Spectral Tuning And Optical Properties Of White Leds For Different Lighting Applications

White Light-emitting diodes?wLED?have increasingly attracted great attentions in a variety of illumination fields.The method where a yellow phosphor is pumped by ae single-chip has become the packaging mainstream for white light illumination due to its simple structure and low cost.A rapid development is seen for wLEDs,which is not only originated from their high efficiecny adn environment friendly,but also from their tunbale emission spectra.As excellent optical properties of wLEDs for general illumination are usually purpsuited,it is thus a hot topic to investigate the light-color regulation of wLEDs.In this study,a spectral control model of phosphor-converted wLEDs?pc-wLED?based on Monte Carlo algorithm was establihsed,with aims to provide guidelines for selecting appropriate phosphors and designing the phosphor combinations.This model is typically applied to wLEDs for road lighting and plant growth lighting,as well as to those with higher R9 for lighting in medical surgeries and supermarkets.The detailed investigations are given as below.1.WLEDs for road lighting are required to have both the high scotopic to photopic ratio?S/P?and color rendering index?CRI?.However,there is a trade-off between S/P and CRI.WLEDs commonly having an S/P of 1.68–2.38 usually exhibit a low CRI.In this work,to provide a best solution to the trade-off problem we proposed an optimal spectral model for pc-wLEDs to figure out an optimal phosphor combination.The Monte Carlo Algorithm was adopted to simulate the optimized CRI and S/P by varying the spectral power distributions of wLEDs through adjusting the spectral parameters.Considering the spectral requirements of pc-wLEDs based on the mesopic vision,we chosed CaAlSiN₃:Eu²⁺and Y₃?Ga,Al?₅O₁₂:Ce³⁺as the red and green-emitting phosphors to prepare wLEDs with both high S/P and CRI,respectively.The simulation based on the optimal spectral model led to an optimal pc-wLED with a high S/P of 2.0–2.14,Ra>80 and CCT of 4000-5000K,which matches very well with the experimental results of S/P=2.064,Ra=93.9 and CCT=4981 K for the two-phosphor converted wLEDs.It implies that the optimal spectral model would be used effectively for the spectral design and phosphor selection for wLEDs.2.The Sr_xCa_1-x-x AlSiN₃:Eu²⁺?x=0.1-0.9?phosphor was synthesized by the high temperature solid phase method.We chosed the phosphor combination of Sr_xCa_1-x-x AlSiN₃:Eu²⁺and Y₃?Ga,Al?₅O₁₂:Ce³⁺,and fabcaited wLEDs by pumping the phosphor blend by the blue LED chip to balance the trade-off relationship between color rendering index?Ra&R9?and luminance efficiency?LER?for lighting in medical surgeries and supermarket where excellent optical quality?especially R9?is required.In this work,we proposed an optimal strategy by combining the red Sr_xCa_1-x-x AlSiN₃:Eu²⁺phosphor with the commercial green phosphor(YAGG:Ce³⁺)by using the optimal spectral model for pc-wLEDs to overcome the trade-off mentioned above.We found that the Sr²⁺doping amount in Sr_xCa_1-x-x AlSiN₃:Eu²⁺?x=0.1-0.9?played a key role in controlling LER and CRIs?Ra and R9?.By using the optimal spectral model for pc-wLEDs,we selected the Sr_xCa_1-xAlSiN₃:Eu²⁺?x=0.8?phosphor,and prepared an ideal wLED by using Sr_xCa_1-xAlSiN₃:Eu²⁺?x=0.8?and the commercial green phosphor.The wLED showed a super-high color rendering index?Ra=94.6,R9=88?and also a high luminous efficacy of¹24 lmW^-1.3.In order to clarify the influence of the Sr substitution on spectral characteristics of Sr_xCa_1-x-x AlSiN₃:Eu²⁺,we calculated electronic structure of Sr_xCa_1-x-x AlSiN₃:Eu²⁺.The2*2*2 supercell model was established for the calculation.The results implied that the spectral tuning of Sr_xCa_1-x-x AlSiN₃:Eu²⁺phosphors could be attributed to two reasons.One is that the band gap of the phosphor was increased with increasing the Sr amount from x=0 to x=0.2.The other one is the Eu-N bond length and the crystal volume were increased with the Sr substitution.4.Adual-chip wLED packaging scheme was proposed in order to improve the color rendering indices?R9&R12?which represent saturated red and blue color in the wLED spectrum for plant lighting.The packaging data indicated the effectiveness of the dual chip to enhance the color rendering performance?R9 and R12?of wLED.In addition,to further explore the high light-color performance wLED for plant growth,the dual-chip pc-wLED spectral model was established.We adopted the phosphor combination of Sr_xCa_1-x-x AlSiN₃:Eu²⁺?x=0.2?and Y₃?Ga,Al?₅O₁₂:Ce³⁺to simulate the wLED spectrum.As a result,the wLED with a high color rendering index?Ra=94?and a high red color saturation?R9=96.2?was obtained.