The flexible encapsulation process is divided into four stages:
① Cell packaging glass manufacturing
② Using glass sealing
③ Place glass
④ Laser seal
In the above steps, you can see the reused words. It's the word "glass". The ordinary OLED (rigid) panel uses glass as a substrate, and the packaging material also uses glass. This is different from the flexible OLED panel, and it does not need to be bent at first, so even if the top and bottom of the panel are glass, there is no problem.
However, the flexible panel should be flexible, so the glass material can not be used on the panel. Although the glass can be processed thin, it is not enough to be named "flexible". Therefore, in order to make flexible OLED, the substrate must be made of flexible materials called PI (polyimide), not glass.
Not only can it flexibly bend, but it uses a film packaging (TFE) method to effectively prevent air and water from infiltrating into the OLED organic layer.
At first glance, it doesn't seem to be much different from the ordinary rigid OLED package. So let's take a look at the cross section.
In general, each unit of the rigid OLED has a weld seal on the frame. As shown in the picture. The top and side parts of the film package are the same material. It is a whole package, because it is not an additional material cover, but a film packaging material on the panel that completes the steaming process. In order to make flexible flexibly flexible displays, it is necessary to encapsulate the material suitable for this material in the way of film.
When you enlarge the cross section of the film package, you can see that the film package is made up of multiple layers. Unlike the traditional OLED, which is covered with single glass, the film is encapsulated with the multilayer structure of inorganic / organic film. By alternately forming an inorganic membrane / organic film, the permeation path of air and water is prolonged, and it is difficult to reach the luminescent layer.
So, why do we use an inorganic and organic membrane structure?
Ideally, it's better to have a single layer package. Because the thickness is thin, the material is less, and the process will become simple. The use of inorganic thin films can prevent hydrate and air infiltration. However, due to the characteristics of inorganic materials, it is easy to produce particle (small dust) or "pinhole", and air and moisture will permeate through this way. Therefore, only 2 layers or more can be effectively prevented from penetrating.
So what is the role of organic film? Organic film is basically a sponge like substance. Organic molecules have larger gaps than water molecules, so organic molecules can not be used to prevent penetration. Organic films are used for leveling organic membranes for leveling (planarization). Organic thin film is deposited on the first inorganic film formed, which helps to form second layers of inorganic thin film with better quality.
Today, we have studied thin film packaging for flexible OLED manufacturing.
If oxygen (O2) penetrates into the gap of the panel, the contact area (Interface) between the cathode part and the EIL layer at the top of EML will oxidize, so that the cathode and EIL will widen and the contact will become very poor. If this happens, the electrons will not be transmitted normally between the cathode and the EIL, causing some pixels to not glow (black spots).
There is also a problem when water permeates. When water (H2O) penetrates the tiny gap of the panel, hydrogen is separated from the oxygen by electrochemical reaction in the water, and the hydrogen (H2) produces bubbles. These bubbles make the cathode layer float, making it difficult for electrons to transfer between the cathode and the EIL. Like oxygen permeation, this also produces black spots.
In particular, when the package does not work properly, oxygen and moisture will not stay in one place, but continue to spread, and the black spots will continue to spread. So it is very important to encapsulate in the manufacturing phase.
The picture is to show that the actual encapsulation is made on the edge of the entire panel. The actual structure may be different from the picture.
So how does encapsulation block these external effects? The typical OLED package, as shown in the previous figure, is covered with the package glass on the OLED panel on the LTPS substrate. The glass is used to prevent the air and water penetration into the layer between the glass and the panel, frit by laser melting and curing to adhesive and glass panel. The upper side is sealed by glass frit seal, oxygen and moisture barrier, the organic material in the OLED panel can play its function without damage.