The reason that OLED has excellent color and quality is its self luminosity. The self emission of the display screen means that both light and color are emitted by the pixels themselves. This concept contrasts with the type that uses like LCD, which receives light from an external light source (Bei Guangyuan) and controls the color of the light through a filter.
In a display, the way the pixels are formed is called color patterning. The sub pixels based on the three primary colors of red, green and blue (usually three sub pixels constitute a pixel) must be patterned without errors, so that the screen of the display can display the content exactly and accurately. So how do you make a self - luminescent OLED pixel?
There are various methods in the industry, and the most commonly used method of mass production is evaporation. At present, the only way to achieve high precision, large capacity OLED microelement color patterning is through evaporation.
Evaporation is one of the core processes of OLED, and it is also the second stage of the five major steps of OLED manufacturing process.
[LTPS] - [steam plating] - [package] - [unit] - [module]
If LTPS (low temperature polysilicon) is used to control the various pixels of the luminescence, the process of evaporation is to produce the self luminescent pixel itself that can produce light and color.
Let's look back.
OLED is the organic light-emitting layer emitting red (R), green (G) and blue (B) on the glass substrate, and the structural composition for protecting the organic light-emitting layer. Carefully observing the organic luminescence layer, it can be seen that the auxiliary layers such as HIL and ETL are combined together. This helps to improve the efficiency of luminescence, making the luminous efficiency higher than the light emitted only by RGB.
The most common way to form an organic layer is "vapour plating". Evaporation is similar to evaporation.
When the water is boiled in the pot, the steam is made into the dew on the lid of the pot. The difference is that the steam plating is used to replace water with organic materials and is heated in a vacuum state rather than under normal atmospheric pressure.
The evaporation must be carried out in a vacuum, that is, in a device called a vacuum chamber. The large LTPS backboard is made in the vacuum chamber for color patterning. (after making the color pattern on this substrate, the cell will be cut and used according to the size of the smart phone.)
Once the LTPS is made well and placed in the vacuum chamber, the precise metal mask (FMM) is placed under the LTPS substrate. A mask is a device with a small hole on a thin plate, so when the organic material is steamed, it can only be deposited in a specific position. If the mask is not used, green and blue will be deposited on the red (R) pixels so that the pure color will not be obtained. Therefore, different templet of RGB corresponding position and shape is used at different time during the evaporation process.
When the mask is ready, the steam source (such as organic materials, such as organic materials) is placed under it and heated to the appropriate temperature. As the heating begins, the small organic molecules in the molecular unit pass through the mask and accumulate to the desired position.
We talked about the concept of steam plating, which is one of the core processes of OLED. Then let's take a closer look at the process of color patterning through the evaporation of the plating.
Below, we will continue to talk about one of the key manufacturing processes of OLED, "steam plating". In the last part of the "first part of the steamed plating", we discuss the concept and principle of the evaporation, and we will discuss the specific process of the evaporation.
The OLED evaporation process first forms the organic layer above the LTPS (low temperature polysilicon). Remember that LTPS is a switch used to control the pixels on the display. In OLED, the luminescent pixels are made up of organic materials, which light and color by electrical signals. The control circuit signal is responsible for the LTPS, so the LTPS should form the connection with the OLED layer, and the formation method is completed in the "evaporation" process.
As shown in the figure above, the EML (emitter layer) exists on the LTPS anode. In the picture, we only draw a red subpixel as an example. Carefully observing the structure, it can be seen that the auxiliary layers are above and below the EML to improve the emission efficiency of EML.
The electron is injected from the cathode to the EIL (electron injection layer) and reaches EML through the ETL (electron transport layer).
Similarly, the holes are injected from the anode into the HIL (hole injection layer) on the opposite side and reach EML through the HTL (hole transport layer). When the electrons and holes in the EML meet, they combine and send out light.
The same structure is not only red, but if green and blue organic light layers can be created, they are combined to form a single pixel.
So, what order is the organic light layer made of, let's see the process through the following animations.
The basic OLED evaporation process begins with the work of removing dirt and impurities on the substrate of the LTPS (containing the anode). After cleaning and drying the substrate, the residual anode material is removed by plasma and the hole injection characteristics from the anode to the HIL are improved.
Then, the HIL (cavity injection layer) is completely evaporated, and then the HTL (hole transmission layer) is vapour plated to form an auxiliary layer.
The next is the actual luminescent EML layer, which requires the use of a mask to selectively deposit the desired position.
Subsequently, ETL (deposited electron transport layer) and EIL (electron implanted layer) are evaporated to form an auxiliary layer for electron transport. Finally, the cathode is evaporated to complete the whole deposition process of the organic emission layer.