Brief introduction of OLED structure and light-emitting principle part two

Cathode (can be transparent or opaque, depending on the type of OLED) - The cathode injects electrons into the circuit when current flows through the device.

OLED is a dual-injection light-emitting device. Driven by an external voltage, the electrons and holes injected by the electrodes recombine in the light-emitting layer to form electron-hole pairs at the bound energy level, that is, excitons. The exciton radiation is de-excited to emit photons. produces visible light. In order to enhance the injection and transport capabilities of electrons and holes, a hole transport layer is usually added between the ITO and the light-emitting layer, and an electron transport layer is added between the light-emitting layer and the metal electrode, thereby improving the light-emitting performance. Among them, holes are injected from the anode and electrons are injected from the cathode. Holes are hop-transported on the highest occupied molecular orbital (HOMO) of organic materials, and electrons are hop-transported on the lowest unoccupied molecular orbital (LUMO) of organic materials.

The light-emitting process of OLED 1.3 usually has the following 5 basic stages:

Carrier injection: Under the action of an external electric field, electrons and holes are injected from the cathode and anode, respectively, to the organic functional layer sandwiched between the electrodes.

Carrier transport: The injected electrons and holes migrate from the electron transport layer and the hole transport layer, respectively, to the light-emitting layer.

Carrier recombination: After electrons and holes are injected into the light-emitting layer, they are bound together by the Coulomb force to form electron-hole pairs, that is, excitons.

Exciton migration: Due to the imbalance of electron and hole transport, the main exciton formation region usually does not cover the entire emissive layer, so diffusion migration occurs due to concentration gradients.

Exciton radiation de-excites photons: exciton radiation transitions, emits photons, and releases energy.

The color of OLED light emission depends on the type of organic molecules in the light-emitting layer. Several organic thin films are placed on the same OLED to form a color display. The brightness or intensity of the light depends on the properties of the light-emitting material and the magnitude of the applied current. For the same OLED, the higher the current, the higher the brightness of the light.