The main materials of organic electroluminescent devices should have good film-forming properties, that is, uniform films can be easily prepared by vacuum evaporation or rotating coating, and should have high quantum properties, as well as certain carrier transport characteristics and good photothermal stability. According to the molecular structure of the compound, the host materials can be divided into small molecule, macromolecule and triplet state materials.
According to the composition and structure of small molecule light emitting materials, the small molecule light emitting materials are divided into organic small molecule light emitting materials, metal complexes and rare earth complexes light emitting materials
Organic small molecule light emitting materials have many advantages. The small molecule material is easy to purify and has high purity. It is easy to prepare high quality films. In general, however, small molecules have limited carrier transport capabilities. Metal complexes are a kind of commonly used small molecular materials, which have high fluorescence quantum efficiency, good carrier transport properties, good optical and thermal stability.
ALq3 is one of the most widely used organic electroluminescent materials, which has many advantages, such as good film quality, high carrier mobility and good stability. It is both an electroluminescent material and an electronic transport material, so it can also be used as an electronic transport layer.
At present, PPV (polyphenylene vinylene) is still the most concerned class of light-emitting polymers. It has strong electroluminescence properties due to its high molecular weight and can form high-quality thin films. Currently, many derivatives have been developed, and fully conjugated polymers without substituents are insoluble. However, by adding elastic side chains to the polymer backbone, the conjugated polymers of aromatic groups can have better processability, and the elastic side chains also increase the steric hindrance of the polymer backbone. The chain can control the effective conjugate length, which can determine the color of the polymer, such as MEM-PPV, BuEH-PPV and other polymers.
In fluorescent materials, the transition from the triplet excited state to the ground state is forbidden, while in some phosphorescent materials, the triplet state can release energy in the form of recombination radiation, so that the highest theoretical quantum efficiency of the phosphorescent device can reach 100%, far exceeding the theoretical limit of 25% for fluorescent devices.
Due to the obvious advantages of phosphorescent devices in energy utilization, phosphorescent materials have become a research hotspot, and many triplet materials have also been reported, such as green materials Ir(ppY)3, Ir(TfmpfpY)2, red materials PtOEP, BtpPt (acac), blue material Ir(dfppY)2(pic), etc. They make full use of the energy of excited triplet states, significantly improve the external quantum efficiency of the device, and are a very promising class of light emitting materials.