OLED is a kind of solid-state device composed of organic molecular flakes, which can emit light after power is applied. OLED can make electronic devices produce brighter and clearer images, and its power consumption is less than that of traditional light-emitting diodes (LEDs) and liquid crystal displays (LCDs).
In this article, you will learn about the working principle of OLED technology, the types of OLED, the advantages and disadvantages of OLED compared with other light-emitting technologies, and some problems that OLED needs to overcome.
Like LEDs, OLEDs are solid-state semiconductor devices that are 100-500 nanometers thick and 200 times thinner than hair. OLED is composed of two or three layers of organic materials; According to the latest OLED design, the third layer helps transfer electrons from the cathode to the emitter layer. This paper mainly deals with the double-layer design model.
1、 Structure of OLED
OLED consists of the following parts:
Substrate (transparent plastic, glass, metal foil) – the substrate is used to support the whole OLED.
Anode (transparent) – the anode eliminates electrons (increases 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 is composed of organic plastic molecules, which transport “holes” from anode. Polyaniline can be used as conductive polymer of OLED.
Emission layer — this layer is composed of organic plastic molecules (different from conductive layer), which transmit electrons from cathode; The luminescence process takes place in this layer. Polyfluorene can be used as the emitting layer polymer.
Cathode (can be transparent or opaque, depending on the type of OLED) – when there is current flowing in the device, the cathode injects electrons into the circuit.
2、 OLED manufacturing
The most important part of OLED production process is to apply organic layer to the substrate. There are three ways to accomplish this work
1. Vacuum deposition or vacuum thermal evaporation (VTE)
The organic molecules in the vacuum chamber will be slightly heated (evaporated), and then these molecules will condense on the lower temperature substrate in the form of thin films. This method has high cost but low efficiency.
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 thin films. Using carrier gas can improve the efficiency and reduce the cost of OLED.
3. Inkjet printing
Using inkjet technology, OLED can be sprayed on the substrate, just as ink is sprayed on paper when printing. Inkjet technology greatly reduces the cost of OLED production, and can also print OLEDs onto films with very large surface area for the production of large displays, such as 80 inch large screen TVs or electronic kanban.
3、 Luminescence process of OLED
OLEDs emit light in a way similar to LEDs, requiring a process called electro phosphorescence.
The specific process is as follows:
1. The battery or power supply of the OLED device will apply a voltage at both ends of the OLED.
2. The 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 conducting layer of organic molecules（ This can be regarded as the anode output holes to the conductive layer, and the two effects are equal.
5. At the junction of the emission layer and the conduction layer, electrons will combine with holes.
6. When an electron encounters a hole, it fills the hole (it falls into an energy level in the atom with the missing electron).
7. When this happens, electrons release energy in the form of photons.
8. OLED lights up.
9. The color of light depends on the type of organic molecules in the emitting layer. Manufacturers 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 applied current. The higher the current, the higher the brightness of the light.
4、 Classification of OLED
Here are several OLEDs: passive matrix OLED, active matrix OLED, transparent OLED, top emitting OLED, foldable OLED, white OLED, etc.
Each kind of OLED has its own unique purpose. Next, we will discuss these OLEDs one by one. First, passive matrix and active matrix OLEDs.
PMOLED has cathode belt, organic layer and anode belt. The anode and cathode are perpendicular to each other. The intersection of cathode and anode forms the pixel, which is the luminous part. The external circuit applies current to the selected cathode band and anode band to determine which pixels emit light and which do not. In addition, the brightness of each pixel is proportional to the applied current.
PMOLED is easy to manufacture, but its power consumption is higher than other types of OLEDs, mainly because it needs external circuits. PMOLED is the most efficient way to display text and icons. It is suitable for making small screens (2-3 inches diagonal), such as those often seen on mobile phones, PDAs and MP3 players. Even if there is an external circuit, the power consumption of passive matrix OLED is less than the LCD currently used in these devices.