The influence of the bending mode on the mechanical behavior of AMOLEDs

Structural optimization has always been the main solution for increasing the stability of flexible screens. The bending method used to test a flexible display screen is an important factor in assessing the functionality and practicability of the electronic device. The bending mode of the display screen impacts the screen service life. In this study, 2D simulation models with different bending radii, bending directions, and bending times were constructed in ABAQUS and compared to determine the effects on the flexible screen from changing the stress value, the location of the stress concentration area and the deformation state of the screen under U- and water-drop-shaped bending. The simulation results show that U-shaped bending severely strains the junction between the bent and unbent areas of a model, whereas water-drop-shaped bending can avoid such problems. In addition, the results of multiple bending experiments were used to determine an appropriate reserved length of r=2.1~2.4 mm for a model under water-drop-shaped bending.

movement mode of a screen considerably influences the recyclability of a flexible screen.
Currently, the U-shaped bending method is generally used for simulation [10][11][12][13][14]. Completely overlapping the two sides of a screen many times to perform U-shaped bending leaves creases in the screen [15,16]. However, using water droplets for bending changes the curve bending trajectory and leaves an unbent area, which helps avoid screen creasing. This novel bending method can extend screen service life and facilitates the design of non-crease flexible display screens.
Using water droplets to bend a screen reduces the linear distance between the bent and unbent areas, and the reduced volume of the folded screen improves portability.
In this work, ABAQUS is applied to establish U-and drop-shaped multiple bending methods on the stress and strain of a flexible panel. Unlike the commonly used single bending model, the simulation results of multiple bending intuitively show that the bending radius directly affects the final screen state after repeated bending. Moreover, not just the maximum stress which is normally used, the recovery degree of the panel group has been an important factor to measure the bending situation in this article and make the results of simulation more reliable.

A. STRUCTURE OF THE SCREEN GROUP
A simulation was performed to analyze the bending of an AMOLED flexible screen, for which the finite element analysis method was used to establish a 2D plane model in ABAQUS. A rectangle with a total length of 15 mm was set as the longitudinal plane of the flexible screen, of which the material properties are listed in Table 1. The 2D model adopted tetrahedral mesh to avoid shear locking, and most of films had been divided into three layers. The element type was CPS4R. The maximal difference in stress distributions along the bending symmetry plane only amounts to 2.6 %, compared to original mesh method. The relative error was far from 5%, which means the result was reliable. (1) In the equation above, J is the ratio of the volume after deformation to that before deformation. I1 is the first strain tensor invariant. N, Ci0 and Di are input parameters.
The viscoelastic behavior of the OCA was determined by the time-varying shear modulus and bulk modulus. These moduli were expressed using the Prony model in the simulation [20]. (2) G0 and K0 denote the instantaneous shear and bulk moduli determined by the instantaneous elastic modulus. and are Prony constants, and τi is a state variable that controls the stress relaxation.

C. THICKNESS OF OCA
To develop a flexible panel with a suitable thickness and performance, four simulation schemes with different thicknesses were proposed, as shown in Table 2. The four schemes were used to simulate U-shaped bending with an inner bending radius of 2.5 mm, and the optimal structure was selected based on the strain changes of the film layer.
The optimal model structure was determined by simulating four structural schemes with different OCA layer thicknesses.
As the strains in the polarizer layer (POL) and the touch screen layer (TP) were less than 0.1, as shown in Fig. 1, the strains in the cover glass layer (cover film), OLED layer, and backplane film (BPF) were carefully observed to select the optimal structure. The model structure for Scheme 1 had relatively small strains in the three functional layers, but the mesh deformation was found to be relatively large during the simulation process. Although the stress on the cover layer for Scheme 2 was quite high, there was no discernible strain in the OLED and backplane layers, and the formed module was relatively thin but able to produce an ideal simulation result.

D. BENDING METHOD
Two bending modes, U-shaped bending and water-dropshaped bending were simulated, as shown in Fig. 2. In the Ushaped bending mode, the reserved length of the bending area was l=π*R , and the bending slip was calculated as ∆l=(π-1)*R/2. For drop-shaped bending, the length of the reserved area was also l=π*R, but the slip was ∆l=π*R/2 .
During the first five seconds, the shaded part of panel rotated clockwise and counterclockwise around the reference point at 0.314 rad/s, while moving upwards at (π-1)*R/2 mm in U-shaped bending or π*R/2 mm in drop-shaped bending, and then back to place for 5 seconds. The whole process would be repeated for 10 times. Furthermore, each analysis step was calculated from 1E-03, the minimum increment size was 1E-15.

A. STRAIN CHANGES AT THE JUNCTION
In the case of U-shaped bending, the maximum force on the screen is exerted at the junction of the bending area and the undeformed area, as shown in Fig. 3  the junction of the screen is correspondingly reduced, as shown in Fig. 3 (d). According to reduces the load at the junction and reduces the strain on the joint compared to U-shaped bending, the maximum strain of the back plate is much higher, as shown in Fig. 3 (e).
Therefore, the state of the backplane should be carefully observed when studying drop-shaped bending.

B. THE INFLUENCE OF BENDING DIRECTION
In water-drop-shaped bending, the central region of the outward bending area of the screen is the area with the maximum strain. As the strain at the junction is not discernible (as shown in Fig. 4 (a)), the overall strain of the module must be determined by carefully observing the changes in the central area of the screen. As there is no obvious difference in the stress and strain of the screen when folded inwards and outwards, the analysis should focus on the change trend for a bending direction. As shown in Fig. 4 (a), the maximum strain of the module gradually decreases as the radius increases. The overall stress change of the panel exhibited the same trend, although the stress dropped sharply in the R=1.5~2.0 mm range, as shown in Fig. 4 (b). Therefore, the ideal bending radius is R>2.0 mm

C. MULTIPLE BENDS
The continuous strain results for a screen that has been bent ten times using different bending radii and bending methods are used to estimate the damage degree for the screen. As shown in Fig. 5 (d) and (e), ten repeated bending cycles for the screen produce wrinkles in unexpected areas.
Under U-shaped bending, the largest deformation is concentrated in the junction area between the cover film and the back plate. However, under water-drop-shaped bending, the OCA layer in the middle of the junction area exhibits the largest deformation. Therefore, under water-drop-shaped bending, the optical clear adhesive layer bears the highest stress. At R=2.4 mm, the screen undergoes a large deformation after ten water drop bends, which is not conducive to long-term screen use (as shown in Fig. 5 (a)).
Bending the screen multiple times, as shown in Fig. 5 (c), also changes the trend in the strain for the area of maximum strain of the screen. This result shows that the wrinkles of the screen group from the bending cycles were superimposed and increasing the bending radius affected the trend in the extension of the maximum strain area of the bent screen.
Under U-shaped bending, the back plate of the screen bears the highest stress, and exhibites the most noticeable deformation after multiple bending cycles. However, 6 VOLUME XX, 2017 changing the bending method to water-drop-shaped bending results in a significant reduction of the strain at the junction of the backplane, and the backplane protrudes upward as the bending radius increases, as shown in Fig. 5 (d). Under water-drop-shaped bending, the adhesive layer is squeezed into the middle area between the outer and inner bending zones due to the influence of the stress directed at both ends. Therefore, the screen group bulges on both sides of the central area after multiple bends, which does not occur under U-shaped bending. In the following experiment, the panel presents the same shape after 200,000 drop-shaped bending, as shown in Fig 6 (d). When the bending radius increases to 3 mm, the layers near cover film will be peeled off, as Fig 6 (b) and (c) shown. Furthermore, the noticeable crease has been left after 100,000 times drop-shaped bending, according to Fig 6 (a). However, reducing the bending radius weakens bulging. Therefore, a small bending radius is more appropriate for simulation design of water-drop-shaped bending. Changing the bending radius also affects the area over which the maximum stress is concentrated, that is, 7 VOLUME XX, 2017 decreasing the bending radius reduces the area over which the force is concentrated. Therefore, if a small bending radius is used in a module design, the thickness of each neutral layer needs to be further optimized.

IV. CONCLUSIONS
A simulation experiment was performed using a 2D model constructed in ABAQUS, the results of the simulation experiments showed that that an appropriate increase in the OCA thickness reduces the strain in the functional layer.
However, an OCA layer with a thickness over 175m would significantly squeeze the functional layer, and the increased film pressure would hinder strain reduction.
The drop-shape bending method significantly reduced the pressure in the boundary area between the unbent and bent areas. However, the water-drop-shaped bending method considerably increased the stress in the central area of the screen compared to that produced under U-shaped bending.
The study on the influence of the reserved length of the bending area on the strain showed that the optimal bending effect was obtained for a bending radius between 2.1 and 2.4 mm, the main force-bearing area of the screen lied within the neutral layer. According to the location of maximum force area, optimizing the mechanical properties of the cover glass may become a good choice to improve the screen life.
However, there is a tradeoff between a low stiffness and reducing screen scratches for practical applications, which should be an important consideration for future optimization studies.