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Flecible AMOLED touch
May 31, 2018

In the flexible panel, it can be divided into three levels: "Bendable", "Foldable" and "Rollable". Flexiability is the most easily realized state of a flexible screen. The display screen is bent at a fixed angle, such as the two mobile phones of Samsung's S6Edge and S7 Edge; and the folding and flexibable screens can be flexed freely on one surface and on any surface, so the two forms of flexible panels will make the manufacturing process and material for the display screen. Put forward new requirements, including the re selection of touch technology solutions. The traditional LCD screen based touch control technology is very different from the AMOLED flexible screen, such as the substrate material, the package material and the overall multi-layer light-emitting device, so the touch control technology of the AMOLED flexible screen will not simply copy the traditional way. Because of its inherent flexibility and flexibility, thin film materials are expected to be reused and developed rapidly in the era of AMOLED flexible screen.

Two, AMOLED flexible screen can not use glass as substrate and packaging material, resulting in changes in touch schemes.

The touch control technology can be divided into external and embedded. External touch technology (Out-cell) is dominated by touch manufacturers, which can be divided into glass and film methods. Later, as the cell phone became thinner, the panel manufacturers launched embedded touch technology to integrate the panel and touch unit. Embedded touch technology can be divided into two categories: On-cell and In-cell. The high-end mobile phone touch program also gradually shifted from plug-in to embedded, such as apple iPhone's In-cell TFT-LCD. Compared with the glass mode and the film mode, the In-cell and On-cell schemes save the glass cost and the fitting cost, and make the module light weight, high transmittance, and meet the quality requirements of the high-end mobile phone.

In 2016, the global touch screen shipped 1 billion 470 million units, including 820 million embedded units, accounting for 55.6%, the first time exceeded the plug-in touch module. Whether it is plug-in or embedded, it is suitable for LCD display touch technology, and manufacturing technology has matured.

Figure 6: the plug-in touch technology of the glass way GG (left) and the film type plug-in touch technology GF (right).

Diagram 7:GG/GF schematic diagram of touch control technology

The external touch control technique of glass mode is to make all the touch sensors on a layer of ITO glass, and the film mode is to replace the ITO glass with ITO film.

Table 1, comparison of glass mode and film mode

For glass, the ITO glass and the glass cover can be integrated into one piece of glass, and the touch sensor is placed directly on the glass, which is also called the "monolithic glass solution" -OGS.

Figure 8: monolithic glass solution OGS and its structural schematic diagram

The In-cell touch mode is to put the touch control module inside the LCD panel, and the On-Cell touch mode is to put the touch sensor outside the LCD module and the polarizer.

Figure 9:In-cell (left) and On-cell (right) embedded touch technology structure schematic diagram

Figure 10: a OLED hard screen (left) coated with glass or metal cover with a flexible polymer material as a substrate and a OLED flexible screen (right) structure packaged with single or multilayer film materials.

For the AMOLED screen, the hard screen uses the glass substrate and the glass cover plate, and the flexible panel is packaged with flexible polymer substrates and thin film because of the glass brittleness. Then there are differences in the choice of touch schemes.

In addition to the In-cell method, the selection of touch technology is similar to that of the LCD screen, which is because In-Cell has to embed a touch sensor in the pixel and must be embedded in it, otherwise it can easily lead to a false touch signal or a large noise, which means the process process. Complexity and difficulty will increase. The liquid crystal layer in LCD adopts the printing coating method, and the yield is high, while the organic layers in the OLED are coated with the evaporation method, the technical requirements are high, the difficulty is great, so the good rate is also low. Adding another layer of touch IC in the evaporating structure will definitely affect the yield. The processing yield of OLED has always been low. If In-Cell is used, it will lead to lower yield. So there is no In-cell touch scheme for AMOLED screen at present.

At present, the touchdown technology of Samsung AMOLED hard screen is On-cell. As Samsung uses the RGB Tri Color Pixel Technology, there is no color filter under the package glass. It can be used to carry on the electroplating of the touch sensing electrode first and then package the OLED light emitting device. Samsung's On-cell touch control scheme is very difficult, and returns to the deep ploughing in the AMOLED display field for many years, and the good rate of its panel production can be controlled between 70%-85%. The probability that other panel manufacturers adopt the On-cell touch scheme is not likely to be affected by the yield. Instead, it will switch to the GF/GG/OGS scheme with high yield. The AMOLED screen has attracted the attention of the market.

Figure 11: schematic diagram of Samsung's hard screen embedded On-cell touch scheme

Three. AMOLED soft screen requires a touch scheme based on thin film materials.

On-cell: because the light-emitting devices in the AMOLED flexible screen are coated with thin film, the touch induction electrodes are not like glass packaging, and then they are plated on the glass before they are returned to the package, but before the induction electrodes are all steamed before the light emitting devices, which will increase the challenge of the good rate again. And steam the plating. The etching process will also damage the encapsulated film. So for AMOLED flexible screens, On-cell is probably not.

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