Development of red phosphor for high efficiency and low light decay LED

Light Emitting Diode (LED) is an energy conversion device that can convert electrical energy into light energy. It has the advantages of low operating voltage, low power consumption, stable performance, long life, strong impact resistance, vibration resistance, light weight, small size, low cost, and fast luminous response. Therefore, it is widely used in display devices and short-distance, low-speed optical fiber communication light sources. In particular, the rapid development of blue, purple and ultraviolet LEDs in recent years has made it possible for LEDs to replace incandescent lamps and fluorescent lamps in the field of lighting.

There are two ways to produce white light LEDs: the first method is to combine red, green and blue LEDs to produce white light; the second method is to use LEDs to excite other luminescent materials to form white light, that is, to use blue LEDs with yellow fluorescent powders, or to use blue LEDs with green and red fluorescent powders, or to use purple or ultraviolet LEDs to excite red, green and blue fluorescent powders.

From the current development trend, the second method is far superior to the first method in terms of feasibility, practicality and commercialization, so it is critical to synthesize special fluorescent powders with good luminous properties. At present, the technology of using blue, purple or ultraviolet LEDs with phosphors to produce white light is relatively mature, but the red phosphors that can be used for LEDs either have low effective conversion efficiency or are unstable and have large light decay. Therefore, the development of high-efficiency and low-light-decay red phosphors for LEDs is becoming a hot spot for research and development by large companies and research institutions at home and abroad.

Our company has been developing phosphors for LEDs since the end of the last century, and has published many academic papers and applied for many invention patents. In addition, the relevant results have formed high-tech products and supplied them to many well-known LED manufacturers at home and abroad, generating good social and economic benefits. This year, based on the original work, we continued to study red phosphors for LEDs in depth and developed three series of high-efficiency and low-light-decay red phosphors.

1 Sulfide series red phosphors

This series of phosphors uses divalent europium as an activator and emits broadband emission with a peak wavelength greater than 600nm under the excitation of ultraviolet, purple and blue light. Figure 1 shows the emission spectrum of sulfide red phosphors at different europium contents. At different europium contents, the shape of the emission spectrum and the position of the emission peak are almost unchanged. However, the emission intensity first increases and then decreases with the increase of europium content, and the europium content is about 0.1% when the strongest emission occurs. Figure 2 shows the excitation spectra of these phosphors. As can be seen from the figure, these phosphors can be effectively excited at 350nm and above 400nm, and the shape of their excitation spectra does not differ significantly with the different europium contents, but the excitation intensity is significantly different. It can be seen that the content of divalent europium, an activator, has a significant effect on the luminous efficiency of the phosphor, but its content has no significant effect on the position and shape of the excitation and emission spectra.

This series of phosphors uses alkaline earth metal sulfides as the matrix, and different alkaline earth metal elements and their contents have different effects on the excitation and emission spectra of the phosphors. Figure 3 shows the emission spectrum of the phosphor when excited by 460nm blue light with different Ca and Sr ratios. With the increase of calcium content, the emission peak moves toward the long-wave direction, and the emission is significantly enhanced. Figure 4 shows the excitation spectrum of the phosphor with different Ca and Sr ratios. The excitation spectrum and the emission spectrum have similar changing trends: with the increase of calcium content, the excitation peak moves toward the long-wave direction, and the peak value is significantly enhanced. These changes expand the application range of the phosphor. According to the needs of different chips and applications, this series of phosphors with different excitation and emission peaks can be selected.

The biggest disadvantage of the sulfide series phosphors is that the properties are not stable enough and the light decay is large. The main reason is that during use, sulfur is easy to precipitate and divalent europium is easy to be oxidized. For this reason, during the preparation process, we conducted an experiment of adding auxiliary agents and carried out surface treatment experiments in the later stage of powder preparation. Through the addition of auxiliary agents and surface treatment, the deliquescence, oxidation and sulfur precipitation of the powder are effectively slowed down, and the stability of the phosphor is greatly improved.

2 Rare earth aluminum (gallium) salt deep red phosphor

The rare earth aluminum (gallium) salt phosphor activated by trivalent cerium is a phosphor that absorbs blue light and emits yellow light. It has been widely used in white light LEDs made of blue light excitation phosphors. Based on the development of rare earth aluminate (gallate) yellow phosphor activated by trivalent cerium, this year we further developed rare earth aluminate (gallate) deep red phosphor co-activated by rare earth and other transition metal elements, laying the foundation for the preparation of white light LEDs with low color temperature and higher color rendering, as well as the preparation of colorful color LEDs. Figure 5 shows the emission spectrum of the rare earth aluminate phosphor at different activator contents. The main emission peak of the aluminate phosphor is above 680nm, while the wavelength of the main emission peak of the gallate is longer, above 700nm. Under different activator contents, the shape of the emission spectrum and the position of the emission peak are almost unchanged. However, the emission intensity shows regular changes with different activator contents. Figure 6 shows the excitation spectrum of the rare earth aluminate phosphor corresponding to Figure 5. Under different activator contents, the shape of the excitation spectrum and the position of the excitation peak are also almost unchanged. However, the excitation intensity also shows regular changes with different activator contents. As can be seen from the figure, this series of phosphors can be effectively excited by blue-violet light in the range of 400-470nm and orange-red light in the range of 560-630nm. Therefore, the phosphor can be used to be effectively excited by two light sources, one is the blue light emitted by the blue LED chip, and the other is the red light emitted by the red LED chip. Under the excitation of the above two light sources, the phosphor emits deep red light. Due to the different luminous colors of the LED chip and the combination of phosphors with other luminous colors, LEDs with different luminous colors and different uses can be prepared. For example, a white light LED with good color rendering can be prepared by using a blue LED chip with green phosphor and the deep red phosphor. Using orange-red LED to excite the phosphor can also be used to manufacture some special monitoring equipment.

3 Alkaline earth and transition metal composite oxide red phosphors

The stability of sulfide red phosphors needs to be further improved, and rare earth aluminum (gallium) salt red phosphors cannot be effectively excited by purple and ultraviolet light. To this end, we have developed a new type of alkaline earth and transition metal composite oxide red phosphor. This series of phosphors can be effectively excited by ultraviolet, violet and blue light. This series of phosphors uses trivalent europium as an activator and emits the characteristic red emission of trivalent europium under the action of the excitation source. Figure 7 shows the excitation spectra of alkaline earth and transition metal composite oxide red phosphors at different europium contents. These phosphors all show strong excitation peaks at around 362nm, 382nm, 394nm, 416nm and 464nm. These wavelengths just cover the emission regions of ultraviolet, violet and blue LEDs, so this series of phosphors can be used in semiconductor lighting devices excited by ultraviolet, violet and blue LEDs. Under different europium contents, the shape of the phosphor excitation spectrum and the position of the excitation peak hardly change. However, the intensity of the excitation peak shows regular changes with different europium contents. Figures 8-10 show the emission spectra of these phosphors under the excitation of different excitation wavelengths. As can be seen from the figure, these phosphors all emit characteristic spectral lines of trivalent europium such as 612nm and 616nm under different excitation wavelengths, and are very good red emitting phosphors. With different europium content, the shape of its emission spectrum has no obvious difference, but the emission intensity is different. It can be seen that the alkaline earth and transition metal composite oxides activated by trivalent europium are suitable for ultraviolet, purple and blue light excitation, and are very good red phosphors. Because it is activated by stable trivalent europium and the matrix is ​​a stable oxide system, this series of phosphors has good stability and small light decay.

4 Conclusion

With the breakthrough of GaN preparation technology, the emergence and discovery of blue, purple and ultraviolet LEDs have increasingly attracted the attention of governments, research institutions and multinational companies. From the current trend, development is inseparable from the progress of phosphor technology. Red phosphor is the bottleneck of LED phosphors. The three phosphors developed in this article will strongly promote the development of red phosphors and lay the foundation for the development of lower color temperature, higher color rendering and higher light efficiency. Welcome relevant device research and production units to join us to jointly promote the progress of the development of red phosphors.