Stable and High Color Rendering Phosphor－A New Application for the LED Industry

Description

The traditional white light-emitting diode (LED) lighting mainly uses blue indium gallium nitride LED chips with yellow light yttrium aluminum garnet phosphor to generate white light. However, the coverage of the white light spectrum is not complete, which leads to poor color rendering. Color rendering is a measure that describes the ability of a light source to present the color of a real object. The higher the color rendering of the light source, the closer its color performance is to an ideal light source or natural light. Therefore, the research team used different sintering methods to synthesize high-intensity and broad emission peak infrared phosphors, and analyzed their structure and emission characteristics, regulated their emission wavelength, and established a mechanism for regulating the wavelength, mixing different proportions of high-intensity infrared phosphors. The team carried out research and development of high-intensity infrared LED devices suitable for detector measurement, and used the space-confinement method to synthesize nano-level phosphors, packaged in mini LED, to evaluate its practical application value. To address the above-mentioned shortcomings of phosphors, this result of high color rendering and high stability nitride-based phosphors will be practically applied in the LED industry. 

 

https://doi.org/10.1021/ja304754b
(https://doi.org/10.1021/ja304754b) 

 

Human lighting tools have undergone countless reforms, from primitive lamps such as torches, kerosene lamps, and candles, to lamps such as incandescent lamps, fluorescent lamps, and light-emitting diode lamps that need to be converted by electricity, which illustrates the change in human demand for lighting. In the second industrial revolution, people already knew how to use electricity and used it as a source of power. Thomas Alva Edison used it as a power source for power generation. He also tried many different heat-resistant materials. In 1880, he borrowed carbonized bamboo silk. As a filament material, the energized filament can generate a lot of light and heat, and its incandescent lamp can be lit for 1200 hours, causing humans to enter the era of electric lighting. However, the incandescent lamp converts excessive electric energy into heat energy, and only less than 10% of the energy is converted into light. People could develop more efficient fluorescent lamps. The glass tube contains inert gas and low-pressure mercury vapor. By increasing the voltage, the filament releases electrons. The mercury excited by the electrons emits ultraviolet light of 253 nm and 185 nm. The fluorescent powder-coating in the tube wall emits visible light by absorbing ultraviolet light, but its shortcomings are mercury pollution, large volume, poor color rendering, flashing, etc., prompting people to pursue better lighting tools, such as light-emitting diodes (LED).

At present, the mainstream of commercial white LED uses a single chip to excite phosphors to mix to produce white light, which is called phosphor-converted LED (pc-LED), which can also be divided into two categories. It is excited by an ultraviolet light chip or blue light chip. It is often used with blue, green, and red phosphors for excitation by ultraviolet light chip. Its color rendering is better. However, the energy of the ultraviolet light chip is too strong, and it is easy to damage the epoxy resin of the packaging material. People also worried about the problem of UV light dissipation. After the development of high-efficiency blue light chips, blue light chips have become the mainstream of current single-crystal diodes. For example, in 1997, Nichia applied for a white light patent of US 5,998,925, borrowed from indium gallium nitride (InGaN) diode excited yellow phosphor yttrium aluminum garnet (Y3Al5O12:Ce3+; YAG, where YAG is called the host lattice and Ce3+ is the luminous center), and subsequently, in response to the needs of various fields, various green and red phosphors were developed to improve color rendering and increase the color gamut. 

Phosphor powder is a substance that produces luminescence, mainly composed of a host lattice and an activator. As shown in Figure 1, the activator is used as the center of light emission, and a small amount of it is doped in the host crystal. In the lattice, the luminescence characteristics of the activator are determined by the coordination environment of the host lattice. Some phosphors are doped with a trace amount of sensitizer, and the energy of the excitation source is transferred to the activator through the energy transfer of the sensitizer in order to improve the luminous efficiency.

Figure 1. Schematic diagram of phosphor host structure and activator.
Figure 1. Schematic diagram of phosphor host structure and activator.

 

Professor Shu-Fen Hu has established a complete phosphor synthesis and characteristic analysis laboratory in the past 10 years. Figure 2 shows a schematic diagram of the instruments and analysis methods used in the laboratory.

Figure 2. Schematic diagram of the instruments and analysis methods of the phosphor characteristics analysis laboratory established by Professor Shu-Fen Hu.
Figure 2. Schematic diagram of the instruments and analysis methods of the phosphor characteristics analysis laboratory established by Professor Shu-Fen Hu.

 

Due to the advancement of human-computer programs and technology, artificial intelligence has developed rapidly and has been applied in many fields, such as medicine, mathematics, neurology, and other fields. Infrared LEDs are a highly anticipated development. With their accuracy and brightness greatly improved, they have been used in many fields, such as health detection, iris recognition, food safety, etc.  Take health testing as an example. In the early days, if you wanted to analyze the basic information of the body, such as glucose concentration and blood oxygen content, you had to use high-precision large-scale instruments and even invasive measurements. However, in recent years, infrared rays have been used to irradiate the body. The absorption value of the wavelength is used to estimate the concentration of the substance to be measured, which greatly reduces the measurement threshold, measurement time, and cost. However, the traditional infrared LED is limited by its narrow half-height width, and the information that a single LED can provide is limited. If you want to measure a variety of information, you must use multiple infrared LED devices. However, this method device will greatly increase the cost. It is also difficult to miniaturize the device, and the circuit design is also more complicated, which limits the application. Therefore, the best solution to this problem is to develop a device that uses a single LED chip to excite infrared phosphors. The infrared LED of this method has to greatly reduce the size of the device, reduce the cost, and reduce the design of the circuit control.

Therefore, in recent years, Professor Shu-Fen Hu has led her team to use different sintering methods to synthesize high-intensity and broad emission peak infrared phosphors, and analyze their structure and emission characteristics, regulate their emission wavelength, and establish a mechanism for regulating the wavelength, mixing different proportions of high-intensity infrared phosphors. The team has carried out research and development of high-intensity infrared LED devices suitable for detector measurement (Figure 3a), and used the space-confinement method to synthesize nano-level phosphors, packaged in mini LED, to evaluate its practical application value (Figure 3b). The team expects to introduce new application directions to the domestic light-emitting diode industry.

 

https://doi.org/10.1021/ja304754b

Figure 3. A conceptual diagram of infrared phosphors with broad emission peaks excited by blue light-emitting diodes. 

 

Source:
Origin of Thermal Degradation of Sr2–xSi5N8:Eux Phosphors in Air for Light-Emitting Diodes, J. Am. Chem. Soc. 2012, 134, 14108−14117.
https://doi.org/10.1021/ja304754b