ITE Technical Report-Information Display-
Robustness of flexible OLED is improved by use of cover film with low Young's modules

August 2, 2019, “ITE Technical Report-Information Display-” was held in Tokyo. In this here, two oral presentations are picked up.

Figure 1. Relationship of thickness and Young's modules of cover film and change of neutral plane1)

Mr. Sugisaki of Semiconductor Energy Laboratory has reported an original flexible OLED with compatibility of robustness and bendability. It has been developed by original concept, first of all, practical robustness is ensured, the next, a certain bendability is obtained to a maximum extent. Generally, relationship of robustness and bendability becomes to be trade-off relationship. For this reason, thickness of the device is minimized because of bendability, the next, a certain robustness is ensured in the past generally. However, this approach is not sufficient for foldable display and so on, because of insufficient robustness. Therefore, the research group tried to develop by above concept.

Concretely, a cover film with very low Young's modules was used as its key material. Its functions are the following:�@Robustness is enhanced by thickening of device, �AShock resistance is enhanced by relaxation of mechanical damage, �BNeutral plane is not almost changed. Neutral plane means to be zero stress state. If device is bended, stress in this plane is not generated, as a result, robustness is greatly enhanced. For this reason, display device would like to be placed near neutral plane.

Figure 2. Cross-section structure and picture of pilot-produced 8.56 inch. flexible OLED1)

Figure 1 shows relationship of Young's modules and thickness and change of neutral plane. If thickness is thick and Young's module is low, neutral plane is not almost changed. In short, if a cover film with low Young's modules (0.01Gpa and under) is used, a flexible display with practical robustness is realized.

Figure 2 shows cross-section structure and picture of pilot-produced 8.56 inch. flexible OLED. In this device, robustness and bendability were ensured at the same time by making use of a cover film with low Young's module. And also, in spite of RGB independent color OEL method, micro color filter (RGB) and black matrix were adopted on the front substrate due to without circle polarizer. By the way, this panel was bended after 100 thousands at R = 3 �o, display property was almost degraded.

A novel reactive monomer is doped into LC due to alignment film-free

Figure 3. Comparison of SVA and PSA2)

On the other hand, Mr. Inoue of DIC reported a novel reactive monomer (RM) for self vertical alignment (SVA), due to alignment film-free, which is essential for the conventional LCDs. In this method, liquid crystal is self-aligned at keeping routine pretilt angle by this new RM. This method has 3 advantages：�@Bezel is easy to narrow because of contact margin-free among seal layer and alignment layer, �ADye series CF and quantum dot material can be used because of annealing process -free of alignment layer, as a result, color reproducibility is spread, �BTotal manufacturing process cost can be decreased.

Figure 3 shows comparison of the conventional polymer sustained alignment (PSA) and this SVA. In the latter, SVA-RM and PSA-RM are doped into liquid crystal host material, and then, it is filled into LC cell, finally, pretilt angle is obtained by polymerization of RM using irradiation of UV light.

Figure 4. Structure of SVA-RM and estimation result2)

Figure 4 shows estimate result of the test panel. In the latest composition RC (C), pretilt angle stability, VHR, and LTS (low temperature stability) were sufficient for practical use. This is reason why polymer becomes to be rigid and strongly fixed on the surface of ITO electrode by introduction of strong polymeric group.

And also, remained RM after UV irradiation was almost zero, which was same as that of PSA. In short, the conventional UV irradiation process can be adopted, too.

Furthermore, if low pressure UV light (peak wavelength：185 nm and 254 nm) was irradiated to ITO electrode, VA alignment property was drastically improved for mere 10 seconds. This process was confirmed to be effective for mass production.

Reference
1)Sugisawa, et.al.：A Flexible OLED Display with Robustness and Bendability, ITE Technical Report-Information Display-, pp.13-16 (2019.8)
2)Inoue, et.al.：A Novel Reactive Monomer for Self Vertical Alignment, ITE Technical Report-Information Display-, pp.9-12 (2019.8)