With the launch of Apple's wearable devices, the technical research and development of silicon-based OLED microdisplays around the world is accelerating continuously.
How to improve the technical maturity of silicon-based OLED microdisplays and further break through the technical performance of high-end OLEDs? How to minimize the design and production costs of silicon-based OLED microdisplays? Solving these problems is not only related to the equipment innovation in relevant industrial fields but also to the development direction of new display technologies for humanity.
What are the difficulties in the technical research and development of silicon-based OLED microdisplays nowadays? Currently, the technical difficulties of silicon-based OLED microdisplays globally are mainly reflected in aspects such as materials, processes, circuit design and integration, and performance improvement.
1. More Stable Materials for Silicon-based OLED Microdisplays
How to further improve the stability of OLED materials is one of the important research and development directions in the field of physical materials science today. Due to the performance degradation of OLED materials during long-term use, problems such as reduced luminous efficiency and color shift occur. In some experiments, after continuous use of ordinary OLED materials for about 1000 hours, the luminous efficiency may decrease by about 20%.
Especially in the tiny pixel structure of silicon-based OLED microdisplays, the requirements for material stability are even higher. However, currently, the material research in this area is in a bottleneck stage. The research and development of several new materials globally require a large amount of cost, so it will take more time to reduce the cost to achieve product popularization.
In addition, to achieve higher-quality display, OLED materials need to be more uniform so that the color and brightness can be more consistent. But due to the small size and high pixel density of silicon-based OLED microdisplays, achieving uniform coating of materials is quite a challenge.
The following are some leading technologies in improving the stability and uniformity of materials for silicon-based OLED microdisplays:
In terms of OLED material research and development, the team led by Wang Suning from Beijing Institute of Technology has studied a new type of blue phosphorescent molecule based on divalent platinum complexes. By using the influence of the ligand structure and substituents on the structural rigidity, photochemical and photophysical properties of divalent platinum complexes, a new type of blue phosphorescent material with high luminous quantum efficiency has been synthesized.
In addition, the Advanced Nanophotonic Materials and Devices Team of Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, has developed a "solvent sieve" that can better remove the thin nanosheet phase and improve the luminous performance of perovskite materials, which is an important technological breakthrough.
2. Lower-cost OLED Manufacturing Processes
Nowadays, the technology of silicon-based OLED microdisplays has gradually matured, and thus the cost is continuously decreasing. However, in the field of high-end silicon-based OLED microdisplays, the challenge of relatively high costs still exists.
Researchers are enhancing the process maturity through three aspects to reduce production costs: high-precision lithography process, thin film deposition process, and packaging process.
To achieve high resolution and high pixel density, silicon-based OLED microdisplays need to adopt high-precision lithography processes to fabricate tiny pixel structures. The smaller the pixel size, the higher the precision requirements for the lithography process. This requires overcoming problems such as the resolution limitations of lithography equipment, the performance of photoresists, and the error control during the lithography process.
In addition, depositing high-quality OLED thin films on a silicon-based substrate is a key process. Precise control of the thickness, composition, and crystallinity of the thin films is required to ensure stable color performance. If there are problems such as uneven thin film thickness and unreasonable cost allocation ratio, it will inevitably lead to a decrease in the yield rate of the equipment. Moreover, if factories want to achieve
large-scale production of such microdisplays, they also need to increase the speed of thin film deposition, which places extremely high requirements on process control.
Furthermore, since OLED materials are very sensitive to moisture and oxygen, the packaging requirements for silicon-based OLED microdisplays are extremely high. Once eroded, the luminous efficiency of OLED materials will decrease rapidly. Therefore, it is necessary to develop better packaging processes to minimize the thickness of the packaging layer and bring the miniaturization of microdisplays to a higher level.
In terms of process improvement, a research team from Shanghai University has analyzed the conductive medium of silicon-based chips, proposed a one-time patterning theory and pixel isolation method, and formed an integrated solution combining the CMOS process and the OLED process, reducing the huge investment cost of process equipment.
3. Circuit Design and Integration
Nowadays, the design technology of CMOS driver circuits is relatively mature, but for the design of circuits with even smaller sizes, the unremitting efforts of researchers are still needed. Among them, achieving lower power consumption and faster response speed is the
main direction of future circuit design. In addition, driver algorithms and AI technologies will be beneficial for accelerating the technical optimization in this area and improving the display refresh rate.
If the design technology of CMOS driver circuits is considered relatively mature, then the high-precision integration of CMOS chips and OLED light-emitting layers is a major technical difficulty. Because scientists need to solve the problems of electrical connection, physical compatibility, and thermal management between the two, and these technologies become more complex as micro-OLEDs become smaller.
In recent years, researchers in Germany have conducted research on directly depositing organic light-emitting diodes (OLEDs) on needle-like CMOS chips. The researchers have carried out transmission electron microscopy analysis of the aluminum contact pads of CMOS chips and achieved ohmic contact conditions, which is convenient for directly vacuum-depositing orange and blue light-emitting OLED stacks.
Researchers from Fraunhofer IPMS in Germany have developed a translucent yellow high-resolution OLED microdisplay, integrating OLED materials directly onto a silicon CMOS driver. This design makes the display much lighter than traditional near-eye systems.
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Conclusion
Currently, there are three key technical research and development directions that urgently need to be broken through for silicon-based OLED microdisplays: research and development of high-stability materials, high-precision circuit design and integration, and improvement of low-cost manufacturing processes.
