First, indium tin oxide (ITO) substrate pretreatment
1. ITO surface flatness: ITO has been widely used in the manufacture of commercial display panels, which have the advantages of high transmittance, low resistivity and high work function. In general, ITO fabricated by RF sputtering is susceptible to poor process control factors resulting in surface irregularities, which in turn produce tipped materials or protrusions on the surface. In addition, the high temperature calcination and recrystallization process also produces a raised layer of about 10 to 30 nm on the surface. The path formed between the fine particles of the uneven layer provides the opportunity for holes to directly hit the cathode, and these intricate paths increase the leakage current. There are generally three ways to solve the effect of this surface layer. U is to increase the thickness of the hole injection layer and the hole transport layer to reduce leakage current. This method is mostly used for PLED and thick hole OLED (~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. The increase of ITO work function: When the hole is injected into the HIL by ITO, the excessive potential difference will produce the Schindler barrier, making the hole difficult to inject. Therefore, how to reduce the potential difference of the ITO / HIL interface becomes ITO. The focus of processing. Generally, we use the O2-Plasma method to increase the saturation of oxygen atoms in ITO to increase the work function. The work function of ITO after O2-Plasma treatment can be increased from 4.8eV to 5.2eV, which is very close to the work function of HIL.
When the auxiliary electrode is added, since the OLED is a current driving component, when the external circuit is too long or too thin, a serious voltage gradient will be caused in the external circuit, so that the voltage actually falling on the OLED component is lowered, resulting in a decrease in panel luminous intensity. Since the ITO resistance is too large (10 ohm / square), it is easy to cause unnecessary external power consumption. Adding an auxiliary electrode to lower the voltage gradient becomes a shortcut to increase the luminous efficiency and reduce the driving voltage. Chromium (Cr: Chromium) metal is the material most commonly used as an auxiliary electrode, which has the advantages of good environmental factor stability and greater selectivity to the etching solution. However, its resistance value is 2 ohm / square at 100 nm of the film layer, which is still too large in some applications, so aluminum (Al: Aluminum) metal with a lower resistance value at the same thickness (0.2 ohm / square ) is another preferred option for the auxiliary electrode. However, the high activity of aluminum metal also causes problems in reliability. Therefore, multi-layered auxiliary metals have been proposed, such as Cr / Al / Cr or Mo / Al / Mo, but such processes add complexity and Cost, so the choice of auxiliary electrode materials has become one of the key points in the OLED process.
Second, the cathode process
In high-resolution OLED panels, the fine cathode and cathode are isolated. The general method used is the mushroom structure approach, which is similar to the negative photoresist development technology of printing technology. In the process of negative photoresist development, many process variation factors affect the quality and yield of the cathode. For example, bulk resistance, dielectric constant, high resolution, high Tg, loss of low critical dimension (CD), and proper adhesion to ITO or other organic layers.
Third, the package
1. Water absorbing material: The life cycle of a general OLED is easily affected by the surrounding water vapor and oxygen. There are two main sources of water and gas: one is to penetrate into the component through the external environment, and the other is the moisture absorbed by each layer of material in the OLED process. In order to reduce the entry of moisture into the module or to exclude moisture 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 remove moisture from the components.
2. Process and equipment development: The process of 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 vacuum environment or fill the cavity with an inert gas, for example. Nitrogen. It is worth noting that how to make the two parts of 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 has become the three main objectives of the development of packaging technology and equipment technology. .
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