Study On Preparation And Properties Of Remote Spectral Conversion Materials For White Light Emitting Diodes

White light emitting diodes(white LED) used as white light sources have attracted more attention owing to their many advantages such as low energy comsumption, long lifetime and environment friendly. Phosphor converted LED(pc-LED) has been the most commonly used technology for white LED lighting applications due to its high efficiency and low cost. In a pc-LED, a film of one or more phosphors is placed on the surface of a blue LED chip, absorbing part of the blue light emitted from the LED chip and reemitting light of longer wavelengths. The white light needed for lighting applications is produced from the mixture of the reemitted light and direct blue light. The traditional packaging method is spraying the mixture of phosphor and transparent silicone on the surface of LED chips. This inevitably suffers from scattering and reabsorption effects. Aiming at the above state, remote spectral conversion materials which included remote phosphor film and phosphor glass-ceramic were prepared. Considering that the color rendering index(CRI) Ra of single phosphor film is low and cannot meet increasing demand of high CRI for LED lighting applications, the remote bilayer and trilayer phosphor films with a new configuration were designed. The laws of spectral conversion from blue LED to white LED were discussed systematically. The relative results are as follows.① The laws of spectral conversion from blue LED to white LED are analyzed by packaging the InGaN LED chips with the remote single phosphor films which include YAG:Ce3+ yellow phosphor film, CaS:Eu2+ red phosphor film and(SrBa)2SiO4:Eu2+ green phosphor film, respectively. Photoluminescence spectrum is composed of directly transmitted spectrum and emission spectrum. The peak of the transmitted spectrum is 455 nm, the peaks of the emission spectra for the yellow, red and green phosphor films are 545 nm、650 nm and 521 nm, respectively. Furthermore, the intensities of emission peaks(yellow peak, red peak or green peak) increase with an increasing amount of phosphor in the films. The peak wavelengths and full widths at half maximum(FWHM) of the transmitted peaks and emission peaks of the films remain unchanged as the phosphor concentrations change. The mathematical models for the three single phosphor films are proposed to describe the relationship between the optical properties of the phosphor/silicone composite films and their compositions when the incident light is from blue LED chips. The simulated spectra based on the mathematical model are found in good agreement with the measured experimentally.② Photoluminescence spectra of remote yellow and red phosphor bilayer films can be separated into three parts, that is, transmitted blue spectrum with a peak at 455 nm and emitted yellow spectrum with a peak at 545 nm as well as emitted red spectrum with a peak at 650 nm. For the Y-R configuration, when the yellow phosphor concentration is fixed and the red phosphor concentration increases, the intensities of blue peak and yellow peak decrease, and the intensity of red peak increases. When the red phosphor concentration is fixed and the yellow phosphor concentration increases, the blue peak intensity decreases, yellow peak intensity increases, and red peak intensity keeps the same. The laws of spectral conversion from blue LED to white LED of R-Y configuration are similar with that of Y-R configuration.③ Compared of the spectra of Y-R configuration and R-Y configuration, the blue peak intensity remains almost unchanged, the yellow peak intensity from Y-R configuration is higher than that from R-Y configuration, and the red peak intensity from R-Y configuration is higher than that from Y-R configuration. Furthermore, for any phosphor concentration, the luminous flux of Y-R configuration is bigger than that of R-Y configuration. The Ra value increases firstly and then decreases as the red phosphor concentration increases. The biggest Ra value is 90.9 of 40 wt.% yellow phosphor with 25 wt.% red phosphor for Y-R configuration. Compared with no red phosphor film, the Ra value improves 13.9%. Comprehensive consideration of both luminous flux and Ra value, the better options of bilayer structured phosphor films is 40 wt.% yellow phosphor with 15 wt.% red phosphor for Y-R configuration, or 40 wt.% yellow phosphor with 10 wt.% red phosphor for R-Y configuration.④ Photoluminescence spectra of remote green and red phosphor bilayer films can be separated into three parts, that is, transmitted blue spectrum with a peak at 455 nm and emitted green spectrum with a peak at 525 nm as well as emitted red spectrum with a peak at 650 nm. For the G-R configuration, when the green phosphor concentration is fixed and the red phosphor concentration increases, the intensities of blue peak and green peak decrease, and the intensity of red peak increases. When the red phosphor concentration is fixed and the green phosphor concentration increases, the blue peak intensity decreases, green peak intensity increases, and red peak intensity keeps the same. The laws of spectral conversion from blue LED to white LED of R-G configuration are similar with that of G-R configuration.⑤ Compared of the spectra of G-R configuration and R-G configuration, the blue peak intensity remains almost unchanged, the green peak intensity from G-R configuration is higher than that from R-G configuration, and the red peak intensity from R-G configuration is higher than that from G-R configuration. Furthermore, for any phosphor concentration, the luminous flux of G-R configuration is bigger than that of R-G configuration. The Ra value increases firstly and then decreases as the red phosphor concentration increases. The biggest Ra value is 89.4 of 40 wt.% green phosphor with 15 wt.% red phosphor for R-G configuration. Compared with no red phosphor film, the Ra value improves 66.5%. Comprehensive consideration of both luminous flux and Ra value, the better options of bilayer structured phosphor films is 40 wt.% green phosphor with 20 wt.% red phosphor for G-R configuration, or 40 wt.% green phosphor with 15 wt.% red phosphor for R-G configuration.⑥ Photoluminescence spectra of remote trilayer films can be separated into three parts, that is, transmitted blue spectrum with a peak at 455 nm and emitted yellow-green spectrum with a peak at 535 nm as well as emitted red spectrum with a peak at 650 nm. When the green and yellow phosphor concentration are fixed and the red phosphor concentration increases, the intensities of blue peak and yellow-green peak decrease, and the intensity of red peak increases. When the green or yellow phosphor concentration increases, the blue and red peaks intensities decrease, and yellow-green peak intensity increases. Comprehensive consideration of both luminous flux and Ra value, the better options of trilayer structured phosphor films is 20 wt.% green phosphor, 25 wt.% yellow phosphor and 20 wt.% red phosphor. The luminous flux is 27.8 lm and the Ra value is 91.9.⑦ YAG:Ce3+ phosphor glass-ceramic is used for high-power white LED. The glass as adhesive is in the gap between the phosphor particles. Photoluminescence spectra are composed of transmitted spectra with the peak at 450 nm and emitted spectra with the peak at 545 nm. The transmitted spectrum intensity increases and emitted spectrum decreases with the glass concentration increases. The thickness has less influence on the structure, morphology and optical properties of YAG:Ce3+ phosphor glass-ceramic.