Research progress of phosphors for single-matrix white light LEDs

introduction

In 1962, Holonyak et al. used GaAsP to prepare the first red-light LED. After more than 30 years of development, the luminous efficiency of LEDs has been greatly improved, and the emission wavelength range has been expanded to green, yellow and blue light regions. In 1993, Nakamura et al. took the lead in making a breakthrough in blue gallium nitride (GaN) LED technology. In 1996, they used Y3Al5O12∶Ce3+ (YAG∶Ce3+) that emits yellow light as a phosphor and coated it on a GaN diode that emits blue light, successfully preparing a white light LED. As soon as the white light LED appeared, it attracted widespread attention. As a light source, it has the characteristics of environmental protection, energy saving, high efficiency, long life, and easy maintenance. It is called the fourth generation of lighting sources that will surpass incandescent lamps, fluorescent lamps and high-intensity discharge lamps (High Intensity Discharge, HID), and is the first choice for today's society that pursues a low-carbon economy [1, 2].

At present, the realization of white light LED mainly adopts a combination of an LED chip and a phosphor. The phosphor converts the short-wavelength light emitted by the chip into part or all of visible light, and finally recombines into white light. The focus is on the research, development and production of phosphors for light color conversion. The most studied and mature one is the blue LED/yellow phosphor system. The phosphor used in this system is Y3Al5-O12∶Ce3+ and phosphors doped with Y3Al5O12∶Ce3+. However, the luminous efficiency of this phosphor is low. In addition, under high current, the electro-optical intensity of the blue light spectrum increases faster than that of yellow light. As the current changes, it will cause spectral mismatch, which can easily lead to changes in color temperature and low color rendering index. The above situation does not exist in the ultraviolet and near-ultraviolet systems. Since the electrical conversion efficiency of the ultraviolet conversion phosphor system is low, it is of great significance to study the near-ultraviolet conversion phosphor [3].

1 Characteristics of phosphors for single-matrix white light LEDs

Near-ultraviolet conversion phosphors can be divided into single-matrix white light phosphors and multiple-matrix white light phosphors. Among them, single-matrix white light phosphors can directly emit white light under the excitation of near-ultraviolet light. Compared with other system phosphors, they have significant characteristics: (1) Due to the insensitivity of vision to near-ultraviolet light, the color of this type of white light LED is determined by the phosphor, so the color is stable and the color reproduction is high; (2) Because it is a single-matrix compound, it can reduce energy loss and help improve luminous efficiency; (3) It can avoid color imbalance caused by the interaction between multiple matrix compounds, which is beneficial to improve color rendering; (4) Cost reduction. Therefore, single-matrix white light phosphors have attracted more and more attention in recent years and have become a research hotspot for the new generation of white light LED lighting. Research on this system material has also gradually deepened.

2 Research status of phosphors for single-matrix white light LEDs

In recent years, a large number of literature reports have been published on the research of single-matrix white-light phosphors, involving a wide range of matrix compounds, including silicates, phosphates, borates, vanadates, aluminates, etc. The main activating ions are Eu2+ and Ce3+, because their electronic configurations have d electrons exposed in the outer layer, which are easily affected by the matrix lattice environment and chemical bond properties, and the f→d transition absorption band and d→f transition emission band are easily broadened. In addition to Eu2+ and Ce3+, Mn2+, Eu3+, Dy3+, Tb3+, etc. are also often used as activating ions in single-matrix white-light fluorescent systems. Among them, the most common are single-matrix white-light phosphors co-doped with two ions, such as Eu2+-Mn2+, Ce3+ -Mn2+, etc.

2.1 Silicate phosphor

The silicate system has some outstanding characteristics, such as resistance to long-term bombardment of ultraviolet photons, stable performance, high light conversion efficiency, excellent crystallization and light transmittance, wide spectrum excitation band, and continuously adjustable emission spectrum. Therefore, silicate phosphor is considered to be a very promising phosphor material [4].

Kim et al. [5-9] synthesized a series of M3MgSi2O8∶Eu2+, Mn2+ (M=Ba, Sr, Ca) phosphors. In this system, M has three lattice positions, 12-coordinated M(Ⅰ) and 10-coordinated M(Ⅱ, Ⅲ). When Eu2+ replaces M(Ⅰ), it emits blue light, when it replaces M(Ⅱ, Ⅲ), it emits green light, and when Mn2+ replaces M(Ⅰ, Ⅱ, Ⅲ), it emits red light. When M=Ba, under the excitation of 375nm near-ultraviolet LED, the phosphor emits three colors of blue, green and red at 440nm, 505nm and 620nm at the same time, and matches with the ultraviolet LED to form white light with color coordinates of (0.38, 0.35). Figure 1 shows the excitation and emission spectra of Ba3MgSi2O8∶Eu2+, Mn2+. When M=Sr, the excitation spectrum broadens at 400nm and the emission is red-shifted. The color temperature and color rendering index of the emitted light can be changed by adjusting the concentration of Eu2+ and Mn2+. The best color temperature can reach 3600K and the color rendering index is 95. Moreover, the color scale is stable to the current, which is far better than the luminescence of the YAG∶Ce3+-InGaN system. When M=Ca, the emission peak continues to redshift. This is mainly due to the change in the crystal field caused by the decrease in the ion radius, which causes the emission peak to redshift. The addition of trace amounts of Al ions will significantly change the relative intensity of the blue and green light of the phosphor, while the intensity of the red light remains basically unchanged. Therefore, the color coordinate position of the phosphor can be adjusted by adding different amounts of aluminum ions [10].

The light-emitting InGaN tube core is made into a white light LED. When the forward drive current is 20mA, the color temperature is 5664K; the color coordinates are (0.33, 0.34), the color rendering index is 85, and the light intensity reaches 8100cd/m2.