Organic light emitting diode, that is, organic light emitting diode (OLED), is also called organic electroluminescent display (OELD). Since 2003, this display device has been widely used in MP3 players because of its light weight and power saving. However, for DC and mobile phones, which are both digital products, only engineering samples with OLED screens have been shown in some exhibitions before, and they have not yet entered the practical application stage. However, the OLED screen has many advantages that LCD can't match.
Overview:
Organic light emitting diode display technology is different from the traditional liquid crystal display mode. It does not need backlight, and uses very thin organic material coating and glass substrate. These organic materials will emit light when current passes through them. In addition, the organic light-emitting diode display screen can be made lighter and thinner, with a larger viewing angle, and can significantly save electricity. At present, among the two major technical systems of organic light-emitting diodes, the low-molecular-weight organic light-emitting diode technology is mastered by Japan, and the so-called OEL of polymer PLEDLG mobile phone is this system, and the technology and patents are mastered by British technology company CDT. Compared with PLED products, colorization is still difficult. Low molecular OLEDs are easier to be colored. Not long ago, Samsung released 65,530 color OLEDs for mobile phones. However, although the OLED with better technology will replace LCD such as TFT in the future, the OLED display technology still has some shortcomings, such as short life and difficult to enlarge the screen. At present, Samsung mainly uses OLEDs, such as the newly listed SCH-X339, which uses 256-color OLEDs. As for OEL, LG is mainly used in its CU8 180 8280, which we have all seen. In order to explain the structure of organic light-emitting diodes, each organic light-emitting diode unit can be compared to a hamburger, and the luminescent material is a vegetable sandwiched in the middle. The display unit of each organic light emitting diode can generate three different colors of light under control. Like LCD, organic light emitting diodes can also be divided into active and passive types. In passive mode, cells selected by row and column addresses are lit. In active mode, there is a thin film transistor (TFT) behind the organic light-emitting diode unit, and the light-emitting unit is lit by the TFT. Passive organic light-emitting diodes save electricity, but active organic light-emitting diodes have better display performance.
Structure and principle:
The basic structure of organic light-emitting diodes is that a thin and transparent layer of indium tin oxide (ITO) with semiconductor characteristics is connected to the positive electrode of power supply, and another layer of metal cathode is wrapped into a sandwich structure. The whole structural layer includes hole transport layer (HTL), light-emitting layer (EL) and electron transport layer (ETL). When the power supply is supplied to a suitable voltage, the positive holes and negative charges in the light-emitting layer will combine to generate light, and three primary colors of red, green and blue RGB will be generated according to different formulas to form basic colors. Organic light-emitting diodes are characterized by their own light emission, unlike TFT LCD, so they have high visibility and brightness, followed by low voltage demand, high energy-saving efficiency, fast response, light weight, thin thickness, simple structure and low cost. It is considered as one of the most promising products in 2 1 century. The light emitting principle of organic light emitting diodes is similar to that of inorganic light emitting diodes. When the element is subjected to direct current (DC; DC), and the applied voltage energy injects driving electrons and holes into the device from the cathode and anode respectively. When they meet and combine in conduction, so-called electron-hole trapping is formed. When a chemical molecule is excited by external energy, if the electron spin is paired with the ground state electron, it is a singlet state, and the light it releases is the so-called fluorescence. On the other hand, if the spins of excited electrons and ground electrons are unpaired and parallel, it is called triplet state, and the light released by it is called phosphorescence. When the state of an electron returns from the excited high energy level to the steady low energy level, its energy will be released in the form of luminescence or heat dissipation, and some photons can be used as display functions; However, triplet phosphorescence cannot be observed in organic fluorescent materials at room temperature, so the theoretical limit of luminous efficiency of PM- organic light-emitting diode devices is only 25%. The light-emitting principle of PM- organic light-emitting diodes is to convert the released energy into photons by using the energy level difference of materials, so we can choose suitable materials as the light-emitting layer or dope dyes in the light-emitting layer to obtain the light-emitting color we need. In addition, the binding reaction between electrons and holes is usually within tens of nanoseconds (ns), so the response speed of PM- OLED is very fast. Attachment: Typical structure of pm-olem. A typical PM- organic light emitting diode consists of a glass substrate ITO (indium tin oxide; ; Indium tin oxide) anode, emitting material layer and cathode, etc. In which a thin and transparent ITO anode and a metal cathode sandwich an organic light-emitting layer, and when holes injected into the anode by voltage are combined with electrons from the cathode, the organic material is excited to emit light. At present, the multi-layer PM- organic light-emitting diode structure with good luminous efficiency and wide application needs to make hole injection layer (hole injection layer; HIL), hole transport layer (hole transport layer; HTL), electron transport layer (electron transport layer; ETL) and an electron injection layer (electron injection layer; EIL), and it is necessary to set an insulating layer between each transport layer and the electrode, so the processing difficulty of thermal evaporation is relatively high and the manufacturing process becomes complicated. Because organic materials and metals are quite sensitive to oxygen and water vapor, they need to be packaged and protected after production. Although PM- OLED needs to be composed of several layers of organic thin films, the thickness of organic thin films is only about 1 0,000 ~1500 A (0. 10 ~ 0. 15 um), and the total thickness of the whole display panel after packaging and adding desiccant is less than 200.
Selection of Organic Luminescent Materials
The characteristics of organic materials deeply affect the performance of photoelectric characteristics of devices. In the choice of anode material, the material itself must have high work function and light transmittance, so ITO transparent conductive film with high work function of 4.5eV-5.3eV, stable performance and light transmittance is widely used in anode. In the cathode part, in order to increase the luminous efficiency of the device, the injection of electrons and holes usually requires low work function metals such as Ag, Al, Ca, In, Li and Mg, or low work function composite metals (such as Mg-Ag-Mg-Ag) to make the cathode. Organic materials suitable for transporting electrons are not necessarily suitable for transporting holes, so different organic materials must be selected for the electron transport layer and hole transport layer of organic light-emitting diodes. At present, the most commonly used materials for making electron transport layers must have high film stability, thermal stability and good electron transport, and fluorescent dye compounds are usually used. Such as Alq, Znq, Gaq, Bebq, Balq, DPVBi, ZnSPB, PBD, OXD, BBOT, etc. The material of hole transport layer belongs to aromatic amine fluorescent compounds, such as organic materials such as TPD and TDATA. Organic light-emitting layer materials must have strong fluorescence in solid state, good carrier transport performance, good thermal and chemical stability, high quantum efficiency and vacuum evaporation. Generally, the material of the organic light-emitting layer is the same as that of the electron transport layer or the hole transport layer. For example, Alq is widely used in green light, and Balq and DPVBi are widely used in blue light. Generally speaking, OLEDs can be divided into two types according to luminescent materials: small molecule OLEDs and polymer OLEDs (also known as PLED). The difference between small molecular organic light-emitting diodes and polymer organic light-emitting diodes is mainly manifested in the different preparation processes of devices: small molecular devices mainly adopt vacuum thermal evaporation process, while polymer devices adopt spin coating or jet printing process. Manufacturers of small molecular materials mainly include Eastman, Kodak, Chu Guang Sheng Xing, Toyo Ink Manufacturing, Mitsubishi Chemical, etc. Polymer material manufacturers mainly include: CDT, Covin, Dow Chemical, Sumitomo Chemical, etc. At present, there are more than 1400 patents related to organic light-emitting diodes in the world, including three basic patents. The basic patents of small molecule organic light-emitting diodes are owned by Kodak Company in the United States, and the patents of polymer organic light-emitting diodes are owned by CDT (Cambridge Display Technology) in the United Kingdom and Uniax Company in the United States.
key technology
1. Pretreatment of indium tin oxide (ITO) substrate (1) Surface flatness of ITO: ITO has been widely used in the manufacture of commercial display panels, with the advantages of high transmittance, low resistivity and high work function. Generally speaking, ITO produced by RF sputtering is prone to uneven surface due to poor process control factors, and then tip substances or protrusions on the surface are produced. In addition, the process of high temperature calcination and recrystallization will also produce a convex layer with a surface of about 10 ~ 30nm. The paths formed between the particles of these uneven layers will provide opportunities for holes to shoot directly at the cathode, and these intricate paths will increase the leakage current. There are generally three ways to solve the influence of this surface layer? One is to increase the thickness of hole injection layer and hole transport layer to reduce leakage current. This method is mainly used for PLED and organic light emitting diodes with thick hole layer (~ 200 nm). Second, the ITO glass is reprocessed to make the surface smooth. The third is to use other coating methods to make the surface flatness better. (2) Increase of ITO work function: When holes are injected into HIL from ITO, too large potential energy difference will produce Schottky energy barrier, which makes hole injection difficult. Therefore, how to reduce the potential difference of ITO/HIL interface has become the focus of ITO pretreatment. Usually, we use O2 plasma to increase the saturation of oxygen atoms in ITO to increase the work function. After O2 plasma treatment, the work function of ITO can be increased from 4.8eV to 5.2eV, which is very close to that of HIL. Adding auxiliary electrodes, because the organic light-emitting diode is a current-driven element, when the external circuit is too long or too thin, it will cause a serious voltage gradient in the external circuit, which will reduce the voltage that really falls on the organic light-emitting diode element and lead to a decrease in the luminous intensity of the panel. Because the resistance of ITO is too large (10 ohm/square), it is easy to cause unnecessary external power consumption. Adding auxiliary electrodes to reduce the voltage gradient has become a shortcut to improve the luminous efficiency and reduce the driving voltage. Chromium (Cr: Chromium) metal is the most commonly used auxiliary electrode material, which has the advantages of good stability to environmental factors and high selectivity to etching solution. However, when the film is 100nm, its resistance value is 2 ohms/square, which is still large in some applications. Therefore, aluminum (0.2 ohm/square) with lower resistance at the same thickness is another better choice for the auxiliary electrode. However, the high activity of aluminum metal also makes it have reliability problems. Therefore, multilayer auxiliary metals, such as Cr/Al/Cr or Mo/Al/Mo, have been proposed. However, these processes increase the complexity and cost, so the selection of auxiliary electrode materials has become one of the keys of organic light-emitting diode technology. Second, cathode technology In the high-resolution OLED panel, the fine cathode is isolated from the cathode. The common method is mushroom structure method, which is similar to negative photoresist development technology in printing technology. During the development of negative photoresist, many process variables will affect the quality and yield of cathode. Such as bulk resistance, dielectric constant, high resolution, high Tg, low critical dimension (CD) loss, and proper adhesion interface with ITO or other organic layers. Third, packaging (1) water-absorbing material: the life cycle of general organic light-emitting diodes is easily affected by surrounding moisture and oxygen. There are two main sources of water vapor: one is that it penetrates into the module through the external environment, and the other is the water vapor absorbed by the materials in the process of organic light-emitting diodes. In order to reduce the water vapor entering the module or eliminate the water vapor absorbed by the process, the most commonly used substance is absorbent. Desiccant can capture freely moving water molecules by chemical adsorption or physical adsorption, so as to achieve the purpose of removing water vapor in the module. ⑵ Process and equipment development: The packaging process flow is shown in Figure 4. In order to place the desiccant on the cover plate and successfully adhere the cover plate to the substrate, it is necessary to dry in a vacuum environment or fill the cavity with inert gas such as nitrogen. It is worth noting that how to make the connection between the cover plate and the substrate more efficient, reduce the packaging process cost and shorten the packaging time to achieve the best mass production rate has become the three main goals of the development of packaging technology and equipment technology.