The materials used in OLED display screens are mainly divided into cathode materials, anode materials, buffer materials, carrier transport materials and luminescent materials by function. Their main features are described below.
- Cathode material
The OLED cathode material is used as the cathode of the display screen. To improve the efficiency of electron injection, the metal material with the lowest work function should be selected, because electron injection is more difficult than hole injection. The size of metal work function seriously affects the luminous efficiency and service life of OLED devices. The lower the metal work function is, the easier the electron injection will be, and the higher the luminous efficiency will be. In addition, the lower the work function, the lower the organic/metal interface barrier, the less joule heat generated in the operation, and the greater the device life.
- Anode material
The anode material of OLED is mainly used as the anode of the display screen, which requires its work function to be as high as possible to improve the injection efficiency of holes. As OLED devices require one side of the electrode to be transparent, ITO conductive glass, a transparent material with high work function, is usually selected as the anode. ITO glass has a transmittance of more than 80% in the wavelength range of 400nm ~ 1000nm, and also has a high transmittance in the near ultraviolet region.
- Buffer layer material
In OLED, the transmission rate of holes is about twice that of electrons. In order to prevent the light quenching caused by the hole transmission to the organic/metal cathode interface, the buffer layer CuPc needs to be introduced in the preparation of devices. As the buffer layer, CuPc can not only reduce the interface barrier between ITO/ organic layers, but also increase the adhesion of ITO/ organic interfaces, increase the hole injection contact, inhibit the injection of holes into the HTL layer, and balance the injection of electrons and holes.
- Carrier transmission material
OLED devices require that the holes injected from the anode and electrons injected from the cathode can be injected into the luminescent layer in a relatively balanced way, which means that the injection rate of holes and electrons should be basically the same, so it is necessary to select the appropriate holes and electron transport materials. In the working process of the device, the heating may cause crystallization of the transmission material, leading to the attenuation of the performance of OLED devices, so we should choose the material with high glass transition temperature (Tg) as the transmission material. NPB is usually used as hole transport layer and Alq3 as electron transport material.
- Luminous materials
Luminescent material is the most important material in OLED devices. Generally, luminescent materials should be characterized by high luminescence efficiency, preferably with electron or hole transport properties or both, stable and uniform films can be made after vacuum evaporation, and their HOMO and LUMO energies should match with corresponding electrodes. Among small molecular luminescent materials, Alq3 is used as luminescent layer directly and alone. Some of them cannot be used as luminescent layer by themselves, but can only be doped in another substrate material, such as red light dopant DCJTB, green light dopant DMQA, blue light dopant BH1, BD1, etc. Alq3 is an organic material that can be used as both luminescent layer material and electron transport layer material.
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