Authors: Xiaodong Liu13 , Xinyuan Zhu3 , Junqiu Guo2 , Honglei Ji2 , Haitao Huang1
1 Huizhou Changed New Materials Co.,Ltd, Huizhou 516000, China
2 TCL Electronics Holdings Limited. , Shenzhen, China
3 Shanghai Jiao Tong University, Shanghai, China
Abstract
Compared with traditional LED backlight source, the quantum dot-based backlight module has recently attracted extensive interest owing to their wider color gamut. Generally, quantum dots are sensitive to the external environment, and quantum dots in current commercial quantum dot film are usually protected by two barrier film layers to isolate the influence of external factors such as water and oxygen. In this report, a new quantum dot plate strategy was developed by a three-layer co-extrusion method without the use of any expensive barrier film. Quantum dots were homogeneous embedded 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. Reliability tests demonstrate that this non-barrier-film quantum dot plate shows satisfactory stability comparable to the high-barrier-material-based quantum dot films under conditions of blue-LED irradiation, high temperature and high humidity.
Author Keywords
Backlight; Quantum Dot; Wide-Color-Gamut, Optical Plate; Extrusion
As a new generation of luminescent materials, quantum dots (QD) have prominent characteristics of narrow half-peak width, adjustable color, high quantum yield and so on.[1] When the quantum dots with a diameter of 2~10 nm are stimulated by light or electricity, these excited crystals emit monochromatic light at a specific frequency, and the type, size and shape of the quantum dots are depended on the light frequency or color to achieve precise tuning all over the visible region. The excitation spectrum of quantum dots is wide, while the emission spectrum is narrow, thus the same quantum dot can be excited by multiple light sources, and different quantum dots can be excited by the same light source. These excellent properties make quantum dots have a broad application prospect in multiple fields such as LED illumination, nonlinear optical evices, bio-labeling, fluorescence detection.[2]
In the field of semiconductor nanocrystal technology, quantum dot display (QLED) technology has changed the screen backlight style. RGB trichromatic quantum dots will greatly enhance the display brightness ontrast 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 llumination can optimize many general specifications, such as brightness, contrast ratio, and color gamut (Fig. 1). 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 advantages of QLED, this backlight technology are attractive for applications of offices, graphics and video games.
Although quantum dots are inorganic, these nanoscale photoluminescence 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 electroluminescent organic small molecules (OLED). In the commercialization process of quantum dots, researchers spend lots of energy and cost to improve water and oxygen arrier efficiency. 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 uantum dot materials to block water and oxygen. Quantum dot films are prepared by coating, laminate, slitting equipment in production lines. It 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. Besides, in the backlight module assembly process, if the QD film is slightly remote from the light source, the repeated reflection times that light passes through the quantum dot layer in optical film is not enough, and the conversion of red and green light is insufficient, thus causing a color shift phenomenon.
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. It is widely used in LCD display, LED lighting and imaging systems. Inorganic or organic light diffusion agent is added to the base materials such as polymethyl methacrylate (PMMA) and olystyrene (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 160 ~ 176°, thus iffusing 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 a novel non-barrier-film quantum dot diffusion plate with high gamut quantum dot luminescence functions in backlight system are prepared. The modified quantum dots were ixed with poly methyl methacrylate (PMMA) materials in the plastic granulator under the protection of argon or nitrogen to prepare QD master batch. Then 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 luorescent stability of the function board in high temperature and high humidity environment (Fig. 2). 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 dots are firstly granulated with base materials by the granulator, and then the QD master batch is transferred to the extruder hopper by the feeding system, and is processed with other substrate and additive in the right proportion at the melting temperature of polymer materials by the co-extrusion molding technique. Specifically, quantum dot powder or quantum dot solution are uniformly mixed with PMMA particles, toughening agent and anti-aging additive by a mixer machine. After setting parameters of the revolving speed, the machine current, and the melt pressure, etc., the mixed materials are fed into the hopper of the granulator, and nitrogen r argon gas is injected when it is necessary to form a protective atmosphere. By steps of plasticizing extruding strips, drawing strips, water cooling, blower drying and particle cutting, quantum dot particles are prepared as the QD master batch, and the concentration of quantum dots in this master batch is relatively higher. After further drying the QD master batch at 60~80 oC for 2~4 hours to remove the residual water, then the QD master batch is ixed with PMMA particles to dilute quantum dots to the required concentration for further use in the next step. The working temperature of the extruder is adjusted by the temperature control meter, when the temperature of the mold barrel is heated to reach the set value of 200~250 oC, the constant temperature should be maintained for 20~30 minutes. The mixed materials of each corresponding substrate layer are put into the feeding ports of he extruder respectively. Then operation personnel replace the filter screens of the extruding machine, click on the metering pump, turn on the screw motor, set parameters of the metering pump pressure, feeding rate and host speed, the quantum dots, dispersing agent, plastic materials and various additives go through the barrels of the extruder and plasticize for extrusion. As the melting materials are extruded into the mold, the components are elted through the mold outlet into an whole body with three functional layers.The upper and the lower surfaces of the plate are hot-pressed into specific microstructures by rollers engraved with micro-templates. According to the production requirements, the product dimensions of the QD plate, and the stacking between the rollers needs accurate adjustion, and the QD plate should be dedusted after trimming treatment, and radually transported to the cutting unit at the back section of the production line by the traction device. Then the QD plate is cut to the required size and grabbed with the mechanical arm, and the operating personnel could clean, stack and pack them into the warehouse.
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, esidence time of mixed material within the mould, and the content of dispersant could be regulated to control parameters of brightness, 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 diffusion particles need to pass strict screening without adversely affecting properties of quantum dots, especially wavelength, half-wave width and the stability performance. Quantum dots are single-level structures. Controlling the size of quantum dots can make the quantum dots emit high-quality pure red- and green monochromatic light after excitation. The fluorescence spectrum is extremely narrow, which can precisely regulate the color to achieve accurate color restoration and higher color purity. In the quantum dot diffusion plate, the used quantum dots in the functional layer can be green quantum dots lone, or applied as a mixture of green fluorescent quantum dots and red fluorescent quantum dots by certain proportions. When using the green quantum dot function board, blue LED chip and red fluoride phosphor powder should be matched ogether. The light emitted by QD, LED chip and phosphor together can constitute primary colors so as to construct the high color gamut display architecture. When the quantum dot function board contains both green and red quantum dots, only blue light is needed for backlight excitation. The two types of quantum dots can be stimulated by blue LED to emit red and green light with higher purity. With the excellent pure color output of quantum dots, the color gamut value and saturation are higher, the color of the display screen can be more exquisite, and the coverage of color gamut can be highly improved.
The quantum dot optical diffusion plate is composed of multi-layer structure, 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. If 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 entioned two layers against water and oxygen, and reduce the use of quantum dots and save production costs. In 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 automated industrial production could be realized.
References
1. Wang, C.; Zhang, Y.; Wang, A.; Wang, Q.; Tang, H.; Shen, W.; Li, Z.; Deng, Z., Controlled Synthesis of Composition Tunable Formamidinium Cesium Double Cation Lead Halide Perovskite Nanowires and Nanosheets with Improved Stability. Chem. Mater. 2017, 29, 2157-2166.
2. Zhou, Q.; Bai, Z.; Lu, W.-g.; Wang, Y.; Zou, B.; Zhong, H., In Situ Fabrication of Halide Perovskite Nanocrystal-Embedded Polymer Composite Films with Enhanced Photoluminescence for Display Backlights. Adv. Mater. 2016, 28, 9163-9168.
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Symp Digest of Tech Papers – 2019 – Liu – P‐8 4 A Novel Non‐barrier‐film Quantum Dot Diffusion Plate in Backlight System
