Although OLED lighting products are still only in the early stages, the brightness, efficiency and longevity of OLED lighting devices have reached a very high level at the research and development level, such as Kodak, UDC, and Philips. Research by Philips, Simmens, Osram and Novaled has yielded good results. In the process of OLED lighting industrialization planning, in addition to continuing to improve the efficiency and longevity of white OLEDs, these enterprises have already lowered the price as the main research and development direction, and even incorporated upstream material suppliers and equipment manufacturers into the plan, in order to form a complete The industrial chain promotes the development and market application of OLED lighting products.
It is understood that Philips, GE, Konica Minolta, Lumiotec, OSRAM and other international companies are expected to launch OLED lighting products in 2010-2012. In fact, these large companies have begun small-scale production of OLED lighting products (the world's major companies plan OLED The mass production roadmap for lighting products is shown in Figure 1). Despite this, OLED lighting has reached the level of fluorescent lamps and is widely used. There are still many problems to be solved, including efficiency, life and production costs, and cost should be a key factor restricting the popularity of OLED lighting.
Figure 1 OLED lighting product production roadmap
OLED lighting should reach the application level of fluorescent lamps in efficiency, and its power efficiency should be at least 70lm/W. Although OLED lighting devices of about 100lm/W have been reported so far, this is manufactured under very extreme conditions, usually The efficiency of the OLED lighting device manufactured in the case is only 30~50lm/W, which is obviously far from the requirement.
OLED high light efficiency needs to solve material and structural problems
OLED lighting should reach the application level of fluorescent lamps in efficiency, and its power efficiency should be at least 70lm/W. Although OLED lighting devices of about 100lm/W have been reported so far, this is manufactured under very extreme conditions, usually The efficiency of the OLED lighting device manufactured in the case is only 30~50lm/W, which is obviously far from the requirement.
From the material point of view, in order to achieve such high efficiency, it is necessary to use a phosphorescent material with a luminous efficiency of up to 100%. At present, green and red phosphorescent materials have no problem, and an urgent need to solve is a blue phosphorescent material that plays an important role. . Nowadays, there is a lack of high-efficiency and high-stability blue-light phosphorescent materials, which is also the main technical difficulty that hinders the efficiency improvement of OLED lighting devices. Therefore, it has become a mass production technology to develop practical high-efficiency and high-stability blue phosphorescent materials. The main research direction of the field.
From the perspective of device structure, effective device structure design is also the key to improve device efficiency. Currently, a good device structure includes a multi-emission layer structure, a single luminescent layer structure, a stacked structure, and a top emission structure. The multi-emissive layer structure is one of the most widely used white OLED devices. The process is relatively mature and the device performance is the best, but the complicated process may affect its yield and production cost of OLED lighting devices in the future. Structural devices sometimes have the disadvantage that the spectral and chromaticity coordinates vary with the drive voltage. However, the multi-emissive layer structure due to good device performance, and the problem of spectral and chromaticity coordinates changing with the driving voltage can be completely solved by the design of the device structure, so if it can be controlled and improved in yield, the structure is still It is a mass production technology, which is currently used by Philips, UDC, and Novaled.
Although the single-emitting structure can avoid the change of color purity with the driving voltage to some extent, it may also simplify the process in the production process, but the unsatisfactory efficiency and stability problems also make this structure not applicable in practical applications. .
The laminated structure has the characteristics of a single light-emitting layer and a multi-light-emitting layer structure, and is a device structure in which a plurality of light-emitting units are connected in series through a charge generating layer, and the laminated device has excellent spectral stability and high light-emitting. The characteristics of high efficiency, high brightness and good stability make the laminated structure device very competitive in practical applications. If the process of complex laminated structure can be further improved, the laminated structure device is expected to be used for lighting. The mainstream technology of white light OLED production.
The top emission structure may also become an important technical development direction of white OLED for illumination due to its advantages in effective light-emitting area and improved efficiency, and the combination of other structures and top emission structures can develop a higher performance white OLED.
Improve light extraction technology while controlling costs
In addition to materials and device structures, there is a technology that is critical to improving the efficiency of white OLEDs, namely light extraction technology, which is the technology that OLED lighting devices must adopt to achieve applicable power efficiency. For the applicable extraction technology, while improving the output light as much as possible, the practicality must be considered. So far, researchers have developed many light extraction technologies, but there are not many applications that can meet the application requirements. The reason is the cost of the process complexity and the problem of large area.
Currently preferred and applicable light extraction techniques include forming a coupling film composed of a prism or a microlens on the outer surface of the substrate, introducing a scattering layer and a high refractive index layer on the outside of the transparent electrode, Philpis, GE, UDC, Novaled, Konica Minolta and other companies have adopted these light extraction methods to greatly improve the efficiency of OLED lighting devices. Combined with high refractive index substrate and microlens coupling structure, the light extraction efficiency has even increased to 2.4 times. It is believed that once this With the development of light extraction technology, substrate suppliers can mass produce such conductive substrates that can effectively improve the output efficiency of OLEDs, meeting the requirements of OLED lighting applications. Before achieving this goal, it is first necessary to solve the low cost of processing large-area substrates. Manufacturing Technology.
Improve the lifetime of OLED products through light attenuation control
If OLED lighting applications are to be popularized, they must further improve their lifespan. The initial goal is to reach the level of fluorescent lamps, that is, 20,000 hours, and the future goal is to reach 50,000 hours. In addition to this, in addition to material In addition to the breakthrough and further improvement of device structure and process, the control method of large area uniformity and the control method of optical attenuation of OLED devices during use are also important to improve the stability of OLED lighting devices in practical applications.
An effective way to improve the large-area uniformity of OLED lighting devices is to introduce some metal auxiliary lines on the substrate to effectively reduce the resistance of ITO (Fig. 2), thus improving the uniformity of current distribution, in order not to affect the effective area of ​​illumination, metal-assisted The area of ​​the line is generally controlled in the range of 10% to 20% of the illumination area. This simple metal guide line method is a good practical method before a better method is found.
Figure 2 Metal Auxiliary Line Structure of OLED Lighting Board
Recently, Kodak has developed another method for assembling large-area OLED lighting devices (Fig. 3). Using a monolithic integrated unit method, a single illuminating unit is connected in series. This method is very practical in practical applications, not only can Improves light uniformity and simplifies power supply design.
Figure 3 Series structure of OLED lighting panels
For the problem of light attenuation of the OLED device during use, the correction signal is generated by the control circuit to ensure a constant light output, that is, a dimming method. In various dimming designs, the feasibility and accuracy of calibration is the key to the practicality of the design. Currently only Philips and Matsushita Electric have developed this circuit for light attenuation control on OLED lighting devices.
Improve the yield and promote mass production under the premise of controlling costs
In fact, the reason why OLED lighting is still not available, the most fundamental reason is its high cost, now the price of OLED is significantly higher than traditional lighting, although the price problem can be improved by improving the brightness of the device, but high brightness Brought the problem of life. What is important is that due to the immature production equipment and the lack of a reliable mass production environment to achieve the OLED process flow, resulting in low yield of OLED, the price of OLED can not be reduced, which in turn affects the mass production of OLED. Promotion. Therefore, to solve the bottleneck of mass production technology of OLED lighting, the primary task is to improve the yield and promote the mass production of OLEDs to form a virtuous cycle.
Figure 4 Manufacturing costs of producing OLED lighting panels
Figure 4 shows the cost involved in the OLED manufacturing process. It can be seen that the reasons for the high price of OLED lighting products are various: (1) small scale makes material cost and manufacturing cost cannot be reduced; (2) evaporation process The utilization rate of materials is relatively low. If the current point source is used, the utilization rate of materials is less than 10%. Even with the improved line source, the utilization rate of materials is less than 40% according to the current size; 3) The synthesis of key materials is complicated, the yield of high-purity sublimation is low, and the price of the material itself is relatively high; (4) The process of production is uncertain, and the production equipment is expensive.
Therefore, to reduce the price of OLED lighting products must address the following key technologies: (1) to expand the size of the substrate; (2) to develop production equipment that is cheap; (3) to increase the automation, reliability and production capacity of the equipment; (4) improve Material utilization, reducing material costs; (5) improving yield; (6) developing low-cost packaging technology.
It is not difficult to see that these links are closely related to raw materials and production equipment. Obviously, we must accelerate the pace of the OLED lighting industry. We must do a good job in industrialization and supporting work such as equipment research and raw material development. In addition, if a simpler and cheaper OLED processing method can be developed, the price of OLED lighting products will be greatly reduced in place of the relatively expensive vacuum evaporation method currently used, one of which is continuous production like a printed newspaper. The roll-to-roll process is most likely to be the production method for future OLED lighting products.
It can be seen that once the OLED lighting device is significantly improved in terms of efficiency, life and cost, and meets the practical application requirements, how to use the advantages of OLED to break through the traditional design concept to obtain novel and unique OLED lighting products will be placed in OLED The problem in front of the lighting designer. Because this directly determines whether OLED lighting products can be accepted by people, and finally become a key factor in the field of lighting.
OLED lighting is a large optoelectronics industry. If OLED technology matures and mass production, it is possible to replace traditional light sources. It can use OLED surface illumination, transparency, and bending to design novel lighting products, such as ceilings. And the large-area installation of OLED on the wall, so that the entire ceiling and wall surface shine, instead of partial lighting like a light bulb, fluorescent light, which makes the light source more gentle and comfortable; or can make hanging lighting fixtures, adjustable The angle of the lighting device, or the use of its light and thin characteristics, the design of the sheet-shaped lighting fixtures, allowing the lighting to show different effects and mood. However, before the OLED lighting products are widely used, it is necessary to solve the low-cost manufacturing technology of OLED lighting products, including key issues such as material production, equipment manufacturing and luminaire design.
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