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Abstract
Development of ceramic phosphors (CPs) featuring small volume and high efficacy is crucial for miniaturization of white LEDs and their integration in solid state lighting. In this study, the chip-level 2.5×2.5 mm Ce:GdYAG CPs with different thicknesses were packaged to the blue chips, and their luminous characteristics were analyzed under the different radiant flux. Notably, when thickness of the CPs was 1.4 mm, a luminous flux of 2000 lm, a correlated color temperature (CCT) of 6266 K and a color rendering index (CRI) of 70 were obtained under 11.0 W blue power (1.76 W/mm2) excitation. Phenomenon of colorimetric drift was explained simultaneously. These results indicate that Ce:GdYAG CPs is a promising candidate for automotive lighting and high-speed rail lighting.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
White light-emitting diodes (LEDs) are considered to be excellent general lighting devices owing to the high brightness, high luminous efficacy and potential for miniaturization [1,2]. Currently, the commercial white LEDs are constructed by blue LED chips and yellow YAG:Ce3+ phosphors-in-silicone (PIS) [3]. Especially, for the applications of automotive lighting and high-speed rail lighting, multiple white LEDs arrays are being used. [4], which requires advanced techniques of beam forming and heat dissipation and leads to higher cost and larger volume. Compared with the multiple white LEDs arrays, the spot light source has better features, such as smaller size, easier modulation and higher brightness, and they are more appropriate for high brightness areas [5]. The color convertor should be made into a chip-level size to match the centimeter level of blue LED chips, and it causes a stricter standard for color convertors because of the high power-density and high operating temperature. Among all the color convertors, ceramic phosphors (CPs) are candidates for spot light source due to their excellent proprieties including high thermal conductivity and size designable, and then they recently attracted a great research interest. [6–11]. However, most of the current researches are focused on the large CPs (d>10 mm), low blue radiant flux (blue powers <4W), and low power density (< 0.05 W/mm2), as listed in Table S1 of the Electronic supplementary information (ESI). Unfortunately, many of the excellent proprieties of CPs including the high thermal conductivity and high brightness could not be thoroughly utilized. In addition, although the reported CPs might have excellent chromaticity parameters by adjusting their thicknesses and concentrations, the luminous flux was not high enough to meet the requirements for the high power lighting (>2000 lm). Therefore, CPs-based white LEDs with excellent chromaticity parameters and high brightness should be constructed, and their luminescence properties should be investigated systematically.
As known, chip-level CPs feature high brightness [12–15]. When they are packaged with blue LED chips for automotive lighting and high-speed rail lighting, the finial lighting source needs to meet a certain color temperature (<6500 K) and a high CRI (>70) [16]. Recently, 1.0×1.0 mm Al2O3-Ce:YAG CPs were successfully fabricated. By combining CPs with high-power LED chips, a highest luminous flux of 639 lm was obtained [6]. With decreasing the thickness of CPs, the corresponding CRI was effectively increased from 65 to 72, but the CCT was higher than 6500 K. Hu et al. systematically investigated the Ce:YAG CPs (d=18 mm) with different Ce3+ ion concentrations and thicknesses, and they found that it was difficult to obtain warm white with high CRI (>70) [17]. Therefore, Ce:YAG CPs are not the optimal materials for automotive lighting and high-speed rail lighting. It has been reported that the incorporation of Gd3+ ions into Ce:YAG CPs is an effective method to obtain higher CRI and appropriate CCT. For instance, the optimized Ce:GdYAG CPs with a diameter of 15 mm acquired a luminous flux of 21 lm, a CCT of 5010 K and a CRI of 71.4 under a 0.26 W blue power excitation [7]. A white LEDs source with the CCT of 5471 K and CRI of 77.1 were constructed by using a 19 mm Ce:GdYAG CPs at a driving current of 350 mA [18]. However, the sizes of these CPs are too large to match the size of integrated chip. Therefore, white LEDs with high luminous fluxes and excellent chromaticity parameters deserve a systematic research by using a small Ce:GdYAG CPs as color convertor.
In this study, the high-brightness white LED devices were encapsulated with chip-level Ce:GdYAG CPs (2.5*2.5 mm), and their chromaticity parameters, such as luminous flux, color coordinates, CRI and CCT were systematically investigated. Additionally, the influence of incident power on the stability of CCT and CRI was studied to compare that of commercial PIS-based white LEDs.
2. Experimental method
Ce:GdYAG CPs ((Ce0.0005Gd0.18Y0.8195)3Al5O12, d=16.0 mm) were prepared, and the detailed fabrication process was described in our previous work [19]. The thicknesses of Ce:GdYAG CPs were processed to 1.8 mm, 1.6 mm, 1.4 mm, 1.5 mm, 1.2 mm and 1.0 mm, respectively, and the final sizes were 2.5×2.5 mm. A PIS based white LED device (CREE, XHP70.2) was also assembled as a reference. The customized blue LED chips were bought from Shenzhen Yinding Technology Co., Ltd (China, 2.3×2.3 mm). The luminous flux, CCT and CRI values under different blue powers were tested using an integrating sphere (HASS-2000, Hangzhou, China). Surface temperature distributions of white LEDs were measured by an infrared camera (Fotric 226s, Fotric, America).
3. Results and discussion
Figure 1 depicts the schematic diagram and prototype of PIS based LED device. The device contained the heat sink, silica, blue LED chips, phosphors layers, lens and other components in Fig. 1 (a, b). The silica with the thermal conductivity of 2.0 W·m-1K-1 was used to conduct the heat from LED chips to the heat sink. Figure 1(c) displays the image of the PIS based white LED, and a near nature white source is obtained when the electric power of the blue LEDs is 27.0 W (blue radiant power=7.66W). The corresponding temperature distribution in Fig. 1(d) shows that the highest temperature of the corresponding white LED device is only 62.9 oC, and it indicates that the current encapsulation builds a stable operation for PIS under high blue radiant power.
Fig. 1. (a) Schematic diagram, (b) the corresponding photograph, (c) the illuminating photograph, (d) the temperature distribution of the PIS packaged LED device under the blue radiant power of 7.66W.
A schematic diagram and prototype of the Ce:GdYAG CPs-based white LED devices are displayed in Fig. 2(a, b). The integrated blue LEDs had the emission wavelength of 450 nm and a maximum blue radiant power of 11.0 W. A thin layer of transparent AB glue with the refractive index of 1.54 was used for light-propagation and heat-diffusion. Figure 2 (c) shows the size of all Ce:GdYAG CPs. The size of 2.5×2.5 mm is suitable for LED chips (2.3×2.3 mm) to avoid the escape of the blue light from the side of Ce:GdYAG CPs [20]. Meanwhile, the images of Ce:GdYAG CPs with different thicknesses (from 1.0 mm to 1.8 mm) are displayed in Fig. 2(d). They absorbed blue light from LED chip to produce yellow emission, and the emitted yellow light was mixed with the residual blue light from LED chip to obtain the white light [21,22].
Fig. 2. (a) Schematic diagram and (b) photograph of the LED device encapsulated by 1.8 mm Ce:GdYAG CPs; (c) the size and (d) the thicknesses of Ce:GdYAG CPs.
Figure 3(a) shows the electroluminescence (EL) spectra of Ce:GdYAG CPs based white LEDs under 1.0 W blue power radiation. All EL spectra consisted of a broad emission band ranging from 400 to 475 nm. It was centered at 450 nm due to the emission of the blue LED chips. Meanwhile, there was a broad emission band from 500 to 800 nm originated from the 5d-4f transitions of Ce3+ icon [23,24]. By decreasing the thickness of CPs, the yellow emission intensity was gradually dropped, and the blue emission intensity increased. As the thickness decreased, the number of Ce:GdYAG molecules in CPs decreased and less blue light could be absorbed and converted into yellow light. The inset photograph shows that the high luminous efficacies (>230 lm/W) are realized exceeding the previously reported results listed in Table S2. The CCT and CRI values of the white LEDs were ranged from 4686 to 5784 K and from 66.0 to 71.7, respectively, in the inset of Fig. 3 (a). Figure 3(b) shows the corresponding CIE color coordinates of all CPs and they are all distributed around the Planckian locus. When the thickness of the CPs was 1.0 mm, the optimized CRI value of 71.7 was obtained. The corresponding CCT and luminous efficacy were 5748 K and 231 lm/W, respectively. It indicated that the CPs based white LED devices were capable in realizing excellent chromaticity parameters and luminous efficacy.
Fig. 3. (a) EL spectra of Ce:GdYAG CPs based white LEDs as a function of thickness under 1.0 W blue radiant power (inset: the corresponding lighting pictures and the detailed chromaticity parameters), (b) the corresponding CIE color coordinates.
The luminous proprieties of PIS and CPs (1.4 mm in thickness) were compared in Fig. 4 (a). As can be seen from the figure, a high luminous flux value of 2000 lm was obtained in CPs and PIS. With increasing the incident blue power, the corresponding luminous efficacy were reduced from 334 lm/W to 264 lm/W for PIS, whereas the luminous efficacy of CPs based white LEDs were dropped from 244 lm/W to 180 lm/W. Under the blue radiant power of 7.6 W, the CPs based white LEDs showed a superior luminous efficacy (only dropped 13.9%) than that of PIS based white LEDs (dropped 21.3%). Because the thermal conductivity and thermal stability of the CPs are better than those of phosphors, and the luminescent behavior of CPs is more stable [25]. In Fig. 5(b), by raising the blue radiant power, the CCT of CPs-based white LED increased from 5000 K to 6266 K, whereas from 6173 K to 7201 K for PIS. It was remarkable to observe that when the blue radiant power was 3.0 W, the value of luminous flux of PIS was up to 1000 lm. However, the corresponding CCT value had exceeded 6500 K, and it was increased continuously with increasing the blue radiant power. This indicates that the PIS is not suitable to be packaged with the high-power blue LEDs for the purpose of generating high luminous flux (>1000 lm) and nature white (< 6500 K) for automotive lighting and high-speed rail lighting. On the contrary, the Ce:GdYAG CPs with the thickness of 1.4 mm showed the excellent luminescence properties under the excitation of high-power blue LEDs. When the blue power was increased from 8.0 W to 11.0 W, the CCT was varied from 5500 K to 6266 K, whereas the corresponding CRI was increased from 67.2 to 70. Meanwhile, when the blue radiant power was 11.0 W (1.76 W/mm2), the white LED had a high luminous efficacy (>180 lm/W). These results indicate that the chromaticity parameters of the CPs-based (1.4 mm) white LEDs are close to that of nature white source, and they are perfect candidates for automotive and high-speed rail lighting.
Fig. 4. Comparisons between the PIS and CPs (1.4 mm thickness) based white LED devices under different blue radiant powers: (a) luminous flux and luminous efficacy, (b) CCT and CRI
Fig. 5. (a) EL and (b) normalized EL spectra of CPs (1.4 mm thickness) based white LEDs as a function of incident blue radiant power (insert: the measured operating temperature of LED chips without and with CPs at 11.0 W blue radiant power)
To obtain a better understanding of CCT and CRI drifting, the EL spectra of the CPs (thickness=1.4 mm) based white LEDs were systematically analyzed in Fig. 5. Figure 5(a) shows the spectral power density of the lighting device under different blue radiant powers. When the blue radiant power exceeded 11.0 W, the yellow region was no longer enhanced, and it resulted in a luminous saturation. It is worth noting that the high blue radiant power leads to a significant rise in temperature in LED chips and Ce:GdYAG CPs [26]. From the insets of Fig. 5 (a), when the blue radiant power was 11.0 W, the temperature of LED chips was around 73.2 °C before they were encapsulated with CPs (1.4 mm). The highest surface temperature of CPs reached 165.6 °C after encapsulation. Therefore, the heat generated from LED chips could be also the reason behind the temperature rise in Ce:GdYAG CPs. Due to the temperature quenching of Ce3+ ion in Ce:GdYAG CPs [27], an obvious decreased light-conversion efficiency of the 1.4 mm Ce:GdYAG CPs was observed when the blue radiant power increases from 0.73W to 11.0 W, in Fig. 5(b). The lower efficiency of light-conversion caused the decrease of yellow emission, and the corresponding CCT was increased from 5000 K to 6266 K.
For CPs-based white LEDs (CPs=1.4 mm), the CRI was decreased from 68.0 to 67.3, and then rose from 67.3 to 70.0 when the blue radiant power increased from 0.73W to 11.0 W. It exhibited a similar variation to that of PIS. At a low incident power (<6.33 W), the blue emission showed a blue-shift, and it caused the decrease of CRI, as shown in Fig. 5(b) and Fig. S1 (ESI). When the blue power exceeded 6.33 W, the blue emission was almost invariant, whereas the yellow emission region displayed a red shift. The red shift resulted in the improvement in CRI [28]. In Fig. 5(b), comparing to standard light source, the emission spectrum from the 1.4 mm Ce:GdYAG CPs based white LEDs had less blue emission under 0.73 W excitation. With increasing the blue radiant power, the yellow region decreased smoothly below 6.33 W. However, due to the significant heat accumulation, the yellow region decreased when the blue power exceeded 6.33 W. Therefore, the spectrum reported here nearly approached the spectrum of the standard white light under a 11.0 W incident power. Meanwhile, the CRI reached the highest value. The decreasing proportions of the yellow emission were summarized in Table 1. Under the incident power of 11.0 W, the fluorescence power (485-800 nm) of the 1.4 mm CPs was declined by 25.13% (10.46% under 6.33W) due to the high operating temperature (165.5 °C). It is beneficial for achieving a proper ratio between the transmitted blue light and emitted yellow light to obtain a high CRI [29,30]. Therefore, the interfering factors of CRI for the CPs-based LED device (CPs=1.4 mm) are focusing on the following aspects. Thermal-induced wavelength shift of LED chips is the main reason for the CRI drifting when the blue power is below 6.33 W. When the LED power increases from 6.63 W to 11.0 W, the main factor behind the CRI drifting is the thermal induced luminous degeneration (including the drop of luminous intensity and the red-shift of yellow emission region) of the Ce:GdYAG CPs.
Table 1. The fluorescence power (485∼800 nm) of the 1.4 mm CPs under different incident power.(Take the 0.39 W fluorescence power generated by the 0.73W incident blue light as a reference.)
To compare the residual blue light of the above LED lighting sources, the special power densities versus different blue radiant powers were shown in Fig. 6(a). The spectra were reduced by N times (N = blue radiant power/0.73). It is interesting to note that the residual blue light in the special EL spectrum shows a similar variation compared to that of CRI. When blue radiant power exceeded 2.83 W, the remaining blue light was increased instead of continuous decrease. More excited photons were effectively absorbed because of the internal scattering, and the decrease in the amount of the transmitted blue light decreased the CRI. However, as the blue radiant power exceeded 2.83 W, the residual blue light was gradually increased and it caused a declining absorption by the Ce:GdYAG CPs (1.4 mm). The high blue radiant power from LED chips and stokes loss from Ce:GdYAG CPs undoubtedly cause the heat accumulation at the Ce:GdYAG CPs. Indeed, the high temperature of the Ce:GdYAG CPs is the main reason for this phenomenon, i.e., the thermal-induced absorption declining [31]. Figure 6(b) shows that the residual blue light of white LEDs (encapsulated with 1.4 mm CPs) was higher in stable illumination (SI, T=165.6 °C) than that in instantaneous illumination (II, T=25 °C). This phenomenon was also observed in other samples. Though the saturation power density of ceramics is more than 50 W/mm2 in the previous literature [8], the absorbing ability of 1.4 mm CPs in this study turns to decline at 1.76 W/mm2 due to the high temperature (165.5 °C) [31], which caused the increase in the residual blue light. The EL spectrum with more residual blue light is closer to the spectrum of standard white lighting source. These results indicate that the decline of absorption of the 1.4 mm CPs partly improved the CRI of white LED device.
Fig. 6. (a) The EL spectra of the CPs (1.4 mm) based white LEDs under different blue powers. The spectra were reduced by n times (n = blue light power/0.73). (b) The EL spectra of the CPs based white LEDs with different thickness at instantaneous illumination (II) and stable illumination (SI).
Although the CRI value of the white lighting source which was encapsulated with CPs (thickness=1.4 mm) could be increased to 70 by increasing the power to 11.0 W, the luminous efficacy was only 180 lm/W, and it was lower than that of the commercial PIS. Therefore, the encapsulation structure requires to be further optimized to improve luminous efficacy of CPs-based white LEDs, such as roughing the surface of CPs or using a dichroic mirror on the LED chips. Meanwhile, optimizing the composition and structure of ceramic phosphor is a better way to improve luminous efficacy, such as composite phosphor ceramics [7], wider spectrum ceramics [32], and so on. Besides, the thermal behavior of Ce:GdYAG CPs based white LEDs was poor in these measurements, a heat conduction channel such as sapphire would be designed between the LED chip and CPs to conduct the heat.
4. Conclusion
In summary, chip-level Ce:GdYAG CPs (2.5×2.5 mm) based white LED devices with high brightness were assembled for automotive and high-speed rail lighting applications. When the thickness of the CPs was 1.4 mm, a luminous flux of 2000 lm, a correlated color temperature (CCT) of 6266 K and a color rendering index (CRI) of 70 were obtained under a 11.0 W blue power (1.76 W/mm2) excitation. Through the detailed analysis of the EL spectra of 1.4 mm CPs based white LEDs, it was found that the main reasons for the chromaticity drift were the thermal-induced wavelength-shift, thermal-induced luminous degeneration and thermal-induced absorption declining. The obtained parameters are expected to advance the applications of Ce:GdYAG CPs-based LED devices in the automotive lighting and high-speed rail lighting.
Funding
National Natural Science Foundation of China (51902143, 61775088, 61971207, 61975070); Priority Academic Program Development of Jiangsu Higher Education Institutions; Jiangsu Provincial Key Research and Development Program (BE2018062, BE2019033); Natural Science Foundation of Jiangsu Province (BK20191467); Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX20_2212); International Science and Technology Cooperation Program of Jiangsu Province (BZ2019063, BZ2020030, BZ2020045); Natural Science Research of Jiangsu Higher Education Institutions of China ((19KJB430018, 20KJA430003); Special Project for Technology Innovation of Xuzhou City (KC19250, KC20201, KC20244); Open Project of State Key Laboratory of Advanced Materials and Electronic Components (FHR-JS-202011017).
Disclosures
The authors declare no conflicts of interest.
Supplemental document
See Supplement 1 for supporting content.
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