Researchers are testing the stressful properties of flexible phone screens

In an article in a recently published open access journal Msubstancesthe researchers presented an analysis of the mechanical characteristics of flexible screens.

Research: Analysis of mechanical operation of flexible screens. Video credit: racksuz / Shutterstock.com

Background

Flexible organic light emitting devices (OLEDs) have become increasingly important in smart clothing, flexible screens, and other industries in recent years. The structural stability and optimization of flexible OLED displays has become an important criterion to take into account the portability and economy aspects as OLEDs are increasingly used in the manufacture of displays.

Due to the narrow bending radius, the device is overloaded during the bending operation, causing it to tear off and cause irreparable damage to the screen. The finite element method can be used to determine the mechanism of damage and peeling of the adhesive layer of a flexible OLED screen, which can give the results of stresses and strains.

Using a new technology (ball milling process) and a green halogen-free solvent, a new hole transfer layer for the base material was produced, which not only improved the photoelectric response of the optical monomer thin film device, but also exposed it. good stability under constant stress. However, there is a gap between the charging structure and the actual flexible display module.

Model of the geometric structure of a U-shaped bending mode.  The left reference point was fixed and the bending was performed by moving the mounting board from position A to position B in a bending time of t seconds.  The lateral gap was pR because the bending angle was p and the bending radius was R.

Model of the geometric structure of a U-shaped bending mode. The left reference point was fixed and the bending was performed by moving the mounting board from position A to position B in a bending time of t seconds. The lateral gap was πR because the bending angle was π and the bending radius was R. Image Credit: New, L et al., Materials

About the Studio

In this study, the authors presented a flexible screen bending model using finite element analysis. An imaging experiment was performed to evaluate the increase in Mises stresses with reduced radius of curvature, normal U-shaped bending, and redistribution of tensile and compressive zones. The water droplet bending regime was also investigated to reduce the possibility of structural collapse.

Using ABAQUS finite element software, the team developed a true loading model for the flexible display module. The mechanical behavior of the OLED flexible screen, the shape of the OCA thickness, the effect of the bending radius, and the bending mode (including water droplet and U-shape) were discussed. An imaging experiment was performed to confirm the conclusions of the analysis.

(has) Result of adapting OCA to the Ogden model; (b) Result of the adaptation of the OCA of the Prony model. Video Credit: New, L, etc., Materials

The researchers simulated the mechanical behavior of a flexible screen using a finite element model developed using ABAQUS. The mounting board and the display module were cemented together, and the movement of the mounting plate caused the screen to bend. The bending radius was set to R, the bending angle to π, the initial lateral gap was πR, and the bending was completed in t seconds using the normal U-shaped bending mode.

Multilayer films with various mechanical properties formed a flexible display module. OCA joined the film layers and coordinated the deformation of each layer during the folding process, which was important for the structure of the flexible screen module. In the 180-degree bending simulation, the bending radii were determined to be 3 mm, 2.5 mm, 2 mm, 1.5 mm, and 1 mm, respectively, and the bending time t = 18 s.

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Based on the analysis, bending would not only be effective at short bending radii, but would also reduce the risk of structural failure. The ratio of the discrepancy between the experimental and the modeled range for the same bending radius was less than 1%, which confirmed the accuracy of the finite element model and the experimental data.

The radius of the spherical droplet was 2.286 mm when the radius of curvature was R = 2 mm. The S1 / S5 maximum increase ratio was 13.07%, the S2 / S5 maximum increase ratio was 18.56%, and the S1 / S5 and S2 / S5 absolute deformation increase ratios were 1.0053% and 1.0042%, respectively.

On the OLED, the light-emitting layer profiles S3, S4, and S5 had similar effects, with the highest difference ratios of S3 / S5 and S4 / S5 being 1.27% and 3.61%, respectively. On the other hand, profiles S1 and S2 increased the maximum OLED layer stress by 13.53% and 24.04%, respectively, compared to S5. When the bending radius was 2.5 mm, plastic tension began on the inner side of the CPI layer, in a compressed location. There was no plastic tension in the outer tension area until the bending radius reached 2 mm. Plastic deformation occurred in the outer stress region when the bending radius was 1.5 mm.

The maximum tensile stress was reduced by 23.99% at R = 2.5 mm, but the maximum compressive stress was increased by 15.82%. At R = 3 mm, the maximum compressive stress increased by 20.22%, but the tensile stress decreased by 27.54%.

(a) experimental scheme;  (b) range before bending (units are shown in µm);  c) range after bending (units are shown in µm).

(has) Experimental scheme; (b) range before bending (units are shown in μm); (against) range after bending (units are shown in μm). Video Credit: New, L, etc., Materials

Conclusions

In summary, this study elucidated a flexible screen bending model using finite element analysis. It was observed that the maximum stress of Mises increases rapidly with decreasing bending radius in the overall U-shaped bend. The stack sequences of the OLED and BP layers were optimized, as well as the most damaging single layer and OCA layer profiles.

The choice of layer material was based on the redistribution of tensile and compression zones according to optimization. An imaging experiment was performed to quantify the maximum slip distance during bending to confirm the analysis.

Source

Niu, L., Ding, J., Liu, W. et al. Analysis of mechanical operation of flexible screens. Materials 15 (8) 2829 (2022). https://www.mdpi.com/1996-1944/15/8/2829

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