Over the last decade, LCDs have firmly established themselves as the dominant technology in both television and mobile electronic markets. Manufacturers worldwide are constantly striving to reduce production costs and expand their market share, aiming to make LCDs a common sight everywhere. However, since Martin Pope’s groundbreaking publication on organic light-emitting displays (OLEDs) in 1963, OLEDs have emerged as a promising contender in the realm of ultra-thin, high-color-gamut flat-panel display technologies. Despite their potential, OLEDs face significant challenges including high manufacturing costs, technological hurdles, low yields, and the inherent instability of organic materials, which continue to hinder widespread adoption.
In contrast, quantum dot display technology has surged in popularity over the past couple of years. With major players in the global TV industry pushing forward with similar flagship high-end series as OLED displays, the quantum dot display offers a unique approach. Unlike OLEDs, quantum dot displays rely on a novel short-wavelength excitation nanometer-specific particle technology, breaking the traditional trade-off between color gamut, cost, and brightness.
Professor Peng Xiaogang, a senior quantum dot expert from Zhejiang University, once remarked, "Quantum dots may well be the most luminous materials ever discovered by humanity."
The quantum dot's tunable size allows it to emit light across the spectrum from blue to green, yellow to orange, and red to red, offering precise and vivid color reproduction. According to the highest BT.2020 standards, Apple's mobile devices only cover around 50% of the color gamut, missing out on half of the colors, whereas quantum dots can achieve a remarkable 100% coverage. This makes quantum dot displays particularly suited for meeting the demanding color gamut requirements of ultra-high-definition Blu-ray standards, ensuring the accurate reproduction of all perceivable colors.
Currently, photoluminescence technology utilizing quantum dot film represents a mature and reliable method within quantum dot display applications. By replacing the white LED light source in traditional LCD displays with a blue LED and adding a nano quantum dot film, the color expression can be significantly enhanced.
In general, the advantages of quantum dot display technology can be summed up as "high, pure, and long." "High" refers to the high color gamut, with coverage reaching up to 110% NTSC; "pure" signifies pure colors, with color purity approximately 58.3% higher than conventional LEDs, allowing for natural color representation; and "long" indicates color durability, thanks to the stable inorganic nanomaterial quantum dots, which ensure color remains vibrant for up to 60,000 hours.
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Due to the quantum dot particle size ranging from 1 to 10 nanometers, the specific surface area is extremely large, making quantum dots susceptible to oxygen and water vapor damage, which can lead to fluorescence quenching. Therefore, the quantum dot film requires a two-layer high-resistance separator and special polymer materials, forming a sandwich structure to protect the quantum dots.
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The quantum dot film structure features an outermost layer with an optical micro/nano structure, which helps reduce Newton's rings, increases the blue return path, and provides threefold homogenization effects. The upper and lower layers of PET serve as substrate layers, with an oxygen barrier layer of SiO2 coating inside the PET substrate. The intermediate quantum dot material layer consists of quantum dots, high molecular polymers, and other formulations. One unique characteristic of quantum dot films compared to other optical films is the invalid margin, caused by the luminescence failure of the quantum dot film layers in free air over time. This indicator holds particular significance in the context of today's trend towards ultra-narrow bezels in larger displays.
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In the application of quantum dot displays, the white LED in the original backlight module is replaced by a blue LED, eliminating the need for blue quantum dots, with all focus placed on red and green quantum dots. Quantum dots utilize about two-thirds of the blue light to generate red and green light.
To achieve the performance metrics of quantum dot displays and accurately express the colors, fine-tuning of the quantum dot layer is essential to match the blue backlight mode with different models and liquid crystal panels, achieving the appropriate color coordinates for the overall quantum dot display. Additionally, the uniformity of the quantum dot layer's thickness is a critical factor influencing the display effect, emphasizing the importance of controlling this thickness.
During the production of quantum dot films, the quantum dots themselves must remain undamaged by external conditions, preserving their original fluorescence efficiency and stability. Utilizing a barrier film to create a sandwich structure becomes the only viable option under realistic conditions.
Due to their sensitivity to water vapor and oxygen, quantum dot materials necessitate structural encapsulation with a high-barrier film. In coating processes, not only must thickness be controlled, but the sandwich structure and total thickness must also be considered. Currently, the thickness of quantum dot film coatings generally ranges from 50 to 100 microns, with larger amounts allowing for embossing, rolling, and slitting.
Comma coating, based on current domestic processing technology for sizes up to 600mm, ensures scraper accuracy at around 1-2 microns. However, scaling this to the 65-inch demand for large-sized quantum dot films results in a width variation of 1500mm reaching 5-10 microns, which directly affects the color coordinate XY values beyond the standard range. Customer specifications often require color coordinate errors not to exceed 0.5%. Moreover, quantum dot glue is prone to aggregation and sedimentation, and comma coating struggles to handle foreign matter and longitudinal streaks.
Similarly, roll coating presents numerous critical issues.
A preferred method is slot die coating. Slot die coating operates on the principle of pumping fluid into a mold that evenly spreads the fluid. It is a closed system, and the fluid is pre-metered using a precision metering pump. These characteristics give slot die coating several advantages over other methods: more uniform coating weight and overall distribution; ease of switching between thick and thin coating processes; minimized volatile emissions, coating contamination, waste of raw materials, and workplace confusion.
Due to its high coating precision, the amount of extrusion can be controlled by the speed of the power motor of the precision metering pump body to achieve a closed-loop system. Under constant system tension, the rotational speed of the feed motor of the slot die coating head is a function of the thickness of the quantum dot film. The thickness of the quantum dot film is detected in real-time online, and after feedback calculation, the glue feeding motor of the slot die coating head is adjusted, enabling precise control of the quantum dot film thickness by altering the rotation speed of the glue feeding motor.
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The production roadmap for quantum dot films involves using a slot die head when filling the quantum dot material polymer glue between two films.
When coating a polymer coating with quantum dot material between two thin-film barrier layers, the amount of the coating directly impacts the final thickness of the quantum dot film while maintaining the adhesive's viscosity. By measuring the thickness of the quantum dot film at the feature site online and feeding back the amount of glue applied, a closed-loop response of thickness and glue amount can be established to precisely control the thickness of the quantum dot film.
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Online thickness measurement monitoring interface with an accuracy of 0.01 µm @ 5m/min.
Control the speed of the glue feeding motor to control the amount of glue flowing through the slot die coating head, further precisely controlling the thickness of the quantum dot film.
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The final product segment calculates the entire length measurement results:
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From the data analysis, the thickness has a uniform distribution in the length direction, with an overall thickness error of +/- 2 µm @ total thickness 300 µm, and ±1 µm @ 75 µm, and a color coordinate deviation of less than 0.2%.
In summary, in addition to the manufacturing base of precision UV molding equipment, combined with more than ten years of precision coating experience, Tianhong employs slot die coating technology and online thickness measurement closed-loop feedback technology to effectively control the thickness of quantum dot coatings. The uniformity index of the quantum dot film has reached international advanced levels. Furthermore, in the design of optical micro-nano structures, Tianhong has its own unique technology and expertise. Through the surface structure of the film, it can enhance light return and improve the excitation efficiency of quantum dots, thereby improving various functional indicators such as light efficiency.
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