Research Progress in the Preparation of Phosphor for White Light Emitting Diode

. As a solid-state lighting source, white light emitting diode (LED) has become the most promising green lighting energy source in the 21st century with its three advantages of energy-saving, environmental-friendly and green-oriented. The study summarized the research progress in the preparation of phosphor for white light emitting diode, analyzed and compared the effects of different materials and synthesis methods on its luminescence performance. The future development trend was also prospected.


The development of light sources in lighting
The development of modern human illuminate lamp is marked by the following important periods. In 1882, Thomas Edison from the United States made a carbon fiber incandescent lamp. In 1898, the British chemist Ramsay accidentally invented the neon lamp when checking whether rare gases conduct electricity. In 1906, the high-pressure mercury lamp brought mankind into the era of gas discharge lamps. In 1962, GE's Holonyak and Bevacqua et al. invented red LEDs using gallium arsenide phosphide (GaAsP) material [1]. In the 1970s, gallium phosphide (GaP) green LEDs and silicon carbide (SiC) yellow LEDs emerged to supplement the orange, yellow and green parts of the LED light spectrum; but at the same time, the LED luminous efficiency was pretty low, and it lacks the blue light sources in the RGB trichromes. As a result, it was always difficult to make the breakthrough in terms of white LEDs [2]. In 1993, Shuji Nakamura and others of Nichia Chemical Company (Nichia) first developed gallium nitride (GaN) blue LEDs, and used indium gallium nitride (InGaN) to prepare ultraviolet, blue and green LEDs with ultra-high brightness; and used aluminum gallium indium phosphide (AlGaInP) to prepare ultra-high brightness red and yellow LEDs [3]. In 1996, the first white LED was ultimately made by using gallium nitride (GaN) blue LED + Y3Al5O12 ∶ Ce3 +yellow phosphors [4]. Since then, the modern light sources in lighting have gone through three stages: incandescent, neon and gas discharge lamps. And then it entered the fourth stage -the white LED as the leading lighting source.

The advantages of white LEDs
LED is now the main light source of semiconductor lighting; and compared with other daily lamps such as incandescent and tungsten halogen lamps, white LED lamps have many advantages [5--6]. In terms of energy consumption, white LEDs have the advantages of using low-voltage power supplies, low energy consumption, and high photoelectric conversion efficiency. In terms of application, white LEDs have high stability, short response time, the ability to emit light in multiple colors, a wide operating voltage range, and flexible materials that can be embedded in any plane. In terms of environmental protection, the average service life of white LEDs is over 100,000 hours, which is over a hundred times more than that of ordinary light sources; and compared to incandescent lamps, which contain harmful elements such as mercury, white LEDs are made without elements harmful to humans and the environment, and are therefore considered one of the most popular new energy sources.

The Implementation Methods and Principles of White LEDs
The main structures of LEDs are a diode composed of a p-type semiconductor and an n-type semiconductor. The carriers of the p-type semiconductor are negatively charged electrons and the carriers of the n-type semiconductor are positively charged holes. When a forward voltage is input, the negatively charged electrons move to the vicinity of the p-n junction and compound with the positively charged holes to produce photons [7].
The bandgap energy E g of electrons and holes and the wavelength of the radiated light satisfy E g =ℎ ⁄ , which also determines that the light emitted by LEDs is monochromatic and cannot emit a continuous spectrum of white light. Therefore, to obtain white light through the LED, it is necessary to mix the single-color LED light source with other light-emitting materials to form white light. The implementation methods of white LED can be divided into single-chip type and multi-chip type according to the number of chips. Single-chip white LED refers to the use of an LED chip to generate light to excite other fluorescent materials to emit light so that a variety of color light mix to form white light, the most commonly used are the following three methods [8].
(1) Blue LEDs are coated with yellow phosphors.
(2) Blue LEDs are coated with green and red phosphors.
(3) UV LEDs are coated with RGB tricolor phosphors or a single white phosphor to produce white light. Multi-chip white LEDs refer to the use of two or more LED chips mixed to form white light, and this method does not require other fluorescent materials.

The Role of Phosphors and the Selection Principles
Phosphors play a key role in the process of producing white light. It is mainly composed of two parts: the matrix lattice and the activator. The matrix lattice commonly has an aluminate system, a silicate system, a molybdate system, a tungstate system, a sulfide system, a nitride and nitrogen oxide system, etc., whose function is to absorb the excitation energy (LED emitted light). The activator is mainly rare earth-like metal ions. In the process of phosphor preparation, it is often at high temperatures to the matrix lattice doping activator and produce impurity defects and thus defective energy levels. Due to the nature of the 4f shell layer electrons of rare earth-like metal ions, when the activator receives the activation energy transferred from the matrix lattice, the electrons in the 4f shell layer will undergo an energy level jump and be excited by the light mixed with the light emitted from the LED, thus achieving white light. The selection principles of phosphors for white LEDs are usually divided into two aspects: in terms of preparation, the preparation of phosphor requires a simple process, lower cost, and produces fewer toxic substances; in terms of performance, they usually regard luminous efficiency, CIE color coordinates, color temperature, color rendering index, and other parameters as measurements. In addition, the selection of phosphor also needs to meet the following principles: the excitation spectrum of the phosphor and the LED emission spectrum should match; the effective excitation position of phosphor should be wide; and phosphor quenching temperature should be high and thermal stability should be good [9]. The yellow phosphor with blue LEDs is currently the most widely used and the most advanced technology to achieve the white LED. The traditional yellow phosphor is mostly used yttrium aluminum garnet (Y 3 A l5 O 12 ) as the matrix lattice. Nichia company initially applied the hightemperature solid-phase synthesis of Y 3 A l5 O 12 ： Ce 3+ (YAG:Ce) to achieve the white LED; it is also the mainstream of the current industrial production solution. This method has a simple structure and low cost, but the disadvantages are the high temperature required for synthesis, the uneven particle size of the product, the partial absence of red light in the synthesized phosphor, and the unsatisfactory color rendering, which makes it difficult to meet the needs of low color temperature lighting [10].

The Preparation Methods of Phosphors
From the perspective of preparation, the phosphor synthesized by the traditional high-temperature solidphase method has the disadvantages of large particle size and difficult encapsulation. Therefore, various synthesis methods, such as the co-precipitation method, sol-gel method, hydrothermal method, microwave radiation synthesis method and spray pyrolysis method, have been tried. The use of co-precipitation method can improve the particle size of phosphors, making it easy to encapsulate, and it is convenient to control the process parameters to get different sizes and shapes of phosphors. The disadvantages of the co-precipitation method are a slight decrease in luminescence intensity and agglomeration of phosphors, which can be improved by adding surfactants [11]. The molten salt-assisted co-precipitation method is based on the co-precipitation method and introduces high-temperature molten salts such as fluorine salts or halide salts as solvents and reaction media, which prepares samples with a smaller particle size and a cubic shape and better phosphor color saturation, improving the phenomenon of red light deficiency [12]. The calcination method is an extension of the traditional high-temperature solid-phase method with a lower synthesis temperature and a smaller particle size of the synthesized phosphor, but the luminescence of the phosphor is greatly affected. So the overall effect is not as good as the high-temperature solid-phase method [13]. The precursors prepared using the sol-gel method have the advantages of high purity, small particle size, and low synthesis temperature, but the disadvantage is the high cost of the production process and certain environmental pollution problems [14]. The hydrothermal method is the use of a high-pressure reactor to prepare precursors. The method synthesis temperature is low but phosphor luminescence performance is not as high as the high-temperature solid-phase method. The microwave method is an emerging synthesis method. It uses polar molecules in the microwave effect of dielectric loss, so that the material heats up to complete the reaction, greatly reducing the reaction energy consumption and speeding up the reaction. Therefore, it has some application prospects [15]. The spray pyrolysis method mainly uses the aerosol process of precursors to prepare spherical particles with a uniform particle size distribution and a large specific surface area. The special advantage of the spray pyrolysis method over other synthesis methods is that the size and size morphology of the luminescent material can be controlled. Spray pyrolysis is a simple technology, environmentally friendly production process, and low cost, so it is expected to replace the hightemperature solid-phase method as the mainstream method for phosphor synthesis in the future [16].

Blue-light-excited Phosphors
Blue LEDs can excite yellow phosphors or red-and-green mixed phosphors to produce white light. Yellow powder excited by blue LEDs has high luminous efficiency but poor color rendering. Red-and-green mixed phosphors excited by blue LEDs can make up for the lack of red light, but still exist many problems. Therefore, people continue to explore new materials to make white LEDs perform better.

Yellow phosphors
The most commonly used material for yellow phosphor is Y 3 A l5 O 12 ：Ce 3+ (YAG:Ce). On this basis, co-doping other ions is often the method to improve the luminous performance. By co-doping with rare-earth ions to replace some of the yttrium ions in YAG, different ratios of YAG:Ce 3+ ,Ln (Ln=Pr 3+ ,Sm 3+ ,Gd 3+ ，Tb 3+ ) are prepared, which can change the emission wavelength of yellow powder and improve the color rendering performance [17]. It was found that when the doping concentration is small, the CIE coordinates move toward the red region, which improves the color rendering to some extent and can meet the demand for warm illumination. But when the doping concentration is larger, the concentration quenching effect is gradually enhanced, which can cause a sharp decrease in the intensity of the emission peak and affect the luminous efficiency. Zhang J et al. compensated for the missing part of red light by doping Cr 3+ into Y 3 A l5 O 12 ： Ce 3+ and using red light excited by Cr 3+ [18]. Yu found that the luminescence intensity was increased by 185% when Li + was introduced with a doping amount of 0.06; and by 170% when Zn 2+ was introduced with a doping amount of 0.1. The emission spectrum peak was red-shifted by 35 nm and the color rendering index was 81.7, which was increased by about 17.7% [19]. There are also many other substrates of yellow powders that have exhibited good performances during the study of new materials. For example, Ca 3 Tb 7 BO 4 (SiO 4 ) 5 O in silicate system with apatite [20] as the matrix doped with Tb particles; LiBa 2 Ga ( P 2 O 7 ) 2 : Dy 3+ and Sr 8 MgAl (PO 4 ) 7 ∶xEu 2+ in phosphate matrix lattice, the color rendering index of the latter has reached 90.1 [21][22]. Among the nitride based phosphors, Ca-α-SiAlON:Eu 2+ emits orange-yellow light at 590 nm and β-SiAlON：Eu 2+ emits yellow-green light at about 550 nm, and the modified Li-α-SiAlON: Eu 2+ has better luminous efficiency and can meet the illumination needs of warm white light. The advantage of nitride phosphors is that they have good thermal and chemical stability, but the disadvantage is that the preparation conditions are harsher and are subject to subsequent development [23].

Red phosphors
To obtain white LEDs with low color temperature and high color rendering index, the method of coating blue LEDs with red and green phosphors is gradually coming into the limelight. The existing sulfide system of red phosphor luminous efficiency is low with poor stability, so the development of new red phosphors is of great significance.
At present, red phosphors are broadly divided into sulfide systems, nitride systems and silicate systems.  [24]. However, phosphors of sulfide systems are still more difficult to solve the problem of light decay, so sulfide phosphors will be gradually eliminated during subsequent studies [25]. In the nitride system, M 2 Si 5 N 8 ：Eu 2+ (M=Ca,Sr,Ba) series phosphors can emit red light with a maximum wavelength of 680 nm [26]. They have good temperature quenching properties and low degree of thermal attenuation. Furthermore, they can also be used to prepare long afterglow materials by co-doping with Nd 3+ , Sm 3+ , Dy 3+ ions. Sr x Ca 1-x AlSiN 3 :Eu 2+ phosphors, developed by Watanabe H, have more excellent temperature quenching properties compared to M 2 Si 5 N 8 ： Eu 2+ [27]. The development of nitride phosphors is still limited by the harsh production conditions and high production costs. The phosphors of silicate systems are gradually gaining attention due to the cheap and easy availability of raw materials and a simple synthesis process. Those with Eu 3+ as an activator include Li 2 ZnSiO 4 : Eu 3+ , Sr 3 B 2 SiO 8 : Eu 3+ , etc. Those with Eu 2+ as activator [28] include (Ba 1.2 Ca 0.74-x ) 2 SiO 4 : xEu 2+ .

Green phosphors
Green phosphors mainly use Eu 2+ as the activator, and the green phosphors studied earlier were mainly sulfide systems, including Ga 2 S 3 ∶ Eu 2+ 、MGa 2 S 4 ∶ Eu 2+ (M= Ca、Sr、Ba) [29], etc., but because of the poor stability and toxicity of sulfides, their promotion and application

UV-excited phosphors
The color of white LED made by UV-excited phosphor is only determined by the phosphor itself, which has overcome the problems of poor color rendering index and that blue LED scheme color temperature is high. Initially, people used UV LED coated RGB tricolor phosphor to produce white light, but a variety of phosphors mixed ratio not only increases the difficulty of packaging but also faces the problem of color re-absorption, which largely affects the luminous efficiency and color reproduction. So the development of a single substrate of white phosphor is of great importance. Single matrix white phosphor, according to the type of activation ion doping, can be divided into two kinds: the first is the white phosphors with only one kind of activation ion-doped. Its light-emitting principle is the use of light-emitting matrix lattice and activation ions mixed to produce white light or regulate the location of activation ions in the lattice, to make it light at different peak positions. The second is a variety of activation ion co-doped white phosphor. Its light-emitting principle is to choose the right number of ions ratio so that a variety of color mixing to achieve white light.

Conclusion and Prospects
After decades of development of rare-earth fluorescent materials, the technologies have become sophisticated and various new fluorescent materials and preparation methods are emerging. But considering various factors such as economic efficiency, synthesis technology and practical performance, it is difficult to find a new solution to completely replace the traditional method of coating YAG：Ce 3+ yellow phosphor for blue LEDs in a short time.
China's rare earth resources is accounted for 41.36%. Therefore, China is a veritable power of rare earth resources, and is also the production and consumption of rare earth light-emitting materials of the first country. But China's overall research level of rare earth light-emitting materials is still in a backward state; most of the technology patents and core intellectual property rights are monopolized by Japan, the United States and other places units. Our researchers should develop innovative and independent capabilities and find a research path suitable for themselves. To fill the gap in the field of fluorescent materials in China, we should start from the following aspects.
(1) To conduct in-depth study of the basic theory, such as rare earth examples of 4f electronic layer of the law of motion, sensitization process, energy transfer process, luminescence quenching process, etc.
(2) To look for new ways to prepare rare earth luminescent materials, such as nitride phosphors with excellent performance (but due to the harsh synthesis conditions, it has not been put into widespread application).
(3) To increase the research in thermal stability as well as conversion efficiency, as our country tends to pay more attention to the color coordinates, color temperature, and color rendering index of rare earth fluorescent materials.
(4) To develop new applications of rare earth fluorescent materials, such as light conversion materials, long afterglow materials, nano-emitting materials, etc.