According to the IHS 2016 Touch Sensing Technology and Market Report analysis, the on-cell touch solution not only advances the display market towards flexible products but is also a fundamental technology for Y-OCTA touch control.
Smartphone OLED panel manufacturers, especially Samsung Display, are continuously working to integrate touch screen structures into displays to achieve film integration and reduce panel thickness. The transition from rigid OLED panels to flexible OLED panels is particularly prominent, enabling the realization of ultra-thin displays that can be folded or even rolled.
According to the IHS 2016 Touch Sensing Technology and Market Report analysis, the on-cell touch solution not only advances the display market towards flexible products but is also a fundamental technology for Y-OCTA touch control. Essentially, Y-OCTA integrates the touch sensing part of the screen to form a separate touch screen layer on the display. Y-OCTA refers to a special structure adopted by Samsung Display in its latest flexible OLED panels, which Samsung has specifically named Y-OCTA.
Rigid OLED: On-cell TSP
The following image shows the traditional structure of a rigid OLED display, where the on-cell touch sensor panel (TSP) can be seen on the surface of the OLED encapsulation glass. Therefore, the rigid OLED panel consists of two glass substrates: the OLED encapsulation glass and the glass substrate on the OLED backplane.
Between the encapsulation and backplane glass, there is a sealed encapsulation (made of a melt or resin); the TSP sensor is located on the surface of the encapsulation glass. In the protective cover section, the OLED polarizer and the protective cover are laminated through an Optical Clear Adhesive (OCA) or Optical Resin (OCR). In this case, the TSP (touch sensor) is located at the upper end of the encapsulation glass.
Overall, the entire OLED module consists of 8 layers, including touch and protective cover. From top to bottom, they are: protective cover, decorative film, OCA/OCR, polarizer, TSP sensor, encapsulation glass, OLED light-emitting layer, and bottom glass substrate. This is a relatively traditional rigid OLED On-cell TSP structure.
Rigid OLED: In-cell TSP
In this structure, the TSP shifts from on-cell to in-cell, so the TSP (touch sensor) is located beneath the encapsulation glass. Apart from the difference in the position of the TSP, the in-cell structure is essentially the same as the rigid OLED on-cell TSP structure.
While the number of layers remains the same at 8, the order is slightly different. From top to bottom, they are: protective cover, decorative film, OCA/OCR, polarizer, encapsulation glass, TSP sensor, OLED light-emitting layer, and bottom glass substrate. In this structure, the in-cell touch screen has higher sensitivity and faster responsiveness.
Flexible OLED: TSP Sensor Placed on Plastic Film
In flexible OLED displays, the glass substrate is replaced by a plastic substrate. Theoretically, due to the different physical properties, it is challenging to laminate the TSP sensor onto the plastic substrate. Therefore, flexible OLED panel manufacturers, especially Samsung Display and LG Display, have developed a special plastic layer called TSP base film. The TSP base film is laminated with the OLED panel through OCA/OCR and serves only to support the touch TSP sensor.
With the addition of the TSP base film, the entire structure reaches 10 layers. From top to bottom, these 10 layers are: protective panel, decorative film, OCA/OCR, TSP sensor, TSP base film, OCA/OCR, OLED polarizer, encapsulation layer, OLED light-emitting layer, and bottom plastic substrate. Due to the increased number of layers and the more complex structure, panel manufacturers are beginning to seek better solutions to achieve a flexible plastic substrate.
Flexible OLED: TSP Sensor Placed on Polarizer
The process of adding the TSP (touch sensor) base film complicates the use of TSP on plastic films, so the new concept is to combine the TSP base film with the OLED polarizer. Through the further development and application of OLED thin film encapsulation (TFE) and the combination of polarizer and TSP base film, the need for OCA/OCR layers during lamination is eliminated. This successfully reduces the number of layers from 10 to 8.
As shown in the image above, this 8-layer structure consists of: protective panel, decorative film, OCA/OCR, TSP sensor, OLED polarizer, thin film encapsulation (TFE), OLED light-emitting layer, and bottom plastic substrate.
Flexible OLED, Y-OCTA: TSP Sensor Placed in Thin Film Encapsulation
As the most advanced structure currently, the special structure named “Y-OCTA” is developed by Samsung Display specifically for flexible displays. As described in the on-cell touch solution, it is a significant advancement in flexible AMOLED development. Its biggest feature is placing the TSP sensor directly on top of the thin film encapsulation (TFE) to achieve the on-cell solution. Compared to the add-on touch structure, the on-cell structure has the following advantages:
Since the touch layer is located beneath the polarizer, the on-cell structure further enhances the optical properties of the touch and curved bending areas, allowing manufacturers to use materials like Indium Tin Oxide (ITO) to create transparent conductive films. The structural advantages are as follows:
1. The polarizer is placed closer to the cover glass, solving some visibility issues at the curved edges of the panel.
2. The on-cell type does not require a supporting film, which negatively affects preventing deformation of the touch layer in the curved bending area during transportation. Eliminating the supporting film helps reduce panel thickness, lower production costs, and simplify lamination processing.
3. Compared to the add-on (external) type, the on-cell structure has fewer layers, making it more suitable for flexible products.
In this case, the layers from top to bottom are: protective panel, decorative film, OCA/OCR, polarizer, TSP placed in thin film encapsulation, thin film encapsulation, OLED light-emitting layer, and bottom plastic substrate.
The Y-OCTA touch structure is used in the flexible OLED panel of the latest Samsung Galaxy Note 7 Edge. “Y” stands for “Youm Samsung’s new panel,” which is Samsung’s name for its flexible AMOLED. Samsung Display has secured a strong foothold in the smartphone market through its OCTA technology and hopes to lead its competitors in the flexible OLED market as well. This is also the main reason for its continued adoption of this flexible touch solution in the Galaxy S6 and subsequent models.
Perhaps Samsung Display merely chose on-cell touch technology as a transitional technology towards Y-OCTA. However, the company seems to have also considered the developmental trends of component suppliers and new panel manufacturers, which is more likely to be a decisive reason for the choice. Furthermore, if manufacturers can reduce the number of polarizers and touch layers, the innovation of the on-cell structure can rival that of Y-OCTA. With advancements in optical film microstructures, IHS also anticipates that the current flexible OLED touch panel structure will undergo many revolutionary changes.
