OLED Process Introduction

OLED Process Introduction

Indium Tin Oxide (ITO) Substrate Pretreatment
　　(1) ITO Surface Smoothness
　　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 prone to surface unevenness due to poor process control factors, which in turn produces surface sharp substances or protrusions. 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 smoother.
　　(2) Increase of 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 at 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 an auxiliary electrode, 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 voltage actually falling on the OLED component to drop, resulting in a reduction in the panel's luminous intensity. Since ITO resistance is too large (10 ohm/square), it is easy to cause unnecessary external power consumption. Adding an auxiliary electrode to reduce the voltage gradient has become a shortcut to increase luminous efficiency and reduce driving voltage. Chromium (Cr: Chromium) metal is the most commonly used material for auxiliary electrodes. It has advantages such as good stability to environmental factors and greater selectivity to 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 makes it have 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 has become one of the key points in OLED technology.

　　Cathode Process

　　In high-resolution OLED panels, the method generally used to isolate the tiny cathode from the cathode is the mushroom structure approach, which 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, the loss of bulk resistance, dielectric constant, high resolution, high Tg, low critical dimension (CD), and appropriate adhesion interface with ITO or other organic layers.
　　Packaging
　　(1) Water-absorbing materials
　　The life cycle of a general OLED is easily affected by the surrounding water vapor and oxygen and is reduced. There are two main sources of water vapor: one is the penetration into the component through the external environment, and the other is the water vapor absorbed by each layer of material during the OLED process. In order to reduce the entry of water vapor into the component or remove the water vapor adsorbed by the process, the most commonly used material is a water-absorbing material (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 carry out 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 process and equipment technology.