Review—A Review on Phosphor in Glass as a High Power LED Color Converter

Encapsulation of phosphor materials with an inorganic glass matrix can provide highly improved thermal and chemical stability, which is the primary purpose of PiG, and promote its commercialization. Compared to phosphor in silicone (PiS), improved thermal quenching property of PiG up to 200°C has been demonstrated and was attributed to thermal conductivity of glass materials (typically 0.5–1.2 W/m⋅K) that was higher than silicone resin (∼0.2 W/m⋅K) (Fig. 3a).^25,31–34 Practical reliability of PiG plate has been clearly demonstrated in terms of luminous flux change with aging time under 85°C/85% humidity conditions with a 1 A operation current and a 150°C junction temperature where PiG maintained ∼97% of its luminous flux even after 1,000 hours, while PiS failed before 500 hours³⁵ (Fig. 3b). The reduced luminous efficacy losses and CCT over time have also been reported, which supported the enhanced reliability of PiG over PiS as shown in Figs. 3c and 3d, respecitvely.^31–34 Although the improved stability has been mostly proved by oxide phosphors such as YAG:Ce³⁺, which has high thermal stability, glass can also provide robustness with other phosphors which have relatively low chemical and thermal stability, such as sulfide^36,37 and nitride phosphors.³⁸ Lee et al. successfully fabricated a PiG using the SrGa₂S₄:Eu²⁺ green phosphor, which easily reacted with the atmospheric moisture and showed improved stability against humidity as well as the thermal stability.^36,37 The photoluminescence (PL) and excitation band (PLE) spectra of PiG maintained up to 97% of the initial intensity while the sulfide powder reduced its intensity down to ∼75% when they were immersed in water at 85°C for 48 hours.³⁷ The moisture sensitive SrLiAl₃N₄:Eu²⁺ phosphor was also stabilized via introduction of a glass matrix.³⁸ SrLiAl₃N₄:Eu²⁺-PiG maintained 90% of its PL intensity while the emission intensity of phosphor powder significantly dropped to 2% when they were immersed in water. The PiG also maintained its intensity up to 96% at 150°C while the phosphor powder showed 90% of its initial intensity proving the improved thermal stability.³⁸ Further enhancement of thermal stability of PiG has been accomplished via effective heat dissipation delivered by the graphene coating on the PiG surfaces.^39,40

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After successful demonstration of PiG with YAG:Ce³⁺, various phosphors have been proposed to fabricate PiG. Garnet based oxide phosphors such as Y₃Mg₂AlSi₂O₁₂:Ce³⁺, Y₃Al_4.6Ga_0.4O₁₂:Ce³⁺⁴¹ and Lu₃Al₅O₁₂:Ce³⁺ (LuAG:Ce³⁺),^42–44 which have a high thermal stability similar to YAG:Ce³⁺, have also been fabricated in PiG plate. Other oxide based phosphors were employed to fabricate PiG: namely, La₂Ti₂O₇:Eu³⁺,⁴⁵ BaMgAl_10-2xO₁₇:xMn⁴⁺, xMg²⁺,⁴⁶ Ca₂LaSbO₆:Eu³⁺,⁴⁷ La₂LiSbO₆:Mn⁴⁺, Mg²⁺,⁴⁸ CaO:xCe³⁺,⁴⁹ Li₃Mg₂SbO₆:Mn⁴⁺,⁵⁰ Li₂MgTiO₄:Mn⁴⁺, Hf⁴⁺,⁵¹ La_0.5Na_0.5TiO₃:Eu³⁺,⁵² BaMgAl₁₀O₁₇:Eu²⁺,⁵³ 3.5MgO-0.5MgF₂-GeO₂:Mn⁴⁺,⁵⁴ ZnGa₂O₄:Cr³⁺⁵⁵ and SrB₄O₇:Sm²⁺.⁵⁶ Nitride phosphors such as CaAlSiN₃:Eu²⁺ (CASN:Eu²⁺),^57,58 SrLiAl₃N₄:Eu²⁺³⁸ and La₃Si₆N₁₁:Ce⁵⁹ have also been successfully embedded within a glass matrix to compose PiG. Oxynitride phosphors based on Ca modified Ca_1.8Si_8.4Al_3.6O_0.45N_15.6:Eu_0.04 (Ca-α-SiAlON:Eu²⁺),³⁵ β-SiAlON:Eu²⁺⁶⁰ and Ca₁₄Mg₂Si₈O_28+δN_4-δ:Eu²⁺⁶¹ were also incorporated into a glass matrix to fabricate a LED. A single phosphor or two to three different phosphors were mixed with a transparent glass powder depending on the desired color coordinate or color properties of the packaged LED and will be discussed in the following sections.

Among various phosphors, commercial phosphors such as YAG:Ce³⁺ (yellow), LuAG:Ce³⁺ (green), CASN:Eu²⁺ (red), Ca-α-SiAlON:Eu²⁺ (amber) and β-SiAlON:Eu²⁺ (green) have been widely used for high power LED applications due to their relatively high thermal stability, because it is crucial to avoid possible thermal degradation of phosphors during the sintering process in order to guarantee the color conversion efficiency of the embedded phosphors. Chen et al.⁶² observed the interaction between YAG:Ce³⁺ and a silicate glass matrix with a high resolution transmission electron microscope (HR-TEM) and found that the color converting efficiency was increased as the sintering temperature decreased. Oxidation of Ce³⁺ and the corrosion of the glass matrix that modified the lattice structure around Ce³⁺ were responsible for the efficiency decrease at an elevated temperature.⁶³ Although YAG:Ce³⁺ can be sintered even up to 800°C, CASN:Eu²⁺ loses its emission intensity significantly when it is heat treated above 550°C.⁵⁷ The quantum yield drop of the PiG with CASN:Eu²⁺ above 600°C has also been observed,⁵⁸ which suggests the importance of thermal stability of the phosphor and sintering temperature.

Transparent glass is highly important for embedding phosphors and replacing conventional organic silicone resin. In order to provide a robust inorganic matrix for phosphors, the glass needs to meet several requirements: 1) high transparency in the visible range, 2) low sintering temperature to avoid thermal degradation of phosphors, 3) high chemical and thermal stability to guarantee long term stability, 4) RoHS (restrictions of hazardous substances)-free composition and 5) no reaction with the embedded phosphors. For commercial application, glass materials must be readily available and cost competitive against conventional high power silicone resin. The glass powder also needs to have a powder size preferably less than 10 μm to be thoroughly mixed with phosphor powders.

Various glass systems which have been proposed for PiG are summarized in Table I. As found in the table, mostly oxide glasses, such as silicates, borate, phosphate, heavy-metal oxide and tellurite glasses, were extensively used for PiG along with oxyfluoride glasses. As discussed earlier, the sintering temperature of the glass is highly important as it determines the available phosphors and production cost. Since the SiO₂-B₂O₃-RO (R = Ba, Zn) system was introduced for Pb-free glass for YAG:Ce³⁺ yellow phosphor with a 750°C sintering temperature,²⁵ various glasses were introduced to reduce sintering temperature as well as to embed other phosphors with lower thermal stability than garnet based phosphors such as CASN:Eu²⁺ (red) which mostly need to be sintered below 600°C. Other than yellow phosphor, the introduction of red and green phosphors which have low stability has been crucial for the control of the LED color properties such as chromaticity, CCT, CRI, and color gamut. SiO₂-Na₂O-RO (R = Ba, Zn) glass was suggested to embed YAG:Ce³⁺ and CASN:Eu²⁺ (red) with the sintering temperature of 550°C and showed that the color coordinate and CCT of LED can be easily controlled by the introduction of CASN:Eu²⁺.⁵⁷ Other silicate glasses based on SiO₂-B₂O₃-ZnO-Na₂O⁸³ with a sintering temperature of 550°C showed that the CRI can be improved up to 93. The sintering temperature of silicate glass was decreased to 500°C using a SiO₂–P₂O₅–ZnO–B₂O₃–R₂O system and achieved a high CRI of 93.⁹³ A recent study on silicate glass revealed that the sintering temperature of the glass should be also carefully managed to avoid possible crystallization, which is detrimental for the viscous flow and transparency of the glass.⁶⁹ Although heavy metal oxide glasses based on Bi₂O₃,^63,105–109 PbO^96–103 and Sb₂O₃^111,112 can provide sintering temperature lower than 600°C as shown in Table I, they possess either RoHS elements or elements with visible absorption, which can vary color coordination of the LED, and thus it is difficult to use them for commercial application. Tellurite glasses can also reduce the sintering temperature down to 520°C,^56,122–134 but they also have visible absorption and the high production cost of TeO₂, which hampers mass production. Borate^60,113,114 and phosphate^115–119 glasses can be sintered below 600°C but they have weak chemical stability against moisture. Low sintering temperature below 400°C has been achieved with oxyfluoride glass based on SnO-SnF₂-P₂O₅^135–138 and Bi₂O₃-B₂O₃.¹⁰⁹ SnO-SnF₂-P₂O₅ glasses were sintered even at 230 and 285°C^135,136 but they easily crystallized during the glass preparation and had weak chemical and thermal stability, which restricted their practical feasibility.

Thus, for the moment, it seems that the silicate glass systems are promising for commercial application of PiG considering the production cost and long-term stability. However, it is very important to find an appropriate glass material that has a sintering temperature lower than 400°C as well as high chemical and thermal stability, in order to provide the enhanced stability in the newly developed phosphors with low thermal stability such as K₂SiF₆:Mn⁴⁺ (KSF).¹³⁹

Although a wLED can be realized only with YAG:Ce³⁺ yellow phosphor, its chromaticity, CRI and CCT are confined by the values which YAG:Ce³⁺ allow. As shown in Fig. 2 and Fig. 5a,⁵⁷ the color coordinate of a PiG-mounted wLED with only YAG:Ce³⁺ is located on the linear line depending on the amount of yellow phosphor or PiG thickness. However, the increased demand on the wLED with a high CRI or a low CCT (warm-white) requires manipulation and tunability of chromaticity to compensate for the limited green or red emission intensity of YAG:Ce³⁺. A high CRI is crucial for lighting applications to obtain an object's natural colors under a wLED similar to daylight conditions under the sun. A warm wLED with a low CCT is also important for indoor applications considering the circadian rhythm.¹⁴⁷ As discussed earlier, incorporation of a commercial red phosphor such as CASN:Eu²⁺ along with YAG:Ce³⁺ was enabled by a glass with a low sintering temperature and it demonstrated chromaticity tunable wLEDs following the Planckian locus as shown in Fig. 5⁵⁷ as well as a high CRI up to 93 and a low CCT below 4,100 K.^83,93,94 YAG:Mn⁴⁺ modified with various dopants such as Gd, Lu, Ga, Li, Na, Mg, Ca and Ge has been also mixed with YAG:Ce³⁺ as a red phosphor to make a color tunable white LED with the improved CRI using tellurite glass matrix.^148–150 As the color conversion spectra of the PiG depend on the embedded phosphors, the proper combination of phosphors can easily tune the chromaticity. For example, chromaticity tuning of the wLED has also been carried out by mixing green (LuAG:Ce³⁺) and red (CASN:Eu²⁺) phosphors.^64,77,107 Other combinations of phosphors are also possible for color tunable wLEDs: Y₃Mg₂AlSi₂O₁₂:Ce³⁺ (orange)+Y₃Al_4.6Ga_0.4O₁₂:Ce³⁺ (green),⁴¹ BaMgAl₁₀O₁₇:Mn⁴⁺, Mg²⁺ (red)+YAG:Ce³⁺ (yellow),⁴⁶ La_0.5Na_0.5TiO₃:Eu³⁺ (red)+YAG:Ce³⁺ (yellow)⁵² and so on. Color tunable wLED under UV-LED incorporating three phosphors – CASN:Eu²⁺ (red), Ba₂MgSi₂O₇:Eu²⁺ (green) and (Sr,Ba)₃MgSi₂O₈:Eu²⁺ has been also reported.³² More combinations of phosphors can be found in a previous review²⁹ and Table I.

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Compensation of colors for the phosphors can be also supplied by the doped rare earth ions such as Eu³⁺ within the host glass matrix.^96,151 Additional glass with Eu³⁺ has also been applied on top of the PiG plate with YAG:Ce³⁺ to tune the resultant CCT.¹⁵² However, due to the limited absorption cross section of rare earth ions within a glass matrix, their color tunability has been restricted compared to that of the PiG with multiple phosphors.

The luminous efficacy (LE) of the wLED represents the ratio of the measured luminous flux (lm) of the LED to the given power (W): It is highly important for reducing power consumption. The LE of wLEDs mounted with various PiGs, which was summarized in a previous review,²⁹ mostly depends on the color conversion efficiency of the PiG plate, which is determined by the content and quantum yield of the embedded phosphors, the transparency of the glass matrix and the light extraction efficiency. The quantum yield of phosphors normally does not vary significantly but can deteriorate when they are thermally degraded during the sintering process due to the reaction with the glass matrix or phosphor surface oxidation as discussed earlier. Under the same conditions as for phosphors, the LE of PiG thus relies on the transparency of the glass matrix and the light extraction efficiency of the PiG plate. As introduced earlier, Kim et al.⁹¹ investigated the effect of transmittance on the luminescence properties of PiG and observed that the transmittance decreased with the trapped pores within the glass because scattering reduced the resultant LE. They also found that the proper management of pore size and porosity also improves the LE due to the optimized scattering paths as illustrated in Fig. 6.⁹² The inherent transparency of the glass can be obtained without pores via additional pressure during the sintering process, such as using GPS to improve the LE.³⁵ External impurities during the grinding and screening process of the glass powders is also a major source for light absorption and should be carefully removed to avoid additional loss of transparency and LE.

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Scattering of the excitation (blue) light and the converted light can also occur at the phosphor/glass interface due to the different refractive indexes (RIs) of the phosphor and glass matrix. For example, the RI of YAG:Ce³⁺ is 1.83 while that of the silicate glass (SiO₂-B₂O₃-RO) is ∼1.57,²⁵ which causes the scattering of the blue light as well as the converted yellow light, which reduces the excitation and light extraction efficiency of the phosphors. Thus, employing a glass matrix with a high RI similar to phosphor can increase the light extraction efficiency as well as the LE of PiG. Seo et al.⁹⁰ confirmed the increase of light extraction efficiency and LE of PiG with an increase in the glass RI when PiG was mixed with YAG:Ce³⁺. Zhang et al.¹²⁹ employed a high RI glass based on Sb₂O₃-B₂O₃-TeO₂-ZnO-Na₂O-La₂O₃-BaO for YAG:Ce³⁺ and obtained a high LE of 124.6 lm/W under a high operating current of 350 mA LED for the first time as depicted in Fig. 7. A similar high quantum yield (93%) and high LE up to 137 lm/W was also observed with TeO₂-ZnO-Sb₂O₃-Al₂O₃-B₂O₃-Na₂O-Eu₂O₃ glass.¹⁵³ Chen et al.¹⁵⁴ used Sb₂O₃ based glass (Sb₂O₃-ZnO-K₂O-B₂O₃) for YAG:Ce³⁺ and obtained a high quantum yield of 94% as well as a high LE of 130 lm/W under a 350 mA operating current.

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The effect of the refractive index of the glass matrix on the light extraction efficiency has also been observed from a screen printed PiG thick film.⁸⁸ For the thick film type PiG, as exhibited in Fig. 8, a multi-layered PiG thick film with glasses with different RIs was proposed to obtain a graded refractive index film and to increase the light extraction efficiency.^89,118 The light extraction efficiency of the PiG thick film can be further improved reducing the total internal reflection at the interface between the glass and air. Wang et al.¹⁵⁵ and Xu et al.⁸² created a textured structure on the surface of a substrate glass via chemical wet etching and confirmed an increase in luminous efficacy.

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The LE of a wLED can also be improved by avoiding the re-absorption of the converted light by the neighboring phosphors. When a green or yellow phosphor is mixed with a red phosphor, the emission can be absorbed by a nearby red phosphor due to a spectral overlap that reduces the emission intensity and color purity. Thus, separation of the phosphor layers has been suggested. Peng et al.⁷⁵ prepared a multi-layered PiG that has different phosphors with varying stacking order such as red (R)-green (G)-blue (B) and G-R-B to fabricate a wLED under UV-excitation as depicted in Fig. 9a. The LE of the multi-layered PiG was improved over that of the PiG with mixed-RGB phosphors due to the reduced re-absorption. Lee et al.¹⁵⁶ and Kim et al.⁷⁸ proposed a segmented PiG plate which is assembled by separately prepared green and red PiGs as shown in Fig. 9b. The segmented PiG showed improved green emission with a marginal increase in LE, which suggested reduced reabsorption compared to the PiG with mixed R-G phosphors. Similar segmented PiGs with various patterns have also been fabricated via screen printing as displayed in Fig. 9c, and they had improved luminous properties.^29,157 Additional introduction of a crater-type resin on top of the patterned PiG further improved the LE as well as the uniformity of the CCT, which will be discussed in the following section (Fig. 9d).¹⁵⁸ Micro-structure array (MSA) on top of the patterned PiG has also been employed to enhance the LE as well as the angular color distribution via broadening the light escaping area (Fig. 9e).¹⁵⁹

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When blue LED light passes through a color converting material, the optical path lengths of the rays are different depending on the outgoing angles of the rays from the blue LED chip. Thus, when YAG:Ce³⁺ is used for a wLED, the light that comes out of the edge of the color converting material experience long interaction length with the phosphor and shifts the chromaticity to yellow with a low CCT value while the center shows the white causing a 'yellow-ring effect'.^160,161 To provide high color and CCT uniformity of a wLED, Yi et al.⁹⁸ introduced ZrO₂ powders within the glass matrix to act as a light diffuser and mounted the ZrO₂ embedded glass plate on top of the PiG plate with YAG:Ce³⁺ as exhibited in Fig. 10a, and thus demonstrated improved angle dependency of the CCT. The angle dependency was also reduced with multi-layered PiG thick films forming a cone shape to compensate for the path length as displayed in Fig. 10b.⁹⁹ A substrate glass with a textured surface that induces scattering also improved the angular color distribution.¹⁵⁵ The patterned structure for PiG plates or screen printed PiGs as shown in Fig. 9 has also been reported to improve color uniformity^78,157–159 via spatial distribution of colored PiG and adjustment of light path lengths or scattering at the surface. The additional layers for color uniformity can reduce the output intensity which needs to be compromised to achieve high quality white light.

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The current liquid crystal display (LCD) adapts the wLED as a white light source in the back light unit (BLU), and its color reproduction range relies on the primary R-G-B colors which are realized by the color filters of the LCD. Thus, the spectrum match of the wLED with color filters determines the R, G, B colors, but the broad emission bandwidth of the conventional wLED, which uses green and red phosphors embedded in silicone resin (RG-LED), restricts the color reproduction range (color gamut) at ∼75% to that of the range defined by the national television standard committee (NTSC).¹⁶² Han et al. thus employed a Nd³⁺-doped glass on top of the conventional RG-LED to modify the emission bandwidth via a strong 4f-4f transition of Nd³⁺:⁴I_9/2→ ⁴G_5/2,²G_7/2 and successfully expanded the color gamut up to ∼ 82% of the NTSC standard.¹⁶² As the previous work used silicone resin with inherent weak long-term stability, Lee et al. doped the silicate glass with the Nd³⁺-ion and fabricated a PiG with LuAG:Ce³⁺ (green) and CASN:Eu²⁺ (red) phosphors.¹⁶³ As shown in Fig. 11, the emission spectrum of the wLED with PiG and R and G phosphors (RG-PiG) has also been successfully modified, and its color gamut has been improved from 66 to 80% of the NTSC standard with an increasing Nd₂O₃ concentration. Similarly, Zhang et al. formed NdF₃ nano-crystals within a silicate glass which was mounted on top of the RG-PiG as a color filter and demonstrated an improved color gamut up to ∼79% of the NTSC standard.¹⁶⁴ The glass with NdF₃ nanocrystals was also used as a host matrix for R-G phosphors but it experienced a limited increase in the color gamut up to 75% of the NTSC standard.¹⁶⁵

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Due to the enhanced long-term stability of PiG under high temperature and humidity conditions and low manufacturing cost, it has been used in high brightness and high power LED applications to replace conventional PiS in applications such as automotive headlamps. While Philips Lumileds released LUXEON, adapting YAG:Ce³⁺ CPP as an inorganic color converter in 2008 and applied it to automotive headlamps, PiG was commercially applied to automotive headlamps by Samsung Electronics for the first time in 2013 employing silicate glass and later by other LED manufacturers. As discussed earlier, reasonable stability and LE of a wLED mounted with PiG at a high operation current have been reported by many researchers.^{29,129,153,154} Similar to LUXEON, PiG with YAG:Ce³⁺ has been packaged with a flip-chip LED and demonstrated its feasibility for automotive headlamps.¹⁶⁶ Lin et al.¹²⁸ demonstrated a high powered wLED based on an AC-LED that minimized its flickering effect with the afterglow of a PiG plate as illustrated in Fig. 12a. Along with the white headlamp, a monochromatic amber LED has also been proposed as a turning signal light using PiG with Ca-α-SiAlON:Eu²⁺³⁵ or mixed phosphors with YAG:Ce³⁺ and CASN:Eu²⁺.⁹⁵ The high power wLED can be also used for a lighting lamp, which was demonstrated by Zhang et al. as shown in Fig. 12b.³⁴

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Recently, a blue laser diode (LD) was considered for the light source of an advanced automotive headlamp with its higher optical power and linearity of light to extend the visible distance. Application of PiG for a LD to produce white light has been reported by several groups.^{34,68,73,79,87,110,143} Zheng et al.⁷³ produced a high luminance of white lighting using PiG and a blue LD. They introduced PiG thick film with YAG:Ce³⁺, YAG:Ce³⁺+α-SiAlON or YAG:Ce³⁺+SCASN to show color tunability and a high luminous flux up to 1,839 lm with a high LE of 210 lm/W under 11.2 W/mm LD excitation (Fig. 13a). Recently, Chang et al.¹⁶⁷ successfully composed a laser headlight module (LHM) with five blue LDs with a total output power of 6 W and PiG with YAG:Ce³⁺ as depicted in Fig. 13b and achieved 1,860 lm with a CCT of 4100 K and a high LE of 310 lm/W, which indicates a high performance LHM. Green phosphors such as β-SiAlON,^60,72 LuAG:Ce³⁺⁴⁴ and La₃Si₆N₁₁:Ce⁵⁹ have also been mixed with glass to prepare a PiG for a thermally robust color converter under a high power blue LD.

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Heat dissipation is highly important, especially for high power applications of LED and LDs. As introduced earlier, materials with high thermal conductivity have been attached to the PiG to effectively dissipate the generated heat at a high operation current such as graphene coating layers on the PiG plate^39,40 (Fig. 14a) and a sapphire substrate for PiG thick films.^87,143 Park et al.⁷⁴ fabricated a phosphor (YAG:Ce³⁺)-aluminum composite via PiG layer formation on the aluminum surface and effectively reduced the maximum surface temperature from 258 to 137°C obtaining 430 lm at 4 W blue LD output power (Fig. 14b).

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Besides the high power LED applications, as discussed earlier, PiG can be used for the light source of displays such as LCDs, although further improvement is necessary for the color gamut to meet the requirements for high picture quality. Due to the robustness of PiG, it can be also applied to other lighting applications such as LEDs for artificial plant growth. Deng et al.⁵⁴ synthesized PiG with 3.5MgO-0.5MgF₂⋅GeO₂:Mn⁴⁺ for deep red emission and observed an improved cultivation effect compared with the conventional LED due to its enhanced color stability as shown in Fig. 15a. Since PiG provides a flat surface along with a color converting property, it can be used as a substrate for organic LED (OLED) fabrication. Kim et al. fabricated a blue OLED on a YAG:Ce³⁺ PiG plate and demonstrated the potential of a color tunable all-in-one OLED as depicted in Fig. 15b.¹⁶⁸ PiG has also been used as an efficient down converter to improve the conversion efficiency of perovskite solar cells with high UV stability.¹⁶⁹

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If PiG contains a phosphor with deep-trap persistent luminescent materials, it can be used for multidimensional and rewritable optical information storage.^117,170 As shown in Fig. 16a, information written in a blue laser is invisible and can be retrieved by high temperature thermal stimulation or photo-stimulation with other laser sources with different wavelengths. Li et al.¹⁷⁰ synthesized a PiG film with Y₃Al_5-xGa_xO₁₂:Ce³⁺,V³⁺ (x = 0-3) and successfully demonstrated the optical writing with a 450 nm blue laser and read-out via heat-treatment at 250°C or photostimulation under 808 nm laser excitation as observed in Fig. 16.

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