cover layer. Index-matching layers may be incorporated in the display cover layer to mitigate visible artifacts caused by materials having different indices of refraction.  FIG.  12    is a cross-sectional side view of a display having a display cover layer with the same arrangement as in  FIG.  11   . However, in addition to the components shown and discussed in connection with  FIG.  11   , the display cover layer of  FIG.  12    includes index-of-refraction-matching layers  80 - 1  and  80 - 2  (sometimes referred to simply as index-matching layers  80 - 1  and  80 - 2 ). 
     As shown in  FIG.  12   , index-matching layer  80 - 1  is interposed between adhesive  72  and heating layer  76 . Index-matching layer  80 - 1  may have an index of refraction that is between the index of refraction of adhesive  72  and the index of refraction of heating layer  76 . Index-matching layer  80 - 2  is interposed between heating layer  76  and transparent dielectric layer  62 - 3 . Index-matching layer  80 - 2  may have an index of refraction that is between the index of refraction of heating layer  76  and the index of refraction of transparent dielectric layer  62 - 3 . The two index-matching layers depicted in  FIG.  12    are merely illustrative. In general, index-matching layers may be incorporated between any two adjacent layers within display cover layer  24 . 
     The position of heating layer  76  in  FIG.  11    is merely illustrative. In general, the heating layer for heating the self-healing material may be incorporated at any desired position within the display.  FIG.  13    is a cross-sectional side view of a display having a display cover layer with the same arrangement as in  FIG.  11   , except for the position of the heating layer. 
     As shown in  FIG.  13   , instead of positioning the heating layer between adhesive layer  72  and transparent dielectric layer  62 - 3  (as in  FIG.  11   ), the heating layer  76  (sometimes referred to as heat-generating layer  76 ) is positioned between self-healing coating  64  and transparent dielectric layers  62 - 1  and  62 - 2 . Positioning the heating layer directly adjacent to the self-healing layer in this way may result in more of the heat generated by the heating layer reaching the self-healing layer. This may result in improved self-healing performance in the self-healing material. However, positioning the heating layer immediately adjacent the self-healing material may require the heating layer to be very flexible. To optimize the flexibility of the display cover layer, the heating layer may be positioned further from the self-healing material (e.g., as in  FIG.  11   ). 
     The examples of  FIGS.  9 - 13    of elastomer layer  74  being formed in the flexible region of the display cover layer are merely illustrative. If desired, a flexible material of another type may replace elastomer layer  74  in  FIGS.  9 - 13   . For example, a plurality of layers of thin plastic (attached together with adhesive layers) may replace elastomer layer  74 . 
     The examples of transparent dielectric layers used in  FIGS.  7 - 13    are also illustrative. It should be understood that, in general, any combination of transparent dielectric layers having uniform thicknesses and transparent dielectric layers having varying thicknesses may be used to form the display cover layer. 
     In  FIGS.  11 - 13   , examples are shown where a transparent conductor is included to form a heating element for heating the self-healing layer of the display cover layer. This type of arrangement may be used when the self-healing process of the self-healing material is initiated or expedited by heat. However, the self-healing material may instead be promoted by a different stimulus such as exposure to light or electric current. 
     In embodiments where the self-healing process is initiated or expedited by exposure to light, any desired light source may be used to provide the exposure to light. If exposure to visible light is a stimulus for the self-healing process, light emitted by the flexible display may be used to provide the stimulus for the self-healing process. Alternatively, a separate light source in the electronic device may be used to provide light as a stimulus for the self-healing process. In some cases, exposure to ultraviolet light may be stimulus for the self-healing process. An ultraviolet light source may be included in the electronic device to provide the ultraviolet light or ambient ultraviolet light may naturally stimulate the self-healing process without external intervention. 
     In yet another possible arrangement, the self-healing process may be initiated or expedited in response to an electric current. Control circuitry may be configured to apply an electric current to the self-healing material. For example, the self-healing material may be coupled to a positive power supply terminal and a ground power supply terminal. 
     Regardless of the type of stimulus used to initiate and/or expedite the self-healing process in the self-healing material, the control circuitry may provide the stimulus in a number of possible ways. The control circuitry may provide the stimulus in response to user input. For example, a user of the electronic device may provide instructions to the electronic device using one or more of the input-output devices in the electronic device (e.g., input-output devices  52  in  FIG.  2   ). The user may, for example, use the input-output devices to provide instructions for immediately starting for the self-healing process. Alternatively, the user may schedule a time or times at which the self-healing process will be initiated by the stimulus. In other arrangements, the control circuitry in the electronic device may determine when to provide the stimulus. For example, the control circuitry may provide the stimulus after a predetermined amount of electronic device usage or according to a predetermined schedule. The control circuitry may analyze the device usage and provide the stimulus based on the device usage. In other embodiments, the control circuitry may provide the stimulus whenever the electronic device is being charged (e.g., when the electronic device receives wireless or wired power to provide power for operating the electronic device or to recharge the electronic device battery) or in other predetermined usage scenarios. 
     In  FIGS.  8 - 13   , examples are shown where separate pieces of transparent dielectric material are used instead of a single, thinned layer of transparent dielectric (as in  FIG.  7   ). In the examples of  FIGS.  8 - 13   , materials such as elastomer  74  and/or self-healing material  64  fill a gap between transparent dielectric layers  62 - 1  and  62 - 2 . There is also a transparent dielectric layer  62 - 3  formed under layers  62 - 1  and  62 - 2  that is uninterrupted in flexible portion  24 B. This example of an arrangement for multiple transparent dielectric layers is merely illustrative. In yet another possible arrangement, a transparent dielectric layer may have a plurality of slits in flexible portion  24 B to promote bending. 
       FIG.  14    is a cross-sectional side view of an illustrative display having a display cover layer formed from a layer of transparent dielectric material with slits. As shown in  FIG.  14   , display cover layer  24  includes transparent dielectric layers  62 - 1  and  62 - 2 . The display cover layer may cover underlying display layers  66  in the display (e.g., substrate  36  and other layers that form the light-emitting diodes and control circuitry of  FIG.  5   ). Transparent dielectric layers  62 - 1  and  62 - 2  may be formed from transparent layers of plastic, glass, sapphire, or any other desired material. 
     Transparent dielectric layer  62 - 2  may have a plurality of slits  82  (which may sometimes be referred to as holes, recesses, grooves, openings, etc.) in flexible portion  24 B to increase the flexibility of the display cover layer. Because transparent dielectric layer  62 - 2  has a pattern of slits in the flexible portion of the display cover glass, transparent dielectric layer  62 - 2  may sometimes be referred to as a patterned transparent dielectric layer or a patterned glass layer. Transparent dielectric layer  62 - 2  is covered by transparent dielectric layer  62 - 1 . The thickness of dielectric layer  62 - 2  may be greater than the thickness of dielectric layer  62 - 1  (e.g., at least two times greater, at least three times greater, at least five times greater, at least ten times greater, at least twenty times greater, etc.). With this arrangement, the dielectric cover layer may have flexibility in flexible region  24 B while maintaining sufficient structural integrity in the adjacent rigid portions  24 A. 
     Index-matching layers may be incorporated in the display cover layer to mitigate visible artifacts caused by materials having different indices of refraction. As shown in  FIG.  14   , the display cover layer  24  includes index-of-refraction-matching layers  86 - 1  and  86 - 2  (sometimes referred to simply as index-matching layers  86 - 1  and  86 - 2 ). Index-matching layer  86 - 1  is interposed between patterned transparent dielectric layer  62 - 2  and display layers  66 . Index-matching layer  86 - 1  may have an index of refraction that is between the index of refraction of transparent dielectric layer  62 - 2  and the index of refraction of display layers  66  and/or may have an index of refraction that is approximately equal to (e.g., within 5% of) the index of refraction of transparent dielectric layer  62 - 2 . Index-matching layer  86 - 2  may fill the slits  82  in transparent dielectric layer  62 - 2 . Index-matching layer  86 - 2  may have an index of refraction that is close to or equal to the index of refraction of transparent dielectric layer  62 - 2  (e.g., within 10% of, within 5% of, within 1% of, within 0.1% of, etc.) to avoid the slits being visible to the user. 
     The nomenclature of  FIG.  14   , with index-matching layers  86 - 1  and  86 - 2  receiving separate labels, is merely illustrative. In one illustrative arrangement, the same material may be used to form both index-matching layers  86 - 1  and  86 - 2 . In this type of arrangement, the index-matching layer may be considered to be a single, integral index-matching layer with a first portion between dielectric layer  62 - 2  and display layers  66  and a second portion that fills the slits in dielectric layer  62 - 2 . 
     The index-matching layers may also increase the flexibility of flexible portion  24 B of the display cover layer. The index-matching layers may be more elastic than dielectric layer  62 - 2  (and optionally more elastic than self-healing layer  64 ). Index-matching layers  86 - 1  and  86 - 2  may be formed from any desired material (e.g., a natural or synthetic polymer). Index-matching layers  86 - 1  and  86 - 2  may have a Young&#39;s modulus that is lower than the Young&#39;s modulus of self-healing layer  64  and/or the transparent dielectric layers  62 . Because the index-matching layers may have elastomeric properties and may increase the flexibility of the display cover layer, the index-matching layers may sometimes also be referred to as elastomeric layers. 
     Transparent dielectric layers  62 - 1  and  62 - 2  may both be glass layers. The glass layers may be coupled together with glass-to-glass welds  84 . The glass-to-glass welds may be formed using laser welding that heats the glass layers and causes the glass layers to fuse together at welds  84 . This example is merely illustrative, and welds  84  may be formed using any desired techniques. 
     Display cover layer  24  may also include a layer of self-healing material  64 , similar to as previously described in connection with  FIGS.  8 - 13   . In  FIG.  14   , the self-healing material is depicted as being formed over the entire display cover layer. This example is merely illustrative. If desired, the self-healing material  64  may be patterned (e.g., only formed in flexible portion  24 B of the display cover layer). The self-healing material  64  from  FIG.  14    may also optionally be omitted from the display cover layer. 
     The example in  FIG.  14    of transparent dielectric layers  62 - 1  and  62 - 2  being attached together by welds  84  is merely illustrative. If desired, an adhesive layer may be used to attach transparent dielectric layer  62 - 1  to transparent dielectric layer  62 - 2 .  FIG.  15    is a cross-sectional side view of an illustrative display cover layer with an adhesive layer  88  between transparent dielectric layer  62 - 1  and transparent dielectric layer  62 - 2 . Adhesive layer  88  may attach transparent dielectric layer  62 - 1  to transparent dielectric layer  62 - 2 . Adhesive layer  88  may be formed from any desired material. In some cases, the adhesive layer may be an epoxy that includes polymer material. In other possible arrangements, adhesive layer  88  may be formed from a pressure sensitive adhesive (PSA), a liquid optically clear adhesive (LOCA), etc. 
     In  FIGS.  14  and  15   , slits  82  extend entirely through transparent dielectric layer  62 - 2 . The slits  82  may sometimes be referred to as through-holes that extend from an upper surface of the transparent dielectric layer to a lower surface of the transparent dielectric layer. This example is merely illustrative. If desired, the slits may extend only partially into the transparent dielectric layer  62 - 2  (e.g., a partial through-hole that does not reach the lower surface of the transparent dielectric layer). 
       FIG.  16    is a cross-sectional side view of an illustrative display cover layer with partial slits in a transparent dielectric layer. As shown in  FIG.  16   , slits  82  may extend only partially through transparent dielectric layer  62 - 2 . The separate slits may merge into a unitary opening  90  at a lower part of the flexible portion of transparent dielectric layer  62 - 2 . Index-matching layer  86  may have a first portion that is formed between transparent dielectric layer  62 - 2  and display layers  66 , a second portion that fills the unitary opening  90  in transparent dielectric layer  62 - 2 , and a third portion that fills the slits  82  in transparent dielectric layer  62 - 2 . These different portions of the index-matching layer may be formed from the same material or from different materials (similar to as discussed in connection with  FIG.  14   ). 
       FIG.  16    depicts transparent dielectric layers  62 - 1  and  62 - 2  as being attached together using welds  84  (similar to as in  FIG.  14   ). However, it should be understood that the dielectric layers may instead be attached using an adhesive layer (similar to as in  FIG.  15   ). 
       FIG.  17    is a cross-sectional side view of a display cover layer showing how three transparent dielectric layers may be used instead of the two transparent dielectric layers of  FIG.  16   . As shown in  FIG.  17   , transparent dielectric layer  62 - 3  may have a single opening  90  in flexible portion  24 B of the display cover layer. Transparent dielectric layer  62 - 2  may have slits  82  in flexible portion  24 B of the display cover layer. Transparent dielectric layer  62 - 1  may cover the entire display. Dielectric layer  62 - 1  may be thinner than both dielectric layer  62 - 2  and dielectric layer  62 - 3  (e.g., by a factor of more than two, by a factor of more than three, by a factor of more than five, by a factor of more than ten, by a factor of more than twenty, etc.). Transparent dielectric layers  62 - 1 ,  62 - 2 , and  62 - 3  may be formed from transparent layers of plastic, glass, sapphire, or any other desired material. 
     A first index-matching layer  86 - 1  may be interposed between transparent dielectric layer  62 - 3  and display layers  66 . A second index-matching layer  86 - 2  may fill opening  90  in transparent dielectric layer  62 - 3 . A third index-matching layer  86 - 3  may fill slits  82  in transparent dielectric layer  62 - 2 . Similar to as discussed in connection with  FIG.  14   , the index-matching layers may be formed from the same material and therefore may be sometimes referred to as a single index-matching layer with various portions. It should be noted that in any of  FIGS.  14 - 17   , self-healing layer  64  may optionally be omitted or patterned. 
       FIG.  17    depicts transparent dielectric layers  62 - 1  and  62 - 2  as being attached together using welds  84  (similar to as in  FIG.  14   ). However, it should be understood that the dielectric layers may instead be attached using an adhesive layer (similar to as in  FIG.  15   ).  FIG.  17    also depicts transparent dielectric layers  62 - 2  and  62 - 3  as being attached together using welds  84  (similar to as in  FIG.  14   ). However, it should be understood that the dielectric layers may instead be attached using an adhesive layer (similar to as in  FIG.  15   ). In general, any interface between adjacent transparent dielectric layers may include an adhesive layer for attaching the two transparent dielectric layers or may include welds for attaching the two transparent dielectric layers. 
       FIG.  18    is a top view showing an illustrative arrangement for slits  82  in the flexible region  24 B of display cover layer  24  (e.g., as in any of  FIGS.  14 - 17   ). As shown in  FIG.  18   , each slit may be an elongated slit that extends along an axis that is parallel to the bend axis of flexible portion  24 B (e.g., bend axis  22  in  FIG.  1   ). The slit pattern depicted in  FIG.  18    is merely illustrative. The openings may not be elongated (e.g., circular holes may be used instead of elongated slits). The slits may include zig-zags (e.g., portions that are angled relative to the bend-axis). In general, any desired number and shapes of openings may be formed in the transparent dielectric layer  62 - 2 . 
       FIGS.  7 - 18    depict a display cover layer for a display that bends along a single bend-axis (as depicted in  FIG.  1   , for example). However, it should be understood that the flexible display cover layer regions shown and described in  FIGS.  7 - 18    may be applied to a display that bends along any desired number of bend-axes. The display cover layer may bend along one, two, or more than two bend-axes. The display cover layer may have a flexible region along each bend-axis. In one possible arrangement, the entire display cover layer may be flexible. 
     The foregoing is merely illustrative and various modifications can be made to the described embodiments. The foregoing embodiments may be implemented individually or in any combination.