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

Sep 28, 2018

      Director of the Luminescence Division of Rare Earth New Materials Co., Ltd.

Key Materials for the 2018 Aladdin Lighting Industry Research White Paper

Rare earth luminescent materials are one of the core materials for current illumination, display and information detectors, and are indispensable for the development of next-generation lighting and display technologies. At present, 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 world's largest producer and consumer of rare earth luminescent materials. In the display field, wide color gamut, large size, high-definition display is an important development trend in the future. At present, there are many ways to achieve wide color gamut, such as liquid crystal display, QLED, OLED and laser display technology, among which liquid crystal display technology is now available. Formed a very complete liquid crystal display technology and industrial chain, with the greatest cost advantage, is also the focus of domestic and foreign display enterprise development. In the field of lighting, full-spectrum illumination like sunlight has become the focus of the industry as a healthier lighting method. As an important development direction of future lighting, laser lighting has attracted more and more attention in recent years, and has been the first to be applied in automotive headlight lighting systems, which can obtain much higher brightness than xenon headlights or LED lights. Lower energy consumption. As an indispensable physical environment factor for plant growth and development, light environment can improve plant growth, reduce plant flowering results, improve plant yield and production capacity through light quality regulation, control plant morphology, and become a global focus. A high-performance luminescent material suitable for plant growth lighting. In the field of information detection, the Internet of Things and biometric (biometric) technologies have a market prospect of trillions of dollars, and the core components of both require NIR sensors using rare earth luminescent materials. With the replacement of lighting and display devices, rare earth luminescent materials, which are core materials, are undergoing rapid changes. The current status and development trends of luminescent materials are detailed below.

1 high-quality display technology with luminescent materials

1.1 Wide color gamut liquid crystal display LED backlight source

In recent years, liquid crystal display (LCD) has been the strongest in flat panel display, becoming the leading technology in the field of flat panel display [1]. Liquid crystal displays based on white light-emitting diode (LED) backlights have outstanding advantages such as good color reproduction, low power consumption, and long life. Currently, the penetration rate in the field of liquid crystal display has exceeded 95%. For the generation method of white LEDs for liquid crystal display, the "blue LED chip + phosphor" method is still the current white LED generation due to the high technical maturity and relatively low cost due to comprehensive consideration of its technology, performance and cost. The mainstream way [2]. For the liquid crystal display LED backlight, the white light generated by the "blue LED chip + phosphor", after filtering and splitting, needs to produce pure red, blue and green light, so the phosphor determines the color of the LED backlight LCD. The key factor of the domain [3].

At present, the phosphors commonly used in LED backlight liquid crystal display are Y3Al5O12: CE (YAG: CE) phosphor system and SIAlON: EU green phosphor (partially using silicate green powder) and nitride red phosphor combination system [4, 5]. Because the spectral peaks of the former are relatively wide and the color purity is not good, the display color gamut display range of the former is about 70% NTSC, while the latter technical scheme shows that the color gamut range can only be increased to 80% NTSC, but the color coordinates of the green powder. The color coordinate x value of the y value and the red powder are both low, and the color gamut display range of the display is difficult to reach 85% NTSC or more, and the light effect is 40% lower than the previous technical solution. The wide color gamut liquid crystal LED display technology refers to a display color gamut with 90% NTSC or more, which can accurately display images and rich colors, and realize the stunning visual effect of restoring the real world. At present, the key implementation of the wide color gamut LED backlight display is the "blue chip + SIAlON: EU green powder + fluoride red powder" system [6]. However, the performance of the new high-efficiency fluoride phosphors for wide-color-domain liquid crystal display LED backlights developed by state-owned research rare earths is at a level comparable to the international level, especially the development of the only commercially available lanthanide fluoride phosphors. Domestic SIAlON: The performance of EU green powder is still far from the foreign countries. Although domestic rare earths can be used to achieve high-volume SIAlON: EU green powder in small batches, its main markets are monopolized by foreign companies.

At present, the industrialization level of liquid crystal display color gamut based on new LED backlights has exceeded 90% NTSC. It is urgent to develop new phosphors and LED backlights, and further increase the liquid crystal display color gamut to 110% NTSC and OLED/QLED technology. Fortunately, the development of narrow-band emission red powder with a longer wavelength than the existing fluoride phosphors and green powder with higher purity than SIAlON:EU green pink has already begun to emerge, and is expected to reach the application level in the next 2-3 years. It will surely build a very complete liquid crystal display technology and industrial chain in China, and seize the commanding heights of the future wide color gamut liquid crystal display technology, and realize a very good material foundation for the breakthrough and catch-up of liquid crystal display technology in China.

1.2 Other emerging display technology luminescent materials

OLED has many advantages such as active illumination, high luminous efficiency, good luminescent color purity, bright color, low power consumption, ultra-thin and flexible device, etc. It is beneficial to full-color display and has good development prospects in the display field. [7]. The application potential of OLED display technology in wearable devices such as TVs, mobile terminals, VRs, watches, etc., as well as the domestic OLED panels are gradually recognized by the market, and will also provide explosive power for the OLED display industry [9]. According to market research, OLED TVs in the US high-end market of 3,000 US dollars, the market share in the first quarter of 2017 reached 65%, 55 inches can reach 100%, the same situation in Europe [12]. Therefore, OLED display technology still has a good application prospect. Luminescent materials (red, blue, green) are important components of OLED display devices, which directly determine device performance and use [8]. Luminescent materials that meet application requirements must have good overall properties, such as high luminance and quantum. Yield; large absorption cross section and wide excitation range under near-ultraviolet or blue light excitation; environmentally friendly; good UV light tolerance; good carrier transport performance; good thermal stability, film formation [10-11], the overall performance of luminescent materials for OLED display still needs to be further improved.

The quantum dot material has excellent luminescent properties, high quantum efficiency, continuously adjustable illuminating wavelength, and narrow half-peak width. The quantum dot is used to replace the traditional phosphor to increase the color gamut of the display to 110% NTSC [13]. However, there are still several bottlenecks in the application of quantum dot luminescent materials that need to be overcome.

Firstly, due to the small size and large specific surface area of nanocrystalline particles, under the action of light, heat and chemistry, nanocrystalline particles are prone to oxidation and decomposition, resulting in a sharp decline in optical performance. The problem of light decay at operating temperature has become a limitation. The main obstacles to the luminous efficiency and lifetime of quantum dot white LEDs.

Secondly, although quantum dots are more easily blended with materials such as encapsulants than conventional rare earth phosphors, due to interfacial compatibility problems, agglomeration and phase separation still exist when nanocrystals are mixed with packaged media, resulting in LED product light. It is difficult to further improve the effectiveness. The use of quantum dot luminescent materials is also an alternative technical approach to the preparation of wide color gamut display devices, but due to the high cost and complexity of components, and the quantum material contains Cd, it has a negative impact on the environment, and because of its cost, it has not been actually Scale application [13].

The stability of quantum dot luminescent materials is the main factor limiting its marketization. Relevant researchers are conducting a series of related research on this issue. With the improvement of material stability, the half-life of quantum dot white LEDs can be predicted within three years. It will reach more than 10,000 hours and the market will be established at the same time [14].

The field of quantum dot display has been in the doldrums of China, the United States and South Korea, and the competition is fierce. Fortunately, China has certain first-mover advantages in core materials, prototypes and 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 "changing overtaking".

2 high-quality lighting technology with luminescent materials

2.1 Luminescent materials for full spectrum illumination

With the accelerated penetration of white LEDs in the field of illumination, the demand for quality of white LED light sources is also increasing. Especially in indoor lighting, the focus on white LED light sources has been purely pursued from the beginning. "Converted to the "high quality" of color performance, such as color rendering index, color temperature, and even the pursuit of full-spectrum illumination similar to sunlight, domestic and foreign packaging companies have accelerated the development of full-spectrum LED products [15-16]. At present, full-spectrum LED implementations mainly include multi-chip type and single-chip type [17]. Among them, single-chip type has the advantages of simple implementation, low cost and more continuous spectrum, which has become the first choice for packaging enterprises. The single-chip implementation is divided into blue chip technology (blue chip + multi-color emission phosphor) and ultraviolet / near-ultraviolet chip (UV / near-ultraviolet chip + multi-color emission phosphor) technology [18-20]. In the blue chip technology, there is a serious spectral loss in the blue-green portion of the device spectrum, and it is theoretically difficult to achieve high-quality full-spectrum healthy illumination. At present, the ultraviolet and near-ultraviolet chip technologies in the third-generation semiconductors that are under development in the country are becoming more and more mature, and the UV/near-ultraviolet chip technology has become the technology of choice for full-spectrum illumination.

The blue-excited phosphor technology has become more mature, but most of these phosphors cannot be excited by violet light. At present, ultraviolet/near-ultraviolet chip excitation is more studied with green, yellow and red phosphors [21-23], but it is common. The problem is that the luminous efficiency is low and it is difficult to meet practical applications. The development of phosphors suitable for high-efficiency excitation of violet light, broad-band emission, and low mutual absorption between phosphors of various colors has become the research focus of the industry, and is also an important force for China to achieve intellectual property breakthrough in the field of lighting in the future. Therefore, in the field of full-spectrum illumination, we will grasp the development opportunities and trends of high-energy and short-wave third-generation semiconductor technologies, and develop new luminescent materials, especially for new UV/NUV chips, which are important for green health lighting. Opportunity.

2.2 High-density energy excitation luminescent materials

LED lighting has become the undisputed mainstream lighting technology, and it is expected that by 2020, only the semiconductor lighting field will form a trillion market size [24]. Compared with the first and second generation semiconductor materials, the third generation semiconductor has the advantages of high breakdown voltage, forbidden bandwidth, high thermal conductivity, high electron saturation rate, strong radiation resistance, and high luminous efficiency and high frequency. It can be widely used in many strategic emerging industries such as semiconductor lighting to promote and support the next generation of industry changes. The third generation of semiconductor materials used in the field of solid-state lighting can greatly improve the light efficiency and light color quality of the device, but the important feature of the third-generation semiconductor illumination source is the increase of current density and the wavelength of the emitted light of the chip to the high-energy short-wave direction [25]. In view of the fact that the luminescent material directly determines the light efficiency and quality of the light source, the excitation characteristics and stability of the existing series of phosphors such as the classic aluminate can not meet the high-energy energy excitation requirements of the third-generation semiconductor, so it is urgent to break through the third-generation semiconductor. A high-density energy source that efficiently excites and forms high-quality white light and a new type of fluorescent material and preparation technology.

Due to the phenomenon of “substantial diminishing efficiency” of LEDs, that is, when operating at high current densities, the internal quantum efficiency will drop sharply. At present, scientists all over the world are looking for a new generation of high-quality light sources, the inventor of blue light-emitting diodes, Nakamura Shuji, In the near future, LED technology will eventually be replaced by laser diodes because of the physical limits of its luminous efficiency. Compared with LED lighting, laser lighting can achieve higher efficiency. Semiconductor laser is considered to be the most promising high-end lighting and high-quality light source for LED after LED. It will become a development trend in the future lighting and display industry. Fluorescence-converted laser display technology has been applied in large-scale display fields such as laser TV, laser projection, and laser cinema [26-27]. Similar to LED illumination, fluorescent conversion materials are also the key materials for achieving white light output in laser illumination. Lasers have higher energy density, and therefore have higher requirements for the ability of fluorescent conversion materials to resist light damage [28]. The development of new rare earth fluorescent materials with high stability and high conversion efficiency and their application technology will be a major challenge for laser illumination in the future, which will lead to the industrialization demand for new rare earth fluorescent materials and their ceramics and crystals.

3 special light source luminescent materials

3.1 Luminescent materials for plant lighting

In recent years, with the development of optoelectronic technology, LED luminous efficiency has been greatly improved, and the application of LED in plant factories has been widely concerned by countries all over the world. LED has the advantages of small size, long life, low heat generation, etc. In addition, its unique wavelength advantage, wide adjustability, etc., is considered to be an effective alternative light source for artificial light plant factories [29]. The market prospects for LED application in plant lighting are quite optimistic, and the market size is expected to grow rapidly. In 2017, the plant lighting (system) market was about 690 million US dollars, including 193 million LED lamps. It is estimated that by 2020, the plant lighting (system) market will grow to 1.424 billion US dollars, and LED lamps will grow to 356 million US dollars. At present, the mode of LED illumination is mainly blue LED chip or ultraviolet LED chip + phosphor. In the future, phosphors for plant lighting will also be one of the important raw materials for realizing plant lighting devices.

The main energy source needed for plant growth and development is light, but the absorption of light by plants is not full-band but selective, while the absorption spectrum of light by different green plants is basically the same [30], chlorophyll is the most light wave. There are two strong absorption zones. One is the blue and violet part with a wavelength of 400-500 nm. The absorption of orange and yellow light is less, and the absorption of green light is the least, so the solution of chlorophyll is green and the other is at wavelength. For the red portion of 640-660 nm, red light is beneficial to the synthesis of plant carbohydrates and accelerates the growth and development of plants. Therefore, efficient plant fill light illumination is generally achieved by a combination of 400-500 nm blue light and 640-660 nm ultra red light and some white light LEDs.

In addition, 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 that absorbs red or far red light. It is extremely sensitive to red and far red light and participates in the entire growth and development of plants from germination to maturity. The phytochrome in plants exists in two relatively stable states: red light absorption (PR, lmax = 660 nm) and far red light absorption (PR, lMAX = 730 nm), which can be transformed into each other, so complete The plant lighting scheme should also have a far red light of 730 nm [31]. Blue powder is excited by UV/near UV chip, mainly based on aluminate, silicate, phosphate and nitride, and EU2+ is luminescent ion [32-33]. Most of the deep red phosphors are obtained by EU, MN or CE plasma or co-doped with MN2+ [34-35]. The research of phosphors in these two bands has been extensive, and the current technology is relatively mature and can be practically applied to plant lighting. However, there are few studies on far-red phosphors for plant phytochrome, and their luminous efficiency is still at a low level, which is difficult to practically apply. Therefore, the development of new far-infrared luminescent materials matching the field of plant lighting, solving its key preparation techniques, as well as the research on the ratio of light for blue, red and far-red phosphors used in illumination, is the contribution of plant lighting to today. The key direction of the development of bio-agriculture.

3.2 Luminescent materials for near-infrared light sources

Near-infrared light refers to electromagnetic waves with a wavelength in the range of 780-2526 nm. In recent years, the application of near-infrared detectors in the fields of facial recognition, iris recognition, security monitoring, laser radar, health detection, 3D sensing, etc. has rapidly developed and has become International research focus [36-38]. It is estimated that the near-infrared detector will reach 25 billion US dollars in the global biometric identification market in 2020, of which only the total output value of iris recognition technology will reach 3.5 billion US dollars. Infrared detectors are an important part of communication and IoT systems, and there is an urgent need for near-infrared (especially 780-1600nm) devices that emit high-efficiency narrowband or special broadband. At present, the patent of infrared chip is mastered by foreign countries, especially the chip with the wavelength above 1000nm is low in efficiency, high in cost and monopolized by foreign patents and technologies. It is urgent to develop a high-energy mature violet-blue chip to stimulate the phosphor-converted high-efficiency infrared device. At the end of 2016, OSRAM launched the first blue-light chip composite near-infrared phosphor near-infrared LED for measuring fat, protein, moisture or sugar content in food. The implementation of the blue chip and the near-infrared phosphor composite package has the advantages of simple preparation process, low cost, high luminous efficiency, and the like, and has been widely concerned internationally. Therefore, it is extremely urgent to develop a new type of near-infrared phosphor for each band of near-infrared LEDs to realize its diversified application requirements.

According to the classification of near-infrared light, the near-infrared long-wave is 1100-2526 nm, and the near-infrared long-wave phosphor mainly uses Er3+ and Ni2+ as the illuminating center. At present, a series of fruitful research progresses have been made in this field [39]. The phosphors of different wavelengths of near-infrared long-wave have been studied, and the energy transfer has been realized by the introduction of sensitized ions, etc., and the luminous efficiency has been greatly improved [ 40].

The near-infrared short-wave is 780~1100nm, and the near-infrared short-wave phosphor is mainly composed of Cr3+, Yb3+, and Nd3+ [41-42]. At present, the industry has obtained a relatively rich material system in the field of near-infrared luminescent materials, but the common problem is that the luminous efficiency is low, and some systems have poor stability, and still cannot meet the market demand. Therefore, the development of a new type of near-infrared luminescent materials, breaking through the violet-blue light-emitting infrared fluorescent powder and its key preparation technology, and constantly improve its light efficiency, and gradually replace the near-infrared chip.

4 Conclusion

In summary, the illumination and display technology based on the high-efficiency and low-cost blue LED chip has been maturely applied. Among them, the performance of the aluminate and nitride system phosphors suitable for blue-light excitation is also improved, but with full-spectrum illumination and large Power lighting technology and application requirements, the development of new phosphors and high-performance fluorescent materials of ceramization or single crystal are urgently needed. In the display field, although QLED, OLED and excitation display technologies are developing rapidly, the development of new phosphors is expected to compensate for the relative lack of color gamut of liquid crystal display. The liquid crystal display backlight technology based on blue LED chips is still extremely large. vitality. In addition, through the innovation of material systems, based on blue LEDs, it is expected to obtain high-efficiency near-infrared and even ultraviolet non-visible light sources. The use of phosphor materials and technological innovations in the above-mentioned fields is an important way to realize the core patent breakthrough and industrial development of China's materials and even photovoltaic devices.