Highly efficient large-area ultrathin phosphor-glass composites fabricated by tape-casting for super-bright LED lights

doi:10.21203/rs.3.rs-4713253/v1

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Highly efficient large-area ultrathin phosphor-glass composites fabricated by tape-casting for super-bright LED lights

https://doi.org/10.21203/rs.3.rs-4713253/v1

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Driven by the demand for super-bright LED lights for cars, buses, and trucks, highly efficient and large-area ultrathin phosphor-glass composites (PGC) with exceptional thermal dissipation capabilities were fabricated by a combined technique of tape-casting + low-temperature cofiring process. Two kinds of ultrathin (100 μm thick) PGC plates uniformly incorporated with YAG:Ce and CaAlSiN₃:Eu²⁺phosphor particles and with a large size of 1044×45 mm were successfully prepared. At room temperature, photoluminescence quantum yields (PLQY) of 98.6% and 80% were achieved for the former and latter kinds of PGC glasses, respectively. Moreover, color tunable emissions were yielded in the ultrathin PGC by varying the weight ratio of different phosphors. Finally, light-emitting diodes (LEDs) encapsulated with different

ultrathin PGC were demonstrated to exhibit outstanding luminous performance. When exposed to blue laser irradiation, the prepared PGC glasses demonstrated a heightened resistance to laser radiation. These unparalleled ultrathin PGC glasses could offer an unprecedented solution for the commercial applications in preparation of super bright car LED lights.

Physical sciences/Materials science/Materials for optics/Lasers, LEDs and light sources/Inorganic LEDs

Physical sciences/Optics and photonics/Lasers, LEDs and light sources/Inorganic LEDs

Recently, there has been a significant surge in the development and application of novel phosphor-glass composites (PGC) ^1–5 in the fabrication of super bright car LED lights since the automotive lights market has been rapidly growing. Currently, phosphor-converted white light-emitting diodes (pc-WLEDs) are mainstream devices of various LED lights^6–7, including car LED lights. They are made up of blue GaN-based LEDs + mixture of phosphor particles and organic resin. However, such lighting devices usually suffer severe performance degradation as the mixture of phosphor and organic resin was heated up to a certain high-temperature by the self-heating of LED chip^8–10. In recent years, traditional doped single-phase transparent ceramics and bulk crystals have emerged as efficient all-inorganic color converters, offering the potential to circumvent the need for organic encapsulation^11–14. However, their costly preparation and limited variety have hindered scalable manufacture and widespread application. PGC luminescent glasses, with excellent luminescence quantum yield and thermal

dissipation capability, have been thus developed for super bright LED lights.

The primary challenge in fabricating PGC is how to produce ultrathin PGC glasses, i.e., glass thickness is thinned to 0.1 mm or smaller. This is crucial to ensure the PGC glasses keep excellent heat dissipation characteristics even under intense excitation from blue light^15–17. Mechanical thinning method is usually used to reduce the thickness of PGC glasses¹⁸. However, the brittleness of glasses remains a problem when they are mechanically thinned to ~ 200 µm¹⁹. Therefore, new technology, such as the tape-casting technique, must be adopted to produce ultrathin PGC, especially when it is combined with a low-temperature cofiring process²⁰. The primary advantage of the tape casting process is its ability to produce large-area, thin, flat PGC green sheets ^21,22. Another challenging issue is strong interfacial interactions between phosphor micro/nano particles and glass matrix, especially at higher temperatures^23,24. The interfacial interactions may cause significant surface corrosion of the embedded phosphor particles, drooping down the luminescence efficiency of PGC glasses. Therefore, choosing low-melting glasses (e.g., borates, tellurites, borosilicate, and phosphates) as the raw matrix offers promise in avoiding the interfacial interactions through consolidation at lower temperatures^25–28. On the other hand, phosphor powders with exceptional chemical and thermal stability, for instance, already-exemplified materials like Ce³⁺ doped Y₃Al₅O₁₂ (YAG:Ce) and Eu²⁺ doped CaAlSiN₃ (CASN:Eu) should be chosen^29,30. Despite the above strategies, nevertheless, maintaining high quantum yield (QY) of the original phosphors in glass matrix still poses a challenge, particularly for red phosphors. In addition, the glass matrixes reported in previous studies exhibit some drawbacks such as limited visible transparency and heavy metal content^31–34, which significantly impedes widespread use of PGC glasses. Consequently, the pursuit of PGC-based color converters with high luminescence quantum efficiency, good chemical and thermal stability, cost-effectiveness, and ultrathin profiles remains as a challenging task.

Herein, we present an innovative approach for fabricating large-area ultrathin PGC glasses, with a thickness of ~ 100 µm, using a tape-casting process combined with a low-temperature cofiring technique. Large-area YAG:Ce-PGC glass with size of 1044×45×0.1 mm has been successfully fabricated. Meanwhile, high luminescence QY values of 98.6% and 80% are achieved for the yellow- and red-emitting PGC glasses incorporated with YAG:Ce and CASN:Eu phosphor particles, respectively. To the best of our knowledge, these results demonstrate the highest QY reported to date for these two kinds of ultrathin PGC glasses, which is evidently supported by measuring both steady-state and time-resolved photoluminescence (PL) spectra of the glasses. Furthermore, color-tunable emissions have been achieved via incorporating two-color phosphors with different weight ratios. In the investigation of luminous performance of PGC-based LEDs, high electric power threshold of > 8 W/mm² is obtained, meaning that excellent heat dissipation capability is achieved for the prepared PGC glasses. Finally, the luminous performance of the prepared PGC glasses under blue laser irradiation is tested. The testing results clearly indicate that the fabricated ultrathin PGC glasses exhibited higher resistance to laser radiation compared to the phosphor counterpart. These advancements hold great promise for future potential applications that require high-performance PGC glasses in super bright lighting fields.

2.1 Design and preparation of both yellow and red ultrathin PGC. Despite the continuous development of novel phosphors exhibiting a range of color emissions for pc-WLEDs^35–38, yellow and red phosphors remain pivotal in enhancing luminosity and fine-tuning color rendering index. As such, this study focuses on the design and fabrication of ultrathin PGC based on YAG:Ce and CASN:Eu phosphors (YAG:Ce-PGC and CASN:Eu-PGC). While the yellow YAG:Ce phosphor boasts thermal stability up to 1700 ℃ with an exceptional quantum yield exceeding 90% at room temperature^39,40, the red CASN:Eu phosphor is prone to structural instability, leading to potential loss of quantum yield through interfacial reactions^41,42. To optimize luminescence efficiency, careful consideration must be given to the composition and structure of the PGC. In this study, we present a novel approach to the fabrication of nearly two-dimensional PGC structures with ultrathin profiles using a combination of tape-casting and low-temperature cofiring techniques. Recognizing the documented reactivity between SiO₂ and the YAG matrix⁴³, we selected an amorphous glass powder with reduced SiO₂ content to facilitate densification while preventing interfacial corrosion during the co-firing process.

Figure 1(a) illustrates the process of preparing ultrathin PGC sheets for emitting yellow and red light. A small amount of yellow phosphor powder (18wt.%) was uniformly mixed with amorphous glass powders through high-speed planetary ball milling for 5 hours, guaranteeing uniform dispersion of particles in the slurry. The identical technique was employed to create the red PGC slurry. Following this, the slurry was deaerated under vacuum conditions utilizing high-speed centrifugation before the tape-casting process.^44–46. To assess the rheological properties of the synthesized slurry, the relationship between shear stress and viscosity at different shear rates was carefully evaluated using a state-of-the-art rotary rheometer (HAAKE, MRSIII, USA), as depicted in Fig. S1. Initially, the shear rate was raised to 300/s before gradually decreasing to zero. Throughout this experiment, a conspicuous thixotropic loop was observed in the flow curve, suggesting the breaking and regeneration of organic molecular chains within the slurry. The viscosity curve displayed a significant drop in viscosity as the shear rate increased until it stabilized at around 370 mPa·s, indicating a distinct shear-thinning behavior. Nevertheless, upon reducing the shear rate to zero, the slurry's viscosity failed to fully restore to its original state and settled at 1.612 Pa·s lower than the initial viscosity. Notably, the blade height was carefully regulated throughout the process, as shown in Fig. 1(a). It is noteworthy that the primary emission wavelengths of the yellow and red phosphors utilized in this study are 558 nm and 645 nm, respectively. The large-area green sheets, characterized by uniform thickness and remarkable flexibility, were successfully produced, underscoring the immense potential of ultrathin PGC in our research. To achieve fully dense PGC, these green sheets, comprising a glass matrix and phosphor powders, were directly sintered at a modest temperature of 700 ℃ without any specialized treatment. During the sintering process, the phosphor particles were consolidated by the melted glass, producing a fully dense microstructure. Figures 1(b) and 1(c) depict images of the prepared green sheets and their corresponding PGC, illustrating the attainment of a fully dense microstructure with some transparency post-sintering. Removing ultrathin PGC from the substrate is typically a challenging task due to the glass melt during the sintering phase. Nonetheless, this obstacle has been effectively overcome, enabling us to create ultrathin PGC on a large scale for the first time. As illustrated in Fig. 1(d), the thickness of the yellow and red PGC measured a mere 52 µm and 111 µm, respectively, a feat that is considerably arduous to achieve via conventional mechanical cutting methods. Considering the size of the LED chips used (1×1 mm in this study), the sintered large-area PGC were precisely sectioned using a UV nanosecond pulse laser cutter guided by a computer program, as shown in Fig. 1(e). Figure 1(f) showcases images of the cut PGC under daylight and UV light, exemplifying the diverse luminous effects of our samples and affirming the effectiveness of the laser cutting technique in producing various sizes and shapes for the crafted ultrathin PGC.

2.2 Structural characterization of PGC. To date, extensive research efforts have delved into exploring the relationship between structure and properties in pure micro/nano-crystalline phosphors, with the goal of achieving high photoluminescence quantum yield (PLQY) and wide-ranging emission. Nevertheless, when phosphor particles are embedded into a glass matrix, a markedly distinct environmental structure emerges, potentially exerting a significant impact on the performance of PGC. To directly observe the impact of glass on the crystal structure of phosphors, X-ray diffraction (XRD) analyses were conducted on YAG:Ce phosphor, glass powder, and our synthesized PGC, juxtaposed against the YAG standard peak, as illustrated in Fig. 2(a). The findings demonstrate that the glass powder utilized in this study exhibits a significant amorphous diffraction background, which is also evident in the XRD pattern of PGC sample. Furthermore, it was noted that all of the diffractive peaks characteristic of the YAG crystal were detectable, albeit with slightly diminished intensity compared to those of the YAG:Ce phosphor. This result suggests that the composite structure of PGC is determined by the combined interaction of glass and phosphor particles. Figure 2(b) presents the scanning electron microscopy (SEM) depiction of our PGC sample, unveiling the dispersion of phosphor particles within the glass framework. Minute pores, as indicated by white dash-line circles, are visible, primarily ascribed to inadequate sintering procedures. Previous studies have demonstrated that a specific number of these pores can serve as scattering sources for luminous emissions⁴⁷, consequently heightening luminous efficacy in LED applications.

To achieve a comprehensive understanding of the lattice environment of PGC, we utilized advanced high-resolution transmission electron microscopy (HRTEM) to capture detailed images with a focal point of approximately 80 nm for further analysis, as depicted in Fig. 2(c). The results obtained unveiled the presence of three distinct regions, namely the crystal, glass, and their interfacial boundary. This observation is further supported by conducting Fast Fourier Transform (FFT) analysis on each domain, as evidenced by the images of selected area electronic diffraction (SAED). Within the crystalline domain, a series of periodic diffraction patterns were discerned in reciprocal space. Conversely, the glass region only exhibited diffraction rings without discrete points. Remarkably, the interface region, with an approximate thickness of 2 nm, exhibited structural characteristics of both crystalline and amorphous state, potentially exerting a profound influence on the luminescent properties of our specimen. Figure 2(d) presents a SEM image of a specific micro-scale region, where elemental mapping for various ions was successfully accomplished using an energy dispersive X-ray spectrometer (EDS). The results obtained from this analysis are shown in Fig. 2(e-g), elucidating a conspicuous detection of the primary elements, notably aluminum (Al) within phosphor particles and silicon (Si) within the glass matrix. The identification of certain dark regions, as drawn by the dash circles in Fig. 2(g), indicates the absence of silicon, suggesting these regions pertain to the phosphor particles, a conclusion that aligns seamlessly with the elemental mapping of aluminum in Fig. 2(f). This finding provides further evidence supporting the existence of a composite structure comprising both phosphor crystals and Si-based glass powder.

2.3 Luminescence properties of the YAG:Ce and CASN:Eu-PGC glasses. As depiected in Fig. 3(a), the excitation and emission spectra of YAG:Ce-PGC exhibit close resemblance to those of the corresponding YAG:Ce phosphor. It is worth noting, however, that we have identified some subtle structural features in the measured PLE of the phosphor itself, which appear to be less pronounced in the case of ultrathin PGC. This phenomenon can be attributed to the smoothing effect induced by the presence of glass on the emission. Furthermore, a slight blue shift was also discerned following the embedding of YAG:Ce phosphor particles into the glass matrix, as illustrated in Fig. 3(b). A comparable observation was made with the CASN:Eu red phosphor and its corresponding PGC (as shown in Fig. S2 of supporting information). This underscores the idea that encapsulating the phosphor particles within the glass matrix may slightly increase the energy gap between the lowest excitation state and the ground energy level for optically active ions. To evaluate the luminescence efficacy of the prepared ultrathin PGC, PLQY values were measured, as depicted in Fig. 3(c). An impressive PLQY of 98.6% was attained for the yellow YAG:Ce-PGC, surpassing that of phosphor itself^48,49. This finding suggests that the interplay between glass and phosphor particles exerts minimal influence on the luminescence of phosphor. Even in the case of CASN:Eu-PGC, a commendable PLQY of 80% was achieved (seen in Fig. S3 of supporting information), representing the pinnacle of achievement for non-substrate red PGC^50,51. To assess the thermal stability of as-prepared YAG:Ce phosphor and its corresponding PGC, we conducted their temperature-dependent PL spectra measurement under 450 nm blue light excitation, as depicted in Fig. 3(d) and (e). The results revealed a sudden escalation in the emission intensity of the phosphor sample within the temperature range of 200 to 240 K, marking an unprecedented occurrence. Conversely, a decline in emission intensity was observed for the PGC within the temperature range of 280 to 300 K. Figure 3(f) and (g) illustrate the integrated PL intensities of both phosphor powder and PGC as a function of temperature. Notably, the luminescence of the phosphor powder exhibited deterioration at temperatures below 200 K and above 240 K. In contrast, the luminescence thermal stability of the PGC remained steadfast at temperatures below 280 K, despite the onset of luminescence thermal quenching at temperatures exceeding 300 K. These intriguing findings suggest that the glass matrix efficiently shields the phosphor particles from the detrimental effects of heat conduction at lower temperatures. Conversely, the abrupt alteration in emission intensity of the PGC is likely attributable to the breakdown of the thermal barrier at elevated temperatures, leading to a pronounced enhancement of nonradiative transition processes. However, the cause of the sudden increase in emission intensity for the phosphor powder remains unknown and requires further investigation in future studies. At elevated temperature of T > 300 K, the decrement in the integrated PL intensity of YAG:Ce-PGC mirrors that of YAG:Ce phosphor powders, retaining nearly 89% of its initial value at 420 K. This signifies that the luminescence properties of YAG:Ce can be effectively preserved at high temperature when the phosphor particles are incorporated into a glass matrix. Moreover, it indicates that the YAG:Ce particles remain intact and well-preserved within the glass matrix during the sintering process.

Furthermore, the luminescence properties of ultrathin red CASN:Eu-PGC sample was also examined. A three-dimensional (3D) PL spectrum was depicted Fig. S4 with temperature ranging from 100 to 500 K. Analogous to the behavior observed in YAG:Ce-PGC, a sudden decrease in emission intensity is noted for CASN:Eu-PGC within the temperature range of 220–240 K, underscoring the notable impact of the glass framework on the luminescence of PGC at low temperature range. To unveil the thermal stability of PGC luminescence, temperature-dependent PL spectra of CASN:Eu red phosphor were further measured for comparison, as shown in Fig. S5. The findings unveiled a conspicuous luminescence thermal quenching phenomenon with increasing temperature from 300 to 500 K^52–54. Based on these findings, the emission intensities were graphed against temperature, as illustrated in Fig. S6. The similar trend in emission intensity with temperature suggests that the luminescence of our red phosphor particles is scarcely affected by the glass matrix at elevated temperatures. To elucidate the underlying mechanism of luminescence thermal quenching, we applied the following equation \(\:I={I}_{0}/(1+A\text{exp}\left(-\varDelta\:E/kT\right))\:\)to plot the normalized emission intensity aganist temperature^55–57. The thermal activation energies (\(\:\varDelta\:E\)) were determined to be 249 meV for PGC and 265 meV for phosphor powder. These outcomes signify that the ultrathin CASN:Eu-PGC demonstrates luminescence thermal stability comparable to that of phosphor powder. Remarkably, the identical \(\:\varDelta\:E\) value was also attained for the ultrathin YAG:Ce-PGC (as shown in Fig. 3(h)), implying that the luminescence thermal quenching of PGC is predominantly governed by the glass component utilized, regardless of the specific phosphor. In contrast, YAG:Ce phosphor demonstrates a thermal activation energy of 206 meV, significantly diverging from that of PGC. Furthermore, an exploration into the luminescence dynamics for both phosphor and PGC was conducted to unravel the underlying influence of the glass matrix on the luminescence. Figure 3(i) shows the luminescence decay results of both phosphor and ultrathin PGC based on YAG:Ce. The measurements were conducted at a temperature of T = 300 K and well fitted using a double-exponential function: \(\:y={A}_{1}\text{exp}\left(-x/{\tau\:}_{1}\right)+{A}_{2}\text{exp}\left(-x/{\tau\:}_{2}\right)+{y}_{0}\)^58–60. Based on the derived results, the luminescence lifetimes for the phosphor powder were calculated to be t₁=1.39 µs and t₂=12.475 µs. For PGC, the lifetimes slightly extended to be t₁=1.519 µs and t₂=14.148 µs. Nevertheless, for CASN:Eu based phosphor and ultrathin PGC, the luminescence lifetimes experienced a marginal decrease from t₁=1.85 µs and t₂=16.42 µs for phosphor powder to t₁=1.45 µs and t₂=13.79 µs for ultrathin PGC (seen in Fig. S7 of supporting information). The minute disparity in luminescence lifetime between the phosphor and PGC indicates that the luminescence kinetics are minimally impacted by the glass matrix at T = 300 K. Additionally, a slight blue shift in emission was observed for CASN:Eu-PGC as temperature rose from 300 to 500 K, as manifested in Fig. S8 of the supporting information. This phenomenon deviates from the typical red-shift observed in most luminescent materials as temperature rises, albeit aligning with the emission characteristics of CASN:Eu phosphor powder. This observation furnishes further validation that the interface between the glass matrix and phosphor particles does not compromise the luminescent properties of the phosphor particles within glass matrix.

2.4 Luminous performance of encapsulated LED devices. In order to showcase the efficacy of YAG:Ce-PGC in practical LED applications, we have successfully engineered a high-power LED device. The electroluminescent (EL) spectra of said devices driven by both high and low electric powers can be observed in Figure S9 of the supplementary materials. When operated at a current of 20 mA, the luminous efficiency (LE) impressively reaches a value of 154.64 lm/W, with the luminous flux (LF) peaking at 455 lm under a current of 2000 mA. However, it is worth noting that the LE diminishes to 52 lm/W due to the phenomenon known as "efficiency droop" when the LED chip functions at high current levels^61–63, as depicted in Fig. 4(a). A similar trend in variation was also observed in the pc-WLED device based on YAG:Ce phosphor coated in resin (YAG:Ce-PCR). Notably, no decline in LF was observed even at elevated power levels, indicating the exceptional resistance of our synthesized YAG:Ce-PGC to high-density blue radiation. This resilience is further supported by the measured EL spectra, shown in Fig. 4(b). Furthermore, we excited our synthesized ultrathin YAG:Ce-PGC using a power-tunable 450 nm blue light source, without necessitating any additional heat sink. The results obtained were remarkable, with no saturation of luminescence even at an electrical power density of 8.73 W/mm². Importantly, this value either equals or surpasses previous findings in this domain^64–65. Additionally, when evaluating the CIE color coordinate of the LED device utilizing YAG:Ce-PGC, only minimal changes were noted as the electric power density increased, as seen in Fig. 4(c), underscoring the exceptional luminous stability of our LED creation.

For comparative purposes, measurements of both LF and LE at varying levels of electric power excitation were conducted for both CASN:Eu-PCR and PGC, as illustrated in Fig. 4(d). The results indicate significantly higher LF and LE values for PGC in comparison to PCR, primarily due to the superior transparency of former relative to the later. This highlights the efficient nature of red PGC as a color converter compared to the corresponding phosphor. Nevertheless, with an increase in electric power density, saturation of LF values for red phosphor and PGC was observed at approximately 9 W/mm². This saturation was accompanied by a decline in LE values, signifying that the red phosphor particles are unable to endure optical power densities as robust as those of YAG:Ce. The EL spectra of red PGC combined with a blue-chip were measured under variable electric power conditions, as shown in Fig. 4(e). As the electric power density increases from 0.05 to 8.19 W/mm², the luminescence intensities in both the red and blue spectral regions exhibited an increase, likely due to saturated absorption of blue light by the red phosphor particles, leading to a shift in the intensity ratio of red to blue light. To evaluate the heat dissipation capabilities of PGC, thermal distribution measurements of phosphor and PGC converted LED devices (pc-LEDs and PGC-LEDs) were conducted under an electric power density of 1.22 W/mm², as depicted in Fig. 4(f). Of these two PGC-LEDs devices, temperature measurements revealed that the maximum temperature reached by CASN:Eu-PGC was 151.8 ℃, while for YAG:Ce-PGC, it was only 91.8 ℃. Given their identical composite structures, this outcome suggests superior heat dissipation capabilities in the latter, possibly due to the higher thermal conductivity of the YAG matrix⁶⁶. This conclusion is reinforced by a laser irradiation experiment showing that YAG:Ce-PGC can withstand higher power laser excitation than CASN:Eu-PGC. Subsequent temperature distribution measurements of pc-LEDs under the same electric power density indicated peak temperatures of 62.0 ℃ for YAG:Ce-PCR and 66.6 ℃ for CASN:Eu-PCR. These results seemingly contradict expectations of higher thermal conductivity in PGC compared to organic resin materials, as they may suggest lower temperatures in PGC-LEDs. Herein, it is noted that temperature measurements were primarily surface-based, with the high temperatures recorded on PGC-LEDs indicating excellent heat dissipation facilitated by the glass matrix, enhancing thermal conductivity. Conversely, lower temperatures in the pc-LEDs likely stem from reduced heat dissipation capabilities of resin materials, acting as heat insulation layers, thereby impeding efficient heat dissipation within the chip and trapping it within the color-converted layer.

2.5 Color tunable emissions of PGC glasses. Utilizing glass components with a low melting point as the matrix materials, a sophisticated low-temperature co-firing technique was employed in our study at T < 700 ℃. This method not only facilitates the densification process of PGC, but also shields the luminescent activators Ce³⁺ and Eu²⁺ from oxidation into quenching species (Ce⁴⁺ and Eu³⁺). Consequently, numerous oxide and nitride phosphor particles are encapsulated within the glass matrix while maintaining exceptional luminescence thermal stability during sintering. This innovative approach holds great potential for achieving color-tunable emission in ultrathin PGC. In the illustration shown in Fig. 5(a) and (b), two distinct phosphors (CASN:Eu and YAG:Ce), emitting red and yellow light respectively, were blended uniformly at varying weight ratios (R). Subsequently, the phosphor blend was integrated into a glass matrix to create ultrathin PGC based on our proposed methodology. The recorded EL spectra of these ultrathin PGC samples reveal a substantial alteration in spectral characteristics by adjusting the weight ratio (R) of YAG:Ce to CASN:Eu, resulting in a shift in the central emission position from red to yellow. This observation is further corroborated by visual evidence from photographs of the ultrathin PGC samples under daylight and blue-light illumination (as seen in Fig. 5(b)). The strong correlation between the luminescence of ultrathin PGC and the R value highlights the potential for color emission tuning, indicating promising prospects for the application of ultrathin PGC in super-bright illumination technologies.

To validate the scalability of our proposed strategy for manufacturing ultrathin PGC, a super-large YAG:Ce-PGC sample measuring 1044×45×0.1 mm was successfully synthesized, as depicted in Fig. 5(c). To the best of our knowledge, these exceptional ultrathin PGC samples possess the largest surface area and thinnest dimensions compared to previous studies on luminescent materials in a phosphor-in-glass matrix. This achievement signifies a bright future for the practical commercialization of our strategy. The results obtained showcase a simple and versatile approach to creating color-tunable ultrathin luminescent materials for various photonics applications, whilst opening up new possibilities for large-scale production of ultrathin PGC in the industry.

2.6 Performance evaluation of laser illumination for PGC. As illustrated in Fig. 6(a), the evaluation of luminous performance for PGC under laser illumination was carried out utilizing a home-made system comprising a 455-nm blue laser as the excitation source, an integrating sphere for the collection of all emission light signals, and a spectrometer linked to a computer. The inset figure illustrates the laser output power under various driving currents, revealing a non-linear correlation. Consequently, the intensity of the stimulated laser can be finely regulated through the adjustment of the driving current. It is notable that transmission light was employed to evaluate the efficacy of our PGC specimen in this investigation, deviating from prior studies^67,68. When subjected to high-power density irradiation from the same blue laser, both yellow and red PCR were noted to endure irreparable harm. Conversely, such impairment

was effectively avoided for both variants of PGC, as depicted in Fig. 6(b). This observation suggests that PGC demonstrate heightened resistance to laser irradiation in comparison with traditional PCR samples. With increasing laser power, the luminous flux of both PGC and PCR exhibits a rise until the laser power surpasses the luminous saturation threshold, as depicted in Fig. 6(c) and (d). Our findings indicate that the luminous saturation threshold is higher for PGC (1.765 W for YAG:Ce-PGC and 1.424 W for CASN:Eu-PGC) than for PCR samples (1.594 W for YAG:Ce-PCR and 1.295 W for CASN:Eu-PCR), regardless of the color converters being yellow or red. Furthermore, we observed that the CIE color coordinates for both types of PGC are strongly dependent on the laser power, as demonstrated in Fig. 6(e). An increase in laser power causes the color coordinate to shift towards the blue region, indicating an increase in the blue component within the spectral range. This tendency is also evident from the recorded EL spectra, exhibited in Fig. 6(f) and (g). Additionally, the broad yellow and red emissions measured for both PGC are influenced by the laser power, especially when it exceeds the luminous saturation threshold, leading to a noticeable luminescence quenching effect. Furthermore, as the laser power intensifies, a blue shift of the primary emission peak of CASN:Eu-PGC is observed, aligning with the temperature-dependent luminescence evaluations for CASN:Eu-PGC. This outcome may furnish us with a pioneering approach for precisely assessing the thermal impact on luminescent characteristics by introducing laser exposure to PGC glass.

In summary, a novel strategy has been developed for the fabrication of highly efficient large-area ultrathin PGC glasses by combining tape-casting process with low-temperature cofiring technique. A super-large size of 1044×45mm was successfully fabricated for PGC glass with typical thickness of ~ 100 µm. Meanwhile, the resulting PGC glasses demonstrate exceptional luminescence properties, bearing a PLQY of 98.6% and 80% for the yellow and red PGC glasses, respectively. Temperature-dependent PL spectra exhibit two opposite abrupt change in emission intensity for both PGC glasses and phosphor powders at T < 300 K. Conversely, a consistent decrease in luminescence is observed as the temperature increases above 300 K for both the PGC glasses and phosphors. This suggests that the high-temperature luminescence of phosphor particles is mostly unaffected by the surrounding glass environment. Utilizing the prepared PGC glass in conjunction with blue LED chips, PGC-LEDs were demonstrated with high-power threshold and exceptional heat dissipation capabilities. Through regulating the weight ratio of the yellow and red phosphors within glass matrix, color tunable emission was achieved for the prepared PGC glass. Furthermore, it is shown that the prepared ultrathin PGC glasses exhibited greater resistance to the blue laser radiation compared to the PCR counterparts. These results indicate the promising prospective of the developed strategy in the manufacture of highly efficient, large-area, ultrathin PGC glasses, which shall greatly promote development of super bright automotive lights.

4.1 Materials: The YAG:Ce and CASN:Eu phosphor powders, commercially available, were purchased from Grirem Advanced Materials Co., Ltd., China, and utilized directly without further treatment. The low-temperature amorphous glass powder, primarily comprising silica and boron elements, was supplied by Corning Incorporated, China. Additionally, the organic components, including anhydrous ethanol, ethyl acetate, oleic acid (OA), polyalkylene glycol (PAG), butyl benzyl phthalate (BBP), and polyvinyl butyral (PVB), were predominantly sourced from Aladdin, China.

4.2 Tape-casting of ultrathin PGC green bodies: Fig. 1a illustrates the detailed manufacturing process of ultrathin YAG:Ce and CASN:Eu-PGC green bodies. Phosphor and glass powders, with the a ratio of 1:9 for YAG:Ce and 3:7 for CASN:Eu-PGC, were thoroughly mixed in a mixture solvent of ethanol and ethyl acetate for 5 hours using a planetary-milling machine. Throughout the ball-milling process, all organic compounds were introduced to fine-tune the rheological properties of the slurry. Fig. S1 presents the rheological characteristics of the synthesized slurry obtained in our study. Subsequently, air was removed from the slurry by employing a vacuum high-speed centrifugal process for a duration of 10 min. The resulting defoamed slurry was then casted into an ultrathin green sheet through a tape-casting process, with the doctor-blade height set at 150 mm with the belt moving at a rate of 0.03 m/min. Following the casting process, the green sheet was dried and stored overnight before undergoing further treatment.

4.3 Preparation of ultrathin YAG:Ce and CASN:Eu PGC：The ultrathin green sheets were placed flat on a specialized quartz plate, and initially heat-treated at 590 ℃ in air for 5 hours with a heating rate of 1 ℃/min using a resistance furnace (TL-1200, Nanjing Boyuntong Instru. Tech. Co.,Ltd. China). This pre-heating process was conducted to remove the organic components within the green sheets. Subsequently, the resulting ultrathin green bodies were further sintered at 750 ℃ for YAG:Ce-PGC and at 800 ℃ for CASN:Eu-PGC in air environment, with a sintering duration of 5 hours. The samples were then allowed to cool naturally to room temperature by cutting off the electric power supply to the furnace. Finally, the produced PGC underwent additional annealing at 350 ℃ for 10 hours prior to characterization.

4.4 Characterization: XRD patterns were obtained on a powder XRD spectrometer (Type D8 Advanced Eco, Bruker, UK) with Cu K_aradiation (l = 1.54 Å) as the radiation source. The continuous scanning rate for the phase determination was 0.1 s/step. The material morphology was studied using a FE-SEM (JSM-8010) equipped with an energy-dispersive X-ray spectroscopy system. HRTEM images of PGC were obtained on a Field emission Transmission Electron Microscope (FEI-Talos F200S) at an accelerating voltage of 200 kV. The PL, PLE and time-resolved PL spectra were measured on a custom-made spectrophotometer with continuous (75 W) and microsecond pulse xenon lamps as excitation sources. Variable-temperature measurements were conducted at 78-500 K by mounting the sample on a cold Cu finger inside a cryostat chamber cooled with liquid nitrogen.

4.5 Fabrication and measurement of encapsulated LED with PGC: To fabricate PGC-LED devices, ultrathin PGC were precisely cut into a circle shape with a dimeter of f=2 mm by using a laser-cutting machine, so that it could match with commercially available 450 blue LED chips. The cut PGC disks were affixed onto the chip, and then heated at 100 ℃ in an oven for 5 h until the PGC tightly adhered onto the chip. After cooling to room temperature, the encapsulated LEDs could be tested for illumination performance. The used blue-chips could be driven by an adjustable electric power, which was used to assess the capability of both yellow and red PGC to withstand high-density radiation. The optical properties of LED devices including the EL spectra, luminous flux, luminous efficiency, CIE color coordinates were measured by an integrated test system (HAAS-2000, Everfine, ATA500).

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

Competing interests

All authors declare that there are no competing interests between them.

Additional information

Supplementary information is available for this paper.

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Highly efficient large-area ultrathin phosphor-glass composites fabricated by tape-casting for super-bright LED lights

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