An OLED is a solid-state device composed of organic molecular sheets that emit light after application of electricity. Oled enables electronic devices to produce brighter, sharper images that consume less power than traditional light-emitting diodes (LEDs) and are smaller than today's liquid crystal displays (LCDs). 
In this article, you will learn about the workings of OLED technology, what types of OLEDs, the advantages and disadvantages of OLEDs compared to other luminescent technologies, and some of the problems that OLEDs need to overcome.
Similar to LEDs, OLEDs are solid-state semiconductor devices that are 100-500 nanometers thick and 200 times thinner than hair. The OLED consists of two or three layers of organic material; in accordance with the latest OLED design, the third layer assists in the transfer of electrons from the cathode to the emissive layer. This paper mainly deals with the two-layer design model.
First, the structure of OLED
OLED consists of the following parts:
OLED structure
Base layer (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.
Organic layer - The organic layer is composed of organic molecules or organic polymers.
Conductive layer - This layer consists of organic plastic molecules that carry "holes" from the anode. Polyaniline can be used as the conductive polymer of the OLED.
Emissive layer - This layer consists of organic plastic molecules (different from the conductive layer) that transport electrons from the cathode; the luminescence process takes place at this layer. Polyfluorene can be used as the emissive layer polymer.
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.
Second, the manufacture of OLED
The most important part of the OLED production process is the application of an organic layer to the substrate. There are three ways to do this:
1. Vacuum deposition or vacuum thermal evaporation (VTE)
The organic molecules located in the vacuum chamber are slightly heated (evaporated) and then the molecules are condensed in the form of a film on the lower temperature substrate. This method is costly but less efficient.
2. Organic vapor deposition (OVPD)
In a low-pressure hot-wall reaction chamber, the carrier gas transports the evaporated organic molecules to the low-temperature substrate, and then the organic molecules condense into a film. The use of carrier gas can increase efficiency and reduce the cost of OLEDs.
3, inkjet printing
The inkjet technology is used to spray the OLED onto the substrate as if the ink was sprayed onto the paper during printing. Inkjet technology greatly reduces the cost of OLED production, and can print OLEDs onto very large surface areas for large displays such as 80-inch large-screen TVs or electronic signage.
OLED manufacturing
Third, the OLED light-emitting process
OLEDs emit light in a manner similar to LEDs and undergo a process called electrophosphorescence.
OLED luminescence process
The specific process is as follows:
1. The battery or power supply of the OLED device will apply a voltage across the OLED.
2. Current flows from the cathode to the anode and through the organic layer (current refers to the flow of electrons).
3. The cathode outputs electrons to the organic molecular emission layer.
4. The anode absorbs electrons from the organic molecular conduction layer. (This can be seen as the anode outputs holes to the conductive layer, and the effects are equal.
5. At the junction of the emissive layer and the conductive layer, electrons will combine with the holes.
6. When an electron encounters a hole, it fills a hole (it will fall into an energy level in the atom of the missing electron).
7. When this process occurs, electrons release energy in the form of photons.
8, OLED lighting.
9. The color of the light depends on the type of organic molecules in the emissive layer. The manufacturer will place several organic films on the same OLED to form a color display.
10. The brightness or intensity of light depends on the amount of current applied. The higher the current, the higher the brightness of the light.
Fourth, the classification of OLED
The following are several types of OLEDs: passive matrix OLED, active matrix OLED, transparent OLED, top emitting OLED, foldable OLED, white OLED, and the like.
Each OLED has its own unique use. Next, we will discuss these OLEDs one by one. The first is passive matrix and active matrix OLED.
Passive Matrix OLED (PMOLED)
Passive matrix OLED structure
The PMOLED has a cathode strip, an organic layer, and an anode strip. The anode strip and the cathode strip are perpendicular to each other. The intersection of the cathode and the anode forms a pixel, that is, a portion where light is emitted. The external circuit applies a current to the selected cathode strip and anode strip to determine which pixels are illuminated and which are not. In addition, the brightness of each pixel is proportional to the magnitude of the applied current.
PMOLEDs are easy to manufacture, but they consume more power than other types of OLEDs, mainly because they require external circuitry. PMOLED is the most efficient for displaying text and icons, and is suitable for making small screens (2-3 inches diagonally), such as those that people often see on mobile phones, palmtops, and MP3 players. Even with an external circuit, the passive matrix OLED consumes less power than the LCDs currently used in these devices.
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