OLED development history and working principle of luminescence

Organic light-emitting diodes can be easily classified into two types, OLED (Organic Light-Emitting Diodes) and PLED (Polymer Light-Emitting Diodes), and mature products have been developed. The main advantage of PLED is its flexible large area display relative to OLED. However, due to product life issues, products currently on the market still use OLED as the main application.
 

OLED development history

The study of OLED technology originated from Dr. Ching Wan Tang, born in Hong Kong, with a BS in Chemistry from the University of British Columbia, and a Ph.D. in Physical Chemistry from Cornell University in 1975.

Deng Qingyun has been involved in research at Kodak's Rochester Laboratory since 1975. He discovered OLED in an accident. One night in 1979, on the way home, he suddenly remembered that something was forgotten in the laboratory and returned to the laboratory. He discovered that an experimental organic battery in the dark was sparkling and began research on OLEDs. In 1987, Dr. Wang Gen-like and his colleague Steven, who belonged to Kodak Company, successfully used a two-layer organic structure similar to a semiconductor PN junction to make a low-voltage, high-efficiency light emitter for the first time.

This implementation in 1987 laid the foundation for Kodak's production of OLED displays. In 1990, the laboratory in Cambridge, England, also successfully developed a polymer organic light-emitting element. The discovery of CDT (Cambridge Display Technology), a Cambridge-based display technology company in 1992, led to a different R&D path for OLED research.

OLED structure and principle of illumination

The basic structure of OLED is to make a layer of tens of nanometer thick organic light-emitting material on the indium tin oxide (ITO) glass as the light-emitting layer. The light-emitting layer has a metal electrode with a low work function to form a sandwich structure.

The basic structure of OLED mainly includes:

Substrate (transparent plastic, glass, metal foil) - The base layer is used to support the entire OLED.

Anode (transparent) - The anode eliminates electrons (increasing electron "holes") as current flows through the device.

Hole Transport Layer - This layer consists of molecules of organic materials that carry "holes" from the anode.

Light Emitting Layer - This layer is composed of organic material molecules (different from the conductive layer), and the light emitting process is carried out in this layer.

Electron Transport Layer - This layer consists of molecules of organic materials that carry "electrons" from the cathode.

The cathode (which may be transparent or opaque, depending on the type of OLED) - the cathode will inject electrons into the circuit when current is flowing through the device.

The luminescence process of OLEDs usually has the following five basic stages:

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

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

Carrier recombination: After electrons and holes are injected into the luminescent layer, they are bound together by the action of Coulomb force to form electron-hole pairs, ie, excitons.

Exciton migration: Due to the imbalance of electron and hole transport, the main formation region of the excitons usually does not cover the entire luminescent layer, and thus diffusion migration occurs due to the concentration gradient.

Exciton radiation repels photons: exciton radiation transitions, emits photons, and releases energy.

The color of the OLED luminescence depends on the type of organic molecules in the luminescent layer. Several organic 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 luminescent material and the magnitude of the applied current. For the same OLED, the greater the current, the higher the brightness of the light.

OLED driving method

PMOLED (English full name Passive Matrix OLED passively driven OLED), PMOLED is simply formed into a matrix by cathode and anode, and the pixels in the array are illuminated in a scanning manner. Each pixel is operated in short pulse mode and is instantaneously high. Brightness illuminates. The advantage is that the structure is simple, and the manufacturing cost can be effectively reduced. However, the driving voltage is high, making the PMOLED unsuitable for application on large-size and high-resolution panels, which is different from the current development.

AMOLED (English full name Active Matrix OLED, is the active drive OLED) AMOLED uses a separate thin film transistor to control each pixel, each pixel can drive light continuously and independently, can use low temperature polysilicon or oxide TFT The driving has the advantages of low driving voltage and long life of the illuminating element. However, the high cost and manufacturing process are more complicated and more difficult to control in terms of cost.

OLED features and key technologies

In the past, OLEDs have not been popularized in the past. The main problem is that most of the OLED samples developed by the earlier technologies are mostly monochromatic. Even with multi-color design, their chromophoric materials and production techniques often limit the diversity of OLED color development. . In fact, the image generation method of OLED is the same as the CRT display, which is formed into a color pixel by three-color RGB pixels. Because the material of OLED is close to linear response to current, it can display different colors and grays under different driving currents. Order.

The characteristic of OLED is that its core can be made very thin, the thickness is one-third of the current liquid crystal, and OLED is an all-solid component, which has good shock resistance and can adapt to harsh environments. OLEDs are mainly self-illuminating, so that they have almost no viewing angle problems. Compared with LCD technology, the display is clearly visible even when viewed at a large angle. The components of the OLED are self-illuminating and are regulated by voltage. The reaction speed is much faster than that of the liquid chip device, and is suitable for use as a high-definition television.

Another characteristic of OLED is its ability to adapt to low temperature. The old liquid crystal technology will break down at minus 75 degrees, and the OLED can display normally as long as the circuit is not damaged. In addition, OLEDs have high efficiency, lower energy consumption than liquid crystals, and can be fabricated on substrates of different materials, and can even be made into bendable displays, and the application range is increasing.

Compared with LCD, OLED is superior to LCD. The service life of OLEDs is still difficult to meet the requirements of consumer products (such as PDA, mobile phone and digital camera). However, in recent years, there have been significant breakthroughs in many mobile phones. The screen has adopted OLED, but it is still much more expensive than LCD in terms of price, which is also waiting for breakthrough in mass production technology in the future.