High color rendering and high-luminance laser lighting using all inorganic nitride phosphor films

Abstract

Despite the huge advances that have been made in the development of ultra-high luminance laser lighting, achieving high color rendering properties in such systems at the same time remains a challenge. Recent studies show that in most cases, the luminous efficacy (LE) of laser lighting is compromised to improve the color rendering index (CRI). In this study, a possible solution to this problem has been proposed by preparing phosphor-in-glass (PiG) films comprised of the yellow-emitting phosphor (LSN:Ce³⁺) and the red-emitting phosphor (CASN:Eu²⁺). The composite material synthesized in this study exhibited outstanding optical and thermal properties. A uniform white light with a high CRI of 80.0 and a high LE of 185.9 lm W^-1 was achieved by optimizing the yellow/red ratio and the emission peak position of the blue laser. Furthermore, it was found that this design enabled the phosphor to restrict the light emission area effectively, thus attaining a high luminous exitance of 1302 lm mm^-2. With their superior optical performance, the PiG films can be regarded as promising color converter candidates for future high-quality laser-based white light sources.

Fig. 1. (a) XRD patterns of LSN:Ce phosphor, CASN:Ce phosphor, PiG-F-5/1∼PiG-F-50/1, and the corundum substrate. SEM graphs of the commercial (b) LSN:Ce and (c) CASN:Eu pristine phosphors.

Fig. 2. (a) SEM images of the LSN&CASN PiG-F samples. (b) Cross-sectional SEM images of LSN&CASN PiG-F-5/1 samples. (c) SEM image showing points 1, 2 and 3 chosen for the elemental analysis. (d–f) EDS spectral analysis at points 1, 2 and 3 as indicated in (c).

Fig. 3. (a) The SEM images of powder particles and (b–i) the related EDS mapping results of the PiG-F-5/1 sample surface.

Fig. 4. (a) PLE spectra of PiG-F-5/1, PL spectra of (b) LSN:Ce PiG-F, LSN:Ce pristine powder and (c) CASN:Eu PiG-F, CASN:Eu pristine powder. (d) The PL spectra of various samples pumped by the 441 nm and (e) 462 nm laser sources, respectively. (f) The obtained CRI and CCT values for different samples pumped by the 441 nm and 462 nm laser sources with various yellow to red phosphors ratios. (g) The EL spectra and optical properties for the LSN:Ce PiG-F pumped by the 441 nm laser source. (h) The EL spectra and optical properties for the PiG-F-5/1 sample pumped by the 462 nm laser source. (i) From points 1 to 3: CIE color coordinates for the LSN PiG-F pumped by 441 nm and the PiG-F-5/1 sample pumped by 441 nm and 462 nm laser sources, respectively.

Fig. 5. The variation in the values of (a) LF and (b) LE as a function of laser power density for different samples pumped by the 462 nm laser source. (c) Temperature-dependent relative integrated emission intensity for LSN:Ce, CASN:Eu and PiG-F. The variation in the values of (d) LF and (e) LE as a function of the laser power density for different samples pumped by the 441 nm laser source. (f) Infrared thermal image of PiG-F-25/1 under laser illumination with a power density of 22.0 W mm⁻².

Fig. 6. (a) Spot size and (b) spot expansion for the PiG-F-5/1, PiG-F-10/1 and PiG-F-50/1 samples as a function of the laser spot size. (c) Luminous exitance for the PiG-F sample as a function of the laser power density under 462 nm laser excitation (the points are experimental data, and the corresponding line is linear regression).