Samsung Electronics is slated to launch a foldable smartphone next year. China is also spurring the development of foldable smartphones, starting with Huawei. The foldable smartphone is a seven-inch smartphone when unfolded into one display and 5 inches when folded. The technological difficulties involved are at a different level compared with existing two-display-equipped foldable smartphones.
Foldable OLEDs differ in structure and materials compared to flexible OLEDs applied to existing edge and full-screen types. The key is the cover window, OCA (Optical Clear Adhesive), and polarizer. The most important point of the foldable OLED is that it should not break even if folded frequently. It must be foldable and able to withstand external impacts.Previously, glass was used as a cover window. Glass is difficult to fold, thus, ultra-thin glass or film material should be used as the cover window.
Currently, the most popular cover material for foldable OLEDs is a transparent polyimide (PI) film. Hard coating is applied to the top and bottom to compensate for the weak hardness. Although the hardness increases as the hard coating thicken, it can also cause it to break easily; thus, it is important to adjust and balance the thickness and hardness of the coating.
Having been recently developed, transparent PI film for the cover window is known to have the hardness of about 4~6H. A hard coating technique is also being announced that achieves a high hardness of 8H or more while maintaining the folding durability. At the same time, because consumers are adapted to glass-like feelings, which requires implementation of glass-like properties.
The second is PSA (Pressure Sensitive Adhesive Application) material. PSA is an adhesive material used for pasting each layer and is used in about 4 to 5 layers in OLED. OCA is a type of PSA that is used to attach a cover window and a polarizer.
In addition to top and bottom bonding, the foldable OLED should also help relieve the stress applied to the cover window and the polarizer when folded then unfolded. The role of OCA is important because the cover window and the polarizers are the thickest and the closest to the outside. The existing flexible OLED OCA is about 100~150 um thick. In the foldable OLED, the optimum material and thickness should be developed to minimize the stress applied to the top and bottom and maintain the adhesive force while lowering the thickness to 100 μm or less. Other than OCA, PSA development for multiple layers is required.
The last one is a polarizer. The polarizer is a material that must be included in OLED. The polarizing plate minimizes the reflection of external light, enhances outdoor visibility, and accurately represents black. The polarizer currently used in mass production is about 100-150 μm thick. Foldable OLEDs should have a thinner polarizer.
It is expected that thin polarizers with a thickness of 70um or less will be applied to the first foldable OLED. As the radius of curvature decreases, the thickness of the polarizer must be thinner. However, existing polarizers have limitations in reducing the thickness. Therefore, a "coated polarizer" for coating a material having a polarizing function on a cover window, in addition to a touch film or a structure without a polarizer, is being developed.
Moreover, the development of new folder structure materials and new structures are underway. In the near future, Samsung Electronics will launch smartphones using Foldable OLED. It is expected that more consumers will select foldable phones when there is not the only development in terms of panels but also design innovation as a finished product.