The Manufacturing Principle and Component Manufacturing of OLED
OLED display components are composed of n-type organic materials, p-type organic materials, cathode metals and anode metals. Electrons (holes) are injected by the cathode (anode), conducted by n-type (p-type) organic materials to the light-emitting layer (generally n-type materials), and emitted light by recombination. Generally speaking, the glass substrate made of OLED components is first sputtered with ITO as the anode, and then plated with p-type and n-type organic materials and metal cathodes with low work function in order by vacuum hot evaporation. Because organic materials are easy to interact with water vapor or oxygen, dark spots are generated and the component does not shine. Therefore, after vacuum coating, this component must be packaged in an environment without water gas and oxygen.
Between the cathode metal and the anode ITO, the currently widely used component structure can generally be divided into 5 layers. From the side close to the ITO, the order is: hole injection layer, hole transport layer, luminescent layer, electron transport layer, electron injection layer.
As for the electron transport layer, it is an n-type organic material with high electron mobility, when the electron from the electron transport layer to the hole electron transport layer interface, because the lowest non-occupying molecular orbital domain of the electron transport layer is much higher than the LUMO of the hole transport layer, the electron is not easy to cross this barrier into the hole transport layer, so it is blocked at this interface. At this time, the hole is transmitted from the hole transport layer to the vicinity of the interface and recombines with electrons to produce excitons, and Exciton will release energy in the form of light and non-light. For general fluorescent material systems, only 25% of the electron-hole pairs are recombined in the form of light emission by the calculation of the selection rate, and the remaining 75% of the energy is dissipated in the form of heat release. In recent years, phosphorescent materials are being actively developed to become a new generation of OLED materials, which can break the limitation of selection rate to improve internal quantum efficiency to close to 100%.
In both layers, the n-type organic material - the electron transport layer - is also used as the light-emitting layer, and its emission 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 emission efficiency, so the current general practice is to doped a high fluorescent organic pigment in the electron transport layer near the hole transport layer, also known as the luminescent layer, with a volume ratio of about 1% to 3%. Doping technology development is a key technology used to enhance the fluorescent quantum absorption rate of raw materials, and the material selected is generally a dye with high fluorescent quantum absorption rate.
Cathode metal materials, traditionally use low-power function metal materials (or alloys), such as magnesium alloys, to facilitate electron injection from the cathode to the electron transport layer, in addition, a common practice, is to introduce an electron injection layer, which constitutes an extremely thin low-power function metal halide or oxide, such as LiF or Li2O, which can greatly reduce the energy barrier of the cathode and electron transport layer, reduce the driving voltage.
Since there is still a gap between the HOMO value of the hole transport layer material and the ITO, in addition, the ITO anode may release oxygen after long-term operation, and destroy the organic layer to produce dark spots. Therefore, between ITO and hole transport layer, a hole injection layer is inserted, and its HOMO value is exactly between ITO and hole transport layer, which is conducive to hole injection into OLED components, and the characteristics of its thin film can block oxygen in ITO from entering the OLED element to prolong the life of the component.
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