Introduction to OLED key processes

Indium Tin Oxide (ITO) Substrate Pretreatment

(1) ITO surface flatness

ITO is currently widely used in the manufacture of commercial display panels. It has the advantages of high transmittance, low resistivity and high work function. Generally speaking, ITO manufactured by RF sputtering is susceptible to poor process control factors, resulting in uneven surfaces, which in turn produce sharp substances or protrusions on the surface. In addition, the high-temperature sintering and recrystallization process will also produce a protrusion layer of about 10 ~ 30nm on the surface. The paths formed between the fine particles of these uneven layers will provide opportunities for holes to be directly ejected to the cathode, and these intricate paths will increase the leakage current. Generally, there are three ways to solve the influence of this surface layer? The first is to increase the thickness of the hole injection layer and the hole transport layer to reduce the leakage current. This method is mostly used for PLED and OLED with thicker hole layers (~200nm). The second is to reprocess the ITO glass to make the surface smooth. The third is to use other coating methods to make the surface flatter.

(2) Increase in ITO work function

When holes are injected from ITO to HIL, the excessive potential difference will produce a Schottky barrier, making it difficult for holes to be injected. Therefore, how to reduce the potential difference of the ITO / HIL interface becomes the focus of ITO pre-treatment. Generally, we use O2-Plasma to increase the saturation of oxygen atoms in ITO to achieve the purpose of increasing the work function. After O2-Plasma treatment, the work function of ITO can be increased from the original 4.8eV to 5.2eV, which is very close to the work function of HIL.

Adding auxiliary electrodes, since OLED is a current-driven component, when the external line is too long or too thin, it will cause a serious voltage gradient in the external circuit, causing the actual voltage on the OLED component to drop, resulting in a reduction in the panel's luminous intensity. Since ITO has a large resistance (10 ohm/square), it is easy to cause unnecessary external power consumption. Adding an auxiliary electrode to reduce the voltage gradient has become a quick way to increase luminous efficiency and reduce driving voltage. Chromium (Cr: Chromium) metal is the most commonly used material for auxiliary electrodes. It has the advantages of good stability to environmental factors and greater selectivity for etching solutions. However, its resistance value is 2 ohm/square when the film layer is 100nm, which is still too large in some applications. Therefore, aluminum (Al: Aluminum) metal (0.2 ohm/square) with a lower resistance value at the same thickness becomes another better choice for auxiliary electrodes. However, the high activity of aluminum metal also causes reliability issues. Therefore, multi-layer auxiliary metals are proposed, such as Cr/Al/Cr or Mo/Al/Mo. However, this type of process increases complexity and cost, so the selection of auxiliary electrode materials becomes one of the key points in OLED technology.

Cathode Process

In high-resolution OLED panels, the mushroom structure approach is generally used to isolate the tiny cathodes from each other. This process is similar to the negative photoresist development technology of printing technology. During the negative photoresist development process, many process variation factors will affect the quality and yield of the cathode. For example, bulk resistance, dielectric constant, high resolution, high Tg, low critical dimension (CD) loss, and appropriate adhesion interface with ITO or other organic layers.

Encapsulation

(1) Water-absorbent material

Generally, the life cycle of OLED is easily affected by the surrounding water vapor and oxygen and thus reduced. There are two main sources of water vapor: one is the water vapor that penetrates into the component through the external environment, and the other is the water vapor that is absorbed by each layer of material during the OLED process. In order to reduce the water vapor that enters the component or remove the water vapor that is absorbed by the process, the most commonly used material is a water absorbent (Desiccant). Desiccant can capture free-moving water molecules by chemical adsorption or physical adsorption to achieve the purpose of removing water vapor in the component.

(2) Process and equipment development

The packaging process is shown in Figure 4. In order to place the Desiccant on the cover plate and smoothly bond the cover plate to the substrate, it is necessary to perform the process in a vacuum environment or fill the cavity with an inactive gas, such as nitrogen. It is worth noting that how to make the process connection between the cover plate and the substrate more efficient, reduce the packaging process cost, and reduce the packaging time to achieve the best mass production rate have become the three main goals of the development of packaging technology and equipment technology.