Nichia's Nakamura et al. first synthesized a white LED using blue-light LED combined with yellow phosphor. The yellow phosphor used by him is Y3Al5O12:Ce3+ (YAG:Ce3+), which has strong broadband absorption near 470nm. Then, yellow light near 540 nm is excited, and the blue light emitted by the LED itself combines with the yellow light excited by the phosphor to form 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.
Going one: 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 YAG:Ce3+ phosphor patent is mainly owned by Nichia [US5998925], and Osram is the Tb3Al5O12: Ce3+ phosphor patent [US 6812500, 6060861, 65267930], TG, LWB and Tridonic hold the patent of Eu2+(SrBaCa)2SiO4Si:Al,B,P,Ge [US6809347], Intematix holds the patent of Eu2+(SrBaMg)2SiO4O:F, Cl,N,S [US20060027781, 200627785, 200628122 In contrast, domestic research on phosphors for LEDs is mostly concentrated in research institutes, mainly on the research of physical properties such as synthesis and luminescence of existing phosphor materials, but not enough in the development of industrial technology.
Going two: 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 influence of the optical properties of phosphors on the performance of white LED package, such as light extraction efficiency, color space distribution and light color quality. 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.
Going three: 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 coats the surface of 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 and other researchers 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. 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. With the deepening of the optical simulation of white LEDs, the far-field excitation scheme of phosphors shows more advantages.
Going four: 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, water cooling, air cooling, etc. to heat the LEDs.
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