Research progress of YAG:Ce³⁺ in laser lighting applications

Lighting and display technology based on laser diodes (LD) represents one of the important future development directions of the semiconductor industry. Fluorescence conversion materials are the core components that determine the energy efficiency of laser lighting and the color quality of display products. The yellow fluorescent conversion material Y3Al5O12: Ce3+ (YAG: Ce3+) is suitable for blue light LD excitation, has high efficiency and is easy to obtain white light, and is still the most widely studied object. The traditional phosphor plus organic silica gel packaging mode has low thermal conductivity and has problems such as ablation, blackening, and failure under LD excitation. The characteristics of LD's high power excitation density have triggered revolutionary changes in fluorescent material packaging technology. For this reason, remote phosphors with multiple shapes and high thermal conductivities have emerged.

Recently, researcher Liu Yongfu of the Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, was invited by the editorial board of Acta Luminescence to write a review paper titled "Research Progress of YAG:Ce3+ in Laser Illumination Applications." This review paper mainly summarizes the preparation methods of YAG:Ce3+-based fluorescent glass, fluorescent films, fluorescent crystals, fluorescent ceramics and other forms and their application performance in LD lighting, and provides an outlook on the development of fluorescence conversion materials and LD lighting.

Background

After long-term development, semiconductor lighting and display technology based on LED chips has been widely used in various lighting and display fields. In comparison, laser diodes (LD) can easily achieve higher brightness in a smaller size, and have higher electro-optical conversion efficiency when driven at high power. Therefore, LD-based lighting and display technology is an important direction for future development.

In 2005, Japan's Nichia Company achieved white light emission by exciting phosphors with fiber-coupled output lasers. In 2011, BMW launched the i8 concept car. The combination of the high-beam LD lighting system and the low-beam LED system greatly increased the effective lighting distance of the car's headlights. In 2014, Audi launched the R8 LMX, a concept car equipped with laser headlights. As people continue to put forward demands for intelligent control, high power, high brightness, etc., LD lighting has entered a period of rapid development. In recent years, laser lighting products represented by car headlights and laser display products represented by laser projection TVs have gradually penetrated into people's lives.

Currently, similar to LED lighting devices, there are three main implementation modes for LD lighting. Among them, the blue light LD chip excites the yellow phosphor mode, and the blue light and yellow light can directly form white light after mixing. This process directly consists of a single blue LD chip and a single yellow fluorescent conversion material, and the solution is the simplest. Blue LD chips and yellow fluorescent conversion materials are relatively mature, and each has high efficiency. Therefore, this solution makes it easier to obtain high-efficiency, high-brightness LD lighting devices.

YAG has a garnet structure, belongs to the cubic crystal system, and the space point group is Ia3d. In 1967, G. Blass and others from Philips Laboratory in the Netherlands first reported the luminescent properties of YAG:Ce. The main excitation peak of YAG: Ce is in the blue light region of 460 nm, which can be effectively excited by blue light. It has high quantum efficiency and good thermal stability. When combined with a blue light chip, white light can be obtained directly. At present, YAG:Ce yellow phosphor has become the most extensive research object and the most mature commercial fluorescent conversion material in white LEDs.

In LD lighting, the LD chip has a high excitation power density and can reach a very high temperature in a very short time. For the traditional YAG: Ce phosphor plus organic silica gel-coated packaging mode, its thermal conductivity is low (0.1-0.4 W m-1K-1). The organic silica gel ages rapidly under irradiation of the LD chip and is even ablated by the laser. The device performance drops sharply or even fails. Therefore, the traditional organic silicone packaging process can no longer meet the service requirements of LD lighting devices, and new packaging processes are in urgent need of development. In high-power LED lighting, there are also problems such as rapid aging of devices and performance degradation caused by a large number of high temperatures. For this reason, remote phosphor packaging modes such as fluorescent glass, fluorescent film, fluorescent crystal, and fluorescent ceramics with high thermal conductivity (1-15 W m-1K-1) have been proposed. These research works have provided rich experience for the development of phosphors for LD lighting.

Drawing on the research experience of high-power LED devices, various fluorescent carriers such as fluorescent glass, thin films, crystals, and ceramics are used in LD lighting research. In 2018, Professor Wang Le from China Jiliang University, Professor Xie Rongjun from Xiamen University, Dr. Li Shuxing and others summarized in detail the research progress of various fluorescent conversion materials for laser lighting. Among them, YAG:Ce has many advantages such as being suitable for blue light excitation and high efficiency. Therefore, combined with blue light LD chips, YAG:Ce fluorescence conversion materials have once again become a research hotspot in LD lighting and have been developed rapidly. During laser irradiation, as the laser power increases, the luminescence of the phosphor reaches a maximum value and then begins to decrease. This phenomenon is usually called the luminescence saturation effect, and the corresponding excitation power is called saturation power, or saturation power density. The higher the saturation power, the more suitable the fluorescent conversion material is for high-power LD to obtain high-brightness lighting. YAG: Ce is the fluorescent host, with phosphor in glass (PiG), film (film), crystal (single-crystal phosphor, SCP), transparent/ceramic (trans parent/ceramic phosphor, TCP/CP) and other forms as carriers (Figure 1), used in laser lighting research. The purpose of developing various phosphors is to improve the thermal conductivity of fluorescent conversion materials, reduce the thermal quenching effect of luminescence caused by laser, and thereby increase the saturation power and luminous brightness. Due to different preparation processes, various types of remote phosphors also have their own characteristics in terms of corresponding luminescence characteristics and applications in laser lighting.

Therefore, this article only reviews the preparation processes of various remote phosphors using YAG: Ce as the main body and glass, thin films, crystals, and ceramics as carriers in recent years and their research progress in laser lighting applications. Through this review, we can have a brief and intuitive understanding of the material performance requirements of laser lighting, the development and application of devices, etc.
Figure 1 Fluorescent crystals, glasses, films and ceramics based on YAG:Ce

Outlook

YAG: Ce is suitable for blue light excitation and has high luminous efficiency. It is still the mainstream fluorescence conversion material for the next generation of laser lighting. Various types of remote phosphors based on YAG: Ce have an emission spectrum dominated by yellow light, with insufficient cyan, green and red light components. This results in the low color rendering index and high color temperature of LD lighting devices, and the quality of white light needs to be further improved. Finding multi-color fluorescent conversion materials suitable for the high power density characteristics of LD light sources and achieving controllable adjustment of the color quality of LD lighting sources is one of the huge challenges currently facing LD lighting technology.

In terms of the development of multi-color fluorescence conversion materials, β-Sialon and LuAG:Ce green phosphors, LSN:Ce yellow phosphors, and CASN:Eu red phosphors have been introduced into LD lighting. In addition, although there are relevant research foundations for 470-500nm cyan luminescent materials, their exploration in laser lighting applications is still blank. Therefore, it is of extremely important value to study cyan phosphors suitable for blue light excitation.

The fluorescent glass film process is highly tolerant, and commercial green, yellow, red and other fluorescent conversion materials can be compounded in the same film. At the same time, it assists the high thermal conductivity substrate to provide a solution for effective control of the color quality of LD lighting devices. Within a relatively small laser power range, this solution is expected to meet the application needs of LD lighting devices. Under larger laser power, the long-term service stability of the fluorescent film needs to be further verified.

Fluorescent crystals and fluorescent ceramics have high thermal conductivity and mechanical strength, good thermal shock resistance, and good stability under various laser power service conditions. However, existing fluorescent crystals and fluorescent ceramics mainly present themselves in a single luminous color. New technical solutions need to be developed in terms of color compounding and white light color quality control. At the same time, existing commercial red fluorescent materials such as nitride and fluoride still have problems such as oxidation, corrosion, and decomposition during the ceramic composite process. New red fluorescent materials that can achieve stable ceramic composites are a direction for future development.

In addition to the luminous color of fluorescence conversion materials, the differences in LD performance testing methods bring difficulties to the unified standard evaluation of laser-fluorescence conversion performance. Laser-fluorescence conversion performance testing has two modes: transmission and reflection. In each mode, factors such as the thickness, surface morphology, roughness, and whether an external heat sink is connected to the fluorescent bulk material will affect the light conversion efficiency of the material. Whether the laser power changes continuously or is collected at a single point, the resulting luminescence saturation characteristics of the fluorescent material are completely different. When evaluating the luminescence saturation characteristics, differences in evaluation methods such as laser power or laser power density, as well as incomplete laser spot size information and errors in spot size measurement, bring difficulties to the unified evaluation of the laser power tolerance characteristics of fluorescence conversion materials. Therefore, the establishment of relevant test standards in laser lighting is also an inevitable problem for the performance evaluation of fluorescence conversion materials used in laser lighting.