There are many kinds of materials for preparing O LEDs , which are mainly divided into anode materials, cathode materials, buffer layer materials, carrier transport materials and luminescent materials.
1, anode material
The anode material of the OLED is mainly used as the anode of the device, and the work function is required to be as high as possible in order to improve the injection efficiency of holes. OLED devices require that the electrodes must be transparent on one side, so a transparent material ITO conductive glass 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 400 nm to 1000 nm, and 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 injection efficiency of electrons, the metal material with the lowest 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 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 more the organic/metal interface barrier Low, the less Joule heat generated during work, the greater the life of the device.
The cathode of an OLED is usually of the following types:
(1) Single layer metal cathode. Such as Al, Mg, Ca, etc., but they are easily oxidized in the air, resulting in unstable devices and shortened service life, so the alloy is selected as a cathode or a buffer layer is added to avoid this problem.
(2) Alloy cathode. In order to improve the luminous efficiency of the device and to obtain a stable device, a metal alloy is usually used as the cathode. When a single metal cathode film is evaporated, a large number of defects are formed, resulting in deterioration of oxidation resistance. When the alloy cathode is vapor-deposited, a small amount of metal preferentially diffuses into the defects, making the entire organic layer stable.
(3) Layered cathode. This cathode is provided with 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. The barrier layer can greatly improve the performance of the device.
3, buffer layer material
The transport rate of holes in the OLED is about twice that of the electron transfer rate. In order to prevent the quenching of light caused by the hole transport to the organic/metal cathode interface, a buffer layer CuPc needs to be introduced in the preparation of the device. As a buffer layer, CuPc can not only reduce the interface barrier between ITO/organic layers, but also increase the adhesion of ITO/organic interface, increase 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 material
OLED devices require that the holes injected from the anode and the electrons injected from the cathode be injected into the luminescent layer in a relatively balanced manner, that is, the injection rates of holes and electrons should be substantially the same, so it is necessary to select suitable hole and electron transport. material. During the operation of the device, because the heat may cause the transmission material to crystallize, resulting in the performance degradation of the OLED device, we should choose a material with a higher glass transition temperature (Tg) as the transmission material. In the test, NPB was generally selected as the hole transport layer, and Alq3 was 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 luminous efficiency, preferably have electron or hole transport properties or both, and can be made into a stable and uniform film after vacuum evaporation, and their HOMO and LUMO energy should match the corresponding electrodes. And other characteristics.
Among the small molecule luminescent materials, Alq3 is a material directly used alone as a light-emitting layer. In addition, it cannot be used as a light-emitting layer alone, and can be doped in another matrix material, such as red light dopant DCJTB, green light dopant DMQA, blue light dopant BH1, BD1, and the like. Alq3 is an organic material that can be used as both a light-emitting layer material and an electron transport layer material.
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