How to actually achieve print display?
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In the past year, printing display technology and flexible display technology have become the development direction of the industry. However, what exactly is the real printed display device? For the electroluminescent display device, if the printing display is to be realized, it is necessary to solve the printing process of the entire production process of the light-emitting display device, including the TFT, the transparent conductive layer, the light-emitting layer, various functional film layers, etc., to realize the printing process. . Achieving the above objectives poses a huge challenge to various materials of the device. It is very difficult to find materials with better solubility or better dispersion and stability, and organic materials are the most reasonable choice.
Printed display technology is not a new concept
In 1977, Ziegler-Natta was synthesized as a conductive polymer polyacetylene.
In fact, print display technology is not a new concept that has existed in recent years. In 1977, Heeger, Macdiarmid and Shirakawa catalyzed the synthesis of conductive polymer polyacetylene with Ziegler-Natta. This original work revealed the fact that organic materials are electrically conductive: organic polymers are electrically conductive after proper doping. Therefore, the three won the Nobel Prize in Chemistry in 2000, and the printed electronics technology has since developed.
In recent years, with the maturity of organic electroluminescent diode (OLED) technology and industry, printed electronics has a better accumulation and development foundation in materials and equipment, and applications and processes have achieved rapid development.
In 1998, Yang et al. exhibited a polymer PLED device using inkjet printing technology at the SID conference. In November of the same year, they successfully produced a two-color PLED device using inkjet printing technology. In 1999, Seiko Epson cooperated with CDT to display the first PLED full color display made by inkjet printing technology on SID. The 16 gray scale can display 4096 colors, up to 120ppi, and adopts active TFT driver. Since then, PLED devices made by inkjet printing technology have developed rapidly, and PLED inkjet printing devices are now commercially available.
Printed display materials are one of the pillars of printed electronics. Printed display materials are not only organic light-emitting materials, but also metal materials and inorganic materials. At present, metal paste is relatively mature, but it is still mainly limited to silver and copper materials; organic materials have been widely used in organic semiconductor devices and organic optoelectronic devices, but there are still “relatively poor reliability” and “life needs to be further improved”. And "low carrier mobility" and other issues.
Adopting OLED materials is a more mature technical solution
Print display technology is the application of printed electronics to the display field.
The printing display technology applies printing electronics to the display field, and refers to a printing method such as spin coating, silk screen printing or inkjet printing, which transfers metal, inorganic materials and organic materials onto a substrate to form a light-emitting display device. The ultimate goal of printed display technology is to realize a full-printing light-emitting display device that achieves low-cost manufacturing in an on-demand feeding mode at normal temperature and pressure. In the current printing display technology, the use of OLED materials to achieve display is the mainstream and relatively mature technology.
In terms of the preparation method, in general, OLEDs can be classified into two types. One is a small molecule luminescent material OLED, which is prepared by an evaporation method; the other is a conjugated polymer luminescent material PLED, which is prepared by a printing method.
Compared with the currently applied vacuum evaporation process for manufacturing OLED displays, the first is that the material utilization rate is as high as 95%, and the vacuum evaporation process material utilization rate is only 20%. Unlike the non-selective deposition of organic materials in the evaporation process, the printing display process only sprays the organic light-emitting materials where needed, greatly improving the utilization of organic materials and being more environmentally friendly; the second is not subject to equipment and large size. The limitation of the fine metal mask, the printing process can prepare a large-size display panel; the third is that the printing process does not require a vacuum evaporation chamber and a precision metal mask, etc., plus material saving and maintenance of vacuum evaporation equipment, Effectively reduce costs. In addition, due to the relatively simple structure of the device, lower power consumption and higher yield, the problems of high cost, low yield and large area preparation that restrict the development of OLED TV have been solved.
Quantum dots are the new research direction of printed electroluminescent devices
Quantum dots are more stable than the chemical structure of organic light-emitting materials.
With the development of OLED technology, researchers have begun to use quantum dot materials instead of organic electroluminescent materials to prepare light-emitting devices, also known as quantum dot electroluminescent devices. Since the OLED luminescent layer is an organic material, the organic substance is very sensitive to oxygen and water vapor, and the stability is poor; the decomposition or modification of the material under the heating effect causes the life of the device to be shortened. Moreover, most organic materials have high hole mobility, but the electron mobility is relatively low, resulting in imbalance of carrier injection, which restricts the improvement of luminous efficiency and causes color shift of chromaticity of light with voltage. , low color purity, etc. Therefore, the introduction of quantum dot materials into OLED devices is expected to compensate for the deficiencies of organic materials. As an inorganic luminescent material, quantum dots are more stable than the chemical structure and lifetime composition of organic luminescent materials.
In theory, QLED's “thin film dot matrix coating” is more suitable for printing technology, the yield rate will be higher, the material cost is 90% less than vacuum evaporation, and the technical difficulty of large size is limited, so it becomes the current A new direction in the research of printed electroluminescent devices. However, inorganic material quantum dots still have a process that is poor in printability, harsh in preparation conditions, and difficult to form a uniform film layer, so development still requires a process.
The working principle of quantum dot electroluminescent diodes is similar to that of organic electroluminescent devices. The basic structure of the QLED electroluminescent device is basically the same as that of the OLED device, that is, the quantum dot material is used to replace the organic light-emitting material in the organic light-emitting device. The quantum dot luminescent material can be used to prepare a display device by a printing process using a suitable dispersion method like a polymer luminescent material and a soluble small molecule luminescent material.
What exactly is a real printed display device?
The use of printing to prepare has always been the goal pursued by researchers.
The fabrication of organic electroluminescent devices by printing processes has been carried out for many years, from OLEDs and PLEDs to today's development of soluble small molecule electroluminescent materials and quantum dot electroluminescent materials. This technology completely eliminates the vacuum evaporation process, and the use of simple process, saving equipment investment and material printing methods to prepare organic electroluminescent devices has been the goal pursued by researchers. If the full printing preparation of the organic electroluminescence display device is to be truly realized, the printing process of the entire production process of the organic electroluminescence display device must be solved.
The organic electroluminescent display device is mainly composed of a thin film transistor TFT control unit and an organic electroluminescence unit. Therefore, both components should be prepared by a printing process to be a true printed display device. Now, for the thin film transistor TFT printing process, one is an organic material OTFT, and the other is an inorganic material TFT printing process.
The organic thin film transistor has the characteristics of "a flexible substrate can be prepared by various means such as a low temperature process and a printing process" and "easy to prepare in a large area". However, the current internationally reported OTFT-OLED devices are mostly limited to OTFT-driven organic small molecule light-emitting diodes. OTFT-PLED devices prepared by printing methods for OTFT-driven polymer light-emitting diodes POLEDs have rarely been reported.
The University of Pennsylvania's team used inorganic materials to produce TFT devices in a printing process. Printing processes such as inkjet printing technology, not only in the field of organic electroluminescent display device manufacturing, but also a new generation manufacturing technology of LCD color filter film, compared with the prior art, in terms of saving raw materials and reducing costs Certain advantages. By comparing the process of fabricating organic material layers with vacuum evaporation, we can see the advantages of printed display technology.