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OLED manufacturing principle and component fabrication

OLED manufacturing principle and component fabrication

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The OLED display panel is composed of an n-type organic material, a p-type organic material, a cathode metal, and an anode metal. Electrons (holes) are injected from the cathode (anode), and are conducted through an n-type (p-type) organic material to a light-emitting layer (generally an n-type material), which is emitted by recombination. Generally, on a glass substrate made of an OLED element, ITO is first sputtered as an anode, and then p-type and n-type organic materials and a metal cathode having a low work function are sequentially plated by vacuum thermal evaporation. Since the organic material is liable to react with moisture or oxygen, a dark spot is generated to make the component not bright. Therefore, after the vacuum coating is completed, the component must be packaged in an environment of anhydrous gas and oxygen.

OLED manufacturing principle and component fabrication

Between the cathode metal and the anode ITO, the currently widely used component structure can be generally divided into five layers. The order from the ITO side is a hole injection layer, a hole transport layer, a light-emitting layer, an electron transport layer, and an electron injection layer.

As for the electron transport layer, it is an n-type organic material, which has a high electron mobility, and the lowest non-occupied molecular track of the electron transport layer when electrons pass from the electron transport layer to the hole electron transport layer interface. The LUMO is much higher than the hole transport layer, and electrons do not easily enter the hole transport layer across the barrier, and the germanium is blocked by this interface. At this time, holes are transferred from the hole transport layer to the vicinity of the interface to recombine with electrons to generate excitons, and Exciton emits energy in the form of light emission and non-light emission. In the case of a general fluorescent material system, only 25% of the electron-hole pairs calculated by the selectivity are recombined in the form of light emission, and the remaining 75% of the energy is dissipated in the form of exotherm. In recent years, phosphorescent materials have been actively developed as a new generation of OLED materials that can break the selectivity limit to increase internal quantum efficiency to nearly 100%.

In the two-layer element, the n-type organic material, that is, the electron transport layer, is also used as the light-emitting layer, and the light-emitting wavelength is determined by the energy difference between HOMO and LUMO. However, a good electron transport layer, that is, a material with high electron mobility, is not necessarily a material with good light-emitting efficiency. Therefore, the current general practice is to dope high-fluorescence organic colorants by electron transport. The portion of the layer adjacent to the hole transport layer, also referred to as the light-emitting layer, has a volume ratio of about 1% to 3%. Doping technology development is a key technology for enhancing the fluorescence quantum absorption rate of raw materials. Generally, the selected material is a dye with high fluorescence quantum absorption rate.

The metal material of the cathode is conventionally a metal material (or alloy) having a low work function, such as a magnesium alloy, for electron injection from the cathode to the electron transport layer, and a common practice is to introduce a layer of electron injection layer. It is composed of a very thin low work function metal halide or oxide, such as LiF or Li2O, which can greatly reduce the energy barrier of the cathode and the electron transport layer and lower the driving voltage.

Since the HOMO value of the hole transport layer material is still different from that of ITO, in addition, after long time operation, the ITO anode may release oxygen and destroy the organic layer to generate dark spots. Therefore, a hole injecting layer is interposed between the ITO and the hole transporting layer, and the HOMO value is just between the ITO and the hole transporting layer, which facilitates hole injection into the OLED element, and the characteristics of the film can block the ITO. Oxygen enters the OLED element to extend component life.

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