1.Definition of OLED
OLED (Organic Light-Emitting Diode), also known as organic electro-laser display, organic light-emitting semiconductor (Organic Electroluminesence Display, OLED).
OLED is a kind of current-type organic light-emitting device, which is a phenomenon of luminescence through the injection and recombination of carriers, and the luminous intensity is proportional to the injected current. Under the action of an electric field, the holes generated by the anode and the electrons generated by the cathode will move in the OLED, and are injected into the hole transport layer and the electron transport layer respectively, and migrate to the light emitting layer. When the two meet in the light-emitting layer, energy excitons are generated, which excites the light-emitting molecules and finally produces visible light.
2.OLED classification
OLED is an organic electroluminescent device, which is composed of relatively special organic materials. According to its structure, it can be divided into four types, namely single-layer devices, double-layer devices, three-layer devices, and multi-layers. Device.
(1) Classification from the device structure
- a. Single-layer device
A single-layer device is to insert a layer of organic layer that can emit light between the positive and negative electrodes of the device, and its structure is substrate/ITO/light-emitting layer/cathode. In this structure, due to the imbalance of electron and hole injection and transport, the efficiency and brightness of the device are low, and the stability of the device is poor.
- b. Double-layer devices
Double-layer devices are based on single-layer devices. Hole transport layers (HTL) or electron transport layers (ETL) are added on both sides of the light-emitting layer, which overcomes the problem of unbalanced carrier injection in single-layer devices and improves The voltage-current characteristics of the device improve the luminous efficiency of the device.
- c. Three-layer device
The three-layer device structure is the most widely used structure, and its structure is substrate/ITO/HTL/light-emitting layer/ETL/cathode. The advantage of this structure is that excitons are confined in the light-emitting layer, thereby increasing the efficiency of the device.
- d. Multi-layer structure
The performance of the multi-layer structure is a relatively good structure, which can play a good role in all levels. The light-emitting layer can also be composed of a multi-layer structure, and since the generator layers are independent of each other, they can be optimized separately. Therefore, this structure can give full play to the role of each organic layer, which greatly improves the flexibility of device design.
(2) Classification from the drive mode
OLEDs are divided according to the driving mode, generally divided into two types, one is the active type and the other is the passive type. The active type is generally an active drive, and the passive type is a passive drive. In the actual application process, active drivers are mainly used for high-resolution products, while passive drivers are mainly used for displays with relatively small display sizes.
(3) Classification based on materials
The materials that make up OLEDs are mainly organic. OLED materials can be classified according to the types of organics. One is small molecules and the other is macromolecules. The main difference between the two devices is the manufacturing process.
The small molecule device mainly uses the vacuum thermal evaporation process, and the polymer device uses the spin coating or spray printing process.
3. OLED structure
OLED devices are composed of substrate, cathode, anode, hole injection layer (HIL), electron injection layer (EIL), hole transport layer (HTL), electron transport layer (ETL), electron blocking layer (EBL), hole blocking Layer (HBL), light-emitting layer (EML) and other parts.
Among them, the substrate is the basis of the entire device, and all functional layers need to be vapor-deposited on the substrate of the device; glass is usually used as the substrate of the device, but if you need to make a bendable flexible OLED device, you need to use other materials, such as plastic, etc.
4. OLED lighting principle
The light-emitting process of OLED devices can be divided into: injection of electrons and holes, transport of electrons and holes, recombination of electrons and holes, and de-excited OLED display light emission of excitons. Specifically:
(1) Injection of electrons and holes.
The electrons in the cathode and the holes in the anode will move to the light-emitting layer of the device under the driving of the applied driving voltage. In the process of moving to the light-emitting layer of the device, if the device contains an electron injection layer and a hole injection layer, then Electrons and holes must first overcome the energy barrier between the cathode and the electron injection layer and the anode and the hole injection layer, and then move to the electron transport layer and hole transport layer of the device through the electron injection layer and the hole injection layer; The electron injection layer and the hole injection layer can increase the efficiency and lifetime of the device. The mechanism of electron injection in OLED devices is still under constant research, and the most commonly used mechanisms are the tunneling effect and the interface dipole mechanism.
(2) Transport of electrons and holes.
Driven by an external driving voltage, electrons from the cathode and holes from the anode will move to the electron transport layer and hole transport layer of the device, respectively, and the electron transport layer and hole transport layer will move the electrons and holes to the device, respectively At the interface of the light-emitting layer; at the same time, the electron transport layer and the hole transport layer will block the holes from the anode and the electrons from the cathode at the interface of the light-emitting layer of the device, so that the electrons and voids at the interface of the light-emitting layer of the device are blocked. The holes are accumulated.
(3) Recombination of electrons and holes.
When the number of electrons and holes at the interface of the light-emitting layer of the device reaches a certain number, the electrons and holes will recombine and generate excitons in the light-emitting layer.
(4) Deexcitation light of excitons.
The excitons generated in the light-emitting layer will activate the organic molecules in the light-emitting layer of the device, which in turn causes the electrons in the outermost layer of the organic molecules to transition from the ground state to the excited state. Because the electrons in the excited state are extremely unstable, they will move to the ground state. During the transition, energy is released in the form of light during the transition, and the device emits light.
5. Features of OLED
The reason why OLED technology can be widely used is that it has the following advantages compared with other technologies:
(1) Low power consumption
Compared with LCD, OLED does not require a backlight, which is a relatively energy-consuming part of LCD, so OLED is more energy-efficient.
For example, the power consumption of a 24in AMOLED module is only 440mw, while a 24in polysilicon LCD module reaches 605mw.
(2) Fast response speed
Compared with other technologies, OLED technology has a fast response speed, and the response time can reach the microsecond level. The higher response speed achieves better moving images.
According to relevant data analysis, its response speed has reached about 1000 times the response speed of liquid crystal displays.
(3) Wide viewing angle
Compared with other displays, because OLED is actively emitting light, the picture will not display distortion in a large viewing angle range. The vertical and horizontal viewing angles exceed 170 degrees.
(4) Able to achieve high resolution display
Most high-resolution OLED displays use active matrix, or AMOLED, whose light-emitting layer can absorb 260,000 true colors with high resolution, and with the development of science and technology, its resolution will be improved in the future. High promotion.
(5) Wide temperature characteristics
Compared with LCD, OLED can work in a large temperature range. According to relevant technical analysis, the temperature can operate normally at -40 degrees Celsius to 80 degrees Celsius. In this way, geographical restrictions can be reduced, and it can be used normally in extremely cold regions.
(6) OLED can realize soft screen
OLED can be produced on different flexible substrate materials such as plastics and resins. The organic layer is vapor-deposited or coated on the plastic substrate to realize a soft screen.
(7) The quality of the finished OLED is relatively light
Compared with other products, the quality of OLED is relatively small, the thickness is relatively small compared with LCD, its seismic coefficient is higher, and it can adapt to harsh environments such as greater acceleration and vibration.
6. Factors affecting the life of OLED
There are many factors that affect the life of OLED devices.
According to the factors that affect OLED devices, the influencing factors can be divided into internal and external factors.
Among them, the internal cause means that the reduction in the life of the device is caused by non-external factors such as the material or structure of the device itself, and the external cause means that the reduction in the life of the device is caused by external factors in the environment in which the device is located.
7. Application fields and the future of OLED
Due to the many advantages of OLED, OLED technology has a wider application range than LCD technology, and can be extended to the fields of electronic products, commercial fields, transportation, industrial control, and medical fields. In addition, in recent years, major international companies have In the continuous strengthening of research on OLED technology, OLED technology will be further improved.
(1). In the commercial field
small-size OLED screens can be installed in POS machines, copiers, and ATM machines. Because of the characteristics of OLED screens such as bendability, lightness and thinness, and strong anti-aging performance, they are both beautiful and practical. Large screens can be used as business promotion screens, as well as advertising screens in stations and airports. This is because OLED screens have wide viewing angles, high brightness, and bright colors, and their visual effects are much better than LCD screens.
(2). In the field of electronic products
OLED is the most widely used in smart phones, followed by notebooks, display screens, TVs, flat panels, digital cameras and other fields. Because OLED displays have more intense colors and can be adjusted in color (different display modes) , So it is very widely used in practical applications, especially today’s curved TV, which is widely praised by the masses.
(3). In the field of VR technology
LCD screens have very serious smears when viewing VR devices, but OLED screens will alleviate a lot. This is because OLED screens light up light molecules, while liquid crystals flow through light. Therefore, in 2016, OLED screens officially surpassed LCD screens and became the new darling of the mobile phone industry.
(4). In the transportation field
OLEDs are mainly used for ships, aircraft instruments, GPS, video phones, car displays, etc., and are mainly small in size. These fields mainly focus on the wide viewing angle performance of OLEDs, which can be clear even if you do not look directly When you see the contents of the screen, the LCD does not work.
(5). In the industrial field
my country's industry is developing toward automation and intelligence, and more and more intelligent operating systems are introduced, which has more demand for screens. Whether it is on touch screen display or viewing display, the application range of OLED is wider than that of LCD.
(6). In the medical field
The impact of medical diagnosis and surgical screen monitoring are inseparable from the screen. In order to meet the wide-view requirements of medical displays, OLED screens are the "one choice."
It can be seen that the development space of OLED display is very high, and the market potential is huge. However, compared with LCD screens, OLED manufacturing technology is not mature enough. Due to the low mass production rate and high cost, only some high-end devices on the market will use top OLED screens.
However, judging from the data in the first half of 2017, various manufacturers have increased their investment in OLED technology research, and many mid-end electronic products in my country have applied OLED displays. From the perspective of the mobile phone industry, since 2015, the proportion of OLED screen applications has increased year by year.
Although there are still not as many LCD products, high-end smart phones have adopted the most advanced OLED screens. Therefore, the development of electronic products such as smart phones is bound to Further promote the development of OLED.
At last
As we said, OLED can be used to create flexible and transparent displays. For consumers, This is quite exciting because it opens up a whole world of possibilities:
● Curved OLED display placed on a non-flat surface
● Wearable OLED
● Foldable OLEDs and scrollable OLEDs can be used to create new mobile devices.
● Transparent OLED embedded in windows or car windshields
There are more that we can't imagine...
Let us embrace OLED technology together！