Electroluminescent Materials

Electroluminescence refers to the phenomenon that when voltage parameters are applied, the electrically charged substance can directly convert electrical energy into the form of light, which is a kind of electric-optical energy conversion characteristic. Materials with this function are called electroluminescence materials, such as light-emitting diodes. Electroluminescence has the characteristics of low power consumption, small volume and surface display. It is an important raw material for instrument lighting and flat display manufacturing.

Electroluminescent materials

According to the structure principle of the electroluminescent device, the main materials used in the device include:

Electron injection material (cathode material)/Hole injection material (anode material)
Both electron injection materials and hole injection materials can be classified as carrier injection materials. The injection balance between the two is one of the key factors for the high efficiency of device luminescence. Electron injection materials are mainly made of low functional metal or alkaline earth metal alloy. The hole injection material uses the most common ITO glass electrode.
Electron transport material
The electron transport material mainly transmits electrons, which is responsible for transferring electron carriers from the metal cathode and injecting them into the luminescent layer. Its excited state energy level can prevent the excitons in the luminescent layer from carrying out reverse energy exchange. Usually, it should have the following characteristics: electrochemical reduction reversible; HOMO and LUMO energy levels are suitable, so that the electron injection barrier is minimal, with low opening and operating voltages, and preferably with hole blocking; High electron mobility; The glass transition temperature and thermal decomposition stability are higher. Amorphous thin film form, thus avoiding light scattering or decay caused by the crystal.
Holes transport material
The ideal hole transport material needs to have the following characteristics: conductive energy level matching the anode, high hole mobility; Good air and thermal stability; Low production cost and good solubility and film formation, etc. Commonly used hole transport materials include polythiophene derivatives (PEDOT: PSS), metal oxides, polymers and small molecular organic materials. Among them, PEDOT: PSS has the advantages of easy preparation at low temperature, high transmittance within the visible light range, good conductivity and excellent film formation, etc., which has been widely used in various photoelectric devices and has been commercialized.
Fluorescent conversion material (luminescent material)
Luminescent materials play a decisive role in electroluminescent devices, which affect the luminescence efficiency, emission wavelength and service life of the devices. It can be divided into inorganic semiconductor materials, organic metal complex materials, organic conjugated small molecule materials and polymer materials with conjugated structure. The first generation of luminescent active materials were fluorescent materials, and the second generation of organic phosphorescent materials were developed due to the low utilization rate of excitons. At present, as the second generation of luminescent materials, organic phosphorescent materials have been widely used and studied. At present, the third generation of high efficiency luminescent materials thermal activation delay fluorescence materials are under full development.

Fig.1 The structure of electroluminescent materials

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