Stress distribution layer for improving film stretchability on a soft substrate having a corrugated surface

Abstract

A flexible thin-film device (10) comprising: - a flexible polymer substrate (5) comprising a corrugated surface structure (8), the corrugated surface structure (8) comprising a wave pattern, the wave being one or more curved lines and / or one or more straight lines; - one or more functional layers (7) located on the corrugated surface structure (8), the one or more functional layers (7) defining an upper surface (9) opposite the corrugated surface structure (8); and - a stress distribution layer (6) located on the upper surface (9), the stress distribution layer (6) comprising a flexible polymer layer.

Description

[0001] Invention Field

[0002] This invention relates to a flexible thin-film device capable of bending and stretching. Specifically, the flexible thin-film device has a design in which, due to the corrugated surface structure of the substrate and the stress distribution layer on the upper surface of one or more functional layers, it is not necessary to place one or more functional layers in a neutral plane. Background of the Invention

[0004] The flexible thin-film device industry is a constantly evolving sector. Perhaps the most prominent example is the bendable smartphone screen, which can be bent in a controlled manner. These smartphone screens are not stretchable and, in fact, brittle under tension.

[0005] The functional layer of these screens is sandwiched between two layers, so that any tension is canceled out at the functional layer. Thus, the forces acting on the functional layer are minimized, hence it is called the neutral layer. The requirement to place the functional layer within or at least close to the neutral layer increases manufacturing complexity. The total film thickness is limited by this requirement to manufacture the layer within the neutral layer; in fact, some material will be outside the neutral plane, and the situation is worse for thicker layers. Furthermore, the "neutral layer" method only improves flexural strength, not tensile strength. Therefore, a flexible thin-film device that is not only bendable but also stretchable is needed.

[0006] It would be an additional benefit if the resulting flexible film could function without utilizing the neutral surface, as this would reduce manufacturing tolerances.

[0007] Purpose of the invention

[0008] One object of the present invention is to provide a flexible thin film device that is both bendable and stretchable, thereby making the flexible thin film device more versatile.

[0009] A further object of the present invention is to provide various applications for flexible thin-film devices.

[0010] Invention Description

[0011] One object of the present invention is achieved by a flexible thin-film device, the flexible thin-film device comprising:

[0012] - A flexible polymer substrate comprising a corrugated surface structure, the corrugated surface structure comprising a waveform pattern, the waveform being one or more curves and / or one or more straight lines;

[0013] - One or more functional layers located on the corrugated surface structure, the one or more functional layers defining an upper surface opposite the corrugated surface structure; and

[0014] - Stress distribution layer, which includes a flexible polymer layer on the upper surface.

[0015] Due to the wave pattern, flexible thin-film devices can be both bent and stretched along at least one axis, or, due to the wave pattern, along two or more axes. Depending on the design of the wave pattern, flexible thin-film devices can also be twisted along a certain axis. However, without a stress distribution layer (SDL), one or more functional layers would fracture in most cases, or at least become very brittle.

[0016] Waveform patterns define one or more axes along which a flexible thin-film device can be both bent and stretched. As an example, a sine wave would cause the flexible thin-film device to be stretchable and compressible in a direction perpendicular to the wavefront of the sine wave, thus also allowing the film to be bent. The sine wave is the simplest example, but more complex waves will have one, two, three, or more axes along which the flexible thin-film device can be stretched and compressed, enabling more complex torsion and / or bending and / or stretching of the flexible thin-film device.

[0017] To test resilience to compressive and tensile forces, several flexible thin-film devices have been fabricated with functional layers for light emission on flat surfaces but without stress distribution layers. However, approximately half of these flexible thin-film devices break before testing can proceed due to slight movement between lab workstations. While this can be reduced in the lab with more refined transport procedures, the fact that flexible thin-film devices on planar substrates (such as silicone substrates) suggests they are not well-suited for commercial applications. In most cases, equivalent flexible thin-film devices without SDL on corrugated surfaces can move between lab workstations without breaking. However, some flexible thin-film devices break during transport, indicating that further improvements in the stability of flexible thin-film devices would be beneficial. Simulations show that adding SDL improves the stability of flexible thin-film devices. The instability of flexible thin-film devices is believed to be at least partly due to the substrate being a soft material such as silicone, which leads to uncontrolled movement of the functional layers.

[0018] The waveform pattern can consist of one or more straight lines. These straight lines can be parallel. The straight lines can be divided into two or more groups of straight lines, where one or more straight lines within a group are parallel to each other. There can be two groups of straight lines, where the straight lines from each group are substantially perpendicular. One or more groups of straight lines can have the same amplitude and period, or the amplitude and / or period can differ between one or more groups.

[0019] The waveform pattern can be one or more curves. These curves can be sinusoidal curves that extend horizontally within a flexible polymer substrate. The waveform, even with one or more curves, still fluctuates between maximum and minimum values, just as if the line were a straight line.

[0020] Waveform patterns can be a combination of curves and straight lines.

[0021] A waveform can have a sinusoidal shape, or a portion of a sinusoidal shape, or be defined by two or more sine waves while still having a smooth, continuous shape.

[0022] The flexible polymer substrate and the SDL can be made of the same material, but the materials can be different. The SDL can have a range of different thicknesses independent of the flexible polymer substrate, because this solution does not require utilizing a neutral plane by placing one or more functional layers in the neutral plane; see Tables 1-7.

[0023] One or more functional layers define the upper surface. The upper surface will vary depending on the type and number of functional layers. However, in all cases, the SDL will be located on the upper surface.

[0024] In one aspect, when the flexible thin-film device is in a relaxed state, the SDL may include a substantially flat or structured top surface. Therefore, the highest points of the corrugated surface structure and one or more functional layers are at least covered by the stress distribution layer, thereby reducing the stress in one or more functional layers.

[0025] In one aspect, the flexible polymer substrate can be made of silicone, wherein the silicone undergoes a surface treatment. This surface treatment alters the corrugated surface structure, increasing the adhesion between one or more functional layers and the corrugated surface structure. Consequently, the flexible thin-film device becomes more stable because the risk of one or more functional layers detaching from the corrugated surface structure is reduced.

[0026] In one respect, silicone can be surface-treated by chemical surface treatments (e.g., acid treatment) and / or by electrons (e.g., electrons from an electron beam). Both proposed methods will increase adhesion.

[0027] Electrons can cause SiOx to form on some silicone surfaces.

[0028] In one aspect, the flexible polymer substrate can be made of silicone, wherein the corrugated surface is plasma-treated to form SiOx. The formed SiOx significantly increases the adhesion between the corrugated surface structure and one or more functional layers.

[0029] In one aspect, the stress distribution layer can have a thickness greater than twice the amplitude of the waveform pattern, where the thickness is measured from the minimum value of the waveform pattern to the top surface of the stress distribution layer. The minimum value of the waveform pattern is the trough of the waveform. A thickness greater than twice the amplitude means that the thickness will be greater than the distance between the crests and troughs projected onto the normal of the flexible polymer substrate, causing the entire structure to be covered by the SDL. The SDL must cover the highest point of the substrate. Simulations have shown that this will reduce the average xx component of the stress tensor obtained by integrating along two intersecting lines on the top surface of the oxide film. If the oxide film is replaced by one or more functional layers, the same or similar results are expected.

[0030] In one aspect, the stress distribution layer can have a thickness at least 50 times, or at least 100 times, or 200 times, or 300 times greater than the amplitude of the waveform pattern, where the thickness is measured from the trough of the waveform pattern to the top surface of the stress distribution layer. Simulations show that the stress-reducing effect persists even when the stress distribution layer material is very thick.

[0031] In one aspect, the waveform pattern can include a period-to-amplitude ratio between 0.3 and 50, or between 1 and 45, or between 5 and 30. These ratios ensure that the rate of change is neither too large (thus hindering good adhesion) nor too small (thus causing the waveform pattern to approximate a flat surface), which would prevent or at least limit the flexibility of the film. If the period-to-amplitude ratio is too large, then the rate of change will be so small that the surface appears flat. A flat substrate surface would result in a non-stretchable device.

[0032] In one respect, one or more functional layers may include one or more of the following:

[0033] - One or more encapsulation layers, such as silicon oxide, silicon nitride, metal oxide, or polymer; and / or

[0034] - One or more transparent electrode layers, such as transparent conductive oxides, nanowire-based solutions, metal mesh designs, ultrathin metal layers, or two-dimensional materials; and / or

[0035] - One or more reflective electrode layers, such as metal layers; and / or

[0036] - One or more organic thin film layers comprising active layers of an OLED stack, the OLED stack including an organometallic layer, an emitter layer, a charge injection layer, a transport layer, and a barrier layer; and / or

[0037] - One or more inorganic thin film layers.

[0038] One or more functional layers can be selected depending on the usage or purpose of the flexible thin-film device.

[0039] In one respect, one or more functional layers may not be located in the neutral plane between the flexible polymer substrate and the stress distribution layer. The flexible thin-film device will function regardless of whether one or more functional layers are in the neutral plane.

[0040] In one respect, the flexible polymer substrate and the stress distribution layer can be symmetrical with respect to one or more functional layers. The flexible thin-film device will function regardless of whether one or more functional layers are in the neutral plane; however, the flexible thin-film device will still function if one or more functional layers are placed in the neutral plane.

[0041] In one respect, flexible thin-film devices can be light emitters and / or sensors and / or optoelectronic devices.

[0042] One object of the present invention is achieved by the use of the flexible thin-film device according to any of the foregoing claims or the embodiments in a stretched state on a surface such as a curved surface. This flexibility allows the flexible thin-film device to be placed or adhered to a surface such as a curved surface in a stretched state.

[0043] One object of the present invention is achieved by the use of the flexible thin-film device according to any one of the preceding claims in a surface in an initial planar state, wherein the surface changes size over time. Most surfaces change, expand, and contract over time due to temperature variations, and such changes can be quite significant within a year. This flexibility increases the mechanical stability of the thin-film device. Some solar cells break after ten to twenty years due to mechanical changes over many years.

[0044] One object of the present invention is achieved by the use of the flexible thin-film device according to any one of the preceding claims on a stretchable surface (such as an expandable polymer or rubber surface). Thus, the flexible thin-film device can be used in fabrics and function as a light emitter, a sensor, or a combination thereof. In embodiments where the flexible thin-film device is used in conjunction with a fabric, the flexible thin-film device can also be twisted. Attached Figure Description

[0045] Embodiments of the present invention will be described in conjunction with the accompanying drawings, wherein: Figure 1 A planar cross-sectional view of one embodiment of a flexible thin-film device in a relaxed state according to the present invention is shown.

[0046] Figure 2 The simulation shows the stress distribution in a small portion of a thin film in a flexible thin-film device subjected to 10% strain, with and without a top stress distribution layer.

[0047] Figure 3 The simulation shows the effect of SDL thickness and material on the stress on the thin film in the flexible thin film device 10. Invention Details

[0049] Figure 1 A planar cross-sectional view of an embodiment of a flexible thin-film device 10 in a relaxed state 12 according to the present invention is shown. The flexible thin-film device 10 includes a flexible polymer substrate 5, which includes a corrugated surface structure 8. The corrugated surface structure 8 includes a waveform pattern, wherein the waveform is one or more curves and / or one or more straight lines. The flexible thin-film device 10 also includes one or more functional layers 7 on the corrugated surface structure 8 of the flexible polymer substrate 5. Figure 1 Two functional layers 7 are shown for illustrative purposes only.

[0050] The flexible polymer substrate 5 can be made of silicone, wherein the silicone undergoes a surface treatment. The surface treatment alters the corrugated surface structure 8, thereby increasing the adhesion between one or more functional layers 7 and the corrugated surface structure 8. The silicone can be surface-treated by chemical methods (e.g., acid treatment) and / or by electron methods (e.g., electrons from an electron beam). Both proposed methods will improve adhesion. Electrons can cause partial SiO2 formation on the silicone surface. x .

[0051] The flexible polymer substrate 5 can be made of silicone, wherein the corrugated surface 8 is plasma-treated to form SiO2. x The formed SiO x This greatly improves the adhesion between the corrugated surface structure 8 and one or more functional layers 7.

[0052] One or more functional layers 7 define an upper surface 9 opposite to the corrugated surface structure 8, and an SDL 6 comprising a flexible polymer layer is deposited on the upper surface 9 of the one or more functional layers 7. The upper surface 9 will vary depending on the type and number of functional layers 7. However, in all cases, the SDL 6 will be located on the upper surface 9.

[0053] When the flexible thin-film device 10 is in a relaxed state 12, the SDL 6 may include a substantially flat top surface 11. Therefore, the highest points of the corrugated surface structure 8 and one or more functional layers 7 are at least covered by the SDL 6, thereby reducing stress in the one or more functional layers 7. The flat top surface 11 further ensures that when the flexible thin-film device 10 experiences tensile or torsional forces, the one or more functional layers 7 will experience similar stresses.

[0054] The flexible polymer substrate 5 and SDL 6 can be made of the same material, but the materials may also be different. The stress distribution layer 6 can have a range of different thicknesses independent of the flexible polymer substrate 5, because this solution does not require utilizing the neutral plane 15 by placing one or more functional layers 7 within the neutral plane 15 (see...). Figure 3 ).

[0055] The thickness of stress distribution layer 6 can be greater than the amplitude of the waveform pattern, wherein the thickness 14 is measured from the neutral plane 15 of the waveform pattern to the top surface 11 of the SDL (see...). Figure 3 B). The thickness of SDL 6 can be at least 50 times, or at least 100 times, or 200 times or 300 times greater than the amplitude of the waveform pattern.

[0056] The waveform pattern can consist of one or more straight lines. These straight lines can be parallel. The straight lines can be divided into two or more groups of straight lines, where one or more lines within a group are parallel to each other. There can be two groups of straight lines, where the lines from each group are substantially perpendicular. One or more groups of straight lines can have the same amplitude and period, or the amplitude and / or period can differ between one or more groups.

[0057] The waveform pattern can be one or more curves. These curves can be sinusoidal curves that extend substantially horizontally within the flexible polymer substrate 5. The waveform, which is one or more curves, still fluctuates between maximum and minimum values, just as if the line were a straight line.

[0058] Waveform patterns can be a combination of curves and straight lines.

[0059] The waveform can have a sinusoidal shape, or a portion of a sinusoidal shape, or be defined by two or more sine waves while still having a smooth and continuous shape.

[0060] The waveform pattern can include a period-to-amplitude ratio between 0.3 and 50, or between 1 and 45, or between 5 and 30.

[0061] One or more functional layers 7 may include one or more of the following: - One or more encapsulation layers, such as silicon oxide, silicon nitride, metal oxide, or polymer; and / or - One or more transparent electrode layers, such as transparent conductive oxides, nanowire-based solutions, metal mesh designs, ultrathin metal layers, or two-dimensional materials; and / or - One or more reflective electrode layers, such as metal layers; and / or - One or more organic thin film layers comprising active layers of an OLED stack, the OLED stack including an organometallic layer, an emitter layer, a charge injection layer, a transport layer, and a barrier layer; and / or - One or more inorganic thin film layers.

[0062] In some embodiments, one or more functional layers 7 may not be located in the neutral plane 15 between the flexible polymer substrate 5 and the stress distribution layer 6. In other embodiments, the flexible polymer substrate 5 and the stress distribution layer 6 may be symmetrical with respect to one or more functional layers 7.

[0063] Due to the waveform pattern, the flexible thin-film device 10 can be both bent and stretched along at least one dimension. However, without the stress distribution layer 6, one or more functional layers 7 would, in most cases, break or at least become very brittle.

[0064] In some implementations, the flexible thin-film device 10 may be a light emitter and / or a sensor and / or an optoelectronic device.

[0065] Figure 2 This paper shows a simulation of the stress distribution of a small portion of the thin film in a flexible thin-film device 10 subjected to 10% strain, with and without the stress distribution layer 6 on top. Figure 2 A is a simulation of a 100 nm SiO2 coating without the stress distribution layer 6 on top, while Figure 2 B is a simulation of a 100 nm SiO2 coating with a stress distribution layer 6 on top.

[0066] When the flexible polymer substrate 5 with the corrugated surface structure 8 is stretched, the wavelength of the surface waveform will increase, accompanied by a corresponding decrease in amplitude. Essentially, when this type of flexible polymer substrate 5 is stretched as a whole, the surface will locally bend to accommodate some strain. However, some portions of the corrugated surface 8 will still experience some tensile strain.

[0067] SDL 6 can be made of the same or a different type of substrate material as the flexible polymer substrate 5; however, in this simulation, it is shown to be the same material as the flexible polymer substrate 5, namely silicone. Figure 2 In A and 2B, a small portion of the flexible polymer substrate 5 with a corrugated surface (100 nm SiO2 coating) 7 is subjected to a 10% strain, i.e., the sample is stretched by pulling along arrow S. The initial unstrained model 2 is represented by a graphical wireframe rendering of vertical lines and sine waves, while the strained model 1 is represented as a black and white (grayscale) surface superimposed on the unstrained model 2. The grayscale on the strained surface 1 represents Von Mises stress, ranging from low 3 to high 4, represented by black 3 and white 4 respectively (grayscale 0 (black) - 5 (white) GPa Von Mises stress). The vertical wireframe lines represent the flexible polymer substrate 5 below the corrugated surface 8 and the stress distribution layer 6 above the corrugated surface 8.

[0068] like Figure 2 As shown in Figure A, under tensile condition 13, the stress in the thin film 7 above the corrugated surface structure 8 is unevenly distributed between the crests and troughs of the corrugations. When the stress distribution layer 6 is sandwiched between the thin film 7 and the corrugated surface structure 8 to enclose the layer, the stress distribution becomes more uniform. Figure 2 B). The corrugated surface 8 has no stress distribution layer 6 above it. Figure 2 A and a stress distribution layer 6 composed of a flexible polymer. Figure 2 Comparing B, the calculated maximum Von Mises stress of the thin film layer 7 is reduced by approximately 50% compared to the maximum Von Mises stress calculated for the structure without the flexible polymer stress distribution layer 6. Therefore, in the case including the stress distribution layer 6 ( Figure 2 B) The stress was reduced by about 50%.

[0069] When added to the corrugated upper surface 9 of one or more functional layers 7, the stress distribution layer 6 reduces surface strain compared to the case without the stress distribution layer 6. Therefore, the thin film will experience a significantly lower level of stress, which will result in a significantly longer lifetime for the flexible thin-film device 10.

[0070] Therefore, the flexible thin film device 10 with stress distribution layer 6 can be mounted on a surface that causes the flexible thin film device 10 to stretch (e.g., a surface with a small radius of curvature) and still maintain its function and lifespan.

[0071] Figure 2 The stress reduction illustrated is a specific example for a SiO2 thin film coating. However, the invention of stress distribution layer 6 can also serve as a stress distributor for other single-layer or multi-layer structures used in OLED technology. These include encapsulation layers (silicon oxide and silicon nitride, metal oxide, polymer), transparent electrode layers (transparent conductive oxide, nanowire-based solution, metal mesh design, ultrathin metal layer, and two-dimensional materials), reflective electrode layers (metal layer), and organic thin film layers (organic metal layer, emitter layer, charge injection layer, transport layer, and barrier layer) constituting the active layers of the OLED stack.

[0072] Figure 3 The simulation shows the effect of the stress distribution layer (SDL) 6 thickness 14 and material on the stress experienced by the thin film 7 in the flexible thin film device 10.

[0073] To test resilience to compressive and tensile forces, several flexible thin-film devices 10 have been fabricated, each having a functional layer 7 for light emission but lacking a stress distribution layer 6. However, all of these flexible thin-film devices 10 broke before testing could be performed due to slight movement between laboratory workstations. An equivalent flexible thin-film device 10 with a stress distribution layer 6 can move between laboratory workstations without breaking.

[0074] The stress distribution layer 6 can have a range of different thicknesses independent of the flexible polymer substrate 5, because this solution does not require utilizing the neutral plane 15 by placing one or more functional layers 7 within it. The thickness 14 of the stress distribution layer 6 is not constant because the flexible polymer substrate 5 contains a corrugated surface structure 8; therefore, the thickness 14 is defined as the distance between the top surface 11 of the stress distribution layer 6 and the neutral plane 15 in the sine function defining the waveform pattern of the corrugated surface structure 8, such as... Figure 3 As shown in B.

[0075] Finite element material analysis of the effects of stress distribution layer 6 shows that having stress distribution layer 6 has structural mechanical advantages over leaving the upper layer 9 of one or more functional layers 7 exposed.

[0076] To compare the effects of various stress distribution layer thicknesses 14 and materials on the stress experienced by the 3D waveform patterned oxide layer 7, a single average value (S0) is extracted from the model for each combination studied. xx The extracted value is the average xx component of the stress tensor obtained by integrating along two intersecting lines on the upper surface 9 of the 3D wave-shaped oxide thin film 7 (see...). Figure 3 (The dashed arrow in A).

[0077] A variety of stretchable materials with different densities and Young's moduli were studied (Table 1).

[0078]

[0079] In calculating S with a selected thickness of 14 xx At this time, it can be seen that the total Sxx value has a small thickness dependence. The thicker stress distribution layer (SDL)6 yields a larger mean stress tensor x component (Table 2-6). However, it should be noted that the dependence on thickness 14 is very small; increasing the thickness by 333 times (from 1.2 μm to 400 μm) only increases S... xx Increase by 9.2%. If there is a negative x-component (compression) contribution to the mean, then the mean S xx The value will decrease, therefore the average of the absolute values ​​of the x-components is also listed in Table 2-6. As can be seen from Table 2-6, the calculated average does indeed have a negative contribution, indicating that certain regions of the 3D wavy oxide film 7 will experience compressive stress when stretched on the flexible polymer substrate 5. xx The absolute value of the average ratio of S xx The 37% increase indicates that approximately two-thirds of the stress borne by the 3D corrugated patterned oxide film 7 is tensile stress, and one-third is compressive stress.

[0080]

[0081] Since the variation in thickness 14 has a relatively small impact on the stress resistance of the 3D wave-shaped oxide film 7, it can be assumed that the stress distribution layer 6 works by filling the gaps between the peaks of the 3D wave pattern with a flexible polymer, with the troughs and peaks receiving equal support from the flexible polymer during stretching. Therefore, the stress distribution layer 6 can provide structural stability within the sinusoidal function defining the 3D wave-shaped oxide film 7.

[0082] Further research focused on materials with very low Young's modulus (MPa), which is common in commercially available flexible substrates. The Young's modulus of the already low-modulus material (Elastosil RT745 (low)) was further reduced, and the S... xx (Table 7). A trend can be observed where a lower Young's modulus is more advantageous, because when reducing the Young's modulus of the same material, the calculated average S... xx The lower the value. However, when comparing similar materials with a Young's modulus of 0.1 MPa and 50 MPa, the Young's modulus has little effect on the stress reduction effect of the stress distribution layer 6. Therefore, a 500-fold increase in the Young's modulus only leads to S xx An increase of 15.4%.

[0083]

[0084] In summary, using finite element analysis, the calculated stress showed only minor differences when the material parameters of the stress distribution layer 6 and the thickness 14 were changed. For higher Young's modulus, the calculated stress on the oxide film surface 7 increased. However, a 500-fold increase in Young's modulus only increased the calculated stress on the oxide film surface 7 by 15.4%. The thickness 14 also affected the stress, as a 333-fold increase in the thickness of the stress distribution layer 6 corresponded to only a 9.2% increase in stress. Therefore, there is strong evidence that the stress distribution layer 6 improves the stability of the flexible thin-film device 10 using multiple materials. Furthermore, it is shown that the effect of the stress distribution layer 6 is achieved by filling the gaps between the peaks of the 3D waveform pattern with a flexible polymer, thereby providing a symmetrical and stable plateau.

Claims

1. A flexible thin-film device (10), comprising: - A flexible polymer substrate (5) comprising a corrugated surface structure (8) comprising a waveform pattern, the waveform being one or more curves and / or one or more straight lines; - One or more functional layers (7) on the corrugated surface structure (8), the one or more functional layers (7) defining an upper surface (9) opposite to the corrugated surface structure (8); and - Stress distribution layer (6), which includes a flexible polymer layer on the upper surface (9).

2. The flexible thin film device (10) according to claim 1, wherein when the flexible thin film device (10) is in a relaxed state (12), the stress distribution layer (6) comprises a substantially flat top surface (11) or a structured top surface (11).

3. A flexible thin-film device (10) according to any one of the preceding claims, wherein the flexible polymer substrate (5) is made of silicone and the silicone is surface treated.

4. The flexible thin-film device (10) according to claim 3, wherein the silicone is surface treated by chemical surface treatment, such as acid treatment, and / or by electrons, such as electrons from an electron beam.

5. A flexible thin-film device (19) according to any one of the preceding claims, wherein the flexible polymer substrate (5) is made of silicone, and wherein the corrugated surface (8) is plasma-treated to form SiOx.

6. A flexible thin-film device (10) according to any of the preceding claims, wherein the thickness (14) of the stress distribution layer (6) is greater than twice the amplitude of the waveform pattern, wherein the thickness (14) is measured from the minimum value of the waveform pattern to the top surface (11) of the stress distribution layer (6).

7. The flexible thin film device (10) according to any one of the preceding claims, wherein the thickness (14) of the stress distribution layer (6) is at least 50 times, or at least 100 times, or 200 times, or 300 times greater than the amplitude of the waveform pattern, wherein the thickness (14) is measured from the trough of the waveform pattern to the top surface (11) of the stress distribution layer (6).

8. The flexible thin-film device (10) according to any one of claims 1 to 7, wherein the waveform pattern includes a period-to-amplitude ratio between 0.3 and 50, or between 1 and 45, or between 5 and 30.

9. The flexible thin-film device (10) according to any one of claims 1 to 8, wherein the one or more functional layers (7) comprise one or more of the following: - One or more encapsulation layers, such as silicon oxide, silicon nitride, metal oxide, or polymer; and / or - One or more transparent electrode layers, such as transparent conductive oxides, nanowire-based solutions, metal mesh designs, ultrathin metal layers, or two-dimensional materials; and / or - One or more reflective electrode layers, such as metal layers; and / or - One or more organic thin film layers comprising active layers of an OLED stack, the OLED stack including an organometallic layer, an emitter layer, a charge injection layer, a transport layer, and a barrier layer; and / or - One or more inorganic thin film layers.

10. The flexible thin-film device (10) according to any one of claims 1 to 9, wherein the one or more functional layers (7) are not located in the neutral plane between the flexible polymer substrate (5) and the stress distribution layer (6).

11. A flexible thin-film device (10) according to any one of claims 1 to 9, wherein the flexible polymer substrate (5) and the stress distribution layer (6) are symmetrical with respect to the one or more functional layers (7).

12. A flexible thin-film device (10) according to any one of the preceding claims, wherein the flexible thin-film device (10) is a light emitter and / or a sensor and / or an optoelectronic device.

13. Use of the flexible thin film device (10) according to any one of the preceding claims in a stretched state (13) on a surface such as a curved surface.

14. Use of the flexible thin-film device (10) according to any of the preceding claims in an initial planar state on a surface, wherein the surface changes size over time.

15. Use of the flexible thin-film device (10) according to any one of the preceding claims on a stretchable surface, such as an expandable polymer or rubber surface.

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