Based on the characteristics of LED high light efficiency, low carbon environmental protection and energy saving, most LCD TVs and mobile phones on the market have adopted LED as backlight. At the same time, in order to meet the high color gamut coverage of LED backlight sources, the industry is gradually introducing some new fluorescent materials with narrow half-wave width, such as fluoride phosphors and quantum dot phosphors. In order to better understand the characteristics of these phosphor materials, we will focus on the introduction and description of fluoride phosphors and their applications.
First of all, before we understand and understand fluoride phosphors, we will introduce some basic knowledge of LED light sources in backlight applications.
1, LCD related knowledge introduction
A, LCD meaning
LCD: Abbreviation for Liquid Crystal Display, the full name of liquid crystal display, which includes TFT, UFB, TFD, STN and other types of liquid crystal display.
LCD TV, mobile phone: TV or mobile phone with LCD display.
B, LCD construction
Taking a TFT type liquid crystal display as an example, the structure mainly includes a backlight, a light guide plate, an upper and lower polarizing plate, a liquid crystal, a color filter, a thin film transistor, etc. (the structure is shown in FIG. 1), and the main structural functions are as follows:
1. Back Light: The principle of LCD display is to block the light and light by the liquid crystal to control the light and dark. It is necessary to have a light source to see the image on the screen, so the backlight is responsible for providing the most basic for LCD display. Light source.
2. Light Guide Plate: distributes light evenly across the entire screen.
3. Up/Down Polarizer: The direction of the light sent by the backlight is inconsistent and radial. If such light passes through the twist of the liquid crystal molecules, we still can't see the image we want to see on the screen. At this time, the polarizing plate below assumes the work of normalizing the direction of the light to the liquid crystal layer.
4. Thin Film Transistor (TFT): Controls the twist angle of liquid crystal molecules.
5. Liquid crystal: This layer of liquid crystal molecules is twisted under the control of the TFT, and the light with the same direction is controlled to be bright, so that the brightness of the light to the rear pixel unit changes.
6. Color Filter: After the white filter is filtered, we can see that the light corresponding to the color of the filter is transmitted, so in the LCD screen, the function of the color filter is to color. .
Figure 1 LCD construction diagram
C, LCD imaging principle
The principle of LCD imaging is to place the liquid crystal between two sheets of conductive glass. By controlling the driving of the electric field between the upper and lower polarizing plates and the upper and lower electrodes, the electric field effect of the liquid crystal molecules is twisted and nematic to control the transmission of the backlight or Masking, combined with other control and auxiliary function layers to achieve the function of restoring the picture.
D, LCD backlight introduction
Since the liquid crystal must be illuminated by an additional light source, the backlights commonly used in LCDs are CCFL (Cold Cathode Fluorescent Lamp), LED (Light Emitting Diode), HCFL (Hot Cathode Fluorescent Lamp), Surface Light Source VFD (Flat Fluorescent Lamp), EL ( Electroluminescent sheet), OLED (organic electroluminescent sheet) and the like. Among them, CCFL is the most commonly used LCD backlight, which is also commonly called traditional backlight.
♦ CCFL vs. LED:
CCFL - made of hard glass and trichromatic phosphors, the tube has a proper amount of mercury and inert gas, the inner wall of the tube is coated with phosphor, and there is one electrode at each end. The disadvantage is that the color displayed is limited. .
LED - is a semiconductor solid-state light-emitting device that uses a solid semiconductor chip as a light-emitting material to recombine excess energy in a semiconductor by carrier to cause photon emission, directly emitting red, yellow, blue, green, cyan, and orange. , purple, white light. Because LED LEDs have good color performance, they have completely replaced the light source of traditional cold cathode fluorescent tubes.
2. Introduction to knowledge about color gamut coverage
A, color gamut coverage meaning
Color gamut coverage: The horseshoe-shaped chromaticity triangle marked with color on the CIE-xy chromaticity diagram is the color area that the human eye can see. If a system can fully reproduce the color in this horseshoe-shaped area, it can be said to be color. The domain coverage is 100%. When the colors are reproduced by the three primary colors of R, G, and B, the triangular regions composed of the three primary color coordinates of R, G, and B are the color reproduction regions determined by the three primary colors, and the ratio of this region to the horseshoe region is the color gamut coverage. Therefore, the gamut coverage ratio is the ratio of the area of ​​the triangular area composed of three basic color coordinates of R, G, and B to the area of ​​the triangular area composed of the three primary color coordinates of the standard R, G, and B.
B, color gamut coverage evaluation criteria
In different fields, the requirements for the three primary color coordinates of the standard R, G, and B are different, which involves different color gamut evaluation criteria. The gamut standards are generally NTSC, ITU-R BT.709, sRGB, Adobe RGB, ITU-R BT.1361, xvYCC, etc. The following three general standards (sRGB, NTSC and Adobe RGB) are briefly introduced:
1) NTSC Standard: In 1953, the National Television Standards Committee (NTSC) developed the NTSC standard based on the CIE1931 chromaticity diagram, which is also a commonly used standard in China.
2) sRGB standard: In 1996, the gamut standard for digital image developed by the International Electrotechnical Commission (IEC) was mainly applied to digital image acquisition equipment, and it was not widely used on the display.
3) Adobe RGB standard: In 1998, Adobe introduced a color space wider than sRGB, which is generally used in printing and publishing, image processing and other fields.
The white light position, color temperature, and RGB primary color coordinates of sRGB, NTSC, and Adobe RGB are shown in Table 1:
Table 1 Related parameters of sRGB, NTSC and Adobe RGB standards
In this paper, the color gamut we are talking about is the color gamut under the NTSC standard, referred to as the NTSC color gamut.
Combined with the concept of color gamut coverage, it can be deduced that the NTSC color gamut is the ratio of the triangle region composed of a certain RGB three primary color coordinates to the triangular region composed of three color coordinates of NTSC standard RGB (schematic diagram is shown in Figure 2). The higher the ratio, the color The better the performance.
Figure 2 Schematic diagram of gamut coverage
C, factors affecting color gamut coverage
From the concept of color gamut coverage and its evaluation criteria, we can also infer that the key factors affecting the value of the RGB three primary colors (ie, backlight, white light LED in this article), CF, standard RGB three colors Coordinates, the known factors are: the standard RGB three color coordinates under the NTSC standard, CF, so the key factor becomes the backlight.
Introduction to CF basic knowledge:
CF is actually composed of R, G, and B filters (the plan is shown in Figure 3). Only the light source close to the filter spectrum can better penetrate the filter. The NTSC value of the source is higher. high.
Figure 3 CF plan
Figure 4 Schematic diagram of CF conversion to light
LED white light passes through CF to get another new white light spectrum. This new white light is actually composed of three independent R, G, and B spectra. The detailed process is as follows:
Figure 5 Schematic diagram of the spectrum after LED white light passes through CF
The characteristics of a CF model can be represented by a curve image (Fig. 6), which is actually composed of three single curves of R, G, and B (Fig. 7). The meaning of each curve can be easily understood. as follows:
♦ Curve—The peak is around 650nm, and the band distribution is 570-780nm, which means that the light in the 570-780nm band can pass, and the other bands will be filtered out. At the same time, in the 570-780nm band that can pass, and because of the wavelength. The difference in energy intensity caused by the difference is also different.
♦ Curve—The peak is around 530nm and the band distribution is 465-615nm, which means that the light in the 465-615nm band can pass, and the other bands will be filtered out.
♦ Curve—The peak is around 460nm and the band distribution is 400-520nm, which means that the light in the 400-520nm band can pass, and the other bands will be filtered out.
Figure 6 CF curve
Figure 7 CF curve exploded view
Table 2 below shows the R, G, and B parameters of a type A CF commonly used by our company:
Table 2 R, G, B parameters of type A CF
From the curve decomposition diagram of CF, it can be seen that in the NTSC standard, in order to make the LED backlight pass through the CF, the NTSC value is high, and the RGB three-color peak constituting the white light is required to be close to the RGB three-color peak of the CF, and at the same time, the RGB three-color half of the white light. The narrower the wave width, the better.
3, high color gamut coverage related knowledge
It is known that the NTSC color gamut is the ratio of a triangle region to a standard triangle region under the NTSC standard. The higher the ratio, the better the color expression, and the high color gamut coverage in the industry refers to the NTSC color gamut ratio ≥ 85%, Table 3 below shows the common white light implementation of different NTSC values.
Table 3 White light implementation schemes with different NTSC values
According to the factors affecting the gamut coverage, under the NTSC standard, the key factor affecting the gamut value is the LED backlight, while the white light with high color gamut coverage is generally realized by the blue chip + green phosphor + red phosphor. Combined with the principle of white light passing through CF, the key to achieving high color gamut coverage lies in the choice of green and red phosphors, and the key influencing factors are the physical parameters (peak, half-width) and CF of green and red phosphors. match. The following table shows the peak wavelength and half-wave width parameters of different system LED phosphors. Combined with the mass production and application status of phosphors in the LED industry, the current green powder in the best solution for achieving high color gamut coverage is β-SiAlON, and the red powder is Fluoride.
Table 4 Peak wavelength and half-wave width parameters of different system LED phosphors
4. Basic knowledge of fluoride phosphors
A, fluoride phosphor concept
The fluoride is a tetravalent manganese-excited fluorosilicate having a molecular formula of AxMFy: Mn4+ (wherein A = Li, Na, K, Ca, Sr, Ba, etc., M = Si, Al, Y, Sc, etc.). Commonly used fluoride phosphors generally contain three systems: KSF, KGF, and KTF. Among them, KSF belongs to cubic crystal system. The lattice diagram is shown in Figure 8. KGF and KTF belong to hexagonal crystal system (lattice diagram is shown in Figure 8). Chinese name It is: tetravalent manganese excited potassium fluorosilicate / bismuth / titanium, the chemical formula is K2SiF6: Mn4 +, K2GeF6: Mn4 +, K2TiF6: Mn4 +, the chemical formula is similar to silicate (silicate chemical formula: XSiO2).
Figure 8 Fluoride lattice diagram
B, fluoride phosphor characteristics
Powder characteristics: the peak wavelength is fixed (630 nm), the half-wave width is narrow (<30 nm), and the appearance of the powder is orange.
Based on the above powder characteristics, in practical applications, with the same band of green powder, the fluoride red powder is higher than the same band of nitride red powder NTSC, the emission spectrum of the fluoride phosphor and the emission spectrum of the same band nitride red powder are as follows:
Figure 9 Fluorescence and fluoride red powder emission spectrum
Stability: Fluoride phosphors are structurally unstable and have a certain hygroscopicity. This property is similar to silicates, and fluoride phosphors are easily decomposed with water to form toxic HF gas when the temperature rises. The reaction is reversible. After exposure to water, the appearance of the powder changes from orange to brown (as shown in Figure 10 below). The chemical reaction formula is:
Figure 10: Color change of powder after fluoride reacts with water
The fluoride phosphor is not resistant to high temperature. In a short time, as the temperature increases, the brightness decay of the phosphor increases, and the attenuation accelerates at a temperature of >150 °C, but the reaction is reversible, and the curve is as follows:
Figure 11 Fluoride powder temperature resistance curve
5. Application scheme of fluoride phosphor powder in high color gamut coverage
In practical applications, fluoride is often used in conjunction with nitride green powder in LED products with high NTSC requirements (NTSC ≥ 85%).
The following table compares the light color parameters of fluoride phosphors and conventional phosphors in the application:
Table 5: Light color parameters of fluoride phosphor powder and conventional phosphor package
Description:
1) Because the half-wave width is narrow, the fluoride has a higher NTSC when it is matched with green powder of different materials in the same band;
2) Due to the narrow half-wave width, the secondary re-absorption of light is reduced, and the fluoride scheme has higher brightness under the same NTSC effect;
The white light spectrum is as follows:
6. Patent status of fluoride phosphor application
Material patents: In 1972, Osram and GE publicly reported fluoride materials. In theory, there is no patent problem in fluoride materials themselves;
Application patent: The original application patent of fluoride is owned by GE. The main patent is US7497973B2. It was applied for February 28, 2006. The application patent is a combination of powder and chip. The applicant countries have been authorized by Europe, Japan and the United States. At the same time, Nichia also claimed to have related patents, and the specific status has yet to be verified.
7. Precautions for fluoride phosphor application
Fluoride phosphors are not resistant to moisture and high temperature. At present, these defects can only be delayed by the coating technology to delay the damage of the powder and the temperature, which cannot be completely solved. Therefore, fluoride needs to match materials with good airtightness and heat dissipation during application, especially the choice of bracket and package glue, and fluoride should not be used in environments with temperatures above 150 °C. At the same time, packaging manufacturers are best to obtain GE's application patent license when using fluoride phosphor to make device packages, otherwise it is easy to infringe GE's packaging application patents.
8, summary
In summary, due to its narrow half-wave width, fluoride has the advantages of high color gamut coverage and high brightness in LCD backlight applications. At the same time, due to its own structural defects, we need special in packaging applications. Pay attention to the selection and matching of the remaining packaging materials. When the packaging materials are properly matched, high color gamut coverage, high brightness and high reliability can be achieved. (Author: Ruifeng, Shenzhen Electronics AG Dong Chunhui)
For more information, please follow LEDinside's official website () or search for WeChat public account (LEDinside).
Corrugated Roofing Roll Forming Machine
Corrugated Roofing Roll Forming Machine is popular in industrial and civil buildings.
The sheet is very beautiful and alike the water wave.We can produce as your requirments.
Roll Forming Machine Working Flow:
Decoiling→Feeding & Guiding→Flattening(If needed)→Pre- Punching/Cutting(If needed)→Roll Forming→Post Step pressing/Punching(If needed)→Post Cutting→Finished Products To Support Table
Corrugated Roofing Roll Forming Machine,Corrugated Metal Roofing Roll Forming Machine,Corrugated Roof Panels Roll Forming Machine,Corrugated Steel Sheet Forming Machine
HEBEI HANMAC MACHINE CO., LTD. , https://www.chinahanmac.com