The materials used in OLED displays are mainly divided into several categories according to their functions, such as cathode materials, anode materials, buffer layer materials, carrier transport materials and luminescent materials. Their main functions are described below
1. Cathode material
The cathode material of the OLED is used as the cathode of the display screen. In order to improve the electron injection efficiency, a metal material with the lowest possible work function should be selected, because the injection of electrons is more difficult than the injection of holes. The size of the metal work function seriously affects the luminous efficiency and service life of OLED devices. The lower the metal work function, the easier electron injection and the higher the luminous efficiency; in addition, the lower the work function, the lower the organic/metal interface barrier. The lower the temperature, the less Joule heat generated during work, and the device life will be greatly improved.
2. Anode material
The anode material of OLED is mainly used as the anode of the display screen, and its work function is required to be as high as possible in order to improve the hole injection efficiency. OLED devices require that one side of the electrode must be transparent, so ITO conductive glass, a transparent material with a high work function, is usually used as the anode. ITO (indium tin oxide) glass has a transmittance of more than 80% in the wavelength range of 400nm to 1000nm, and also has a high transmittance in the near ultraviolet region.
3. Buffer layer material
The transport rate of holes in OLEDs is about twice that of electrons. In order to prevent light quenching caused by the transport of holes to the organic/metal cathode interface, a buffer layer CuPc is required to be introduced when preparing the device. As a buffer layer, CuPc can not only reduce the interface barrier between ITO/organic layers, but also increase the bonding degree of ITO/organic interface, increase the hole injection contact, inhibit the injection of holes into the HTL layer, and make electrons and The injection of holes is balanced.
4. Carrier Transport Materials
OLED devices require that the holes injected from the anode and the electrons injected from the cathode can be injected into the light-emitting layer in a relatively balanced manner, that is, the injection rates of holes and electrons should be basically the same, so it is necessary to choose a suitable hole and electron transport. Material. During the working process of the device, heat may cause the crystallization of the transport material, resulting in attenuation of the performance of the OLED device, so we should choose a material with a higher glass transition temperature (Tg) as the transport material. In the test, NPB is usually used as the hole transport layer, and Alq3 is used as the electron transport material.
5. Luminescent material
The luminescent material is the most important material in OLED devices. Generally, luminescent materials should have high luminous efficiency, preferably have electron or hole transport properties or both, and can be made into stable and uniform films after vacuum evaporation, and their HOMO and LUMO energies should match the corresponding electrodes and other characteristics. Among the small molecule luminescent materials, Alq3 is directly used alone as the material of the luminescent layer. There are also those that cannot be used as a light-emitting layer alone, and can only emit light when doped in another host material, such as red dopant DCJTB, green dopant DMQA, blue dopant BH1, BD1, etc. Alq3 is an organic material that can be used not only as a light-emitting layer material, but also as an electron-transporting layer material.