There are many kinds of materials for preparing OLEDs, which are mainly divided into anode materials, cathode materials, buffer layer materials, carrier transport materials and luminescent materials.
1, anode material
OLED anode material is mainly used for the anode of the device, requiring its work function as high as possible in order to improve the hole injection efficiency. OLED devices require the electrode must have a side is transparent, so usually choose a high function of transparent material ITO conductive glass for the anode. ITO (indium tin oxide) glass in the wavelength range of 400nm ~ 1000nm transmission rate of 80% or more, and in the near ultraviolet region also has a high transmittance.
2, cathode material
OLED cathode material is mainly used for the cathode of the device, in order to improve the efficiency of electronic injection, should use the work function as low as possible metal materials, because the injection of electrons is more difficult than the injection of holes. The higher the work function, the lower the metal work function, the easier the electron injection, the higher the luminous efficiency; In addition, the lower the work function, the organic / metal interface barrier Low, the work will produce less Joule heat, the device life will have a greater increase.
OLED cathode usually use 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, shortened life, so choose the alloy to do the cathode or increase the buffer layer to avoid this problem.
(2) alloy cathode. In order to improve the device's luminous efficiency, but also to get a stable device, usually using metal alloy as the cathode. In the evaporation of a single metal cathode film, will form a large number of defects, resulting in deterioration of oxidation resistance; and vapor deposition alloy cathode, a small amount of metal will be preferentially spread to defects, so that the entire organic layer becomes very stable.
(3) layered cathode. The cathode is a layer of barrier 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
The transfer rate of holes in the OLED is about twice that of the electron transport rate. In order to prevent the quenching of the light caused by the hole transport to the organic / metal cathode interface, the buffer layer CuPc is introduced in the preparation of the device. CuPc as a buffer layer not only reduces the interfacial barrier between ITO / organic layers, but also increases the degree of adhesion of ITO / organic interface, increases hole injection contact, inhibits the injection of holes into HTL layer, Hole injection is balanced.
4, carrier transport materials
OLED devices require that the holes injected from the anode and the electrons injected from the cathode are relatively balanced into the light-emitting layer, that is, the injection rate of holes and electrons should be substantially the same, so it is necessary to select the appropriate hole and electron transport material. In the process of the device, due to heat may cause the transmission material crystallization, resulting in OLED device performance attenuation, so we should choose a higher glass transition temperature (Tg) material as a transmission material. NPB is usually used as the hole transport layer, and Alq3 is used as the electron transport material.
5, light-emitting materials
The luminescent material is the most important material in OLED devices. General luminous materials should have high luminous efficiency, preferably with electron or hole transport properties or both, vacuum deposition can be made after the stable and uniform film, their HOMO and LUMO energy should be matched with the corresponding electrode And other characteristics.
In small-molecule luminescent materials, Alq3 is a material that is used alone as a light-emitting layer. And can not alone as a light-emitting layer, doped in another matrix material to light, such as red dopant DCJTB, green dopant DMQA, blue dopant BH1, BD1 and so on. Alq3 is an organic material that can be used as both a luminescent layer and as an electron transport layer.