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Electroluminescence and its principle


ElectroluminescenceEL is the luminescence phenomenon caused by matter excited by corresponding electric energy under the action of a certain electric field. Electroluminescent EL is a phenomenon that converts electrical energy directly into light energy. As early as the early 20th century, Joseph isford discovered the luminescence of SiC crystals under an electric field. Electroluminescence as a planar light source has aroused people's great interest. People attempt to realize the revolution of lighting source from point light source, line light source to surface light source. Electroluminescence has been widely used in many fields since the invention of zinc sulfide and gallium phosphor arsenide in inorganic luminescent plates. Although powder electroluminescence was first discovered in 1937, it was not until the 1950s that it became stable by coating zinc sulfide and organic media on transparent conductive glass with a second electrode and an ac voltage. More and more attention has been paid to OLED, a new type of organic electroluminescent device with better performance and simpler technology.

1. Electroluminescent materials from the point of view of luminescent materials, electroluminescence can be divided into non-electroluminescent and organic electroluminescent. Non - electroluminescent materials are generally equal - semiconductor materials. Organic electroluminescent materials can be divided into small molecules and macromolecules according to their molecular weights. Polymer OLED materials take conjugated or non-conjugated polymers as luminescent materials. Typical polymer luminescent materials are PPV and its derivatives. Organic electroluminescent materials can be classified into hole injection layer HIL, hole transport layer HTL, luminous layer EML, electronic transport layer ETL, electronic injection layer EIL and other materials according to their different functions and structures in OLED devices. Some of the luminescent material itself has a hole transport layer or the function of the electron transport layer such luminescent material also giving priority to light emitting material layer is often called a small amount of doping of organic fluorescence or phosphorescence dye can be independent of light energy transfer and accept after carrier capture carriertrap mechanism and different colors of light that doped luminescence materials is usually referred to as the guest lights or doped luminous body English expressed as Dopant. Electroluminescence can be divided into high - field electroluminescence and low - field electroluminescence.

2. According to the principle of luminescence, electroluminescence can be divided into high-field electroluminescence and low-field electroluminescence. High field electroluminescence is an in vivo luminescence effect. Luminescent material is a semiconductor compound doped with appropriate impurities that are introduced into the luminescent center or form a dielectric state. When it comes into contact with an electrode or other medium the barrier is in the opposite direction and the electrons from the electrode or interface state are accelerated into the high field of the luminescent material and become superheated electrons. It can collide with the luminescent center to excite or dissociate or dissociate the lattice and so on. The electrons glow from the excited state back to the ground state through a series of energy transport processes. Low - field electroluminescence (low - field electroluminescence) is also known as injection luminescence mainly refers to semiconductor LED. In 1960, it was found that the p-n junction diode of GaAs was injected with minority carriers under forward bias and the two carriers were combined around the p-n junction to emit light. Because the semiconductor material has a narrow band gap, it emits infrared light. Subsequently, GaPGaInPGaAlAsGaN, a semiconductor material with a wide gap band, was used to develop red, yellow, green and blue leds. In recent years, organic thin films have emerged in the field of electroluminescence. It is generally believed that the organic electroluminescent thin film process consists of the following five steps: 1 carrier injection. Under the action of an applied electric field, electrons and holes are injected from the cathode and anode to the organic functional thin film sandwiched between the electrodes. Electrons are injected from the cathode into the lowest unocessional molecular orbital of the organism and holes are injected from the anode into the HOMO of the highest occupied molecular orbital of the organism. 2 carrier migration. The injected electrons and holes migrate from the electron transport layer and the hole transport layer to the luminous layer respectively.

3.The composition of 3 carriers. Electrons combine with holes to produce excitons.

4 migration of excitons. Excitons continue to diffuse freely in organic solid films and are inactivated by radiation or no radiation.

5. Electroluminescence. When the exciton returns to the ground state by a radiative transition from the excited state, it can be observed that the color of the emitted light is determined by the energy level difference from the excited state to the ground state. The basic structure of an electroluminescent device is a sandwich structure in which the excitation layer is sandwiched between two electrodes and a transparent electrode is formed on the middle side to obtain surface luminescence. Due to the high anode work function, the cavity injection efficiency can be improved, so the anode used in general is indium oxide - tin oxide ITO. A single or multilayer film was prepared on ITO by evaporation or rotary coating. The film was made with metal negative electrode on top of it. Since the work of electrons escaping from metal affects the injection efficiency of electrons, the work function was required to be as low as possible. This paper takes the hot organic electroluminescent devices as an example to illustrate that most organic electroluminescent materials are unipolar and have equal cavitation and electron transport properties at the same time. In order to increase the compound probability of hole and electron, improve the efficiency and life span of OLED devices, the structure of OLED devices has developed from simple single-layer devices to double-layer devices, 3-layer devices and even multi-layer devices. Because this kind of monopole organic matter is used as the luminescent machine material of single-layer device, the composite of electrons and holes will be naturally close to an electrode. The closer the composite area is to the electrode, the easier it will be quenched by the electrode. Such quenching will damage the effective luminescence of organic matter, thus reducing the luminous efficiency of OLED. However, the double-layer, 3-layer or even multi-layer OLED structure can fully play the role of various functional layers to regulate the rate of holes and electrons injected into the luminous layer. Only by combining the injected electrons and holes in the luminous layer can the luminous efficiency of the device be improved. The total thickness of the organic film should not exceed several hundred nanometers, because most organic materials are of infinite boundary and can only make carriers flow from one molecule to another at a very high electric field intensity. Otherwise, the driving voltage of the device will be too high and the practical application value of LED will be lost. 3. Application of vision electroluminescent EL display is characterized by active luminescent cold light source surface luminescence and uniform luminance without speckle power consumption with a small life of up to 5000h wide operating temperature range -4070℃ ultra-thin shape and size can be arbitrarily cut according to the requirements with good impact resistance and shock resistance. EL electroluminescent screen is widely used in active display or backlight display of LCD module, mobile phone, IC card telephone, magnetic card phone, battery-powered display screen, BP machine, wrist watch, automobile dashboard, audio and TV remote controller, handheld GPS receiver, portable computer, etc. With the development of technology, the emergence of dot matrix module, EL big screen display will develop rapidly in the advertising industry, traffic key, conference display, etc. In recent years, OLED technology has made great progress, including Pioneer of Japan, Sanyo, Kodak of the United States, UDC, Samsung of South Korea, and LG, which have been developing products and entering the market. New and improved materials are constantly being discovered, and inventions have greatly improved the stability and life span of components. A recent report by iSuppli/StanfordResource, a research firm, suggests that the OLED market will grow rapidly from $215m in 2003 to $3 billion in 2008. Therefore, with the rapid development of OLED technology in the next few years, the prospect of OLED will bring more convenient life to human beings is expected.


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