Uncovering the five major materials that make up OLED

　　1. Anode material

　　The anode material of OLED is mainly used as the anode of the device, 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 selected as the anode. The transmittance of ITO (indium tin oxide) glass is more than 80% in the wavelength range of 400nm to 1000nm, and it also has a high transmittance in the near-ultraviolet region.

　　2. Cathode material

　　The cathode material of OLED is mainly used as the cathode of the device. In order to improve the efficiency of electron injection, metal materials with the lowest work function should be selected, because electron injection is more difficult than hole injection. The size of the metal work function seriously affects the luminous efficiency and service life of the OLED device. The lower the metal work function, the easier the electron injection and the higher the luminous efficiency; in addition, the lower the work function, the lower the organic/metal interface barrier, the less Joule heat will be generated during work, and the device life will be greatly improved.

　　The cathode of OLED usually adopts the following types:

　　(1) Single-layer metal cathode. Such as Al, Mg, Ca, etc., but they are easily oxidized in the air, resulting in device instability and shortened service life. Therefore, alloys are selected as cathodes or buffer layers are added to avoid this problem.

　　(2) Alloy cathode. In order to improve the luminous efficiency of the device and obtain a stable device, a metal alloy is usually used as the cathode. When evaporating a single metal cathode film, a large number of defects will be formed, resulting in poor oxidation resistance; while when evaporating an alloy cathode, a small amount of metal will preferentially diffuse into the defects, making the entire organic layer very stable.

　　(3) Layered cathode. This type of cathode is a barrier layer added between the light-emitting layer and the metal electrode, such as LiF, CsF, RbF, etc., which form a double electrode with Al. The barrier layer can greatly improve the performance of the device.

　　3. Buffer layer material

　　In OLED, the transmission rate of holes is about twice that of electrons. In order to prevent holes from being transmitted to the organic/metal cathode interface and causing light quenching, a buffer layer of CuPc needs to be introduced when preparing the device. CuPc as a buffer layer can not only reduce the interface barrier between the ITO/organic layer, but also increase the adhesion of the ITO/organic interface, increase the hole injection contact, inhibit the injection of holes into the HTL layer, and balance the injection of electrons and holes.

　　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 select suitable hole and electron transport materials. During the operation of the device, heat may cause crystallization of the transport material, resulting in the degradation of the performance of the OLED device, so we should choose materials with a higher glass transition temperature (Tg) as the transport material. In the experiment, NPB is usually selected as the hole transport layer, and Alq3 is selected as the electron transport material.

　　5. Luminescent materials

　　Luminescent materials are the most important materials in OLED devices. Generally, luminescent materials should have high luminescence efficiency, preferably electron or hole transport properties or both, can be made into stable and uniform thin films after vacuum evaporation, and their HOMO and LUMO energies should match those of the corresponding electrodes.

　　Among small molecule luminescent materials, Alq3 is used directly as a luminescent layer material. Some of them cannot be used as a luminescent layer alone, and can only emit light when doped in another matrix 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 a luminescent layer material and an electron transport layer material.