DE102022202180A1 - Flexible display and method of making a flexible display - Google Patents

Abstract

The invention relates to a method for producing a flexible display device, comprising providing at least two material layers (210, 220, 260) which are to be connected to one another over a large area to form a layered material, the at least two material layers (210, 220, 260) at least one imaging layer (220) and a flexible backing layer (210); generating a temporary change in length in the direction of the surface expansion in at least one of the material layers, the change in length being produced by thermal and/or mechanical action; and joining the at least two layers of material (210, 220) together to form a layered material while the change in length is present in at least one of the layers of material, such that after the change in length has subsided, a prestress (270) is created within the layered material.

Description

The present invention relates to a flexible display and a method for manufacturing a flexible display.

Background of the Invention

Electronic display devices or displays are used today in a wide variety of application areas, shapes and sizes. While such displays were usually rigid and flat in the past, flexible display technologies are now increasingly being used with which, for example, rollable displays, foldable smartphones or display elements integrated into clothing (wearables) can be realized. However, the desired mechanical flexibility places very high demands on the display technology. High-resolution displays require complex control of individual pixels, in the case of active matrix displays, for example, by means of column and row connections. The required electrical lines must therefore be designed to withstand a large number of rolling and bending cycles without damage. The same is true for other elements of a display, such as the different layers of an OLED (Organic Light Emitting Diode) display.

In the case of an OLED display in particular, the organic light-emitting layer components should be protected from external influences. Chemical reactions with water or oxygen should be avoided, as these would reduce or completely destroy the efficiency of the emission. While a protective seal between e.g. two glass surfaces is easily achieved with a conventional flat display, the protective layers of a flexible display and the active image-generating components of the display must remain undamaged even under permanent or recurring stress. Such displays are then usually designed to be as thin as possible so that the bending forces acting in the various layers are minimal. The selection of materials such as adhesive layers must also be adapted to the desired flexible application.

Disclosure of Invention

According to the invention, a flexible display and a method for producing a flexible display with the features of the independent patent claims are proposed. Advantageous configurations are the subject of the dependent claims and the following description.

In particular, a method for producing a flexible display device is proposed, in which at least two material layers are initially provided, which are to be connected to one another over a large area to form a layered material, the at least two material layers comprising at least one image-forming layer and a flexible carrier layer. A temporary change in length, i.e. compression or stretching, is then produced in at least one of the material layers in the direction of the surface expansion in at least one of the material layers, the change in length being produced by thermal and/or mechanical action. Finally, the at least two layers of material are joined together to form a layered material while the change in length is present in at least one of the layers of material, so that after the change in length has subsided, a prestress occurs within the layered material. The prestress can at least partially compensate for forces that occur later during operation of the display, in particular tensile and compressive forces that arise when the flat material bends, so that the particularly sensitive layers of a display can be protected from damage.

In one possible embodiment, the temperature of at least one of the material layers can be changed as part of the method in order to produce the change in length. Then the at least two material layers can be connected to each other at the respective changed temperature, the changed temperature for the material layers being selected such that after the drop to, for example, room temperature as the usual operating temperature, a prestress occurs within the layer material. Due to the different selected temperatures and/or due to different coefficients of expansion and different thermal behavior of the material layers, these are bonded stress-free with a changed length, but essentially return to their original length when they return to the operating temperature, so that internal prestressing occurs in the layers.

When changing the temperature, for example, a first material layer can be heated and a second material layer can be cooled. However, the same temperatures can also be selected for a number of layers, all layers can be heated or cooled to different temperatures, or the processes of changing the temperature and joining together for a number of layers can be carried out one after the other.

In particular, for example, a backing layer, which is often made of a more resilient material, could be heated and at least one adhesive layer placed between the backing layer and the imaging layer to bond the layers of material together.

Additionally, the method may include determining an expected tensile and/or compressive stress on at least one of the material layers of the bonded sheet material in an end use of the display device, determining a required prestress in the bonded sheet material to at least partially meet the expected tensile and/or compressive stress compensating, and generating the transient length change such that the determined required prestress is achieved. In particular, the position of a neutral fiber in the connected layered material can also be determined, i.e. the position of a layer or partial layer in the layered material that does not change in length when bent, and then the prestressing within the layered material can be generated in such a way that the position of the neutral fiber is shifted into a given one of the material layers. The layer of the neutral fiber is then preferably placed in the particularly sensitive material layers, such as the image-forming layers of a display.

The joining of the two or more layers of material can be done, for example, by gluing, such as with an optical glue.

In particular, the method presented is suitable for a display device that includes an OLED display, with the image-generating layer including an OLED layer that is attached to the carrier layer by means of an adhesive layer.

In addition, a flexible display device made of a layered material with two or more surface-connected material layers is proposed, the material layers comprising at least one image-forming layer and a flexible carrier layer, and wherein there is a mechanical prestress within the layered material, the layered material using a method such as is produced as described above.

In such a display device, the mechanical prestress can be designed in such a way that an expected tensile and/or compressive load on at least one of the material layers of the connected layer material in an end application of the display device can be at least partially compensated by the prestress. This protects the sensitive layers of the display from damage in the final application, e.g. for a scrollable display element. In particular, the imaging layer can comprise an OLED layer, but can also comprise other flexible display technologies.

Finally, a flexible display device as described can be used in an electronic device for displaying information to a user, where the display device is rollable and/or foldable and/or bendable during operation. For example, smartphones with a foldable and therefore larger display area, displays that can be rolled up to save space, or displays that are integrated into flexible objects or clothing can be implemented.

Further advantages and refinements of the invention result from the description and the attached drawing.

The invention is shown schematically in the drawing using exemplary embodiments and is described below with reference to the drawing.

Brief description of the drawings

• 1 a shows schematically the structure of an exemplary OLED display with protective layers;
• 1b shows the course of a neutral axis in a material during bending;
• 2a until 2c outlines a possible embodiment of a flexible display and its manufacture; and
• 3 shows a flow chart of exemplary method steps of a manufacturing method for a flexible display.

Embodiments of the invention

Flexible displays are manufactured on different technical bases, all of which face the challenge of achieving maximum flexibility without damaging the active components and the protective casings.

OLED technology is to be considered here as an example, in which encapsulation via thin layer stacks is usually implemented for protection. 1a shows a schematic structure of an exemplary OLED display element 100. In this case, the active, image-generating OLED element 120 is arranged on a carrier substrate or TFT array (thin film transistor) 110, with a multiplicity of red 120a, green 120b and blue 120c sub-elements can be provided. Other components of the OLED layer 120, such as an anode, a cathode, a conductive layer, and the organic light-emitting layer, are not detailed here. These layers 120, which are to be protected from external influences, can be encapsulated by vapor deposition of thin-layer systems, which can alternately comprise inorganic 130 and organic 140 layers. These layers are applied by means of various vapor deposition processes, eg CVD (chemical vapor deposition), ALD (atomic layer deposition) or also by printing processes. The organic layers 140 serve as a flexible element, while the inorganic materials 130 usually consist of metal oxides, which are intrinsically brittle and inflexible, but are essential for protection against moisture and oxygen. Only the combination of many such layers allows secure encapsulation, which enables flexible use of the display. Possible defects (holes, cracks) 150 in the inorganic layers 130, which would allow the undesired molecules to penetrate, are compensated for by this multi-layered system. Molecules that penetrate at one point would have to travel long distances to reach the organic emitting layers to be protected. When using a flexible carrier substrate and suitable materials, such an OLED display can be designed as a flexible display that allows bending or rolling.

When an at least partially flexible element is deformed, stresses in the form of various forces and stresses arise, namely tension/compression, bending, shearing and torsion. In order to avoid damage to a display, these forces should be minimized on the relevant sensitive components. An essential aspect is the neutral axis or zero voltage line. In strength theory, this refers to the fiber or layer of a layered material that is neither shortened nor lengthened when bent, i.e. retains its length. In this layer there are no tensile or compressive forces due to stretching or compression. 1b shows an example of a cross section through a thin, flat layer element 10 which is bent over a roller 20. FIG. In the case of a homogeneous material, the neutral fiber 16 runs through the center of gravity of the cross section, ie for example in the middle of the layer 10, as indicated in the figure. The layers above are stretched in the bent area, 12, while the layers below are compressed, 14.

According to the preferred embodiment of the invention shown here, the neutral fiber or layer in a display arrangement can now be shifted in a targeted manner so that it is in the area of particularly sensitive components. This can be achieved by introducing mechanical prestresses into part or all of the components of the layered system before they are bonded together (e.g. by gluing). The preloads should be designed in such a way that they compensate for the compressive and tensile forces that occur later in operation.

In this way, the position of the neutral axis can be shifted and, for example, a system can be designed in such a way that it is ideally free of tensile and compressive forces in the desired plane when rolled up.

In particular, the state in which the display will usually be can be taken into account. For example, in the case of a foldable smartphone, it can be assumed that the display is usually, and therefore most of the time, in a folded or bent state and is only open and flat for short phases of use.

A bias can be achieved in different ways. In particular, at least parts of the layer system can be thermally or mechanically braced when they are assembled.

As an example, in 2 Consider a layered material made up of three layers 210, 220, 260, in which a thermally induced prestress is introduced. This can be, for example, a display system in which at least one image-generating layer 220, eg an OLED layer, is provided as the upper layer, while a lower layer is formed by a supporting element or carrier substrate 210. An adhesive layer 260 can then be provided between these two layers, such as an OCA layer (Optical Clear Adhesive). The OLED layer 220 can be configured as shown in FIG 1a , i.e., comprising within themselves a plurality of organic and inorganic layers for imaging and for protecting the light-emitting elements, not individually shown herein; however, other variants or entirely different imaging layers are possible.

The individual layers or layer elements therefore consist of different materials or compositions and therefore also have different mechanical properties. For example, they usually differ in thermal expansion coefficient, elastic modulus, and other parameters.

In the present example, it should be assumed that the upper, image-forming layer 220 is to be specially protected, so that stresses in this layer are to be minimized during actual operation. However, any other layer in a layered material system could also be specified here as a layer to be protected.

2a shows the three layers before joining. In this state, the different materials can now be brought to different temperatures. As an example, the first layer 220, e.g. B. a flexible OLED layer, are cooled while the bottom layer 210, such as the carrier substrate, is preheated. The intermediate adhesive layer 260 is not tempered in this example, but could also be cooled or heated. In this state, ie with altered temperatures, the layers are then joined together. In particular, this can involve any type of gluing process, for example with a prefabricated adhesive layer or adhesives applied in liquid form.

The pre-tempering of the materials is preferably carried out immediately before the joining process; in principle, however, there can also be further steps or a longer period of time in between.

Since the materials usually show different temperature behavior, it is also possible not to use different temperatures, but to bring two or more layers to the same temperature, deviating from the operating temperature (e.g. to cool or heat), at about this temperature to join together and also to achieve a prestress in the overall layer system through the different expansion or contraction of the respective layers as soon as the assembled material is brought back to operating temperature. It goes without saying that the “operating temperature” is understood to mean the temperature range in which the display layer system is usually to be used, ie for example temperature ranges around room temperature for conventional electronic devices. For specific applications, however, the final temperature and the associated temperatures when the layers are joined can also be selected differently. Depending on how much time elapses between the heating or cooling of the layers and the joining process, the layers will not have exactly the same initial temperatures at the moment of joining, but these effects can also be taken into account accordingly.

2 B now shows the layers connected to one another over a large area by suitable joining techniques, in particular gluing, to form a combined layered material 2a . Due to the different temperatures and/or material properties of the layers, after the temperatures have been equalized or after a return to the operating temperature, a permanent prestress 270 - indicated by the arrows in the figure - occurs in the layer material, which may be released later when the display is used (e.g by rolling up) can compensate for compressive or tensile stress.

2c shows the layer schematically 2 B in a severely bent or coiled state. The imaging layer 220 on the outside here is stretched 280 by the bending; however, the existing pretension 270 due to the described thermally influenced adhesive process counteracts the bending force in this layer. With a suitable design of the bias voltage 270, a stress-free state or a state with minimal stress can be achieved in the desired layers as a result.

It goes without saying that the example shown can also be transferred to materials, in particular displays, with more than three layers. All or just some of the various assembled components can be pre-tempered. It is also possible for a plurality of layers to be joined in a plurality of successive steps, with parts that have already been joined being brought to a specific temperature for the first time or again. In addition, the layers shown here themselves can in turn comprise several layers or components; for example, an OLED element is made up of at least partial elements such as anode, cathode, light-emitting layer and conductive layer and can have further protective layers thereover for encapsulation. The considerations mentioned can also be applied within such sub-layers in order to reduce the forces on certain areas in later operation by targeted prestressing.

In addition to influencing the prestress thermally, it is also possible, particularly in the case of elastic materials, to achieve mechanical prestress or compression before the materials are joined or glued. The desired compensating force can also be achieved in this way in the equilibrium state at the operating temperature.

In principle, it is also possible to combine both methods, i.e. both thermally as well as mechanically introduced prestresses.

3 shows an example of a process sequence for producing such a prestressed layer material, in particular a flexible display. In this case, the layered material should comprise a plurality of layers of different materials that are joined together over a large area.

In this case, it is first determined in step 300 which layer of the display is to be protected against strong tensile or compressive stress, or in which layer stresses are to be minimized.

In the next step 310, depending on the area of application, it is checked which states are to be expected in the later application, e.g. bending at a specified bending edge, or flexible rolling up of the entire material surface.

It can also be taken into account how long or how often the respective state is likely to exist.

From this information it is then determined in step 320 which stresses and forces will occur in this state at least in the layer to be protected. This can be determined, for example, by suitable simulations and models.

Then, in step 330, it is checked how large the mechanical prestress must be for compensation, in which areas the prestress should be present and how this can be achieved. The respective coefficients of expansion, moduli of elasticity and other relevant parameters of the materials can be taken into account for thermal prestressing, for example.

Finally, the components as previously associated with 2 described thermally or mechanically prestressed and assembled. In the case of thermal prestressing, in step 340 one or more material layers that have not yet been joined can be heated and/or cooled. After the desired temperatures are reached, the layers can be joined together in a suitable manner in step 350, eg glued. Finally - for example by simply waiting - the entire layer system is brought to operating temperature in step 360 and can then be used in an associated device, e.g. B. be installed on a smartphone. In principle, it is also possible to install the display components before they have cooled down. Optionally, measures can also be taken to control the course of cooling or heating over time, for example by using temperature-controlled underlays.

Claims (12)

A method of making a flexible display device, comprising: Providing (300) at least two material layers (210, 220, 260), which are to be connected to one another over a large area to form a layered material, the at least two material layers (210, 220, 260) having at least one image-forming layer (220) and a flexible carrier layer ( 210) include; Generating (340) a temporary change in length in the direction of surface expansion in at least one of the material layers, the change in length being produced by thermal and/or mechanical action; and Joining (350) the at least two material layers (210, 220) to form a layered material while the change in length is present in at least one of the material layers, so that after the change in length has decreased, a prestress (270) is created within the layered material. procedure after claim 1 , the method further comprising: changing (340) the temperature of at least one of the layers of material (210, 220) from an operating temperature to produce the change in length; Joining (350) the at least two material layers (210, 220) at the respective changed temperature, the changed temperature for the material layers being selected such that after the return to the operating temperature the prestress (270) arises within the layer material. procedure after claim 2 wherein when the temperature is changed (340), a first layer of material (220) is cooled and/or a second layer of material (210) is heated. procedure after claim 2 or 3 wherein the backing layer (210) is heated and wherein at least one adhesive layer (260) is placed between the backing layer (210) and the imaging layer (220) to join the layers of material together. A method according to any one of the preceding claims, further comprising: determining (310, 320) an expected tensile and/or compressive load (280) on at least one of the material layers (210, 220, 260) of the bonded sheet material in an end use of the display device; determining (330) a required preload (270) in the bonded sheet material to at least partially compensate for the expected tensile and/or compressive stress, and creating (340) the transient length change such that the determined required prestress (270) is achieved. Method according to any one of the preceding claims, further comprising: determining the location of a neutral axis in the bonded sheet material, and Generating the prestress within the layer material in such a way that the position of the neutral axis is shifted into a predetermined one of the material layers. A method according to any one of the preceding claims, wherein the two or more layers of material are joined together by gluing (260). A method according to any one of the preceding claims, wherein the display device comprises an OLED display, the imaging layer comprising an OLED layer (220) attached to the support layer (210) by an adhesive layer (260). Flexible display device made of a layered material with two or more surface-connected layers of material, the layers of material comprising at least one image-forming layer (220) and a flexible support layer (210), and wherein a mechanical pretension (270) is present within the layered material, the layered material has been produced by a method according to any one of the preceding claims. display device claim 9 wherein the mechanical preload is configured such that an expected tensile and/or compressive load (280) on at least one of the material layers of the bonded sheet material in an end use of the display device is at least partially compensated by the preload. display device claim 9 or 10 , wherein the imaging layer (220) comprises an OLED layer. Electronic device with a flexible display device according to any one of claims 9 until 11 for displaying information to a user, the display device being rollable and/or foldable and/or bendable in use.

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