Authors: Honglei Ji1,2*, Huaishu Xu2 , Junqiu Guo2
1. University of Chinese Academy of Sciences, Zhejiang, Ningbo,China
2. TCL Electronics Holdings Limited, Shenzhen, China
*Corresponding author, jihl@tcl.com
Abstract
This paper reports a novel QD diffusion plate for liquid crystal display by using a three-layer co-extrusion method without any expensive barrier film. Quantum dots were homogeneous em- bedded in the polymeric matrix of the middle layer, and the upper and lower layer act as protective layers to avoid the destruction of external environment to quantum dots. The QD diffusion plate can realize 105% NTSC1931 color gamut with the same brightness as the backlight using conventional QD film. Reliability tests demonstrate that the quantum dot diffusion plate shows satisfactory stability comparable to the conventional quantum dot films under conditions of blue-LED irradiation, high temperature and high humidity.
Author keywords
Back1ight, Quantum Dot Diffusion p1ate, Extrusion
From colorless Tv to color Tv, from cRT to LcD, from current LcD to OLED/QLED in the future, the driving force behind dis- play technology progress is high picture quality[l]. The change of each era brings about great improvement of picture quality, and the elimination of each technology is because of the insurmountable gap with the new technology in picture quality. similarly, in to- day Is world of new display technologies, to decide which display technologies will dominate the industry in the future, will base on whether this technology can form a technical barrier that other technologies cannot achieve in the field of high picture quality.
In the field of semiconductor nanocrystal technology, quantum dot display (QLED) technology has changed the screen backlight style[2]. RGB trichromatic quantum dots will greatly enhance the display brightness contrast and color gamut display effect at a lower cost. By blue backlight excitation module, quantum dots can produce green and red light. compared with ordinary monitor display, the application of the QLED illumination can optimize many general specifications, such as brightness, contrast ratio, and color gamut . The QLED color performance is more bright and pure, the saturation degree is higher, specific light is transported accurately with more continuity chromatography, and power consumption also can be greatly reduced. Based on these ad- vantages of QLED, this backlight technology are attractive for applications of offices, graphics and video games[3.4].
Although quantum dots are inorganic, these nanoscale photolu- minescence materials are actually quite sensitive to the external environment. They are not only as sensitive to high temperatures as phosphors, but also as sensitive to water and oxygen as elec- troluminescent organic small molecules (OLED)[3]. ,n the com- mercialization process of quantum dots, researchers spend lots of energy and cost to improve water and oxygen barrier efficiency [5]. currently, the quantum dot optical film sold on the market is relatively expensive, and the cost of the used barrier film almost accounts for half of the whole optical film, which is due to the need for quantum dot materials to block water and oxygen. Quantum dot films are prepared by coating, laminate, slitting equipment in production lines. ,t is difficult for quantum dots to achieve full dispersed state in Uv light curing glue as aggregation, settlement and floating phenomenon are inevitably existed. QD films are found to contain some coarse quantum dot particulate matters after amplifying product details.
2. QD diffusion plate structure and manufacturing method
It is well known that when light traveling through media with different refractive index, refraction, reflection and scattering phenomenon happen. The light diffuser plate can fully scatter the incident light and achieve a softer and more uniform irradiation effect. ,t is widely used in LcD display, LED lighting and imaging systems. ,norganic or organic light diffusion agent is added to the base materials such as polymethyl methacrylate (pMMA) and polystyrene (ps), polycarbonate (pc) polypropylene (pp), and particular type of micro feature structures are set on the surface of the optical base materials to adjust the route of light and change the beam angle to be l60 ~ l76。, thus diffusing incident light sufficiently and realizing the optical diffusion effects. conventional optical diffusion plate has a single-layer structure. Although it has good light transmittance, shading effect and weather ability, its function is simple, and its assembly cost is relatively high when it is combined with other functional optical devices, which limits its further application.
In order to expand the application scope of quantum dot materials and avoid the defects in application performance of traditional quantum dot displays, the preparation methods of optical diffusion plates and application of quantum dots integrated and improved, and the quantum dot diffusion plate with high gamut quantum dot luminescence functions in backlight system are prepared. The modified quantum dots were mixed with poly methyl methacrylate (pMMA) materials in the plastic granulator under the protection of argon or nitrogen to prepare QD master batch. As shown in figure l at the polymer melt temperature, a diffusion plate with multilayer structures and luminescent properties can be obtained using the co-extrusion technology which moulds the multilayer together into a whole product by three extruders. The core screw extruder pelletizer is composed of twin screws, and quantum dots evenly dispersed in the middle function layer. The upper and lower diffusion layers of the QD diffusion plate have good resistance to the water and oxygen environment, which fully ensure the quantum dot fluorescent stability of the function board in high temperature and high humidity environment . compared to traditional quantum dot film products, the manufacturing method can avoid some drawbacks and problems encountered in the QD coating process, such as QD aggregation in the Uv glue, equipment blocking and uneven QD distribution. Thus the uniformity and luminescence properties of quantum dots in the product have greatly improved, and tedious process steps of coating, laminating and Uv-light curing are avoided, which simplify the preparation methods.
The quantum dot optical diffusion plate is composed of multi-layer structure as shown in figure 2, and the composition and size can be adjusted flexibly, which can meet the functional requirements of high brightness, concealment and luminescence at the same time. ,f the upper substrate layer surface of the quantum dot diffusion plate is treated with structural disposal, it is more favorable for post processing. For example, the lamination of brightening film or diffusion film with QD diffusion plate can reduce the light loss, and achieve higher brightness and better shading. The upper and lower diffusion layers of the QD plate can be set relatively thicker, and the functional layer can be set thinner, which can increase the blocking efficiency of the mentioned two layers against water and oxygen, and reduce the use of quantum dots and save production costs. ,n addition, no barrier film is used in the production of QD functional plate, and in this fully robotic operation there is no need to clean the diffusion plate and quantum film respectively when assembling the backlight module, which greatly save the cost of raw materials and manufacturing time, and the large-scale auto- mated industrial production could be realized.
In the discussed extrusion technology scheme, quantum dots are evenly distributed in polymer materials in the functional layer of the QD diffusion plate. The concentration of the quantum dots,mixing proportion, function layer thickness, residence time of mixed material within the mould, and the content of dispersant could be regulated to control parameters of brightess, color gamut, color coordinates, color temperature, color rendering, light transmittance and haze. Anti-aging additive, toughening agent,diffusion particles or similar functional materials should be added in the as described quantum dot function layer and diffusion layers.which ensure the service life, bearing strength, toughness and luminous performance of the plate. It is important to note that these anti-aging auxiliaries, toughening agents and diffsion par-ticles need to pass strict screening without adversely affecting properties of quantum dots,especially wavelength, half wave width and the stability performance.
3. QD diffusion plate samples and test results
we have prepared quantum dot diffusion plate samples by the above method. As shown in the figure 3, (a) picture is a positive view and (b) is a side view. we can clearly see that the internal structure of QD diffusion plate is three layers of powder, which is in accordance with our theoretical design.
After obtaining the sample, we have carried on the optical test and the reliability test to the sample respectively, the experimental result is shown in the table 1. Through the analysis of the test results of the current quantum dot diffusion plate samples, we can see that the optical performance of the QDs diffusion plate is not much different from that of the current mainstream QD films, and the reliability is also in line with the needs of our current products as shown in figure4. we tested the sample for more than 1000 hours under conditions of blue-LED irradiation, high temperature and high humidity , the fluctuation of the brightness and the chromaticity of the sample is kept within 10% . Through the analysis of the reliability results, the key problems of temperature resistance and water-oxygen barrier of quantum dots used in diffusion plate process are solved.
4. opportunities and challenges
Quantum dot diffusion plate has attracted more and more attention in the display industry because of its simple process, very low cost. As quantum dot technology is spreading from high-end TV products to low-end products, we judge that quantum dot diffusion plate will grow explosively in 2020. As a new technical form, quantum dot diffusion plate still faces some challenges. At present, quantum dot diffusion plate products are based on pMMA materials. As we all know, pMMA materials have high price and high expansion rate, so they are faced with some structural design limitations in mass production applications. The conventional diffusion plates are all based on pS materials, so the development of pS quantum dot diffusion plates in the future is the general trend of the industry. ,n the quantum dot diffusion plate, the quantum dot is dispersed in the space of 0.4mm thickness, and its unit concentration is four times lower than that of the traditional quantum dot film, so it is certain that the self-absorption effect of the red and green quantum dot is greatly reduced, and so the quantum dot diffusion plates still have room for improvement in EQE.
5. summary
The QD diffusion plate can realize 105% NTSc1931 color gamut with the same brightness as the backlight using conventional QD film. The performance of quantum dot diffusion plate technology is the same as that of the mainstream quantum dot film, and it has reached the level of mass production application. Because of its simple producing process and very low cost, it will be widely used in the market and can help quantum dot television to achieve rapid growth in the future. Quantum dot light conversion technology can even be popularized as an alternative technology of fluorescent powder light conversion.
References
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Symp Digest of Tech Papers – 2021 – Ji – 28 2 Invited Paper A new generation of QD Diffusion Plate Technology for TV
