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Abstract
Phosphor-converted blue laser diodes are regarded as the next-generation high-brightness solid-state lighting sources. However, it is difficult to obtain white light with high angular color uniformity due to the Gaussian distribution of the laser light sources. Meanwhile, laser excitation power density of the light source is high, which would bring serious heating effects to the phosphor layers. In this study, a strategy has been proposed to solve the problem by using remote sediment phosphor plates. In detail, we have compared the effects of remote sediment/non-sediment phosphor plates to the phosphor-converted blue laser diodes on the overall light output characteristics, angular optical distribution properties, as well as their thermal performance. The emission from sediment phosphor samples has been found more divergent, and angular deviation in the correlated color temperature of the emitted light could be greatly reduced from 1486 to 294 K, yet with only 5% luminous flux loss, as compared to non-sediment phosphor samples. Most importantly, the sediment phosphor sample pushes the power damage threshold up to 588.1 W/cm2 (non-sediment sample: 512.3 W/cm2). Our work has demonstrated the sediment phosphor plates would ameliorate the angular color uniformity for the laser-based lighting source, while extending its lifespan with improved thermal stability.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Thanks to low cost, high efficiency, high reliability and environmental friendliness, solid state lighting sources have been widely used [1–6]. At present, the most conventional strategy is to use a InGaN-based blue light-emitting diode (LED) to excite yellow phosphors, generating yellow light mixed with the remaining blue light to produce white light [7–10]. However, LEDs are always suffering severe efficiency droop effect in the circumstances of high injection current densities. As an alternative, the use of blue laser diodes (LD) as the excitation source has been booming for high power white light sources most recently, which raises the threshold of the occurrence of “efficiency droop” with a lower beam divergence [11,12]. Hence, blue laser lighting as an emerging technology holds huge potential for widespread applications in solid-state lighting. In commercial phosphor-converted light emitting diodes (PC-LED) products, phosphors are usually uniformly dispersed in silicone [13–15]. However, the density of phosphor particles (4800kg/m3) is greater than that of silicone (1120kg/m3) [16], whereby the phosphor particles will slowly settle down over time because of gravity. In this regard, there have been a few reports on the phosphor sedimentation. For instance, Lee et al. have studied the influence of phosphor sedimentation on the white LEDs with different chip configurations, and the difference in luminous efficacy can reach up to 19% for vertical chips [17]. Hu et al. have conducted experiments based on the packaging process to study the effects of phosphor sedimentation on light extraction efficiency, correlated color temperature (CCT), and angular color uniformity (ACU), by which they have found the effect of phosphor sedimentation on optical performance can be ignored during the packaging process [18]. Yu et al. have studied the effect of dynamic phosphor sedimentation on the optical performance of PC-LEDs and found that CCT is more sensitive outside the LED chips [19]. Kim et al. found that the phosphor sedimentation samples exhibited better optical performance and hydrothermal reliability than the phosphor dispersed samples [20]. Phosphor particles are a kind of photoluminescent materials, and strong light scattering occurs when the light of incidence enters the phosphor layer [21,22]. One efficient method to improve the ACU involves optimization of patterned phosphor layers [23, 24] and nanoparticle-mixed phosphors [25,26]. In addition, the effect of phosphor sedimentation on the luminescence stability of the LED has been studied under varying temperatures, and it is found that the color coordinates of phosphor sedimentation LEDs are more stable and have less blue shift compared with phosphor non-sedimentation LEDs [27]. As it will cause almost 60% of the light to be reflected back to the chip by coating phosphors directly on the LEDs, which would increase light loss and heat [28–30]. Up now, the optical and thermal properties of the remote sediment/non-sediment (S/non-S) phosphor layers on the emerging high-power laser lighting have not been studied in depth.
In this study, we fabricated the S/non-S PC-LD samples. The optical performance, including spectrum, luminous flux, phosphor conversion efficiency, CCT and color coordinates were measured. Also, the angle-dependent illuminance, irradiance, CCT and color coordinates were measured to compare the spatial photochromic performance. At last, the thermal performance of phosphor plates was tested as excited over a wide range of high laser power densities by using a IR camera.
2. Experiment
In order to investigate the influence of remote S/non-S phosphor plates on the optical and thermal properties, remote phosphor plates with a 20wt% concentration were prepared, which included both S/non-S samples. For non-S samples, phosphors (YAG:Ce3+) and silicone matrix (Dow Corning) were uniformly mixed to form phosphor gel, which was loaded into a defoaming vacuum machine to remove bubbles generated during the stirring process. To shape the remote phosphor gel, a dam with a diameter of 2 cm and a height of 1 mm was enclosed on the quartz glass using a dispenser. Afterwards, the phosphor gel was filled into the dam. The samples were then put into an electro-thermal blowing dry box and cured at 150°C for 30 minutes. For S samples, the phosphor gel was dispersed on the quartz glass for 2 hours, and then the phosphor was settled down by gravity over a long period of time. The schematic diagram depicting the manufacturing process of the remote phosphors is shown in Fig. 1(a). Figure 1(c) and (e) shows the distribution of phosphor particles on the bottom surface near the quartz glass for the non-S sample and the S sample, respectively. It can be seen that the phosphor particles are denser for the S sample, as they have settled down over time. Figure 1(d) and (f) present cross-sectional view optical images of the non-S sample and the S sample, respectively, which reaffirms that the phosphor particles have settled down to the quartz glass. The laser excitation source used in the experiment was a commercial Nichia TO-56 packaged laser with a peak wavelength of 450nm, and was mounted onto a thermoelectric cooling (TEC) controller (ITC4005, Thorlabs, USA). Then an reflector cup was mounted on the LD. After that, the remote phosphor plate was fixed on the reflector cup. The reflector cup is conducive to the light extraction of the white light source. The transmittance and reflectance spectra of the remote-phosphors were collected by a UV-vis spectrophotometer in an integrating sphere (TU-1950, Persee, China). The optical power of the blue LD as driven under different currents were probed by a laser power meter (S142C, Thorlabs, USA). The light intensity distribution was collected with a beamprofiler (Unice e-Pro, China). The optical performance of the LD white light source with remote phosphor plates were measured in the integrating sphere by a photoelectric analysis system (LHS-1000, Everfine, China). Angular illuminance, irradiance and correlated color temperature (CCT) were recorded with a spectral flickering irradiance meter (SFIM-300, Everfine, China). Here, the angular illuminance or irradiance were measured by fixing the light source on the motorized rotation stage, while a spectral flickering irradiance meter is mounted on the fixed stage. By tuning the angle of the rotating stage precisely, so as to change the emitting direction of the light source, the illuminance and irradiance at different angles can be obtained. An infrared thermal imager (T620, Flir, USA) was used to measure the surface temperature of the remote phosphor plates as excited by the blue laser. The test configuration used in the experiment is shown in Fig. 1(b).
Fig. 1. (a) Fabrication process of phosphor plates, (b) the measurement setup for the laser lighting. Optical images from the bird view (c) and (e), and the cross-sectional view (d) and (f) of the phosphor distribution for the non-S/S samples, respectively.
3. Results and discussion
The blue LD is mounted onto a TEC to control its operating temperature and enable a stable laser output. Figure 2(a) shows the light output power and the operating voltage of the LD as a function of current injection. The current is tuned from 0.45A to 3.25A with an interval of 0.05A, and the highest light output is about 4.2W at the injection current of 3.25A. The normalized laser emission spectrum is shown in Fig. 2(b). The laser peak wavelength is 450nm, and the full width at half maximum (FWHM) of the laser spectrum is about 3nm. The laser spot is elliptical, which is out of its TE-mode dominant far-field Gaussian distribution, with a major axis of approximately 1.2 mm and the minor axis length of 0.35 mm, respectively, at the plane where the phosphor plates are placed.
Fig. 2. (a) The power-current-voltage (P-I-V) curves of blue LD, (b) the luminescence spectra of the blue LD. The inset picture shows the laser spot of the blue LD where the remote phosphor plates are located.
The typical normalized emission spectra of the S/non-S remote phosphor plates under laser excitation are shown in Fig. 3(a). It is evident that the remaining blue light for the non-S remote phosphor plate is more. In specific, at an injection current of 1A, the remaining blue light power of non-S sample is 1.73 times that of S sample. As shown in Fig. 3(b), the total luminous flux of the S/non-S sample shows a continuous rise as the laser power density increasing from 0 to 350 W/cm2, which is caused by the increase in the number of electrons pumped to the excited state of Ce3+ [31]. By contrast, the S sample exhibits a 5% reduction at 320.2 W/cm2 compared to the non-S sample. It is attributed to the fact that the phosphor concentration is higher close to the quartz glass, and the reflectivity of the S sample is higher than that of the non-S sample (see, Fig. 4(a)). Meanwhile, the increase in backscattering will increase the re-absorption of light by remote phosphor plates and the wall of the reflector, which would induce light output loss. Therefore, although phosphor sedimentation is beneficial to increase the blue light absorption, the optical backscattering at the same wt% will also increase correspondingly, which slightly reduces the overall luminous flux eventually.
Fig. 3. (a) Emission spectra of S/non-S samples, (b) luminous flux, (c) phosphor conversion efficiency and (d) CCT of the remote phosphor plates as a function of the incident blue laser power density. (e) CIE color coordinates of the remote phosphor plates under 309.2W/cm2 blue laser excitation. The insets are the corresponding luminescent images of the S/non-S PC-WLDs.
Figure 4(a) shows the transmittance/reflectance spectra of the S/non-S phosphor samples over the wavelength range of 380∼800nm. The absorption at 450nm is observed in the transmittance spectra, which can be attributed to the 4f→5d transition of Ce3+ in YAG [5]. As compared to the corresponding non-S sample, the transmittance of the S sample is lower than that of the non-S sample (∼5.93% at 550nm), which confirms the probability of the multiple light scattering effect of the phosphor particles increases due to the phosphor sedimentation [32, 33]. Obviously, the reflectance of the S sample is higher than that of the non-S sample, which can also be mainly attributed to the larger scattering coefficient of the S sample [34], which is beneficial to obtain a stronger reflection. At a wavelength of 550nm, the reflectance of the remote S/non-S phosphor plates are 45.9% and 44.1%, respectively. This would explain the origin of power loss for S samples as excited by the blue laser. Figure 4(b) shows the absorptance spectra of S/non-S remote phosphor samples. In the range of 360-700 nm, the absorptance of S/non-S samples are almost identical. Figure 3(c) illustrates the conversion efficiency of S/non-S remote phosphor samples as a function of laser power density. Here, the conversion efficiency is defined as the ratio of the converted yellow light to absorbed blue light. Here it should be pointed out that, due to the concentration of phosphor on the surface of the S phosphor plate near the light source increases, which not only enhances light scattering, but also creates a refractive index difference. The phosphor non-S sample reveals approximately 14.9% higher conversion efficiency at laser power density of 320.2 Wcm-2 as compared to the S sample.
Fig. 4. (a) Transmittance and reflectance spectra of S/non-S remote phosphor samples, (b) absorptance spectra of S/non-S remote phosphor samples
To investigate the color performance of the PC-WLD, the total CCT of remote phosphor plates were measured by increasing the laser power density. As plotted in Fig. 3(d), the CCT shows a slight reduction trend with the increase of laser power density. It is easy to find that the CCT of the S PC-WLD (∼3877K) is slightly lower than that of the non-S PC-WLD (∼3972K). The remaining blue light of the S remote phosphor plate is less, which thus reduces the CCT (Fig. 3(a)). The calculated CIE color coordinates of the S and non-S PC-WLD are (0.4251, 0.5098) and (0.4140, 0.4915) at laser power density of 320.2 Wcm-2, respectively. The inset in Fig. 3(e) gives the corresponding luminescent photographs of S/non-S PC-WLDs.
Relying on the spectrum, luminous flux, phosphor conversion efficiency, CCT and color coordinates, it still may be not straightforward enough to evaluate the optical performance of the remote phosphor plates vividly by using the integrating sphere. Figure 5(a) shows the contour images corresponding to the far-field light intensity distribution at 320.2W/cm2 to investigate the light scattering characteristics for both S/non-S remote phosphor plates. All the counter images show a near Gaussian emission profile (see, Fig. 2(b)). The light spot for the S sample has a clear boundary but with a low central intensity. By contrast, the emission for the non-S sample is more centralized and its boundary is broader and indistinct. As phosphor particles settle down to the quartz glass, the concentration of the phosphor declines from the bottom up. The improved emission uniformity and reduced luminous exitance observed for S samples in this work should be attributed to mutual impact of the scattering effect and gradient refractive index [24,35].
Fig. 5. (a) The light speckles for the S/non-S phosphor plates as excited by the blue LD, (b) schematic diagram depicting optical paths in S (left) and non-S samples (right). Angular-dependent (c) irradiance, (d) CCT and (e)-(f) color coordinates of the S/non-S PC-WLD.
Figure 5(c) plots the angle-dependent illuminance and irradiance distributions from 0° to 90° for S/non-S samples under 320.2 W/cm2 laser excitation. At a normal incidence, the non-S sample reveals higher illuminance (1567 lx) than the S sample (1490 lx). Meanwhile, the non-S sample exhibits higher irradiance (4.5728 W/m2) versus the S sample (3.6686 W/m2). When the angle increases from 0° to 90°, the deviation illuminance of S/non-S sample is 1424.6 lx and 1508.2 lx, respectively, revealing 5.87% lower for the S sample. The angular CCT distributions for the S/non-S samples are shown in Fig. 5(d). Great fluctuation can be observed in the angular CCT distribution of the non-S sample, whereas the S sample are more stable, suggesting the angular color uniformity of the latter is better. In detail, the angular CCT of the non-S sample exhibits a severe drop from 5436K to 3950K (ΔCCT=1486K), which is much larger than the S sample dropping from 4210K to 3916K (ΔCCT=294K). This can be explained by the following reasons: (1) the blue LD is a Gaussian source, the blue light in the center is much stronger, especially for the non-S sample. Therefore, the blue light intensity would decrease sharply from the center to the periphery; (2) the S phosphor plate can better scatter the blue light from the normal direction to the periphery, leading to the severer CCT reduction at the center. This effect has been strengthened by a fact that the remaining blue light for the non-S remote phosphor plate is more, as can be observed from the emission spectra in Fig. 3(a), which further reduces the CCT values both at the normal direction and periphery of the light spot. Additionally, the CCT slightly rises in the viewing range of 75°-90°, attributed to the leakage of blue LD emission in the large angles from the side of the quartz glass substrate into the air without passing through the phosphor layer.
The angle-dependent CIE 1931 color coordinates are shown in Fig. 5(e) and Fig. 5(f). As the viewing angle increases from 0° to 90°, the color points created by the S/non-S sample gradually shifted from white region to yellow region. It can be seen that sedimentation of phosphor helps to confine the color coordinates. The color uniformity of the lighting sources can be consequently improved, so that the human eye would feel more comfortable. The color coordinates of the S sample shifted from (0.3898, 0.4495) to (0.4286, 0.5215), and its counterpart shifted from (0.3339, 0.3452) to (0.4265, 0.5179). Although the angle varies widely, the color coordinates at the same laser power density are close. This is different in the case for the non-S PC-WLD where a slight change in the angle leads to a significant variation in color points.
Further analysis on the thermal management of the remote phosphor plates was conducted under the laser irradiation. The duration time of laser irradiation is set as 60s, and the data is recorded by a FLIR IR camera under the condition that the background surface temperatures of the phosphor plates are basically the same. As shown in Fig. 6, the operating temperatures of both S and non-S samples increase slowly with increasing the excitations and then increase sharply as the power density exceeds the damage thresholds, defined as the laser power densities that the remote phosphor plate can maximally withstand. As the laser power density further increased beyond the threshold, the remote phosphor plates will be damaged permanently. Throughout all laser power densities, the operating temperatures of S sample are much lower than that of non-S sample. The deviation in the temperatures gets larger as increasing the laser power density. Specifically, the S sample reveals a higher power damage threshold (588.1 W/cm2) than the non-S sample (512.3 W/cm2). On the threshold power density, the operating temperature of S sample is about 135℃, which is 73℃ lower. A possible reason is that the thermal conductivity of the S phosphors near the quartz glass side is greater, whereas the threshold for reaching the damage threshold is higher [36–39]. The infrared thermal images in the inset show the temperature distribution as excited at a laser power density of 320.2 W/cm2. The temperature difference at the exit surface of the phosphor plate for the S/non-S samples is as high as 22.9°C, as the temperature was measured in the central points..
Fig. 6. (a) Infrared camera captured operating temperatures of S/non-S remote phosphor plates as increasing the laser power densities. Inset shows the infrared images of remote phosphor plates under 309.2W/cm2 blue laser excitation.
4. Conclusion
In this study, we have investigated the influence of remote phosphor sedimentation/non-sedimentation on the optical and thermal properties of high power laser-based light sources. The sediment PC-LD exhibited better color uniformity, the angular CCT deviation of which has been significantly suppressed to 294K, with only 5% luminous flux loss, as compared to its non-sediment counterparts. Both samples could achieve high photochromic quality through optimization of phosphor parameters. As for the thermal performance, the sediment phosphor samples exhibited higher power damage threshold up to 588.1 W/cm2, compared to 512.3 W/cm2 of the non-sediment samples. These results are helpful for achieving high ACU and thermal stability for laser-driven lighting applications.
Funding
National Key Research and Development Program of China (2017YFB0403200, 2017YFB0403201).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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