Principle and device structure of electroluminescence

Electroluminescence can be divided into high field electroluminescence and low field electroluminescence. High field electroluminescence is an in vivo luminescence effect. A luminescent material is a semiconductor compound doped with appropriate impurities to introduce a luminescent center or to form a dielectric state. When it comes into contact with an electrode or other medium and its barrier is in the opposite direction, electrons from the electrode or interface state enter the high field of the luminescent material, are accelerated and become superheated electrons. It can collide with the luminescent center to excite it or ionize it, or ionize the lattice, etc. Through a series of energy transport processes, the electrons return from their excited state to their ground state and emit light. Low field electroluminescence, also known as injection luminescence, mainly refers to semiconductor light-emitting diodes (LED). In 1960, it was found that the p-N junction diode of GaAs, under the forward bias, has minority carrier injection, and near the P-N junction, the two carriers combine and emit light. Because the semiconductor material has a narrow band gap, it emits infrared light. Subsequently, by using this principle, the semiconductor materials GaP, GaInP, GaAlAs, GaN and so on have been developed into red, yellow, green and blue light-emitting diodes. In recent years, organic thin film electroluminescence is a new force in the field of electroluminescence. It is generally believed that the organic thin film luminescence process consists of the following five steps:

(1) Injection of carriers. Under the action of an applied electric field, electrons and holes are injected from the cathode and anode respectively into the organic functional film layer sandwiched between the electrodes. Electrons are injected from the cathode into the lowest unoccupied molecular orbital (LUMO) of the organism, while holes are injected from the anode into the highest unoccupied molecular orbital (HOMO) of the organism.

(2) Carrier migration. The injected electrons and holes migrate from the electron transport layer and the hole transport layer to the luminescence layer respectively.

(3) recombination of carriers. Electrons and holes combine to produce excitons.

(4) Exciton migration. Excitons are continuously free-diffused in organic solid films and inactivated in radiative or non-radiative ways.

(5) electroluminescence. Electroluminescence is observed when the exciton returns to the ground state by radiative transition from the excited state. The color of the emitted light is determined by the energy level difference between the excited state and the ground state.

The basic structure of the electroluminescent device is a sandwich structure, the excitation layer is sandwiched between two electrodes, one side is transparent electrode to obtain surface luminescence. Because the high anodic work function can improve the efficiency of hole injection, indium oxide – tin oxide (ITO) anodes are generally used. A single or multilayer film is prepared on ITO by evaporation or rotating coating method. Above the film is a metal negative electrode. Since the electron escape work of metal affects the electron injection efficiency, the work function should be as low as possible. In this paper, the thermal organic electroluminescent devices are taken as an example

Most organic electroluminescent materials are unipolar, and there are few organic compounds with equal hole and electron transport properties, generally only hole transport properties or electron transport properties. In order to increase the probability of hole and electron recombination and improve the efficiency and life of the device, the structure of OLED has been developed from simple single layer device to double layer device, three layer device and even multi-layer device. Because this kind of unipolar organic matter is used as the luminescent material of the monolayer device, the composite of electrons and holes will be naturally close to an electrode, and the closer the composite area is to the electrode, the easier it is to be quenched by the electrode, and this quench is harmful

The effective luminescence of organic matter, thus reducing the luminous efficiency of OLED. The OLED with double layer, three layer or even multi-layer structure can give full play to the role of each functional layer and adjust the rate of hole and electron injection into the luminous layer. Only when the injected electrons and holes are combined in the luminous layer, can the luminous efficiency of the device be improved. Because most organic matter has insulation, only in the very high electric field intensity can make the carrier flow from one molecule to another molecule, so the total thickness of the organic film can not exceed several hundred nanometers, otherwise the device driving voltage is too high, will lose the practical application value of LED.