Electrode materials and carrier transport materials for flexible OLEDs

The proper selection of organic materials can greatly improve the luminescence performance of flexible OLED devices. In recent years, people have invested a lot of energy in developing various new materials in order to develop EL devices with better performance, thereby achieving full-color display. Considering the structure of flexible OLED devices, flexible organic electroluminescent materials can be divided into: electrode materials, carrier transport materials and luminescent materials.

1. Electrode materials

Electrode materials are divided into anode materials and cathode materials. For anode materials, in addition to requiring them to have good conductivity and stability, in order to improve the hole injection efficiency, their work function is required to be as high as possible. When used as the anode of a downlight or transparent device, it is also required to have high transparency in the visible light region. Therefore, the anode of a flexible OLED device generally uses a transparent ITO conductive film with a high work function. The cathode of an organic electroluminescent device mainly uses metals with low work functions, such as Ca, Mg, Al or their alloys. In order to improve the electron injection efficiency, the lower the metal work function, the better; however, metals with low work functions are relatively active and are easily affected by the surrounding environment and undergo chemical reactions, which can lead to device failure.

2. Carrier transport materials

Carrier transport materials can be divided into two categories: hole transport layer materials and electron transport layer materials according to their different roles in flexible OLED devices.

1) Hole transport materials

Hole transport materials generally have strong electron-donating properties, relatively low ionization energy and high hole mobility. Traditional hole transport materials are aromatic polyamine materials, such as aromatic diamines TPD and NPB. The presence of aromatic amine moieties can make the molecule have good electrochemical stability and can also adjust the ionization energy of the material.

2) Electron transport materials

Electron transport materials are electron-deficient systems in molecular structure. Most of them have strong electron-accepting ability and can effectively transfer electrons under a certain forward bias. They also need to have good film-forming properties and stability. Ideally, the electron mobility of ETM should be equivalent to the hole mobility of HTM, but in fact, the electron conduction rate of organic materials is much smaller than the hole conduction rate. Electron transport materials are all planar aromatic compounds with large conjugated structures.