In 1996, Nichia's Nakamura and others first used blue LEDs combined with yellow phosphors to convert white LEDs. The yellow phosphor used by him is Y3Al5O12:Ce3+ (YAG:Ce3+). This phosphor has strong broadband absorption near the 470nm band, and then excites yellow light near 540nm. The blue light emitted by the LED itself and the phosphor are excited. The combination of yellow light forms white light. Later, YAG: Ce3+ became the mainstream technology of white light because of its high conversion efficiency, good thermal stability and wide excitation band.
Trend 1: Development of new phosphors
YAG: Ce3+ is one of the first phosphors widely used in white LED technology, but due to its low red component in the emission spectrum, it is difficult to obtain white LEDs with higher color rendering index and low color temperature. On the other hand, semiconductor lighting The continued development has led to the development of phosphors with higher conversion efficiencies. In the early days, high color rendering index and low color temperature were achieved by adding (Ca,Sr)S:Eu2+, (Ca,Sr)Ga2S4:Eu2+ red-green phosphor to YAG:Ce3+, but due to such alkaline earth metal sulfides The physicochemical properties are unstable, deliquescent, volatile and corrosive, and cannot meet the needs of the LED lighting industry. Recently, a red phosphor with excellent thermal stability and chemical stability has been developed, which can completely replace alkaline earth metal sulfide to achieve a high color rendering index, low color temperature white LED, which has a silicon nitride (oxygen) tetrahedral structure. Known as nitrogen oxides, it has higher excitation efficiency.
At present, foreign companies are mature in the technology of LED phosphors and hold most important patents. They occupied the LED market through the control of phosphor patents. The patent for YAG:Ce3+ phosphors is mainly owned by Nichia [US 5998925], and Osram is the patent for phosphors of Tb3Al5O12: Ce3+ [US 6812500, 6060861, 65276930], TG , LWB and Tridonic hold the patent of Eu2+(SrBaCa)2SiO4Si:Al,B,P,Ge.. [US 6809347], Intematix holds the patent of Eu2+(SrBaMg)2SiO4O:F,Cl,N,S.. [US 20060027781, 200627785, 200628122]. In contrast, domestic research on phosphors for LEDs is mostly concentrated in research institutes, mainly on the synthesis of existing phosphor materials and the study of physical properties such as luminescence, but not enough in the development of industrial technology.
Trend 2: The establishment and development of white LED optical model
The application of phosphors to white LEDs depends on the specific needs of the LEDs, such as the particle size of the phosphors. The research on phosphors mainly focuses on the effects of the optical properties of phosphors on the performance of white LED packages, such as light extraction efficiency [1-3], color space distribution [4, 5] and light color quality [6, 7]. In these studies, the Monte Carlo ray tracing method was used to simulate the optical properties of the LED package structure using optical software, and the phosphor layer was processed into a Mie scattering material, so that the excitation and emission characteristics of the white LED could be obtained by optical simulation, but The simulation did not take into account the specific scattering characteristics of the phosphor and lacked experimental verification. Narendran [8], Zhu [9], Kang [10] and Z. Liu [11] studied the optical properties of phosphors in detail through experiments.
Trend 3: New phosphor coating method
The traditional phosphor coating method is a powder-powder mode, in which a mixture of a phosphor and a colloid is filled into a chip holder cup and then heat-cured. The amount of phosphor in this coating mode is difficult to control, and the white LED is prone to light color unevenness such as yellow spots or blue spots due to different excitation light. Philips Lumileds has proposed a conformal coated phosphor coating method that covers a flip-chip LED chip with a uniform thickness of phosphor film to improve the color stability of white LEDs. There are also companies that use a method of depositing a layer of phosphor on the surface of the chip to achieve excitation. These coating methods are all in contact with the phosphor. The optical simulation results of H. Luo et al. show that the near-field excitation method of the phosphor in contact with the chip increases the backscattering loss of the excitation light and reduces the light extraction efficiency of the device [1]. Australia's Sommer used a numerical simulation to simulate Philips Lumileds' phosphor conformal coating structure. The results show that this coating method does not provide better angular uniformity [4]. With the deepening of the optical simulation of white LEDs, the far-field excitation scheme of phosphors shows more advantages.
Trend 4: High current injection and heat dissipation structure
In order to meet the demand for high-light flux of general illumination, people have increased the driving power of a single chip. In the past, a 1W high-power chip was injected to 3W, 5W, or even higher. This makes the thermal problem of white LEDs more and more serious. People use various heat dissipation technologies, such as heat pipes, micro heat pipes [12], water cooling, air cooling and other methods to heat the LEDs. (Unfinished, please refer to the "Semiconductor Lighting" magazine 2010 total ninth edition editor: maysoong)
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