Organic light-emitting diode display technology has the characteristics of self-luminescence, which uses very thin organic material coating and glass substrate. When the current passes through, these organic materials will emit light, and the organic light-emitting diode display screen has a large viewing angle, which can save electricity. Since 2003, this display device has been applied to MP3 players.
According to the organic light-emitting materials used in organic light-emitting diodes, one is a small molecule device system with dyes and pigments as materials, and the other is a polymer device system with conjugated polymers as materials. At the same time, because organic electroluminescent devices have the characteristics of light-emitting diode rectification, small molecule organic electroluminescent devices are also called organic light-emitting diodes (OLEDs), and polymer organic electroluminescent devices are called PLED. Small molecule organic light-emitting diodes and polymer organic light-emitting diodes can be said to have their own advantages in material characteristics, but in terms of the development of the prior art, such as the reliability, electrical characteristics and production stability of displays, small molecule organic light-emitting diodes are in a leading position, and all the organic light-emitting diode components currently put into mass production use small molecule organic light-emitting materials.
structure
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 the photon part can be used as a display function; 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.
Typical structure: pm- organic light emitting diode. 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.
Multi-layer PM- organic light emitting diode structure with good luminous efficiency and widely used 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.
material
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. 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.
craftsmanship
Pretreatment of indium tin oxide (ITO) substrate
(1) ITO surface flatness: 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.
cathode precess
In high-resolution OLED panels, mushroom structure is usually used to isolate the fine cathode from the cathode, which is similar to the negative photoresist development technology of 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.
package
⑴ Water-absorbing material: In general, the life cycle of organic light-emitting diodes is easily affected by the surrounding water vapor 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 needs to be carried out in a vacuum environment, or by filling the cavity with inert gas, such as nitrogen, so as to place the desiccant on the cover plate and successfully adhere the cover plate to the substrate. 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.
Colorization technology
Full-color display is an important symbol to test whether the display is competitive in the market, so many full-color technologies have also been applied to organic light-emitting diode displays. According to the type of panel, there are usually three types: RGB pixel independent light emission, color conversion and color filter.
RGB pixels emit light independently.
Using luminescent materials to emit light independently is the most widely used color mode at present. It uses the precise metal shadow mask and CCD pixel alignment technology to prepare the luminous centers of red, green and blue, and then adjust the color mixing ratio of the three color combinations to produce true color, so that the three-color organic light-emitting diode elements can emit light independently to form a pixel. The key of this technology is to improve the color purity and luminous efficiency of luminescent materials, and the etching technology of metal shadow mask is also very important.
Organic small molecule luminescent material AlQ3 is a good green luminescent small molecule material with good green purity, luminous efficiency and stability. However, the best small molecular material for organic light-emitting diodes, with luminous efficiency of only 3 1mw and lifetime of 1000 hours, is also very slow and difficult to develop small molecular materials for blue light. The biggest bottleneck of organic small molecule luminescent materials lies in the purity, efficiency and life of red and blue materials. However, blue light and red light with good color purity, luminous efficiency and stability have been obtained by doping the main luminescent materials.
The advantage of polymer luminescent materials is that their luminescent wavelength can be adjusted by chemical modification. At present, various colors covering the whole visible light range from blue to green to red have been obtained, but the life span is only one tenth of that of small molecular luminescent materials, so the luminous efficiency and life span of polymer luminescent materials need to be improved. It should be an arduous and long-term task for material developers to continuously develop luminescent materials with excellent performance.
With the color, high resolution and large area of OLED display, the etching technology of metal shadow mask directly affects the quality of display panel, so more stringent requirements are put forward for the dimensional accuracy and positioning accuracy of metal shadow mask graphics.
Light-color conversion Light-color conversion is a combination of blue organic light-emitting diodes and light-color conversion.
Thin-film array: firstly, organic light-emitting diodes emitting blue light are prepared, and then red light and green light are obtained by exciting color conversion materials with blue light, thus obtaining full color. The key of this technology is to improve the color purity and efficiency of light-color conversion materials. This technology does not need metal shadow mask alignment technology, but only needs to evaporate blue organic light-emitting diode elements, which is one of the most potential full-color technologies for large-size full-color organic light-emitting diode displays in the future. However, its disadvantage is that the light-to-color conversion material easily absorbs the blue light in the environment, which leads to the decrease of image contrast, and at the same time, the light guide will also cause the problem of image quality degradation. Japan Quanseiko Co., Ltd., which has mastered this technology, produced 10 inch OLED display.
Color filter film
In this technology, white organic light emitting diodes are combined with color filter films. Firstly, devices emitting white organic light-emitting diodes are prepared, then three primary colors are obtained through color filter films, and then the three primary colors are synthesized to realize color display. The key of this technology is to obtain high efficiency and high purity white light. Its manufacturing process does not need metal shadow mask alignment technology, but can adopt mature color filter manufacturing technology of liquid crystal display LCD. Therefore, it is one of the potential full-color technologies for large-size full-color organic light-emitting diode displays in the future, but the light loss caused by the color filter film is as high as two-thirds. TDK in Japan and Kodak in the United States use this method to manufacture organic light-emitting diode displays.
RGB pixels emit light independently, light-color conversion and color filter are three full-color technologies for manufacturing organic light-emitting diode displays, each of which has its own advantages and disadvantages. It can be determined according to the process structure and organic materials.
Drive type
The driving modes of organic light-emitting diodes are divided into active driving (active driving) and passive driving (passive driving).
Passive driving (PM organic light emitting diode)
It is divided into static driving circuit and dynamic driving circuit.
⑴ Static driving mode: On a statically driven organic light-emitting display device, the cathodes of organic light-emitting pixels are generally led out together, and the anodes of each pixel are led out separately, which is the * * * cathode connection mode. If a pixel emits light, as long as the difference between the voltage of the constant current source and the voltage of the cathode is greater than the luminous value of the pixel, the pixel will emit light driven by the constant current source. If a pixel does not emit light, its anode is connected to a negative voltage, and it can be turned off in turn. However, when the image changes greatly, cross effect may appear. In order to avoid this, we must adopt the form of communication. Static driving circuits are generally used to drive segmented display screens.
⑵ Dynamic driving mode: On the dynamically driven organic light-emitting display device, people make the two electrodes of the pixel into a matrix structure, that is, one horizontal group displays the electrodes of the same nature of the pixel, and the other vertical group displays the electrodes of the same nature of the pixel. If a pixel can be divided into n rows and m columns, there can be n row electrodes and m column electrodes. The rows and columns correspond to the two electrodes of the light-emitting pixel, respectively. Namely a cathode and an anode. In the process of actual circuit driving, pixels should be lit row by row or column by column, usually row scanning, and row scanning and column electrodes are used as data electrodes. The implementation method is: pulse is applied to each row electrode circularly, and at the same time, all column electrodes give driving current pulses to the pixels in this row, thus realizing the display of all pixels in a row. In order to avoid "cross effect", pixels in the same row or column are not displayed by applying reverse voltage. This scanning is done line by line, and the time required to scan all lines is called frame period.
The selection time of each line in the frame is equal. Assuming that the number of scanning lines in a frame is n and the time for scanning a frame is 1, then the selection time occupied by a line is 1/N frame time, which is called the duty ratio coefficient. Under the same current, the increase of scanning lines will reduce the duty cycle, which will lead to the effective reduction of current injection on organic electroluminescent pixels in one frame and reduce the display quality. Therefore, with the increase of display pixels, in order to ensure the display quality, it is necessary to appropriately increase the driving current or adopt a double-screen electrode mechanism to improve the duty ratio.
In addition to the cross-effect caused by electrode sharing, the mechanism of positive and negative charge carriers recombination in organic electroluminescent display screen makes any two luminescent pixels, as long as any functional film that constitutes their structure is directly connected together, there may be crosstalk between them, that is, one pixel may emit light and the other pixel may emit weak light. This phenomenon is mainly caused by poor thickness uniformity and poor lateral insulation of organic functional films. From the driving point of view, in order to slow down this unfavorable crosstalk, it is also an effective method to adopt the reverse cut-off method.
Display with gray scale control: the gray scale of the display refers to the brightness level between black and white images. The more gray levels, the richer the levels from black to white, and the clearer the details. Gray scale is an important index for image display and colorization. Generally, the screens used for gray scale display are mostly dot matrix displays, and their driving is mostly dynamic. Several methods to realize gray scale control are: control method, spatial gray scale modulation and temporal gray scale modulation.
Second, the active drive (AM organic light emitting diode)
Each pixel that is actively driven is equipped with a low-temperature polysilicon thin film transistor (LTP-Si TFT) with switching function, and each pixel is equipped with a charge storage capacitor. The peripheral driving circuit and the display array are integrated on the same glass substrate. The same TFT structure as LCD can not be used in organic light emitting diodes. This is because LCD is driven by voltage, while OLED is driven by current, and its brightness is proportional to the current. Therefore, in addition to the addressing TFT for on/off switching, a small driving TFT with low on-resistance is needed, which can allow enough current to pass.
Active drive is a static drive mode with memory effect, which can be driven with 100% load. This driving is not limited by the number of scanning electrodes, and each pixel can be selectively adjusted independently.
Active driving has no duty cycle problem, and the driving is not limited by the number of scanning electrodes, so it is easy to achieve high brightness and high resolution.
Active driving can adjust the brightness of red and blue pixels independently, which is more conducive to the realization of colorization of organic light-emitting diodes.
The driving circuit of active matrix is hidden in the display screen, which makes it easier to realize integration and miniaturization. In addition, the connection problem between the peripheral drive circuit and the screen is solved, and the yield and reliability are improved to some extent.
Third, the comparison between the two.
Passive active type
Instantaneous high-density luminescence (dynamic driving/selective) continuous luminescence (steady-state driving)
Design of TFT driver circuit with IC chip added outside the panel/built-in thin film driver IC
Scan line by line and erase data line by line.
Gray scale control is easy to form organic EL pixels on TFT substrate.
Low cost/high voltage drive low voltage drive/low power consumption/high cost
Easy design change, short delivery time (simple manufacturing) and long life of light-emitting elements (complicated manufacturing process)
Simple matrix drive+organic light emitting diode LTPS TFT+ organic light emitting diode