Lightness, thinness, low power consumption, active light emission, no viewing angle limitation...In the display field, organic light-emitting diodes (OLED) have high expectations due to these characteristics and have become the mainstream of smart phone displays. However, OLED panels at this stage generally adopt a vacuum evaporation preparation process, and their strict process requirements and high finished product prices limit their large-scale popularization and application in larger-size display products.
Inkjet printing can quickly and continuously prepare red, green, and blue pixel arrays on a larger-sized glass substrate under normal temperature and pressure, creating conditions for the preparation of large-sized OLED displays. However, OLED printing technology has only just started, and the industrial printing of OLEDs still faces many challenges.
There is a long way to go, but the light is already here. Recently, a reporter from Science and Technology Daily learned from the High-Tech Research and Development Center of the Ministry of Science and Technology that the “Strategic Advanced Electronic Materials” key special project-the National Key R&D Program 8.5-generation printed OLED display industrialization demonstration "based on industrialized printing materials devices and processes A series of results have been achieved on the subject of "Development", such as the development or evaluation of more than 10 new light-emitting layer, hole transport layer, and electrode layer materials for the printing process; the preparation of a fence that allows the pixels to be evenly distributed and is resistant to dissolution by printing solvents Materials; developed a large-area luminescent film with a film surface roughness of less than 5 nanometers.
Just like printing a document, marking the luminescent material into a pixel pattern
"China has long been behind in the field of information display, and the birth of the third-generation display technology represented by OLED has provided us with an opportunity for overtaking in the display field." At the Institute of Advanced Materials, Nanjing University of Technology, Jiangsu Professor Qin Tianshi, the leader of the project at the Provincial Key Laboratory of Flexible Electronics, took out two small bottles filled with OLED powder while talking.
This is the yellow light OLED material that his research team is developing. Under the light of the flashlight, the powder in the bottle is emitting a dazzling yellow light. "The yellow light material can increase the color saturation of the display panel, while effectively reducing the panel's energy consumption." Qin Tianshi said.
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OLED display technology is an active light-emitting technology. When driving voltage is applied to the pixels of the three primary colors of red, green and blue, the display panel will emit red, green and blue light, and then through different permutations and combinations, all colors visible to the human eye are generated. Compared with passive liquid crystal display technology, OLED technology does not require a backlight, has high brightness, low energy efficiency, large viewing angles, and bright colors. It has become the next-generation display technology with the most development potential.
At present, there are two main manufacturing processes for OLED panels, one is a vacuum evaporation process, and the other is an inkjet printing process.
"There is a big difference between these two processes, mainly in the equipment and materials used to prepare OLED displays." Qin Tianshi said that the current mass production of OLED screens is mainly prepared by vacuum evaporation process, that is, in a high-vacuum chamber. , Place the OLED light-emitting material on the evaporation source and heat it at high temperature, the material is heated and sublimated into gas, and then condenses on the substrate again to form an OLED pixel array. However, the evaporation process also has some insurmountable problems. During the evaporation process, a lot of material remains on the mask around the pixel, causing material waste and chamber pollution.
"More importantly, the manufacturing of evaporation equipment is difficult and costly, and more stringent conditions are required to achieve a high vacuum environment. Therefore, the evaporation process is difficult to produce large-size OLED TV screens on a large scale. Even if individual manufacturers use the evaporation process to prepare The cost and price of some high-end products are far beyond what the public can accept." Qin Tianshi said.
Pick one from a hundred, select the best luminescent material
Compared with evaporation technology, inkjet printing technology can greatly improve production efficiency and utilization of luminescent materials, significantly reduce the direct cost of manufacturing OLED displays, and effectively prepare large-area OLED displays.
However, building a printed OLED production demonstration line faces many challenges. This is also the meaning of the industrialization project of printed OLED display.
OLED luminescent materials are the source of display technology. "Currently mature OLED light-emitting materials, mainly small molecule light-emitting materials of vacuum evaporation process, are not necessarily suitable for printing processes. Therefore, how to use molecular design and solvents to dissolve OLED materials into ink, and compatible with the printing preparation process, This is one of the difficult points that need to be tackled in this subject." Qin Tianshi said, more importantly, most of the OLED materials compatible with the printing process are developed on the patented material system of Europe, America, Japan and South Korea. The development of luminescent materials with China's core technological advantages is imminent.
Since 2017, the Nanjing University of Technology team where Qin Tianshi works has begun to develop new luminescent materials with independent intellectual property rights. "In our research, we found that most of the current commercial luminescent materials are based on the rare earth element iridium and ruthenium complex system. These two elements are scarcely distributed in nature and the raw material costs are extremely high."
Therefore, the team tried to develop metallic platinum-based luminescent materials. Although platinum is a precious metal, the market price is much lower than the above two rare earth elements. In addition, the chemical stability of platinum is better than that of iridium and ruthenium, which means that if it is made into a display device, its service life will be longer than that of iridium and ruthenium-based complex materials. At the same time, platinum-based materials are also easier to break through foreign patent barriers. In this regard, they are looking for hope in a little bit of groping.
"For example, traditional iridium-based luminescent materials use a six-coordinate chemical structure. We initially designed platinum-based materials to be six-coordinated, but their luminous efficiency is not high. With the deepening of research, we found that It is prepared into a four-coordination structure, the material can emit very good dark blue phosphorescence, and the luminous efficiency is three times that of the previous." Qin Tianshi said that in three years, the team designed and developed more than 100 kinds of luminescent materials, and finally prepared one High-efficiency deep blue phosphorescent material is one of the best in a hundred.
At present, the team has developed or evaluated 3 kinds of light-emitting layer materials suitable for printing process, 3 kinds of charge transport layer materials suitable for printing process, and 5 kinds of new light-emitting metal complexes and electrode materials.
"Strong" pixel fence, let the luminescent material refer to where to play
The pictures we see on mobile phones and TVs are composed of many colored particles that are hard to detect with the naked eye. These particles are pixels. Each pixel includes three colors of red, green and blue, red, green and blue through different light-emitting combinations to form all colors. The smaller the pixel volume and the tighter the arrangement, the more delicate and realistic the resulting picture.
How to make the luminescent material form pixels, evenly distributed on the display screen, is a big challenge to the printing process.
"In inkjet printing, it is necessary to use a solvent to dissolve the OLED luminescent material to form a formula ink, and then distribute the red, yellow and blue formula inks in a certain array and print them in the corresponding positions. Therefore, before printing the ink, it is necessary to print on the substrate Spread a layer of'fence' material on top, so that every drop of ink printed can accurately enter the preset fence to achieve high-resolution full-color display. However, the current fence materials still rely on imports, and the solvent resistance of domestic materials is poor. When printing OLED ink, the fence is easily dissolved, resulting in uneven film thickness and color distortion of the display panel. For example, pixels that display blue may incorrectly display red or green. In the past few years, we have been optimizing the process. The fence'becomes strong'." Zhang Dongyu, a senior engineer at the Printing Electronics Center of the Suzhou Institute of Nanotechnology and Nano-Bionics, Chinese Academy of Sciences, said that based on domestic photosensitive glue, they deployed ink, controlled the temperature and time of exposure and development, and optimized the drying process. Dry and other technology enhances the solubility resistance of domestic fencing materials.
After the preparation was completed, the team placed the fence in a highly soluble acetone solution and ultrasonically treated it for 20 minutes, and found that the performance and height of the fence remained unchanged.
In addition, the R&D team also designed fences with different diameters and spacings, for example, circular fences with diameters of 80 microns, 100 microns and 150 microns. Zhang Dongyu said: "Fences of different diameters can control the number of pixels in the fence, so that it is convenient to make displays of different resolutions according to needs in the future."
Uniform film formation, surface roughness is less than 5 nanometers
The uniformity of printing is very important to the printing effect. The fence will be a combination of luminescent materials and solvent inks, circled in a circular fence, but also ushered in another challenge-the "coffee ring".
People who like to drink coffee will find an interesting phenomenon. After a drop of coffee evaporates, a dark ring that is much darker than the middle area will be formed on the edge of the drop. This uneven deposition phenomenon is the "coffee ring effect."
When a coffee ring appears on the display film, it will mean that the color of the displayed image is uneven. In severe cases, the image cannot be displayed in these areas.
"This puts high demands on the ink formulation, the distance between the ink droplets, the processing temperature of the substrate and the film layer. In order to make the film uniformity better, we have tried three to four decades in two years. A kind of small molecule and polymer additives, hoping to dope them in the ink, make the ink more diffusive, to form a uniform film." Project team member, Nanjing University of Posts and Telecommunications Professor Chen Shufen said that they systematically studied the solvent and The influence of the solute formula on the quality of the printed film and the performance of the OLED device. Based on the density, surface tension and viscosity of the ink droplets, several ink solvent formulas that are more in line with the needs of the printing process are finally determined, and patterned light-emitting films are prepared by printing. , And the surface roughness of the film is less than 5 nanometers.
"We hope that in the future, we can overcome the bottleneck of key materials and common technologies for printing OLED, and introduce more and more domestic materials and equipment with independent intellectual property rights into the production line of printing OLED TVs, and contribute to the localization of OLED TV screens. "Qin Tianshi said.