Quantum Dot Film:"Advanced Cosmetics" in the Display World

 

LCD has always dominated the TV and mobile electronics markets.Manufacturers' focus on continually reducing LCD manufacturing costs and expanding the market has made them a ubiquitous commodity.Since Martin Pope published the first article on organic light-emitting display (OLED) in 1963, OLED has gradually become a hot research topic as an ultra-thin, high-color gamut flat-panel display technology.However, due to high cost, high technical difficulty in development, low yield and instability of organisms, there is still a long way to go before large-scale popularization.

 

Quantum dot display has been booming in the past two years.Under the layout of global color TV giants, there is quite a trend of "the back wave of the Yangtze River pushes the front wave", positioning the flagship high-end series of products.Compared with OLED display, quantum dot display is a display technology based on unique short-wavelength excitation of nano-scale special particles, which breaks the balance of "color gamut and cost and brightness are contradictory".

 

Professor Peng Xiaogang, a global NO.8 scientist, also once said that "quantum dots may be the best luminescent materials ever discovered by mankind".

 

 

So what is a quantum dot film? Don't underestimate this film.The quantum dot film is a wide-color gamut special optical film made of quantum dots, polymer resin and optical-grade water-oxygen barrier film as the main raw materials, combined with high-precision coating technology.The structure is similar to a sandwich.Multilayer composite structure.

 

Why do I need a barrier film? The stability of quantum dots will be affected by the barrier rate of the barrier film, the external ultra-high energy and the properties of the material itself; the failure mechanism as a whole is the destruction of chemical bonds, oxidation and hydrolysis.Therefore, it is necessary to use two layers of high barrier films and special polymer materials to wrap quantum dots to solve these problems.At present, the most mainstream packaging method is quantum dot film (QLCF, QD on Surface).

 

Is the quantum dot film a film attached to the surface of the screen?

 

is not.The usage of quantum dot film "external application" is invalid and needs to be "internally administered".

 

It is an optical film applied to the backlight part of the display.The quantum dot film can not only be used as a light conversion layer, but also has a high degree of haze.Acne light shadow and other pictures).In order to achieve the best light conversion rate, its position needs to be placed at the bottom of the optical film, which can replace the diffusion film in the original LCD structure.

 

What are the advantages of many well-known brands choosing quantum dot technology?

 

 

Wide color gamut.The wider the color gamut, the larger the displayed color range, and the intuitive feeling is that the displayed color is richer, the color transition is more natural, and the picture is more realistic.Under the NTSC standard, the color gamut of ordinary LCD TVs (YAG powder) is only 72%, the color gamut of ordinary high-color gamut TVs (RG powder) is only 82%, and the color gamut of OLED can reach 100%, while the color gamut coverage of quantum dot TVs is as high as 110%.

 

Pure color.The RGB spectrum emitted by quantum dots has a narrower half-peak width, higher color purity, very little stray light, and can accurately present natural colors.

 

Comparison of spectrum between quantum dot screen (left) and ordinary LCD screen (right)

 

The color is long.Quantum dots are inorganic nanomaterials.Compared with organic materials, they have stronger stability and can ensure long-lasting, dazzling and bright colors, and the color lasting stability can reach 50,000 hours.

 

Low power consumption.According to the mechanism of visual neurology, using quantum dot display technology, the color saturation of the picture is higher, and a better display effect can be obtained with lower brightness and less power consumption.

 

Quantum dot film, as a new nanomaterial functional film with unique optical properties, can accurately and efficiently convert blue light into bright red light and bright green light.Through fine adjustment of the backlight, the color gamut can be greatly improved performance, make the color more vivid and bright.

 

 

In order to make the quantum dot display achieve various performance indicators, more accurately present the displayed colors, and give full play to the advantages of the quantum dot display, it is necessary to do fine formula work on the quantum dot layer to match the blue color of different models.Color backlight module and LCD panel, so that the overall quantum dot display can achieve appropriate color coordinates.At the same time, the thickness uniformity of the quantum dot layer is also a key indicator that affects the display effect of the quantum dots, so the control of the thickness of the quantum dot layer is very important.

 

In the production of quantum dot films, it is necessary to keep the quantum dots themselves from being damaged by external conditions and maintain the original fluorescence efficiency and stability.The use of barrier films to produce quantum dot films with sandwich structure has become the only choice under realistic conditions.

 

Quantum dot materials have to be structurally encapsulated with high-barrier films due to their special properties of sensitivity to water vapor and oxygen.When coating, not only the control of the coating thickness, but also the compounding into sandwiches must be considered.The total thickness after the structure.At present, the coating thickness of quantum dot film is generally about 50-100um.This relatively large coating amount can be used in several ways such as comma, roll coating and slit.

 

Comma coating, on the basis of the current domestic processing technology, for the size below 600mm, the accuracy of the scraper can be guaranteed to be about 1-2um, enlarged to the 65-inch requirement of large-size quantum dot film, and the unevenness in the width of 1500mm It will reach an error of 5-10um.This thickness difference will directly affect the value of the color coordinate XY beyond the standard range.It is still too large for the tolerance that the customer requires that the color coordinate error should not be greater than 0.5%.In addition, quantum dot glue is easy to agglomerate and precipitate, and comma coating is difficult to deal with the problems of foreign matter and longitudinal drawing.

 

Similarly, there are several fatal problems with the roller coating method.

 

The ideal coating method is slit coating.The operating principle of slot coating is to pump the fluid into a mold that can spread the fluid evenly with a certain amount of pump.It is a closed system, and secondly, it pre-meters the coating material through a precision metering pump.Based on these two points, the slot coating method has some advantages over other coatings:more uniform coating weight and overall distribution; easy to switch between thick and thin coating processes; minimized Volatile emissions, coating contamination, raw material waste, and workplace clutter.

 

Due to the high coating precision, the extrusion volume can be controlled by the rotational speed of the power motor of the precision metering pump body to realize a closed loop.Under the condition of constant system tension, the rotational speed of the glue feeding motor of the slit coating head is a function of the thickness of the quantum dot film.Through the online real-time detection of the film thickness of quantum dots, through relevant calculations, it is fed back to the glue feeding motor of the slit coating head.By changing the speed of the glue feeding motor, the thickness of the quantum dot film can be precisely controlled.

 

Slot coating head is used when pouring quantum dot material polymer glue between two layers of film

 

When the polymer coating with quantum dot material is applied between two thin film barrier layers, the amount of glue applied directly affects the thickness of the final quantum dot film while keeping the viscosity of the glue unchanged.By measuring the thickness of the quantum dot film at the feature site online and feeding it back to the amount of glue applied, a closed-loop response between the thickness and the amount of glue applied can be established to precisely control the thickness of the quantum dot film.

 

On-line thickness measurement monitoring interface, with an accuracy of 0.01 micron@5m/min, the amount of glue flowing through the slit coating head can be controlled by controlling the speed of the glue feeding motor, and the thickness of the quantum dot film can be further accurately controlled.

 

According to the data analysis, the thickness is uniformly distributed in intervals in the length direction, the overall thickness error is +/-2um @ total thickness 300um, and ± 1um @ 75um, and the color coordinate deviation is less than 0.2%.

 

In summary, the use of slit coating technology and online thickness measurement closed-loop feedback technology can effectively control the thickness of the quantum dot coating, and the uniformity index of the quantum dot film can reach the international advanced level.In addition, certain processes and technologies are adopted in the design of optical micro-nano structure, through the surface structure of the film to increase the return of light to improve the excitation efficiency of quantum dots, which can better improve various functional indicators such as light efficiency.