OLED core materials mainly include anode, cathode, transport layer materials and light-emitting layer materials, as well as film materials and packaging materials.
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 efficiency of hole injection. At the same time, OLED devices require that one side of the electrode must be transparent. Therefore, Au, transparent and conductive are generally used. Polymer (such as polyaniline) and ITO conductive glass, ITO glass is commonly used.
The cathode material of OLED is mainly used as the cathode of the device. The lower the metal work function of the cathode material, the easier the injection of electrons, the higher the luminous efficiency, the less Joule heat generated during operation, and the longer the life of the device. The improvement.
The cathode of OLED usually adopts the following types:
Single-layer metal cathode. Such as Ag, Al, Li, Mg, Ca, In, etc., but they are easily oxidized in the air, resulting in unstable devices and shortened service life.
Alloy cathode. Such as Mg:Ag (10:1), Li:Al (0.6%Li) alloy electrode, the active low work function metal and the chemically stable high work function metal are evaporated together to form a metal cathode, which improves the quantum efficiency of the device. stability.
Layered cathode. Add a barrier layer between the light-emitting layer and the metal electrode, such as LiF, CsF, RbF, etc., which form a double electrode with Al, which can obtain higher luminous efficiency and better I-V characteristic curve.
Doped composite electrode. Sandwiching an organic layer doped with a low work function metal between the cathode and the organic light-emitting layer can greatly improve the device performance, such as ITO/NPD/AlQ/AlQ(Li)/Al.
Transmission layer material
OLED devices require that the injection rate of holes and electrons into the light-emitting layer should be basically the same, so it is necessary to select appropriate hole and electron transport materials.
In the working process of the device, heat may cause the transmission material to crystallize and cause the performance of the OLED device to degrade. Therefore, a material with a higher glass transition temperature should be selected as the transmission material. In the experiment, NPB is usually used as the hole transport layer, and Alq3 is used as the electron transport material.
Light-emitting layer material
Light-emitting materials are the most important materials in OLED devices. Generally, light-emitting materials should have high luminous efficiency and good electron or hole transport properties. According to the molecular structure of the compound, organic light-emitting materials are generally divided into two categories:
High polymer. It is usually conductive conjugated polymer or semiconductor conjugated polymer, which can be formed into a film by spin coating. It is simple to produce and low in cost, but its purity is not easy to improve, and it is inferior to small molecular organic compounds in terms of durability, brightness and color.
Small molecular organic compounds. The film can be formed by vacuum evaporation method, and is divided into organic small molecule light-emitting materials and complex light-emitting materials according to the molecular structure.
Organic small molecule light-emitting materials are mainly organic dyes, which have the advantages of strong chemical modification, wide selection, easy purification, high quantum efficiency, and can produce red, green, blue, yellow and other color emission peaks, but most of them are in solid state. There are problems such as concentration quenching.
The complex luminescent materials are between organic and inorganic materials. They have both the high fluorescence quantum efficiency of organic materials and the high stability of inorganic materials. They are regarded as a kind of luminescent materials with great application prospects.