Quantum dot film coating precision process control

For more than a decade, LCDs have 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.

 

But since Martin Pope published his first article on Organic Light Emitting Displays (OLEDs) in 1963, OLEDs have gradually become a research hotspot 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.

 

The quantum dot display has been booming in the past two years.Under the layout of the global color TV giants, there is a trend of "the back wave of the Yangtze River pushes the front wave", and the same as the OLED display, it is positioned as a flagship high-end series of products.The difference is that the quantum dot display is based on a unique short-wavelength excitation nanoscale special particle display technology, which breaks the balance of "color gamut and cost and brightness are contradictory".

 

Professor Peng Xiaogang, a professor at Zhejiang University and a senior expert on quantum dots, once said that "quantum dots may be the best luminescent materials ever discovered by mankind".

 

The size of quantum dots is continuously adjustable, which can realize blue to green, to yellow, to orange, to red emission, and the color is accurate and pure.Its color effect if in accordance with the highest BT.According to the 2020 standard, the Apple mobile phone is only about 50%, that is, half of the colors cannot be displayed, but quantum dots can achieve 100% of the color gamut.Corresponding to the high color gamut requirements of the Ultra HD Blu-ray standard, quantum dot displays have the ability to reproduce all colors we can perceive.

 

The current photoluminescence technology using quantum dot film technology is a mature and reliable technology in quantum dot display.The traditional LCD display can achieve excellent color expression ability as long as the white LED light source in the backlight is replaced with a blue LED light source and a film of nano quantum dots is added.

 

In general, the advantages of quantum dot display technology can be summarized as "high, pure, long" three aspects.

 

"High" means high color gamut, with a color gamut coverage of 110% NTSC;

 

"Pure" means pure color, the color purity is about 58.3% higher than that of ordinary LED, and it accurately presents natural colors;

 

"Long time" means long-lasting color.The stable quantum dots of inorganic nanomaterials can ensure that the color will not fade for a long time, and the color lasting stability can reach 60,000 hours.

 

Since the particle size of quantum dots is between 1 and 10 nm and the specific surface area is very large, oxygen and water vapor can easily damage the surface of quantum dots, resulting in fluorescence quenching.Therefore, quantum dot films need to use two layers of high barrier films and special high Molecular polymeric material wraps the quantum dots to form a sandwich structure.

 

The quantum dot film structure shows that the outermost layer is a surface layer with an optical micro-nano structure, which has the triple effect of reducing the support of Newton's rings, increasing the blue light return path, and homogenizing light.The upper and lower PET layers are the substrate layers.The inside of the PET substrate has a SiO2 coating that blocks oxygen and water.The middle quantum dot material layer is composed of quantum dots, high molecular polymers and other formula materials.

 

The quantum dot film has a special indicator that other optical films do not have—the ineffectiveness margin, which is mainly due to the failure of the quantum dot luminescence produced by the quantum dot film layer over time in free air.This indicator is emphasized today.It has special significance in the environment of ultra-narrow bezels.

 

In the application of quantum dot display, the white light LED in the original backlight module is replaced by a blue light LED, and there is no blue light quantum dot.The key to everything lies in the red light and green light quantum dots.Quantum dots use about 2/3 the blue light to produce red and green light.

 

In order to make the quantum dot display reach various performance indicators, more accurately present the 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 in different models.Backlight module and liquid crystal panel, so that the overall quantum dot display can reach the 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 quantum dot film coating The method is slit coating.

 

Slot coating is that its operating principle 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.

 

A slit coating head is used when pouring and sandwiching the quantum dot material polymer glue between the two films.

 

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.

 

Controlling the speed of the glue feeding motor can control the amount of glue flowing through the slit coating head, and further accurately control the thickness of the quantum dot film

 

From the data analysis, the thickness is uniformly distributed at 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%.