Following the thin film transistor liquid crystal display (TFT-LCD), active organic light-emitting diode (AMOLED) is regarded as the best technology for the next generation panel, in which the organic light-emitting diode (OLED) is a solid-state self-luminous display with simple structure and self-luminescence. It does not require backlights, wide viewing angles, good image color and power saving, and is expected to be widely used in the small and medium panel market.
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At present, AMOLED technology is not easy to make a large-sized panel, and the blue portion of the backlight has a short life span, but it is usually used on a smart phone, and the replacement rate of such a mobile phone is quite high, so that the technical problem of short blue light life can be avoided. As long as the color advantages are maximized during use, and the mobile phone panel is not too big. At present, AMOLED has been applied to small and medium-sized display products such as mobile phones, MP3s, digital cameras, and digital photo frames. According to DisplaySearch data (Figure 1), the global OLED panel production value will increase from 600 million US dollars in 2008 to 7.1 billion US dollars in 2016, with a compound annual growth rate of 36%. This high growth trend is mainly due to the growth of AMOLED. In the first quarter of 2009, the panel output value has surpassed PMOLED for the first time. It is expected to surpass PMOLED in 2010 and become the main force driving the luminescence of the OLED market.
Although the price of AMOLED panels is still about 50~100% higher than that of TFT LCD, it is expected to fall to 10% in the next 2-3 years, becoming an alternative technology. In the future, the size and brightness of AMOLED panels will be improved to varying degrees. The current size will be enlarged from the current 2.2~2.8 inches to more than 3 inches, and the brightness will be further increased from the current 200nits to 300nits to highlight the OLED color vividness and high contrast. Self-illumination advantages. These features increase the opportunity for AMOLEDs to be applied to high-end smartphone panels.
AMOLED manufacturers are currently based on South Korea's Samsung Mobile Display (SMD), with shipments of approximately 2 million units per month. The company announced on April 20, 2010 that it will invest in next-generation production lines in Cheonan, Chungcheongnam-do, South Korea. The factory invested in the production of a 5.5-generation specification line for larger-size TVs and larger-size active-matrix OLED substrates. It is expected to officially enter mass production in January 2011; Samsung Corning, which is responsible for producing glass substrates at the same time, is scheduled to be in Korea in 2010. Investment in glass substrate production line for OLED. In short, the Samsung Group is ready to meet the advent of the active matrix OLED market. LG Display (LGD) is expected to expand AMOLED production this year and will increase its overall shipments to 5-10 million units per month in the second half of the year.
According to the OLED Association evaluation, AMOLED panels will start from small and medium-sized panels. After the scale of production is enlarged, it is possible to produce large-size TVs that are competitively priced, and the image quality of large-size TVs will be the killer application of AMOLEDs. The output value of small and medium-sized products is expected to reach US$3 billion to US$4 billion in 2015. In the future, if large size is added, it can further grow to US$12 billion. After SMD and LGD expanded the AMOLED display panel manufacturing capacity, AUO also announced that it will mass-produce small-size AMOLED panels at the end of 2010 to meet the strong demand for the panel in the mobile phone market in 2011, in order to catch up with the large number of OLED panels used in smartphones. The trend.
In addition to many smart phones starting to adopt active matrix OLED panels, iPad-led tablet PCs will enter the OLED era from 2011. The table on the right is the layout of active matrix OLEDs for different manufacturers in the world. The market performance of the size is expected.
Development status of soft AMOLED
At present, most monitors use TFT LCD. If you want to be light, thin, and easy to carry around, the amount of information displayed on the screen is not enough. If you want to display enough information at a time, the volume will be too large to carry, and it may consume electricity. Too big. For future displays, consumers hope that they can have the characteristics of large amount of information, convenient collection, low power consumption, unbreakable, bendable folding or easy winding.
In addition to excellent image quality, the transistor-driven AMOLED has all the qualities that the above consumers want, and is fully in line with the future information society's demand for mobile device displays. Compared with the TFT-LCD technology, the AMOLED has a simple structure, and does not require complicated components such as a backlight, a diffusion plate, an alignment film, a spacer, and the like, and the mechanical characteristics of the organic light-emitting layer are closer to the flexible substrate, so it is more suitable for making a bendable A rollable soft display.
Many research units are currently working on the development of flexible AMOLED panels and showcasing relevant technologies in professional journals and international seminars. For example, Sony in Japan showed a 4.1-inch QVGA soft AMOLED display at 2010 SID. The display is fabricated on a PES substrate with an organic transistor (OTFT) backplane and a top-emitting OLED structure with a pixel size of 432×240 and a resolution of 121 ppi. 80 μm, radius of curvature is 4 mm.
In addition, Korean manufacturer Samsung exhibited 6.5-inch and 2.8-inch WQVGA soft AMOLED displays at 2010 SID. The active backplanes are Metal-Oxide TFTs (MOx TFTs) and LTPS-TFTs. The 6.5-inch AMOLED is a metal oxide transistor back plate fabricated on a plastic substrate at a process temperature of 250 to 300 ° C, and is equipped with a light-emitting OLED structure. The Oxide TFT (17.8cm2/vs) component is an active component with higher carrier mobility and driving stability than the a-Si TFT and OTFT after the LTPS-TFT, and thus is suitable for driving a current component such as an OLED.
Oxide TFTs are used in a wide range of applications, including flexible displays and transparent displays. Taking a flexible display as an example, since the Oxide TFT can be fabricated by a low temperature process, a flexible display can be formed with a plastic substrate. Therefore, Samsung will also introduce this technology in the development of flexible displays. The sample pixels are 160×272, the resolution is 52ppi, and the curvature radius of curvature is up to 10mm.
Another Korean manufacturer, LGD, is also very active in investing in AMOLED. In addition to the upcoming commercial 15-inch AMOLED TV products at the Yokohama Photonics Show 2009, the 4.3-inch HVGA soft AMOLED panel was also demonstrated at the 2010 SID. The panel adopts an opaque stainless steel metal substrate, and a low-temperature amorphous silicon transistor (a-Si TFT) back plate is fabricated on the substrate, and is matched with an inverted top-emitting OLED structure, and the pixel is 480×320, the resolution is 134 ppi, and the thickness is 0.3 mm. The bending radius of curvature is less than 50 mm.
Soft AMOLED key technology
The key development technologies of soft AMOLED include flexible substrate, flexible TFT backplane, flexible OLED light-emitting layer and package and protective layer. The following key technologies are discussed in depth.
The flexible substrate includes a metal foil, a thinned glass, and a plastic substrate. Metal foils are more limited in their use due to opacity; the biggest problem with thinned glass is fragility; replacing traditional substrates such as glass and metal with soft plastic substrates faces two major technical problems, first the substrate itself. The heat resistance, optical, mechanical and water-blocking oxygen properties can meet the needs of component applications, followed by compatibility with TFT and OLED processes.
The heat-resisting characteristics of the plastic substrate include thermal cracking temperature, glass transition temperature, thermal expansion coefficient, etc. In the TFT process, the plastic substrate must undergo a plurality of temperatures of at least 200 ° C and a plasma bombardment during vacuum coating, and has heat resistance. Stability and essential characteristics are the key to success; optical properties include light penetration, light color, refractive index, etc. As the display surface substrate of the display, excellent optical properties are important factors in image quality; mechanical properties include surface Flatness and roughness, surface hardness, mechanical strength, etc., because the display must be able to withstand the harsh environment of human touch, storage and carrying, so it is also an important part of the life without damage and good life; As for the water-blocking oxygen transmission characteristics, including water vapor transmission rate (WVTR) and oxygen permeability (OTR), these two parameters are most often used to explain the water-blocking ability of the substrate and the film package. The lifetime of the OLED should be more than 10,000 hours, the WVTR of the film package should be less than 1×10-6 g/m2/day, and the OTR should be less than 1×10-5 g/m2/day.
The brightness of OLED is extremely sensitive to moisture and oxygen. The traditional AMOLED technology can effectively block the damage of water vapor and oxygen to OLED components by using glass substrate. The current water-proof oxygen characteristics of plastic substrates are all above 1g/m2/day, so it is soft. The AMOLED must be combined with a barrier structure and a thin film encapsulation technology to achieve effective water-blocking oxygen transmission characteristics. How to manufacture a high-reliability package structure similar to a glass package on a flexible display and to have flexural characteristics is the most important issue for improving the life of a soft AMOLED.
With existing semiconductor process technology, it is still quite difficult to directly fabricate electronic components on a plastic substrate. It is easier to attach the plastic substrate to the supporting glass carrier and perform subsequent assembly. Possible techniques include attaching and direct coating. The attaching method is to attach a plastic substrate to a glass carrier by using a bonding material or electrostatic adsorption, and the usable substrate includes PC, PET, PEN, PES, PI, etc.; the direct coating method uses a coated PI plastic substrate. It is directly coated on the glass carrier board, and the release layer is sandwiched in the middle to facilitate the removal of the plastic substrate after the assembly process is completed. Each of the above technologies has its own advantages and disadvantages. The difference is that the electrostatic adsorption method requires the entire process to use the printing method to make the components, and the process is more limited. The attaching method must use the bonding glue, and the rubber material is susceptible to deformation caused by the process temperature of the component, resulting in components. The alignment error affects the characteristic performance of the electronic component; the direct coating method can utilize the good adhesion characteristics between the plastic substrate and the glass carrier, so that the adhesive material is not required, but the problem arises after the release layer and the electronic component are completed. How to completely remove the panel, the current successful combination of Philips's sacrificial layer and laser removal method and the special release layer structure of the Institute and the direct cutting quick removal method.
In addition, the flexible TFT backplane is the most critical technology for driving flexible AMOLED panels. The current research and development technologies include silicon-based transistors (Si TFTs), organic transistors (OTFTs), and recently popular metal-oxide-semiconductor transistors. All of the above three technologies can be fabricated at different low temperatures, so they can be matched with the aforementioned flexible plastic substrate.
The development of flexible TFT backplanes needs to solve two technical bottlenecks, namely low-temperature process and stress-free film technology. On the one hand, in order to fabricate the TFT on a flexible substrate, the process temperature must conform to the temperature tolerance that the substrate can withstand, and the low-temperature process TFT will pose great challenges to the component characteristics; on the other hand, the low-growth film has an intrinsic stress. Small, more suitable for soft components. However, low-temperature deposited films have higher film defects and higher temperature deposited films, which affect the electrical and reliability performance of the components. Therefore, the process temperature must also take into account the electrical properties of the film. It cannot be reduced without limit.
The soft substrate must take into account the stress influence of the film of each layer of the component. Therefore, the development of the stress-free thin film technology has become a priority for the fabrication of the flexible TFT backplane. Reducing the stress between the interfaces can improve the TFT characteristics. Therefore, in the development of the flexible TFT backplane technology, not only the substrate material, the process temperature, and even the intrinsic stress generated in the substrate processing and the process must be matched to produce a flexible TFT backplane suitable for a flexible display.
Since silicon-based technology is more mature than other materials, soft silicon-based TFT backplane technology is still the most common. Among them, a-Si TFT has the advantages of simple process and superior component uniformity, but the current drive reliability is poor. In addition, the development of high carrier mobility technology will contribute to the high stability current required for AMOLEDs, such as microcrystalline silicon TFTs and low temperature polysilicon (LTPS) TFTs, which have better silicon crystal quality in terms of materials. However, it is necessary to overcome the problem of compatibility between process temperature and plastic substrate process.
Another research and development focus is on the critical voltage drift phenomenon of soft TFTs under flexing operation. The main reason for this phenomenon is firstly caused by external stress causing deep defects in the gate insulating layer and the active layer, and secondly, the characteristics such as carrier mobility and subcritical slope are less changed by applied stress. The change of these TFT characteristics is very important for integration into the flexible TFT backplane. How to avoid the electrical variation of the TFT backplane due to soft deflection will be the focus of the soft AMOLED panel structure design and drive circuit design.
Conclusion
Compared with other soft display technologies such as EPD and Ch-LC electronic paper, the soft AMOLED display has the advantages of full color display and high contrast in display quality, and can play dynamic video content such as multimedia audio and video, suitable for high-end intelligent mobile devices. Applications, the bendable flexible panel is significantly different from existing glass substrate displays. DisplaySearch's forecast of the growth trend of global soft OLED displays (Figure 4) shows that related products will enter the market in 2011, shipments will exceed 10 million in 2013, and in 2015, soft OLED panels will reach ten. The scale of output value of billions of dollars.
Although soft OLEDs are still in the stage of technology research and development and sample development, with the development of related technologies such as flexible substrates, TFT backplanes, OLEDs and packaging, in addition to the active development of technology by Japanese and Korean manufacturers, Taiwanese companies continue to increase. Relevant research and development efforts, this means that the possibility of future soft AMOLED productization is increasing. In addition, the soft AMOLED will help the continuous upgrading of the midstream and downstream industries on the display in Taiwan. In addition to the transformation of the less competitive five-generation production line to the development of high-value-added soft AMOLED displays, it can also assist other industries in Taiwan. For example, plasticization, equipment and other aspects of the electronic display field, and accelerate the material manufacturers to enhance the technical competitiveness, bring new opportunities for intelligent life and energy saving carbon for the display industry.
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