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OLED is called organic light-emitting diode, also known as organic light-emitting semiconductor. OLED display technology is a new generation of flat-panel display technology after liquid crystal display (LCD). Compared with LCD, OLED technology has the advantages of power saving, thin, large viewing angle, flexibility, and so on, gradually becoming the mainstream solution for small and medium-sized display panels.
OLED is an organic light-emitting device that converts electric energy directly into light energy based on a variety of organic materials. Its basic structure includes a cathode, anode, auxiliary layer, and light-emitting layer . Under the action of the applied electric field, the electrons and holes are injected from the cathode and anode, respectively, then the injected electrons and holes will migrate from the electronic auxiliary layer and the hole auxiliary layer to the light-emitting layer.
When electrons and holes are injected into the luminescent layer, they form electron-hole pairs, namely excitons, which radiate transition excitons and emit photons to make OLED glow. It can be seen that OLED materials directly determine the luminous characteristics of the panel, including light-emitting efficiency, life, and so on.
Structurally, OLED organic light-emitting materials include the hole injection layer (HIL), hole transport layer (HTL), emitting layer (EML), electron transport layer (ETL), and electron injection layer (EIL). The EML is used to convert electrons into the light source, and the other electron/hole organic layer helps the electrons/holes flow smoothly.
OLED, an organic light-emitting device, is composed of special organic materials. Structurally, it can be divided into four types: single-layer, double-layer, three-layer, and multi-layer device.
(1) Single-layer
Single-layer device means that a light-emitting organic layer is connected between the positive and negative electrodes of the device, and its structure is substrate /ITO/ light-emitting layer/cathode. In such a structure, the efficiency and brightness are low and the stability is poor due to the imbalance of electron, hole injection, and transport.
(2) Double-layer
Based on the single-layer device, the two-layer device adds the hole transport layer (HTL) or the electron transport layer (ETL) on both sides of the light-emitting layer, which overcomes the unbalanced carrier injection problem in the single-layer device, improves the volt-current characteristics, and enhances the light-emitting efficiency.
(3) Three-layer
The three-layer device structure, substrate /ITO/HTL/ light-emitting layer /ETL/ cathode, is the most widely used. The advantage of this structure is that the excitons are confined to the light-emitting layer, thus further improving the efficiency of the device.
(4) Multi-layer
The multi-layer structure has better performance and can play the role of each layer well. The luminescence layer can be composed of multi-layer structures, which can be optimized separately due to the independence of each layer, greatly improving the flexibility of device design.
According to the driving mode, OLED is generally divided into two types: active-matrix OLED and passive-matrix OLED. In the practice, the active-matrix OLED is mainly used for high-resolution products while the passive-matrix OLED is applied in the small-size displayer.
OLED can be classified into small molecules and polymers as its major materials are organic matter. The main difference between these two devices is in the production process. Specifically speaking, the small molecule device uses the vacuum thermal evaporation process while the polymer device utilizes the rotary coating or spray printing process.
From the perspective of the production process, the generation of OLED material needs to go through four links: chemical raw material, intermediate, crude monomer, and terminal material.
The OLED industry chain involves upstream supply materials and equipment as well as downstream application fields. In detail:
The upstream of OLED includes the production of raw materials, equipment manufacturing, and assembled parts and components. Each sector has segmented products, e.g. raw materials like glass substrate, organic materials, polarizers, and packaging adhesive.
OLED downstream is the applications, including smartphones, smart TVs, VR/AR, wearable electronic devices (smartwatches, etc.), computers, vehicle displays, lighting, and other fields. Among them, smartphones, TV, and VR top the OLED application list.
The midstream is OLED production and sale. At present, leading enterprises are Samsung, LG, BOE, Changhong, and Huaxing Optoelectronics, among which Samsung is the world's largest manufacturer of small and medium-sized OLED panels while LG focused on large-size OLED at the beginning. Considering the trend of small-screen electronic products, LG gradually shifts to small-size OLED production.
OLED organic light-emitting materials are one of the fields with the highest technical barriers in the industrial chain. AMOLED (active-matrix OLED) panel requires the evaporation of more than ten layers of organic materials, and the evaporation thickness and uniformity are the core indicators. The thickness of each layer varies from a few nanometers to hundreds of nanometers. Although the materials are used sparingly, their cost accounts for about 10% of the total cost of panel production, second only to equipment manufacturing.
Compared with the last two generations of display technology (CRT, LCD), the OLED display panel has the features of lightness, thinness, fast response, transparent display, flexibility, and foldability. Its special structure does not need heavy vacuum devices, additional screen backlight, and liquid crystal phase. It only needs to deposition the light-emitting material onto the thin ITO glass plate, and then it can glow and display color with electricity. At the same time, OLED has a wider color gamut, viewing angle, operating temperature range, and lower power consumption.
OLED is broadly used in the terminal market and recent years witnessed its fast advancement in the areas of smartphones, TV, VR, vehicle, wearable devices, lighting, and others. With the establishment of more OLED panel production lines, Apple iPhone X began to use OLED display screens in 2017, and then Samsung, Huawei, Xiaomi, MOTOROLA, and other mobile phones launched foldable OLED screens successively in 2019. Another example is wearable devices. Due to its characteristics of wide viewing Angle, lightweight, and flexible folding, OLED panel is highly competitive in wearable devices. These all mark the success of OLED commercialization.