OCA challenges OLED foldable screens, what material is it?

Some people say that 2021 was a year of explosive growth for foldable smartphones. In addition to Huawei, Samsung, Motorola, and Rouyu, which have already ventured into foldable screen smartphones, it is said that OPPO, vivo, and Xiaomi will also join the battle.

Although foldable screen phones are still a relatively niche product, do these powerful manufacturers entering the market one after another mean that the "foldable future" is becoming more and more within reach?

It cannot be denied that with the early exploration and careful iteration of manufacturers such as Huawei, Samsung, Motorola, and Rouyu, foldable screen phones on the market have become increasingly sophisticated and practical.

For those manufacturers who later entered the market, they have a greater advantage as latecomers, fully drawing on the experience of those pioneers, especially in how to reduce creases.

I don't know if you still remember that two years ago, Samsung's first foldable screen phone had a large number of design defects when it was first launched. Hurriedly recalling and patching, and hastily relisting.

Samsung Galaxy Fold screen quality issues

Although many "friendly merchants" were gloating at the time, to be fair, it is unfair to solely blame the engineering design for the screen issue this time. If we have to blame it, we can only blame the mobile phone manufacturers for having too "rich" ideals, but the reality level of material technology is relatively "bone like"!

Don't be fooled by the fact that a "foldable screen" is a pure brand electronic product, but what truly makes it a "foldable screen" are several important material technologies! The most crucial among them is having bendable performance Optical adhesive filmOCA technology!

The 'Heavenly Mandate' falls upon OCA

Don't be fooled by the word "glue" in the name of Optical Adhesive Film (OCA), its main task is to make the screen more "beautiful" compared to the "adhesive" function.

Since the refractive index is basically the same as glass (1.47-1.48), applying this OCA adhesive film between the cover glass and the display module can significantly reduce the reflected light at the interface, thereby greatly improving brightness and contrast.

The working principle and effect of Optical Adhesive Film (OCA)

That's why in those years when mobile phone screens were not yet "bent", it can be said that OCA film only needed to lie there and do nothing to instantly enhance the "appearance" of the display screen!

But unfortunately, the world is changing rapidly, and no one expected that mobile phone manufacturers would suddenly need screens that can be bent or straightened in just one second. So the comfortable days of OCA adhesive film being "out of work without effort" came to an end!

Don't be fooled by the fact that the "folding screen" module is just a layer of film thinner than paper, but if you enlarge the cross-section by 1000 times, you will find that it is actually a "thousand layer cake" composed of n layers of functional films!

The AMOLED film is responsible for emitting light, the touch film is responsible for collecting pressing signals, the polarizer increases brightness, and the thin film cover is responsible for resisting external impacts... And there must be OCA adhesive film between these functional films.

Construction of Folding Screen (Simplified)

(Foldable OLED Displays Whitepaper_Joel T. Abrahamson)

Considering that foldable screens need to withstand hundreds of "bending and straightening" times a day, the OCA film not only improves the display effect of the screen, but also provides mechanical support for various fragile functional layers. No matter what kind of damage the folding screen encounters, it is necessary to ensure the structural stability of each layer of functional film!

However, judging from the performance of the first generation foldable screen phones in the past year, there are still many problems with the foldable OCA film. The hardest hit area is the "crease" area that all foldable screens cannot get rid of.

The "creases" on the screens of three mainstream models (Zhihu @ Dieci)

If you open Station B to search for "folding screen", you can see that a large number of UP owners are particularly prone to bad spots at the crease position of the roast screen!

The main reason for this is Folding screenThe mechanical performance of the bending position is very complex, and traditional OCA adhesive film is completely inadequate! And the new foldable OCA adhesive film has a short working time, which is bound to cause many bonding failures and stress damage problems!

At this point, it is necessary to introduce a concept from material mechanics - the "neutral layer"

neutral layer

 

In fact, the mechanical structure of a "folding screen" is similar to that of a magazine. If we bend the magazine, each page will experience displacement under external pressure, with the paper closer to the inner side experiencing more displacement.

Stress caused by bending on objects (source: official account @ adhesive film matrix)

This phenomenon tells us that external forces will decompose into two opposing forces at the bending position——

• The outer side of the bending part decomposes into "tensile stress" pointing to both sides, and this area is called the "tensile stress layer";

• The inner side of the bending part decomposes into a "compressive stress" pointing towards the middle position, which is called the "compressive stress layer".

Tensile stress layer vs compressive stress layer

(Module Mechanics Study of Flexible AMOLED Display - Party Pengle)

When the various functional films of the "folding screen" are firmly adhered into a whole by OCA adhesive film, then in a certain layer area in the middle, the "extrusion stress" and "tensile stress" will cancel each other out. This area is called the 'neutral layer'!

Neutral layer - safe zone with zero stress

(Module Mechanics Study of Flexible AMOLED Display - Party Pengle)

The 'neutral layer' is a safe area where there is basically no stress acting, thus greatly reducing the probability of failure!

But the functional membranes outside the 'neutral layer' are more difficult to handle: if OCA adhesive film adhesionIf the functional film is too strong, it is prone to fracture due to the inability to release tensile stress in the "tensile stress layer"; However, if the adhesive strength of OCA film is weak, the film in the "compressive stress layer" will peel off and delaminate under the action of compressive stress.

Tensile stress layer vs compressive stress layer

(Module Mechanics Study of Flexible AMOLED Display - Party Pengle)

As an OCA adhesive film, it is mission impossible to simultaneously meet these two completely opposite demands! No wonder the crease position of foldable screen phones is so prone to problems!

Since the cause of the problem has been analyzed, the solution to the problem is also very clear - creating multiple neutral layers!

Folding screens are prone to breakage, OCA should strive for self-improvement

 

Because if the overall "neutral layer" of the folding screen is dispersed on each functional film, it means that the functional films of each layer are in the "safe zone". The originally irreconcilable contradiction has been resolved invisibly, and there is no longer the problem of you being crushed by compressive stress, while I am torn apart by tensile stress.

Disperse the neutral layer and create multiple 'safe zones'

(Optically Clear Adhesives for OLED_Joel T. Abrahamson)

The key to implementing this plan lies in making OCA film have higher shear strain! The general meaning is that once the folding screen is bent, the OCA film will have a significant amount of shear deformation. Try to ensure that each functional layer is not constrained by adjacent layers and has space to achieve relatively independent sliding.

Shear strain of OCA adhesive film

(Optically Clear Adhesives for OLED_Joel T. Abrahamson)

The advantage of this is that the stress on each functional layer is greatly reduced, and for each functional film, it has an independent neutral layer.

After discussing theory for a long time, we still need some hardcore data to connect with reality.

OCA8211 is 3M ™ The classic ultra-thin optical adhesive film produced by the company has a thickness of only 25 microns and high bonding strength, making it very suitable for bonding glass cover plates; And 3M ™ CEF3501 is an optical film product specifically designed for foldable screens, with the selling point being its very low storage modulus! The translation is that this adhesive film is particularly soft and can easily have significant shear strain.

Comparison of Physical Properties of Two OCA Films

(Optically Clear Adhesives for OLED_Joel T. Abrahamson;  3m.com)

By using FEA mechanical simulation software, it can be seen that the folding screens with CEF3501 and OCA8211 optical adhesive films exhibit relative displacement of the functional layer.

Shear strain of CEF3501 optical adhesive film

(Optically Clear Adhesives for OLED_Joel T. Abrahamson)

However, due to the ultra-low storage modulus of CEF3501, which is 60% lower than OCA8211, the larger shear strain of CEF3501 causes a greater relative sliding of the folding screen functional layer. And this key performance directly determines the mechanical performance of the folding screen during the bending process!

Mechanical behavior of tensile stress layer

The tensile stress layer, which is the outermost part of the bending, is the focus of everyone's attention. From the FEA simulation results, it can be seen that the bending point of the screen using OCA8211 bears much greater stress than CEF3501.

Mechanical behavior of tensile stress layer in CEF3501 vs OCA8211

(Optically Clear Adhesives for OLED_Joel T. Abrahamson)

Sure enough, the amount of screen stretching using OCA8211 is actually 200% of that of CEF3501.

Stretch deformation rate of folding screen after using different OCA adhesive films

(Optically Clear Adhesives for OLED_Joel T. Abrahamson)

Such a large tensile deformation can lead to severe material fatigue, and the tensile stress layer of the screen only has one outcome after multiple bending - fracture!

The tensile stress layer fractures due to tensile stress after repeated bending

(Module Mechanics Study of Flexible AMOLED Display - Party Pengle)

Mechanical behavior of compressive stress layer

The mechanical situation of the compressive stress layer is quite complex, generally subjected to stress in both horizontal and vertical directions:

• Horizontal compressive stress pointing from both sides towards the center;
• After the compressive stress meets, it synthesizes a vertical peeling force!

The stress here is not only about squeezing and deforming the functional film, but also about tearing it off from the OCA adhesive film.

The compressive stress layer delaminates and delaminates due to compressive stress after repeated bending

(Module Mechanics Study of Flexible AMOLED Display - Party Pengle)

From the FEA simulation results, it can be seen that after bending and unfolding, the inner compressive stress layers of both groups undergo warping deformation. However, due to the lower storage modulus of CEF3501 and the greater stress changes, the degree of warpage of the test sample is much lower than that of OCA8211.

Mechanical behavior of tensile stress layer in CEF3501 vs OCA8211

(Optically Clear Adhesives for OLED_Joel T. Abrahamson)

From the product parameters, it can be seen that CEF3501 has the same adhesive strength as OCA8211! Maintaining the same adhesive strength with a significant reduction of modulus by 60% means that CEF3501 has significantly improved its adhesive performance.

Comparison of Physical Properties of Two OCA Films

(Optically Clear Adhesives for OLED_Joel T. Abrahamson;  3m.com)

So far, through the testing data of 3M's CEF3501, we have seen the technological direction to make "foldable screens" more "sturdy and resistant to manufacturing". (Reference source for the above adhesive material content: I choose adhesive)

Now that we know that the biggest problem with folding is creases, which come from OCA and stress, let's take a look at how to evaluate and quantify the bending stress risk of a folding screen product?

Let's take a closer look at the folding belowBending stress simulation of OLED screens:

Flexible OLED screenDuring the bending process, it is easy to cause device damage, adhesive layer peeling, and other phenomena. The main way to solve this problem is to adjust the stress neutral layer position of the display layer and the strain of the optical transparent adhesive (OCA) layer.

This article establishes a bending simulation model for flexible OLED screens, focusing on exploring the factors that affect the position of the stress neutral layer at the display layer, in order to make adjustments and avoid excessive damage to the display layer caused by the bending process. At the same time, it reduces the strain of the adhesive layer and improves the problem of adhesive layer peeling.

Simulate and analyze the bending process of flexible OLED screens, and compare the results when bent 90 ° and left to stand for 300 seconds,

The stacking structure of each layer of materials, the stiffness of the protective cover plate, the thickness of the OCA adhesive layer, the thickness of the backing plate, and the bending radius have an impact on the stress of the display layer and the strain of each OCA adhesive layer,

By adjusting material parameters and structures, the goal is to improve and eliminate the above-mentioned defects.

• Simulation model establishment

Geometric model establishment and boundary condition setting

The flexible OLED screen is composed of multiple layers, with a screen length of 150mm and a rigid body set below. Its size parameters are shown in Figure 1 (a).

 

Figure 1 Geometric Model

When bending, the screen is driven to bend by the rotation of the rigid body. In order to form a circular arc at the bending part and minimize the force, the distance between the axis of symmetry and the reference point is set to R * π/4. When the radius is 5mm, the distance is 7.85mm. In addition, the reference point needs to move to the left (π/4-1) * R, which is 2.85mm, to finally reach the position shown in Figure 1 (b).

The screen structure, from top to bottom, consists of a protective cover plate OCA、 Touch layer OCA、 Polarizing film OCA、 Display layer, substrate OCA、 The thickness of each part of the backboard is shown in Figure 2.

 

Figure 2 Screen Structure

This article uses Abaqus simulation software to simplify the simulation model into a two-dimensional plane strain problem. The given boundary conditions are: within the first 1 second, the rigid body rotates counterclockwise around the reference point at a speed of 1.57 rad/s, while moving to the left at a speed of 2.85 mm/s; Subsequently, let it stand for 300 seconds to simulate actual usage.

Mesh

The analysis model is divided into tetrahedral grids with a length dimension of 0.025 mm, and all structures in the thickness direction are divided into three layers. Select the type of grid element as plane strain, hybrid, or reduced integral element.

Material model

The elastic modulus of the protective cover plate is 5.6 GPa; The elastic modulus of the backplate is 4.076 GPa; The selected OCA adhesive material has super elasticity and viscoelasticity, and is an incompressible material. Among them, hyperelasticity describes the nonlinear elastic behavior during the deformation process; Viscoelasticity describes the relationship between material properties and strain rate.

There are various forms of hyperelastic constitutive models, such as Ogden, Mooney Rivlin, Van der Waals, Yeoh, etc. Due to the good fitting results between the Yeoh model and experimental data, as well as strong computational convergence, this paper adopts the Yeoh model, with specific parameters shown in Table 1.

Table 1 Yeoh Model Parameters

C10	C20	C30
0.010 605 3	-0.000 120 6	1.731 8×10-6

The viscoelastic parameters can be obtained by normalizing the shear relaxation experimental data and inputting it, as shown in Figure 3.

Figure 3 Viscoelastic data

• results and discussion

Comparison of different stacking structures

When exploring the influence of different stacking structures on the stress neutral layer of the display layer, as the display layer and substrate are integrated, the display layer and packaging layer were selected as blank controls. Based on this, other materials were added to compare 5 different structures:

1. Display layer substrate;

2. Polarized OCA display layer substrate;

3. Touch layer OCA polarizer OCA display layer substrate;

4. Protective cover OCA touch layer OCA polarizer OCA display layer substrate;

5. Full structural model.

Comparing the stress neutral layer changes of five structural display layers when bent at 90 ° and left to stand for 300 seconds, the results are shown in Figure 4.

Figure 4: The Influence of Different Stacking Structures on the Stress of the Display Layer

so

• On the basis of the display layer and substrate, adding a polarizer significantly shifts the stress neutral layer upward;

• After adding the touch layer, the stress neutral layer continues to move upward;
• The addition of the protective cover did not have any impact;
• But after adding the back plate, the position of the stress neutral layer changed significantly, with a large degree of downward movement, and basically returned to its initial position.

It can be observed that structural changes and adjustments have a significant impact on the position of the stress neutral layer.

Comparison of different stiffness of protective cover plates

The protective cover plate belongs to the membrane layer that is relatively easy to adjust in flexible screen structures. Based on this, the material parameters of the protective cover plate are adjusted to explore the effect of its stiffness on stress and strain.

Comparative analysis was conducted on protective cover plates with elastic moduli of 2.8 GPa, 5.6 GPa, 11.2 GPa, 28.0 GPa, and 56.0 GPa to discuss their effects on the stress neutral layer of the display layer and the strain of the OCA adhesive layer. The results are shown in Figure 5:

Figure 5 Comparison of different stiffness of protective cover plates

From Figure 5, it can be seen that:

• At different stiffness levels, the stress curves of the display layer basically overlap, indicating that the influence of the protective cover plate stiffness on the stress neutral layer of the display layer can be ignored;
• The strain of OCA adhesive layer changes significantly with the change of stiffness ->the elastic modulus of the protective cover plate increases, and the strain of each OCA adhesive layer also increases to varying degrees.

The stiffness change of the protective cover plate only has a significant impact on the stress of this layer. As the stiffness increases, the stress of this layer increases significantly, so the strain of the adjacent OCA adhesive layer also changes, indirectly affecting the strain of other adhesive layers. Properly reducing the stiffness of the protective cover plate is beneficial for optimizing the screen bending process.

Comparison of different thicknesses of OCA adhesive layer under protective cover plate

OCA adhesive layer is used for bonding between different film layers in the screen, and its thickness is easy to adjust. Therefore, it is necessary to explore the influence of OCA adhesive layer thickness on stress-strain.

For comparison purposes, only the thickness of the OCA adhesive layer under the protective cover was changed, with thicknesses of 25 μ m, 50 μ m, 75 μ m, 100 μ m, and 125 μ m, respectively. The results are shown in Figure 6.

Figure 6 Comparison of OCA thicknesses under protective cover plate

From Figure 6, it can be seen that:

• At different OCA thicknesses, the stress curves of the display layer basically overlap, indicating that the influence of OCA adhesive layer thickness on the stress neutral layer of the display layer can be ignored;

• The strain of each OCA adhesive layer is closely related to the thickness variation of the OCA adhesive layer

• As the thickness of the OCA adhesive layer under the protective cover increases, the strain of this layer decreases significantly, and the strain of other layers also decreases slightly. The increase in thickness of OCA adhesive layer only has a significant impact on the adjacent protective cover and touch layer.

On the one hand, the protective cover plate has an increased curvature during bending due to the increase in adhesive layer thickness, which leads to an increase in stress and suppresses the reduction of adhesive layer strain; On the other hand, as the thickness of the adhesive layer increases, the stress on the adhesive layer per unit length decreases, which promotes the reduction of strain and has a more significant effect. Therefore, the strain of the adhesive layer ultimately shows a decreasing trend.

To reduce the risk of peeling off the protective cover, the thickness of the OCA adhesive material at the bottom can be appropriately increased.

Comparison of different thicknesses of backboards

The back panel is often located at the bottom of the flexible screen, providing protection and support for optical components. Therefore, the impact of the back panel on the neutral layer of the display layer also needs to be explored.

Comparative analysis was conducted using backplates with thicknesses of 25 μ m, 50 μ m, 75 μ m, 100 μ m, and 125 μ m, as shown in Figure 7.

Figure 7 Comparison of Backboard Thicknesses

so

• The thickness of the backplate has a significant impact on the stress neutral layer position and adhesive layer strain of the display layer.

• The increase in the thickness of the back panel leads to an increase in the bending curvature of the display layer and a shift in the position of the stress neutral layer.

Therefore, increasing the thickness of the backplane is beneficial for reducing the tensile stress on OLED devices and providing protection for the display layer, but it is not conducive to reducing the strain of the adhesive layer, and needs to be adjusted comprehensively.

Comparison of different bending radii

Flexible OLED screenThe bending radius is a factor that must be considered in design. When studying the effect of bending radius on screens, comparisons were made using radii of 3mm, 4mm, 5mm, 6mm, and 7mm, as shown in Figure 8.

Figure 8 Comparison of bending radii

It can be seen that:

• As the bending radius increases, the stress in the display layer decreases, and the strain change in the adhesive layer is not significant.

• The increase in bending radius improves the overall stress situation of the structure.

• conclusion

This article simulates and analyzes the stress-strain of flexible OLED screens during bending, and discusses four factors: stacking structure, glass cover stiffness, OCA adhesive layer thickness under the glass cover, bending radius, etc. The effects of these factors on the position of the stress neutral layer in the display layer and the thickness of each OCA adhesive layer are compared and analyzed. The following conclusions are drawn:

(1) Different stacking structures have a significant impact on the position of the stress neutral layer in the display layer, which is an important factor causing changes in the position of the stress neutral layer.

(2) When the stiffness of the protective cover plate increased from 2.8 GP to 56 GPa, the position of the stress neutral layer in the display layer remained basically unchanged, but the strain of each OCA adhesive layer increased.

(3) When the thickness of the OCA adhesive layer under the protective cover plate increased from 25 μ m to 125 μ m, the position of the stress neutral layer in the display layer remained basically unchanged, but the strain in this layer decreased significantly.

(4) When the thickness of the backplate increases from 25 μ m to 125 μ m, the stress neutral layer position of the display layer shifts downwards, and the strain of the adjacent OCA adhesive layer increases.

(5) When the bending radius increases from 3 mm to 7 mm, there is a significant decrease in the stress of the display layer, and the strain change of the OCA adhesive layer is not significant.

Based on the above, it can be concluded that:

Appropriately reducing the stiffness of the protective cover plate and the thickness of the back plate, and increasing the thickness of the OCA under the protective cover plate, are all beneficial for improving the phenomenon of adhesive layer peeling; Increasing the thickness of the back panel is beneficial for reducing the tensile stress of the display layer, and increasing the bending radius is beneficial for optimizing the overall stress of the structure, which can provide protection for the display layer.

(The above simulation content material reference: LCD and Display, Volume 33, Issue 7, 2018, 555-560)

Regarding the issue of creases in foldable screens, it can be said that since the birth of foldable phones, various manufacturers have been striving to overcome this difficulty. In addition to improving the internal structure and materials, there are also many detailed improvement methods.

For example, Samsung uses UTG glass as a folding material in its latest generation products, and the hinges can still maintain a certain folding angle for the screen in a closed state, further reducing creases compared to the previous generation products;

Huawei, on the first two generations of foldable screen phones, reduced creases through external folding and clever internal mechanical structures.

Perhaps soon, people will see various new forms of products such as scroll screen phones.