Apple said it will use OLED screens on the iPhone 8 this year, and its demonstration effect in the field of electronic products such as smart phones will be an important driving force for OLED. With the explosion of new electronic products such as VR (virtual reality) and wearable devices, OLED will bring new entry points. The market for OLED terminal products has exploded rapidly, which will drive the growth prospects of OLEDs. In general, this year is likely to become the “first year of OLEDâ€.
This article starts with OLED, introduces the development history and development prospects of OLED, etc., and follows the small series into the world of OLED. .. ..
Organic Light Emitting Diode (OLED) is a patented display technology developed by Kodak that uses organic polymeric materials as semiconductor materials in light-emitting diodes. The polymeric material may be natural or synthetic, and may be very large in size or small in size. Proteins and DNA are examples of organic polymers. OLED display technology is widely used in mobile phones, digital video cameras, DVD players, personal digital assistants (PDAs), notebook computers, car stereos and televisions. OLED displays are thin and light because they don't use backlights. The OLED display also has a wide viewing angle of up to 160 degrees and operates at two to ten volts. A new OLED-based technology is Soft Organic Luminescence Display (FOLED), which has the potential to enable highly portable, folded display technologies in the future.
Development HistoryDeng Qingyun, a Chinese-American professor born in Hong Kong in 1947, discovered organic light-emitting diodes, or OLEDs, in the laboratory. He began research on OLEDs. In 1987, Professor Deng Qingyun and Van Slyke used ultra-thin films. The technique uses a transparent conductive film as an anode, Alq3 as a light-emitting layer, a triarylamine as a hole transport layer, and a Mg/Ag alloy as a cathode to form a two-layer organic electroluminescent device. In 1990, Burroughes et al. discovered an OLED with a conjugated polymer PPV as a light-emitting layer, which has since ignited a worldwide OLED research boom. Professor Deng is also known as the "father of OLED."
Among the two major technical systems of OLED, low-molecular OLED technology is mainly concentrated in the three regions of Japan, South Korea and Taiwan, while the PLED of polymer is mainly developed by European manufacturers. In addition, OEL of LG mobile phones is also using OLED technology. OLED technology and patents are controlled by the British technology company CDT. Compared with the two major technical systems, there are still difficulties in the colorization of PLED products. Low molecular OLEDs are easier to colorize.
However, although OLEDs with better technology will replace LCDs such as TFTs in the future, organic light-emitting display technologies have shortcomings such as short lifetime and large screen size.
To illustrate the OLED configuration, each OLED unit can be compared to a hamburger, and the luminescent material is the vegetable sandwiched between them. The display unit of each OLED can be controlled to produce three different colors of light. Like LCDs, OLEDs are also active and passive. The unit selected by the row and column address in the passive mode actively emits light. In the active mode, there is a thin film transistor (TFT) behind the OLED unit, and the light emitting unit is lit under the TFT driving. Active OLEDs save power compared to passive OLEDs and offer better display performance.
Product FeaturesOLED display technology has self-illuminating properties, using a very thin coating of organic materials and a glass substrate. When there is current, these organic materials will emit light, and the OLED display screen has a large viewing angle and can save power. This display device was introduced on the MP3 player in the beginning of the year.
In view of the organic luminescent materials used in OLEDs, one is a small molecular device system using dyes and pigments, and the other is a polymer device system using conjugated polymers as materials. At the same time, since the organic electroluminescent device has the characteristics of rectifying and emitting light of the light emitting diode, the small molecule organic electroluminescent device is also called OLED (Organic Light EmitTIng Diode), and the polymer organic electroluminescent device is called PLED (Polymer). Light-emitTIng Diode). Small molecule and polymer OLEDs can be said to have different materials in terms of material properties. However, in terms of the development of existing technologies, such as the reliability of monitors, electrical characteristics, and production stability, small molecule OLEDs are in a leading position. The OLED components currently in mass production are all using small molecule organic luminescent materials.
The basic structure of an OLED is composed of a thin, transparent semiconductor-tin indium tin oxide (ITO) connected to the positive electrode of electricity, and another metal cathode, which is sandwiched into a sandwich structure. The entire structural layer includes a hole transport layer (HTL), a light emitting layer (EL), and an electron transport layer (ETL). When the power is supplied to an appropriate voltage, the positive hole and the cathode charge are combined in the light-emitting layer to produce light, and three primary colors of red, green, and blue RGB are generated according to the formulation to form a basic color. The characteristics of OLEDs are self-illuminating. Unlike TFT LCDs, which require backlighting, they have high visibility and brightness, followed by low voltage requirements and high power saving efficiency, plus fast response, light weight, thin thickness, simple structure and low cost. Etc. is considered one of the most promising products of the 21st century.
The principle of luminescence of organic light-emitting diodes is similar to that of inorganic light-emitting diodes. When the component is subjected to a forward bias derived from direct current (DC), the applied voltage energy will drive electrons (Electron) and holes (Hole) from the cathode and the anode, respectively, when the two meet in conduction. , combined, to form the so-called Electro-Hole Capture. When a chemical molecule is excited by external energy, if the electron spin (Electron Spin) and the ground state electron are paired, it is a singlet (Singlet), and the light released is so-called fluorescence (Fluorescence); The state electrons and the ground state electron spins are unpaired and parallel, which is called a triplet, and the light released is so-called Phosphorescence.
When the state of the electron returns from the excited high energy level to the steady state low energy level, its energy will be emitted as Light Emission or Heat DissipaTIon, respectively, where the photon portion can be utilized as a display function; The organic fluorescent material cannot observe the triplet phosphorescence at room temperature, so the theoretical limit value of the luminous efficiency of the PM-OLED element is only 25%.
The principle of PM-OLED illumination is to use the material energy level difference to convert the released energy into photons, so we can choose the appropriate material as the light-emitting layer or doping the dye in the light-emitting layer to get the desired color of the light. In addition, the general combination of electrons and holes is in the tens of nanoseconds (ns), so the response speed of the PM-OLED is very fast.
1. S.: Typical structure of PM-OLED. A typical PM-OLED is composed of a glass substrate, an ITO (Indium TIn oxide) anode (Anode), an organic light-emitting layer (Emitting Material Layer), and a cathode (Cathode), among which a thin and transparent ITO anode The organic light-emitting layer is sandwiched with the metal cathode as a sandwich, and when a hole injected with a voltage from the anode and an electron from the cathode are combined with the organic light-emitting layer, the organic material is excited to emit light.
In addition to the glass substrate, the yin and yang electrodes and the organic light-emitting layer, a hole injection layer (HIL) and a hole transport layer (Hole) are required to be fabricated. Transport Layer; HTL), Electro Transport Layer (ETL) and Electron Inject Layer (EIL), and an insulating layer is required between each transport layer and the electrode, so Evaporate The processing difficulty is relatively increased and the production process is complicated.
Since organic materials and metals are quite sensitive to oxygen and moisture, they must be packaged and protected after fabrication. Although the PM-OLED needs to be composed of several layers of organic thin films, the thickness of the organic thin film layer is only about 1,000 to 1,500 A (0.10 to 0.15 um), and the entire display panel (Panel) is packaged with a desiccant (Desiccant). The thickness is less than 200um (0.2mm), which is light and thin.
materialThe properties of organic materials profoundly affect the photoelectric properties of the components. In the choice of anode material, the material itself must have a high work function and opacity, so it has a high work function of 4.5eV-5.3eV, stable and transparent ITO transparent conductive film. It is widely used in anodes. In the cathode portion, in order to increase the luminous efficiency of the element, the injection of electrons and holes usually requires a low work function of Ag, Al, Ca, In, Li, and Mg, or a low work function composite metal. A cathode (for example, Mg-Ag magnesium silver) is produced.
The organic material suitable for electron transport is not necessarily suitable for transporting holes, so the electron transport layer and the hole transport layer of the organic light-emitting diode must use different organic materials. At present, the materials most commonly used to make electron transport layers must have high film stability, thermal stability, and good electron transport properties. Fluorescent dye compounds are generally used. Such as Alq, Znq, Gaq, Bebq, Balq, DPVBi, ZnSPB, PBD, OXD, BBOT and the like. The material of the hole transport layer belongs to an aromatic amine fluorescent compound such as organic materials such as TPD and TDATA.
The material of the organic light-emitting layer must have strong fluorescence in the solid state, good carrier transmission performance, good thermal stability and chemical stability, high quantum efficiency and vacuum evaporation. Generally, the material of the organic light-emitting layer is usually used. The electron transport layer or the hole transport layer are made of the same material, for example, Alq is widely used for green light, and Balq and DPVBi are widely used for blue light.
In general, OLEDs can be classified into two types according to luminescent materials: small molecule OLEDs and high molecular OLEDs (also known as PLEDs). The difference between small molecule OLED and polymer OLED is mainly reflected in the different preparation processes of the device: small molecule devices mainly adopt vacuum thermal evaporation process, and polymer devices adopt rotary coating or spray printing process. Small molecule material manufacturers mainly include: Eastman, Kodak, Idemitsu Kosan, Toyo INK, Mitsubishi Chemical, etc.; polymer materials manufacturers mainly include: CDT, Covin, Dow Chemical, Sumitomo Chemical. There are more than 1,400 patents related to OLED in the world, and three of the most basic patents. The basic patent for small molecule OLED is owned by Kodak Company of the United States. The patent for polymer OLED is owned by Cambridge DisPlay Technology of the United Kingdom and Uniax of the United States.
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