Application Status And Trend Of Luminescent Materials For High Quality White LED Light Source

- Sep 29, 2018-

Director of lighting department, Limited by Share Ltd

Rare earth luminescent material is one of the core materials of current lighting, display and information detectors, and also an indispensable key material for the future development of new generation of lighting and display technology. At present, the R&D and production of rare earth luminescent materials are mainly concentrated in China, Japan, the United States, Germany and South Korea. China has become the largest producer and consumer of rare earth luminescent materials in the world. In the field of display, wide color gamut, large size and high definition display will be the important development trend in the future. At present, there are many ways to realize wide color gamut, such as liquid crystal display, QLED, OLED and laser display technology. Among them, liquid crystal display technology has formed a very complete liquid crystal display technology and industrial chain, with the largest cost optimization. Potential is also the focus of enterprise development at home and abroad. In the field of illumination, full spectrum illumination, which is similar to sunlight, has become the focus of attention in the industry as a healthier way of illumination. As an important development direction of future lighting, laser lighting has attracted more and more attention in recent years, and has taken the lead in the automotive headlamp lighting system to obtain higher brightness and lower energy consumption than xenon headlamp or LED lamp. As an indispensable physical environmental factor for plant growth and development, light environment can promote plant growth, shorten the time of flowering and fruiting, and increase plant yield and productivity. It is urgent to develop high-performance luminous materials suitable for plant growth and lighting. . In the field of information detection, the Internet of Things (IOT) and biometrics (biometrics) technology have a market prospect of trillions of scales, and the core components of both require the application of near infrared sensors based on rare earth luminescent materials. With the renewal of lighting and display devices, rare earth luminescent materials, as the core materials, are changing rapidly. The status and development trend of luminescent materials are described in detail as follows.——LED chip

1 luminescent materials for high quality display technology

1.1 wide gamut LCD display luminescent material for LED backlight

In recent years, liquid crystal display (LCD) has become the leading technology in the field of flat panel display. Liquid crystal display based on white light emitting diode (LED) backlight has the outstanding advantages of good color reduction, low power consumption, long life and so on. At present, the penetration rate of liquid crystal display has exceeded 95%. Considering the technology, performance and cost, blue LED chip and phosphor is still the mainstream of white LED because of its high technology maturity and low cost. For LCD LED backlight, white light produced by "blue LED chip + phosphor" needs to produce pure red, blue and green light after filtering and splitting, so phosphor is the key factor to determine the color gamut of LCD backlight.——LED chip

Y3Al5O12: CE (YAG: CE) phosphor system and SIAlON: EU green phosphor (partially silicate green) and nitride red phosphor combination system are widely used in LED backlight LCD display at present. Because of the wide spectral peak and poor color purity of the former, the gamut display range of the display made by the former is about 70% NTSC, while the latter shows that the gamut range can only be increased to 80% NTSC, but the green and red color coordinates y and X are low, and the gamut display range of the display is difficult to reach 85% NTSC. And the luminous efficiency decreased by 40% compared with the previous technology. Wide-gamut LCD LED display technology refers to the display gamut with more than 90% NTSC, through which can accurately render images, rich tones, to restore the real world's excellent visual effect. At present, the key way to realize wide color gamut LED backlight display is "blue chip + SIAlON: EU green powder + fluoride red powder" system [6]. However, the performance of new fluoride phosphors for wide color gamut LCD LED backlight developed by state-owned R&D Institute is at a fairly high level with the international level, especially the only germanium fluoride phosphors that can be supplied in large quantities in the industry. The performance of domestic SIAlON:EU green powder is still far from that of foreign countries. Although rare earth can be prepared in small batches in high-band SIAlON:EU green powder, the main market of domestic SIAlON:EU green powder is monopolized by foreign enterprises.——LED chip

At present, the industrialization level of LCD color gamut based on new LED backlight has exceeded 90% NTSC. It is urgent to develop new phosphors and LED backlight, and further enhance the color gamut of LCD display to 110% NTSC, which is comparable to OLED/QLED technology. Fortunately, the narrow-band emission red powder with longer wavelength than the existing fluoride phosphor is needed. The development of green powder with higher purity than SIAlON:EU green pink powder has already begun to appear, and is expected to reach the application level in the next 2-3 years. It will be sure that a very complete LCD technology and industrial chain will be built for the efficient utilization of our country, seizing the commanding point of the future wide-color-domain LCD technology, and realizing the LCD technology in our country. The breakthrough and catching up of the operation laid a very good material foundation.——LED chip

1.2 other emerging technologies for luminescent materials

OLED has many advantages, such as active luminescence, high luminous efficiency, good luminous color purity, bright color, low power consumption, ultra-light and thin devices, flexible and so on. It is conducive to panchromatic display. OLED has good development prospects in the display field and is favored by the industry. OLED display technology in television, mobile terminals, VR, watches and other wearable equipment application potential, as well as domestic OLED panel gradually recognized by the market, will also provide an explosive force for the OLED display industry [9]. According to market research, the market share of OLED televisions in the US $3,000 high-end market reached 65% in the first quarter of 2017 and 100% in 55 inches, the same situation in Europe [12]. Therefore, OLED display technology still has a good application prospect. Luminescent materials (red, blue and green) are important components of OLED display devices, which directly determine the performance and application of devices [8]. Luminescent materials must have good comprehensive properties to meet the application requirements, such as high luminous brightness and quantum yield; under near-ultraviolet or blue excitation, they have large absorption cross-section and wide excitation. The comprehensive properties of luminescent materials for OLED display still need to be further improved.——LED chip

Quantum dots (QDs) have excellent luminescent properties, such as high quantum efficiency, continuous tunable luminescent wavelength, narrow half-peak width and so on. Quantum dots have replaced the traditional phosphors, so that the color gamut of the display can be improved to 110% NTSC [13]. However, there are still several bottlenecks to overcome in the application of quantum dot luminescent materials.——LED chip

Firstly, due to the small size and large specific surface area of nanocrystalline particles, nanocrystalline particles are easy to oxidize and decompose under the action of light, heat and chemistry, resulting in a sharp decline in their optical properties. The optical decay at operating temperature has become a major obstacle to the improvement of luminous efficiency and lifetime of quantum dot white LED.——LED chip

Secondly, although QDs are easier to blend with encapsulation materials such as adhesive than traditional rare earth phosphors, there are still some problems such as agglomeration and phase separation when nanocrystals are blended with encapsulation media due to interface compatibility, which makes it difficult to further improve the light efficiency of LED products. The use of quantum dot luminescent materials is also an alternative way to fabricate wide color gamut display devices. However, due to the high cost and complexity of components, quantum materials containing Cd have a negative impact on the environment, and because of its cost problems, it has not been applied in actual scale [13].——LED chip

The stability of quantum dot luminescent materials is the main factor restricting its marketability. Researchers are launching a series of related studies on this issue. With the improvement of the stability of materials, it is predicted that the half-life of quantum dot white LED will reach more than ten thousand hours within three years, and the market will be established at the same time.——LED chip

The display of quantum dots has shown a strong competition among China, the United States and South Korea. Fortunately, China has some advantages in core materials, prototype devices and manufacturing processes. It is expected to provide a good opportunity for China's display industry to break through the patent blockade of foreign technology routes and realize "lane change and overtaking".——LED chip

2 luminescent materials for high quality lighting technology

2.1 luminescent materials for full spectrum lighting

With the accelerated penetration of white LED in the lighting field, the market demand for the quality of white LED light source is getting higher and higher. Especially in indoor lighting, the focus of white LED light source requirements has been changed from the original pursuit of "high brightness" to "high quality" which takes into account the color rendering index, color temperature and other light and color properties. Pursuing full spectrum illumination similar to sunlight, packaging companies at home and abroad have accelerated the development of full spectrum LED products [15-16]. At present, there are two main ways to realize full spectrum LED: multi-chip and single-chip. Single-chip has become the first choice for packaging enterprises because of its simple implementation, low cost and more continuous spectrum. The implementation of single chip can be divided into blue chip technology (blue chip + multi-color emission phosphor) and ultraviolet / near ultraviolet chip technology (ultraviolet / near ultraviolet chip + multi-color emission phosphor) [18-20]. In the blue-light chip technology, there is a serious spectrum loss in the blue-green part of the device spectrum, so it is difficult to achieve high-quality full-spectrum illumination in theory. At present, the ultraviolet and near-ultraviolet chip technology of the third generation semiconductor developed by the state is becoming more and more mature, which makes the ultraviolet/near-ultraviolet chip technology become the preferred technology for full spectrum illumination.——LED chip

Blue-excited phosphors are becoming more and more mature, but most of these phosphors can not be excited efficiently by violet light. At present, green, yellow and red phosphors for UV/near-ultraviolet chip excitation have been studied extensively [21-23]. However, the common problem is that the luminous efficiency is low and it is difficult to meet the practical application. Developing phosphors suitable for purple light with high efficiency excitation, wide band emission and low mutual absorption among colored phosphors has become the research focus of the industry, and also an important starting point for China to make intellectual property breakthroughs in the field of lighting in the future. Therefore, in the field of full spectrum illumination, it is an important opportunity to grasp the development opportunities and trends of the third generation semiconductor technology with high energy and short wave, and develop new luminescent materials, especially new phosphors suitable for ultraviolet/near ultraviolet chips.——LED chip

2.2 luminescent materials for high-density energy excitation

LED lighting has become an indisputable mainstream lighting technology, it is expected that by 2020, semiconductor lighting alone will form a trillion market size [24]. Compared with the first and second generation semiconductor materials, the third generation semiconductor has many advantages, such as high breakdown voltage, band gap, high thermal conductivity, high electron saturation rate, strong radiation resistance and so on. At the same time, it also has the characteristics of high luminous efficiency and high frequency. It can be widely used in many strategic emerging industries, such as semiconductor lighting, promoting and supporting. A generation of industrial transformation. The third generation semiconductor materials used in the field of solid-state lighting can greatly improve the light efficiency and color quality of the devices. However, the important characteristics of the third generation semiconductor lighting sources are the increase of current density and the shift of the wavelength of the chip emission to high energy and short wave [25]. Since the luminous materials directly determine the light efficiency and quality of the light source, the existing ones The excitation characteristics and stability of classical phosphors such as aluminates can not meet the needs of the third generation semiconductor high-density energy excitation. Therefore, it is urgent to break through the new fluorescent materials and preparation technology of the third generation semiconductor high-density energy source.——LED chip

Because of the phenomenon of "drastic drop in efficiency" in LED, that is, the internal quantum efficiency will drop sharply when working with high current density, scientists in various countries are looking for a new generation of high-quality light source, blue light-emitting diode inventor Nakamura proposed that in the near future, LED technology will be subject to the physics of its luminous efficiency. The limit will eventually be replaced by laser diodes. Compared with LED lighting, laser lighting can achieve higher efficiency. Semiconductor laser is considered as the most promising high-end lighting and display high-quality light source after LED. It will become a development trend of future lighting and display industry. At present, fluorescent conversion laser display technology has been used in laser television, laser projection, and so on. Laser theater and other large size display areas have been applied to [26-27]. Similar to LED lighting, fluorescent conversion materials are also the key materials to achieve white light output in laser lighting. Laser has a higher energy density, so the ability of fluorescent conversion materials to resist light damage has been put forward higher requirements [28]. The development of new rare earth fluorescent materials with high stability and conversion efficiency and their application technology will be a great challenge for laser lighting in the future, which will promote the industrialization of new rare earth fluorescent materials, ceramics and crystals.——LED chip

3 luminescent materials for special light sources

3.1 luminescent materials for plant lighting

In recent years, with the development of optoelectronic technology, the luminous efficiency of LED has been greatly improved. The application of LED in plant factories has gradually attracted widespread attention all over the world. LED has many advantages, such as small size, long life, low calorific value and so on. In addition, its unique wavelength advantage and wide-range adjustability are considered as an effective alternative light source for artificial light plant factory [29]. The market prospect of LED applied to plant lighting is quite optimistic. It is expected that the market will grow rapidly. The plant lighting (system) market in 2017 will be about $690 million, of which $193 million will be LED lamps. It is estimated that the plant lighting (system) market will grow to $1.424 billion by 2020 and the LED lamps will grow to $356 million. At present, blue LED chips or ultraviolet LED chips plus phosphors are the main modes of LED luminescence. In the future, phosphors for plant lighting will also be one of the important raw materials for plant lighting devices.——LED chip

Light is the main energy source for plant growth and development, but the absorption of light by plants is not full-band but selective. At the same time, the absorption spectra of light by different green plants are basically the same [30]. There are two areas where chlorophyll absorbs the strongest light, one is the blue and purple parts with wavelength of 400-500 nm. The chlorophyll solution is green because it absorbs less orange and yellow light and less green light. The other is in the red part of 640-660 nm wavelength. Red light is beneficial to plant carbohydrate synthesis and accelerates plant growth and development. Therefore, efficient plant supplementary illumination is generally achieved by the combination of 400-500 nm blue light, 640-660 nm ultra-red light and some white LED.——LED chip

In addition to the above two types of light that plants must absorb, plants also have photoreceptor systems (photoreceptors). The most important photoreceptor in plants is the phytochrome which absorbs red or far red light. It is extremely sensitive to red and far red light and is involved in the whole growth process of plants from germination to maturity. Photochromes in plants exist in two relatively stable states: red light absorption (PR, Lmax = 660 nm) and far red light absorption (PR, lMAX = 730 nm). These two states can be transformed each other, so the complete plant lighting scheme should also have 730 nm far red light [31]. Blue powder is excited by ultraviolet/near ultraviolet chips, mainly with aluminate, silicate, phosphate, nitride as the matrix, EU2+ as the luminescent ion [32-33]. The deep red phosphors are mostly EU, MN or CE plasma or Co doped with MN2+ to get [34-35]. The research of these two bands of phosphors has been very extensive, and the technology is relatively mature, which can be applied to plant lighting. However, there are few studies on far-red phosphors for phytochrome, and their luminescence efficiency is still at a low level, which is difficult to be applied in practice. Therefore, the development of new far-infrared luminescent materials matching the field of plant illumination, the key preparation technology, and the study of the light proportion of blue, red and far-red phosphors used for illumination are the key directions for plant illumination to contribute to the development of bio-agriculture.——LED chip

3.2 luminescent materials for near infrared light sources

Near-infrared (NIR) light refers to electromagnetic waves with wavelengths ranging from 780 nm to 2526 nm. In recent years, the application of NIR detectors in face recognition, iris recognition, security monitoring, lidar, health detection, 3D sensing and other fields has developed rapidly, and has become the focus of international research [36-38]. Near infrared detectors are expected to reach US$25 billion in the global biometric market by 2020, of which only the total output value of iris recognition technology will reach US$3.5 billion. Infrared detectors are an important part of communication and physical systems. It is urgent to use near infrared (especially 780-1600nm) devices with high efficiency and narrow band or special wide band emission. At present, the patents of infrared chips are mastered by foreign countries. Especially, the chips above 1000 nm band have low efficiency, high cost and are monopolized by foreign patents and technologies. At the end of 2016, Osram launched the first near infrared LED with a blue-light chip and near-infrared phosphor to measure the content of fat, protein, water or sugar in food. Blu-ray chip and near-infrared phosphor composite packaging has attracted wide attention in the world because of its simple preparation process, low cost and high luminous efficiency. Therefore, it is urgent to develop new near-infrared phosphors for near-infrared LEDs in various bands to realize their diversified applications.——LED chip

According to the classification of near-infrared light, the near-infrared long-wave luminescence is 1100-2526nm, and the near-infrared long-wave phosphors are mainly Er3+ and Ni2+ as luminescence centers. At present, a series of fruitful research progresses have been made in this field [39]. Phosphors with different bands of near infrared long wave have been developed, and the energy transfer has been realized by introducing sensitized ions, and the luminescence efficiency has been greatly improved [40].——LED chip

Near-infrared short-wave luminescence is 780-1100 nm, and near-infrared short-wave phosphors are mainly composed of Cr3 +, Yb3 +, Nd3 + [41-42]. At present, the industry has obtained more abundant materials system in the field of near infrared luminescent materials, but the common problems are low luminescent efficiency, poor stability of some systems, still unable to meet market demand. Therefore, new near infrared luminescent materials have been developed to break through the purple-blue excitation of high-efficiency emission of infrared phosphors and its key preparation technology, improve its luminous efficiency, and gradually replace the near infrared chip.——LED chip

4 Conclusion

In summary, the illumination and display technology based on high-efficiency and low-cost blue LED chips has been mature and applied. Among them, aluminate and nitride phosphors suitable for blue-light-excited illumination are becoming more and more perfect. However, with the development of full-spectrum illumination and high-power illumination technology and application requirements, it is urgent to develop new phosphors and phosphors. Ceramic or single crystal high performance fluorescent materials. In the field of display, although QLED, OLED and excitation display technology are developing rapidly, the development of new phosphors is expected to compensate for the relatively low color gamut of LCD. The LCD backlight technology based on blue LED chip still has great vitality. In addition, through the innovation of material system, high efficiency near infrared and even ultraviolet non-visible light sources can be obtained based on blue LED. The material and technological innovation of phosphors used in the above fields is an important way to realize the breakthrough of core patents and industrial development of materials and even optoelectronic devices in China.——LED chip

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