The main materials of OLED display

2021.09.15

The materials used in OLED display are mainly divided into cathode materials, anode materials, buffer layer materials, carrier transport materials and luminescent materials according to their functions. Their main features are described below

1. Cathode material

The cathode material of OLED is used as the cathode of the display screen. In order to improve the efficiency of electron injection, the metal material with the lowest work function should be selected, because the injection of electron is more difficult than the injection of hole. The size of metal work function seriously affects the luminescence efficiency and service life of OLED devices. The lower the metal work function is, the easier the electron injection is and the higher the luminescence efficiency is. In addition, the lower the work function, the lower the barrier at the organic/metal interface, the less joule heat will be generated in the operation, and the device lifetime will be greatly improved.

2, anode material

The anode material of OLED is mainly used as the anode of display screen, and its work function is required to be as high as possible in order to improve the efficiency of hole injection. OLED devices require that one side of the electrode must be transparent, so ITO conductive glass, a transparent material with 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 ~ 1000nm, and it also has a high transmittance in the near ultraviolet region.

3, buffer layer material

The hole transport rate in OLED is about twice that of electron transport rate. In order to prevent the hole transport to the organic/metal cathode interface and cause light quenching, the buffer layer CuPc should be introduced when the device is prepared. As a buffer layer, CuPc can not only reduce the interface barrier between ITO/ organic layer, but also increase the adhesion degree of ITO/ organic interface, increase the hole injection contact, inhibit the hole injection into HTL layer, and balance the injection of electrons and holes.

4, carrier transport materials

OLED devices require that the electron energy injected from anode and cathode should be relatively balanced into the luminescent layer, that is, the injection rate of holes and electrons should be basically the same, so it is necessary to select appropriate holes and electron transport materials. During the working process of the device, the heating may cause crystallization of the transmission material, leading to the performance attenuation of the OLED device, so we should choose the material with higher glass transition temperature (Tg) as the transmission material. NPB is usually used as the hole transport layer, while Alq3 is used as the electron transport material.

5. Luminescent materials

Luminescent material is the most important material in OLED device. General luminescent materials should have the characteristics of high luminescence efficiency, preferably electron or hole transport performance or both, stable and uniform films can be made after vacuum evaporation, and their HOMO and LUMO energies should match the corresponding electrodes. Among the small molecule luminescent materials, Alq3 is directly used as a luminescent layer. Some cannot be used as luminescent layer alone, but can only emit light when doped in another matrix material, such as red dopant DCJTB, green dopant DMQA, blue dopant BH1, BD1, etc. Alq3 is a kind of organic material which can be used as luminescent layer material and electron transport layer material.