Recent Advances in Preparing Transparent Phosphor Ceramics for High-Index Color Rendering and High-Power Lighting

Abstract

In recent years, high-power white light-emitting diode (wLED)/laser diode (wLD) lighting sources based on transparent phosphor ceramic (TPC) materials have attracted increasing application interest in automotive headlights, projection displays, and space navigation lighting due to their superior brightness, lighting distance, compactness, lifespan, and environmental resistance compared with the widely used phosphor-converted wLEDs. However, preparing TPC-converted wLEDs/wLDs with high color rendering index (CRI) remains a huge challenge, which limits their widespread application. In this review, we summarize the recently adopted strategies for constructing TPCs to develop high-power wLEDs/wLDs with high CRI values (>75). The construction protocols were categorized into four groups: host regulation, red-emitter doping, host regulation/red-emitter doping combination, and composite structure design. A comprehensive discussion was conducted on the design principles, photoluminescent properties, and device performances for each strategy. The challenges and future trends of high-power and high-CRI wLEDs/wLDs based on TPCs are also discussed toward the end of this review.

1. Introduction

The first decades of this century have witnessed an extraordinary revolution in the lighting industry, as blue light-emitting diodes (LEDs) were cleverly combined with yellow Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce³⁺) phosphors dispersed in organic resins to generate white light [1]. Due to their advantages of low energy consumption, non-pollution, and long service life, these phosphor-converted white LEDs (pc-wLEDs) have quickly replaced traditional incandescent and fluorescent lamps in a short time to become the mainstream lighting sources [2,3,4]. However, the thermal conductivity of organic resins is low, and thermal degradation occurs under the accumulated thermal radiation of the LED chip, which inevitably reduces the luminous efficiency (LE) and color quality of the pc-wLED devices during long-term use [5,6,7]. As the power of LEDs increases, this problem will worsen significantly. In order to achieve high-power lighting, alternative color converters with high thermal conductivity and strong resistance against thermal degradation are required.

Recently, a range of color converters for high-power wLEDs or white laser diode (wLD) lighting devices have been reported, including phosphor films [8,9,10,11,12], phosphors in glass [13,14,15,16,17,18,19,20,21,22], glass ceramics [23,24,25,26,27], single crystal phosphors [28,29,30,31,32,33], and transparent phosphor ceramics (TPCs) [34,35,36,37,38]. Among them, TPCs have gained particular attention due to their high thermal conductivity, excellent heat resistance, high mechanical strength, high homogeneity, and high LE [34,35,39,40,41]. The first TPC-converted wLED was reported in 2008 [42], in which YAG:Ce³⁺ ceramic was used. Since then, YAG:Ce³⁺ TPCs have been widely studied, and have shown broad application prospects in automotive headlights, projection displays, space navigation lighting, and other fields. However, the wLEDs/wLDs constructed with YAG:Ce³⁺ TPCs show low color rendering index (CRI) due to the lack of red emission component [34,38,40,43], which restricts their practical application.

To address the issue, many approaches to enhance the CRI values of YAG:Ce³⁺ TPC-converted wLEDs/wLDs have been explored in recent years. However, a systematic summary of this topic is still lacking. With the growing research interest in high-power and high-CRI lighting [16,44,45,46], a summary of the latest research results in this aspect is meaningful for junior researchers who are going to start their research in this area. Therefore, this review comprehensively summarizes the reported preparation methods for TPCs for constructing wLED/wLD lighting sources with high power and high CRI values (>75). The construction strategies were divided into four general categories: host regulation, red-emitter doping, host regulation/red-emitter doping combination, and composite structure design. For each strategy, design principles, photoluminescent (PL) properties, PL tuning mechanisms, and device performances were extensively discussed. Finally, the current challenges and future prospects of high-power and high-CRI wLED/wLD lighting systems are also presented. We believe that a comprehensive summary of construction strategies from foundation to application will benefit junior researchers in developing advanced lighting systems.

2. Modifying the YAG Host to Redshift the Emission or/and Widen the Emission Band of Ce³⁺

As a well-known phosphor host, YAG has a garnet structure (Figure 1) with a general formula of Y₃Al(1)₂Al(2)₃O₁₂, where Y, Al(1), and Al(2) are coordinated with 8, 6, and 4 oxygen atoms to form a dodecahedron, octahedron, and tetrahedron, respectively. The most widely used yellow phosphor YAG:Ce³⁺ is obtained by replacing a proportion of the Y³⁺ ions with Ce³⁺. Through further substitution/co-substitution of proper cations on the Y, Al(1), or Al(2) sites, the crystal field of the host lattice can be tailored, which results in PL tuning of YAG:Ce³⁺. Notably, the cation substitution occurs only when the coordination numbers (CNs) are equal and the ionic radii are similar. In order to better understand all material systems in this review, the ionic radii for a given CN of all relevant ions are listed in

Table 1. The ionic radii (Å) for a given CN of all relevant ions in this review refers to the YAG lattice.

Ions	CN = 4	CN = 6	CN = 8
Y3+	—	—	1.019
Al3+	0.39	0.535	—
Ce3+	—	1.01	1.143
Gd3+	—	0.938	1.053
Tb3+	—	0.923	1.04
Lu3+	—	0.861	0.977
Mg2+	0.57	0.72	0.89
Si4+	0.26	0.4	—
Cr3+	—	0.615	—
Mn2+	0.66	0.67	0.96
Pr3+	—	0.99	1.126
Sm3+	—	0.958	1.079

.

For YAG:Ce³⁺-based photo-converters, an effective way to add red emission components to their PL spectra is to adjust the crystal field of the 5d orbital of Ce³⁺, which is capable of shifting the emission peak of Ce³⁺ to longer wavelengths or/and broadening the full width at half maximum (FWHM) of the emission band. This can be accomplished by modifying the YAG garnet host via two general technical approaches: one is to replace the dodecahedral Y³⁺ site with ions with larger ionic radii, such as Gd³⁺ or Tb³⁺; the other is to replace octahedral/tetrahedral Al³⁺ with Ca²⁺-Si⁴⁺, Mg²⁺-Si⁴⁺, or Mg²⁺-Ge⁴⁺ ion pairs with longer bond lengths [48,49,50,51,52,53]. Using this strategy, a series of YAG:Ce³⁺-based ceramics with high CRI have been reported.

In 2011, Nishiura et al. [54] synthesized a series of transparent (Gd_γY_1-γ)₃Al₅O₁₂:Ce³⁺ (γ = 0–0.75) ceramics with thicknesses of 0.1–1.1 mm through the vacuum sintering technique. The 5d₁ → 4f transition of Ce³⁺ led to broad PL emission bands for all samples. As the content of Gd³⁺ increased, the peak of the emission bands shifted from 530 nm to 560 nm (Figure 2a), and the sample color changed from greenish yellow to yellow, which can be attributed to the increase in crystal field splitting (CFS) upon the substitution of Y³⁺ with Gd³⁺. Under the excitation of a 465 nm LED, the color coordinates shifted from blue to the yellow or orange region upon increasing the thickness of the samples from 0.1–1.1 mm (Figure 2b). For samples with same thickness, the color coordinates redshifted with the increase of Gd³⁺ content. When connecting the color coordinates of the sample with the same Gd³⁺ content with a straight line, it was found that the straight lines of (Gd_γY_1−γ)₃Al₅O₁₂:Ce³⁺ (γ = 0.25–0.75) passed through the theoretical white point of (0.33, 0.33), while that of the YAG:Ce³⁺ ceramic did not. In other words, by the substitution of Gd³⁺ for Y³⁺, the emission color can be adjusted to pure or warmer white upon optimization of the sample thickness. As summarized in Figure 2c, with the increase of the Gd³⁺ content, the CRI value of the samples increased from 65 to 81, indicating their suitability as promising candidate color converters for high-power wLED/wLD lighting.

To further investigate the LE of the material, in 2015, Chen et al. [55] also introduced Gd³⁺ into YAG:Ce³⁺ ceramics. The extent of redshift of the PL peak was proportional to the doping concentration of Gd³⁺. When combined with the InGaN blue chip, a CRI of up to 78.1 and a LE of 128.15 lm/W were obtained at an optimal Gd³⁺ concentration of around 10%.

Taking advantage of the rich red emission components and readily adjusted luminescent properties of (Gd,Y,Lu)₃(Al,Ga)₅O₁₂:Ce³⁺ garnets [56], Liu et al. [57] designed and synthesized a series of highly transparent Gd₃Al₄GaO₁₂:xCe³⁺ (GAGG:xCe³⁺, x = 0.25–1.00%) ceramics with thicknesses of 0.45, 0.65, 0.85, and 1.05 mm by sintering under an oxygen atmosphere. The broad emission bands (FWHM = 133–137 nm) and abundant red emission components (λ_em = 568–574 nm) of the GAGG:xCe³⁺ ceramics enable warm white light upon excitation by blue LED or LD chips (Figure 3a–h,a’). According to Figure 3b’–h’, with the increase of the concentration of Ce³⁺ or thickness of the samples, more and more blue light emitted from the chips was absorbed, resulting in an increase in the emission ratio of GAGG:Ce³⁺. As a result, the white emission color shifted from cool to warm and the CRI reached 78.9. When fabricating GAGG:0.75%Ce³⁺ ceramic (thickness = 1.05 mm) with a 445 nm blue LD of 2 W, the strong blue light (Figure 3i, left) changed to warm white with a CCT of 3053 K (Figure 3i, right), indicating the feasibility of the material for high-power wLEDs/wLDs with a comfortable warm white light.

In 2020, Sun et al. [58] selected Lu₂Mg₂Al₂Si₂O₁₂:Ce³⁺ (LMAS:Ce³⁺) phosphor with a broad emission band (λ_em = 575 nm, FWHM = 144 nm) [59] and synthesized LMAS:Ce³⁺ phosphor ceramic using the vacuum sintering technique. As shown in Figure 4a, a series of LMAS:xCe³⁺ (x = 0.02–0.12) ceramic plates (diameter of 11 mm, thickness of 1 mm) was prepared. The samples showed broad emission bands spanning 475–800 nm when excited at 455 nm (Figure 4b), which could be ascribed to the 5d¹ → 4f transition of Ce³⁺ ions (occupying the dodecahedral Lu³⁺ sites). When x = 0.08, the emission intensity reached the maximum value, displaying a broad yellow-orange peak at 565 nm with a FWHM of 130 nm. According to the thermal quenching properties shown in Figure 4c, the emission intensity of the LMAS:0.08Ce³⁺ ceramic plate at 150 °C decreased to 81.0% of the intensity at room temperature, while the pure LMAS:0.08Ce³⁺ phosphor decreased to 74.7% [59], indicating superior thermal stability of the ceramic plate compared with the phosphor. wLD light was obtained when LMAS:Ce³⁺ ceramic plates were fabricated with a blue LD in reflection mode (Figure 4d). When the laser power increased from 1 W to 3.3 W, the LMAS:0.08Ce³⁺ ceramic plate exhibited high CRI values (76.0–77.7) in consequence of the broadband emissive characteristic (Figure 4e,f).

3. Co-Doping Red-Emitting Cr³⁺, Pr³⁺, or Mn²⁺ Ions into YAG:Ce³⁺

Another approach to supplement the red-emission component of YAG:Ce³⁺ ceramics is to co-doping red-light-emitting ions such as Cr³⁺, Pr³⁺, or Sm³⁺ into the phosphor ceramic material.

3.1. Co-Doping Cr³⁺ Ions into YAG:Ce³⁺

In 2021, Lu et al. [60] prepared Ce³⁺ and Cr³⁺ co-doped Al₂O₃-YAG eutectics through the micro-pulling-down method. X-ray diffraction (XRD) and scanning electron microscopy and energy dispersive spectroscopy (SEM-EDS) results showed substitution of Y³⁺ with Ce³⁺ and occupation of octahedral Al³⁺ sites by Cr³⁺. The co-luminescence of Ce³⁺ (λ_em = 543 nm, 5d₁ → 4f transition) and Cr³⁺ (λ_em = 694 nm, ²E → ⁴A₂ transition) ions in Al₂O₃-YAG:Ce³⁺,Cr³⁺ significantly enhanced the red-emission component. However, since the emission band of Ce³⁺ overlapped with the absorption band of Cr³⁺, energy transfer occurred from Ce³⁺ to Cr³⁺, which quenched the yellow emission of Ce³⁺. TPC converted LED based on Al₂O₃-YAG:Ce³⁺,Cr³⁺ emitted only blue light from the LED chip and red light from Cr³⁺ ions. To address the issue, an Al₂O₃-YAG:Ce³⁺ and Al₂O₃-YAG:Cr³⁺ stacked TPC converted WLED was fabricated, the CRI of which reached 78.7.

In 2022, Liu et al. [61] further investigated the influence of Cr³⁺ doping concentration on the luminescent properties of Ce/Cr co-doped Al₂O₃/YAG eutectic phosphor ceramics. The ceramics were prepared and fabricated through high-temperature sintering and the laser floating zone melting method. Upon adjustment of the concentrations of Cr³⁺ (0.2–0.8 wt%), the Ce³⁺ → Cr³⁺ energy transfer and luminescent properties of the ceramics were finely tuned. As shown in Figure 5a, after Cr³⁺ doping, strong red light emission centered at 694 nm appeared, which increased with the increase in Cr³⁺ concentration. At the same time, the emission intensity of Ce³⁺ around 550 nm decreased dramatically due to enhanced energy transfer from Ce³⁺ to Cr³⁺. As the concentration of Cr³⁺ increased, CRI first increased and then gradually decreased (Figure 5b), which can be rationalized by the suppression of yellow light emission. When the concentration of Cr³⁺ was equal to 0.4 wt%, CRI reached a maximum value of 80. Despite the improvement in CRI, overall LE declined (Figure 5b).

In 2023, Li et al. [62] prepared Y_2.94Al_5−xO₁₂:0.06Ce³⁺,xCr³⁺ (x = 0.004, 0.029, 0.054, 0.079, 0.104 mol) ceramics, in view of the wide emission band of Cr³⁺ spanning from 650 to 750 nm. As shown in Figure 6a, weak emission bands from 675 to 720 nm appeared upon the doping of Cr³⁺ into YAG:0.06Ce³⁺ ceramics. With the increase in the Cr³⁺ content, the CRI displayed a remarkable increase from 65.1 to 79.4 (Figure 6b). Therefore, the Ce³⁺, Cr³⁺ co-doped garnet ceramics are promising candidates for high-CRI lighting applications of wLEDs/wLDs.

3.2. Co-Doping Mn²⁺ Ions into YAG:Ce³⁺

As summarized in Table 1, Mn²⁺ possesses multiple coordination numbers (CN) and feasibly occupies the tetrahedral Al³⁺ site, octahedral Al³⁺ site, and the dodecahedral Y³⁺ site in YAG host. Due to the spin-forbidden ⁴T₁ → ⁶A₁ d-d transition of Mn²⁺ into different occupation sites in the YAG lattice, the emission of Mn²⁺ can span from green to deep red depending on the specific coordination environment. In particular, Mn²⁺ at the octahedral position emits orange-red light, which is beneficial for promoting CRI. In this regard, Ao et al. [63] introduced Mn²⁺ into YAG:Ce³⁺ ceramics by preparing a series of Y_2.994Ce_0.006Al_5−2xMn_xSi_xO₁₂ (YAG:Ce³⁺,xMn²⁺,xSi⁴⁺) (x = 0, 0.01, 0.02, 0.04, 0.08, 0.16) ceramics via solid-state reaction under vacuum conditions (Figure 7a). The co-doping of Si⁴⁺ was for charge compensation. XRD data, together with the PL properties of Y₃Al_4.98Mn_0.02O₁₂, indicated that Mn²⁺ ions tended to occupy the position of octahedral Al³⁺, while Si⁴⁺ ions were located at the position of tetrahedral Al³⁺. As shown in Figure 7b, upon the doping of Mn²⁺/Si⁴⁺ pairs, the green emission band of Ce³⁺ (535 nm) as well as orange (587 nm, Mn²⁺ occupying the octahedron Al³⁺ site) and red (740 nm, Mn²⁺ occupying the dodecahedron Y³⁺ site) emission bands were observed, indicating the successful complementing of the red emission. It is worth noting that the emission band centered at 740 nm exceeds the cutoff wavelength (690 nm) of CIE 1931 color matching function, as the human eye cannot effectively perceive anything beyond 690 nm. Therefore, in wLEDs/wLDs, only the orange-red emission at 587 nm has significance in improving the CRI. As the concentration of Mn²⁺/Si⁴⁺ pairs increased, the emission intensity of Ce³⁺ decreased, while the emission intensity ratio of Mn²⁺ to Ce³⁺ increased (Figure 7b). The results indicate efficient energy transfer from Ce³⁺ to Mn²⁺, which is further evidenced by the shortened lifetime of Ce³⁺ in fluorescence lifetime decay studies. This energy transfer facilitates the realization of tunable emission color from yellow toward the orange-red region, as demonstrated by the changes of CIE chromaticity coordinates (x, y) in Figure 7c,d. When x = 0.16, a CRI as high as 75 was obtained, which is promising for warm white light application.

3.3. Co-Doping Cr³⁺ and Pr³⁺ Ions into YAG:Ce³⁺

Pr³⁺ ion is a well-known orange-red light emitter upon irradiation of blue LEDs. Therefore, similar to the Ce³⁺,Cr³⁺ co-doped garnet ceramics, the doping of Pr³⁺ into YAG:Ce³⁺ ceramic can also supply the red spectral component and improve CRI performance. In 2017, Feng et al. [64] introduced both Cr³⁺ and Pr³⁺ into YAG:Ce³⁺ ceramics to expand the emission spectrum. The YAG:Ce³⁺,Pr³⁺,Cr³⁺ ceramics were prepared through a solid-state reaction–vacuum sintering approach. The introduction of Pr³⁺ and Cr³⁺ changed the yellow color of YAG:Ce³⁺ ceramic to orange-yellow and yellow-green (Figure 8a). Analysis of the XRD diffraction patterns confirmed that Ce³⁺ and Pr³⁺ occupied the position of Y³⁺, and Cr³⁺ resided on the position of octahedral Al³⁺. Under 450 nm excitation, characteristic emission peaks of Ce³⁺, Pr³⁺, and Cr³⁺ at 530, 609, and 689 nm appeared, enabling wide emission spectra spanning 500 to 750 nm (Figure 8b). Investigation of the fluorescence decay curves confirmed energy transfers of Ce³⁺ → Pr³⁺ and Ce³⁺ → Cr³⁺, consistent with the studies by Lu et al. [60]. Through systematic optimization of the doping concentration of Ce³⁺, Pr³⁺, and Cr³⁺, high-quality white light with CRI reaching 78 (Figure 8c) was obtained by fabricating the YAG:Ce³⁺,Pr³⁺,Cr³⁺ ceramics with blue LED.

3.4. Co-Doping Pr³⁺ and Mn²⁺ Ions into YAG:Ce³⁺

In view of the orange light emission from Mn²⁺ and red light emission of Pr³⁺, Ma et al. [65] doped Mn²⁺ and Pr³⁺ synchronously into YAG:Ce³⁺ ceramics using a solid-state vacuum sintering technique. Due to the similarity of ion radii, in the YAG lattice, Pr³⁺ (1.126 Å, CN = 8) occupied the position of dodecahedral Y³⁺ (1.019 Å, CN = 8), while Mn²⁺ (0.67 Å, CN = 6) took up the position of octahedral Al³⁺ (0.535 Å, CN = 6), which was further confirmed by XRD patterns. The as-prepared single-structured YAG:Ce³⁺,Mn²⁺,Pr³⁺ ceramics exhibited typical emission peaks of Ce³⁺ (545 nm), Mn²⁺ (580 nm), and Pr³⁺ (609 nm) in the fluorescent spectra (Figure 9a). As the concentrations of Mn²⁺ and Pr³⁺ were adjusted, the FWHM of the emission spectrum increased from 91.7 nm to 102.2 nm (Figure 9b), which was conducive to achieving high CRI. When YAG:Ce³⁺,Mn²⁺,Pr³⁺ ceramics were assembled with a blue LED, the obtained CRI of the wLED device reached 84.8 for Pr02Mn08 (Figure 9c).

4. Combining the Strategies of 2 and 3

As presented above, the red emission composition of YAG:Ce³⁺ ceramics can be increased by adjusting the YAG lattice to modulate the crystal field of the Ce³⁺ 5d orbit, or by adding red-light-emitting ions through synchronous doping. Attempts were also made to explore the synergistic effects of these two strategies.

4.1. Co-Doping Mn²⁺ Ions into TAG:Ce³⁺

In 2021, Ma et al. [52] prepared a series of (Y,Tb)₃(Al,Mn)₅O₁₂:Ce (YTAMG:Ce³⁺) ceramics with different concentrations of Tb³⁺ and Mn²⁺ (Figure 10a) through a vacuum sintering technique. Due to the larger ionic radius of Tb³⁺ (1.04 Å, CN = 8) compared with that of Y³⁺ (1.019 Å, CN = 8), replacing Y³⁺ with Tb³⁺ will strengthen the crystal field around the Ce³⁺ ion. As confirmed by the refinement of the crystal structure of YTAMG:Ce³⁺ ceramics, the CFS of Ce³⁺ was enhanced via Tb³⁺ doping (Figure 10b,c). The promotion of CFS resulted in redshift of the emission band of Ce³⁺. In addition, the orange-red emission of Mn²⁺ (occupying the octahedral Al³⁺) further compensates for the red emission component. As a result of the dual effects, the emission band of the YTAMG:Ce³⁺ ceramics exhibited a redshift of 12 nm and an increase in FWHM (16.2%) compared with that of the YAG:Ce³⁺ ceramic (Tb00Mn00) (Figure 10d). When YTAMG:Ce ceramics were fabricated with blue LED chips, the resulting wLED exhibited a CRI of 79.8 for Tb15Mn08.

In another work presented by Duan et al. [66], Y³⁺ was not partially but instead completely replaced with Tb³⁺. They prepared a series of Tb_2.997Al_5−2xSi_xO₁₂:0.003Ce³⁺,xMn²⁺ (S0–S4) phosphor ceramics with different contents of Mn²⁺-Si⁴⁺ (Figure 11a) via high-temperature solid-state reaction under oxygen. The synergistic effect of Tb³⁺ and Mn²⁺ resulted in orange-red emission in the case of S4, with an emission peak reaching 605 nm (Figure 11b). It is worth noting that wLED devices with high CRI (76.8–84.4) have been achieved by using a double-layer ceramic structure, where S0 is installed on the blue LED chip and S1–S4 are located on S0 respectively (Figure 11c).

4.2. Co-Doping Mn²⁺ Ions into LuAG:Ce³⁺

Although modifying the YAG lattice with ions with larger ionic radii or ion pairs with longer bond lengths can redshift and/or widen the emission band of Ce³⁺, the thermal stability of the ceramic may be reduced if there is a large ion mismatch between the doped ions and the Y³⁺/Al³⁺ ions in the YAG lattice. In contrast, if the Y³⁺ ions in the YAG are completely replaced with Lu³⁺ ions with smaller ionic radius to form Lu₃Al₅O₁₂ (LuAG), the thermal stability can be significantly improved [67]. In addition, Lu_3−xAl₅O₁₂:xCe³⁺ (LuAG:Ce³⁺) exhibits greater absorption coefficient and higher quantum yield than YAG:Ce³⁺ [68,69,70,71]. Therefore, LuAG:Ce³⁺ ceramics have excellent heat-quenching performance and broad application prospects in automotive headlights, biomedical equipment, and projection systems. However, as LuAG:Ce³⁺ ceramics emit green light, the lack of red emission components in the PL spectra severely limits their practical application.

Since hexacoordinate Mn²⁺ ions display orange-red emission (⁴T₁ → ⁶A₁ transition of Mn²⁺) [72,73] and are feasible to be doped into LuAG:Ce³⁺ ceramics [70,74], Yang et al. prepared a series of LuAG:Ce³⁺,Mn²⁺ ceramics through a solid-state vacuum sintering technique [75]. The occupation of the octahedral Al³⁺ sites by Mn²⁺ ions enables orange light emission peaking at around 588 nm due to the strong crystal field (Figure 12a). As the content of Mn²⁺ increases, a small number of Mn²⁺ ions occupy the dodecahedron Lu³⁺ sites, resulting in far-red emission light peaking at 750 nm. In addition, due to the efficient energy transfer from Ce³⁺ to Mn²⁺ (5d₁ → ⁴T₁), with the increase of Mn²⁺ concentration, the emission intensity of the Ce³⁺ ions (5d₁ → 4f, occupying the dodecahedral Lu³⁺ sites) at 505 nm decreases, while the emission intensity of Mn²⁺ at 588 nm increases (Figure 12a, inset). As a result, by controlling the doping concentration of Ce³⁺ and Mn²⁺-Si⁴⁺ (Si⁴⁺ ions as charge balancer to replace tetrahedral Al³⁺ sites), the emission spectrum of Lu_2.98Al_5−2xSi_xO₁₂:0.02Ce³⁺,xMn²⁺ (x = 0–0.12) can be adjusted in the range of 500 to 800 nm (Figure 12a), almost covering visible light. Under ultraviolet irradiation, the samples emit color from green to yellow and orange (Figure 12b). In addition, the materials exhibit excellent thermal stability. When x = 0.01, the PL properties remain at 99% of the initial value at 425 K. When assembled with high-power blue LDs, the sample (x = 0.12) has a high CRI of 80.1 (Figure 12c), a correlation CCT of 3298 K, and an LE of 68 lm/W, showing good application potential in museum lighting.

To further improve the LE and CRI of the above-mentioned LuAG:Ce³⁺,Mn²⁺ ceramics, Ling et al. [76] also co-doped Ce³⁺ and Mn²⁺ into LuAG-based ceramics. A series of Lu_2.998Al_5−2ySi_yO₁₂:0.002Ce³⁺,yMn²⁺ (y = 0, 0.01, 0.03, 0.05, 0.07, 0.09) was synthesized via solid-state vacuum sintering, denoted as Ce02Mn0, Ce02Mn1, Ce02Mn3, Ce02Mn5, Ce02Mn7, and Ce02Mn9, respectively (Figure 13a). By carefully adjusting the occupied sites of Mn²⁺, the proportion of tricolor (orange, green, and blue) components can be changed, and the red emission centered at 590 nm and 750 nm can be effectively compensated. As a result, CRI achieved significant improvement. After assembling the LuAG:Ce³⁺,Mn²⁺ ceramics with blue LED, the variation of chromaticity parameters with Mn²⁺ doping concentration was systematically investigated (Figure 13b,c). When ceramic Ce02Mn7 was applied, pure white light with a CRI of up to 91.0 and an LE of up to 85.07 lm/W was obtained. Unfortunately, the reason for the improvements in CRI and LE of Lu_2.998Al_5−2ySi_yO₁₂:0.002Ce³⁺,yMn²⁺ (y = 0.07) compared with Lu_2.98Al_5−2xSi_xO₁₂:0.02Ce³⁺,xMn²⁺ (x = 0.12) [72] was not elucidated.

4.3. Co-Doping Sm³⁺ and Mn²⁺ Ions into LuAG:Ce³⁺

Due to the ⁴G_5/2 → ⁶H_J (J = 5/2 − 11/2) transitions of the 4f electrons of Sm³⁺ near-ultraviolet excitation, orange-red emission (>560 nm) may occur. However, Sm³⁺ is difficult to be excited by blue LD, limiting its applications in high-power wLD lighting. With this in mind, Sun et al. [77] doped Ce³⁺ and Sm³⁺ synchronously into LuAG to form highly transparent Lu_2.98−xCe_0.02Sm_xAl₅O₁₂ (LuAG:0.02Ce³⁺,xSm³⁺) (x = 0.001–0.01) ceramics through vacuum sintering. The XRD pattern, the lattice parameter of LuAG:0.02Ce³⁺,xSm³⁺ ceramics versus Sm³⁺ concentration, and the retrieved refinement of LuAG:0.02Ce³⁺,0.04Sm³⁺ ceramic have all evidenced the substitution of Ce³⁺ and Sm³⁺ for Lu³⁺ in the LuAG:Ce³⁺,Sm³⁺ ceramics. In addition, the spectral overlap between the PLE of LuAG:0.04Sm³⁺ and the PL of LuAG:0.02Ce³⁺ (Figure 14a), as well as the variation in emission intensity of Ce³⁺ (540 nm) and Sm³⁺ (618 nm) in (LuAG:0.02Ce³⁺,xSm³⁺) with Sm³⁺ concentration (Figure 14b), confirmed the effective energy transfer from Ce³⁺ to Sm³⁺. To overcome the narrow bandwidth of Sm³⁺, Mn²⁺/Si⁴⁺ were further incorporated, taking advantage of the orange emission of Mn²⁺ and charge compensation of Si⁴⁺. The as-prepared single-structured Lu₃Mn_0.04Al_2.92Si_0.04O₁₂:0.02Ce³⁺,0.04Sm³⁺ (LuAG:0.02Ce³⁺,0.04Sm³⁺,0.04Mn²⁺) ceramics displayed characteristic emission of Ce³⁺ (530 nm, 5d₁ → 4f transition), Sm³⁺ (618 nm, ⁴G_5/2 → ⁶H_7/2 f-f transition), and Mn²⁺ (590 nm, ⁴T₁ → ⁶A₁ d-d transition), resulting in a broad emission spectrum spanning from 500 nm to 750 nm (Figure 14c). The spectral overlap between the PLE of LuAG:0.04Sm³⁺ and the PL of LuAG:0.04Mn²⁺, and the lifetime-decay studies of LuAG:0.02Ce³⁺,0.04Sm³⁺ and LuAG:0.02Ce³⁺,0.04Sm³⁺,0.04Mn²⁺ suggest multiple energy-transfer processes of Ce³⁺ → Mn²⁺, Ce³⁺ → Sm³⁺, and Mn²⁺ → Sm³⁺ (Figure 14d). When the LuAG:0.02Ce³⁺,0.04Sm³⁺,0.04Mn²⁺ ceramic was coupled with a 450 nm blue LD, the resulting wLD exhibited a CRI of up to 78.5.

5. Designing Composite Structural Ceramics

In addition to the above strategies, the design of composite structure ceramics, that is, the design of a double-layer converter with at least one layer of phosphor ceramic, can also broaden the emission spectrum, provided that the components of each layer and the structure between layers are subtly designed. In composite structural ceramics, the emission spectra range and color ratio of the two layers can be flexibly tuned, allowing improvement of the CRI. Moreover, they maintain the good thermal and mechanical performances of the parent ceramics. Therefore, the design of composite ceramics is a feasible way to achieve ceramics with high CRI values.

5.1. Combining Two Phosphor Ceramic Layers

To systematically investigate the influence of components and structure design on the properties of composite structural ceramics, Huang et al. [78] prepared double-layered YAG:Cr³⁺/YAG:Ce³⁺ (Cr01/Ce02) and YAG:Ce³⁺,Cr³⁺/YAG:Ce³⁺ (Ce01Cr01/Ce01) ceramics, as well as red-light-emitting Cr³⁺ co-doped YAG:Ce³⁺,Cr³⁺ (Ce02Cr01) single-layer ceramics (Figure 15a) through the vacuum sintering technique. As shown in Figure 15b, two characteristic emission bands centered at 540 nm and 687 nm appeared in the PL spectra for all ceramics containing Cr³⁺, which were attributed to the emission of Ce³⁺ and Cr³⁺, respectively. After the ceramics were packaged with 460 nm (2 W) blue LED chips, wLED devices were obtained and their electroluminescence spectra are shown in Figure 15c. In the yellow emission region (500–650 nm), the emission intensity of the single-layer Ce02Cr01 ceramic is significantly lower than that of the composite double-layer Cr01/Ce02 and Ce01Cr01/Ce01 ceramics. The reduced LE of the Ce02Cr01 ceramic can be ascribed to the energy consumption caused by the sensitization effect between Ce³⁺ ions and Cr³⁺ ions in this co-doping system. In the red emission region (650–750 nm), the emission intensity of Cr01/Ce02 is lower than that of Ce01Cr01/Ce01, even though they contain the same Cr³⁺ content and have the same double-layer structure. The results show that the combined strategy of “co-doping” and “composite double layer structure” is more effective to improve the excitation efficiency of Cr³⁺, because the emitted light from the lower layer (Ce01) and the local layer (Ce01Cr01) of the Ce³⁺ ion can both be absorbed by the Cr³⁺ ion. This is further verified by the shorter lifetime of Ce01Cr01/Ce01 (54.213 ns) than that of Cr01/Ce02 (66.349 ns); in other words, the energy transfer efficiency of Ce³⁺ → Cr³⁺ increased in Ce01Cr01/Ce01. The CRI values of all the ceramics containing Cr³⁺ were elevated compared with YAG:Ce³⁺ (Ce02Cr00), and both Ce02Cr01 and Ce01Cr01/Ce01 ceramics exhibited CRI values beyond 75. The lower CRI of Ce01Cr01/Ce01 (75.2) than Ce02Cr01 (82.7) is due to mismatched red–green–blue ratio.

In another study reported by Lu et al. [60], the Al₂O₃-YAG eutectic was doped with Ce³⁺ and Cr³⁺ via a micro-pull-down method, respectively. When the Al₂O₃-YAG:Cr³⁺ was superimposed on the Al₂O₃-YAG:Ce³⁺ eutectic, the CRI of the Al₂O₃-YAG:Ce³⁺ and Al₂O₃-YAG:Cr³⁺ stacked LED reached 78.7.

Zhou et al. [79] also prepared LuAG:Ce³⁺,Cr³⁺ ceramics via solid-state sintering technique under vacuum condition. By combining YAG:Ce³⁺ ((Y_0.995Ce_0.005)₃Al₅O₁₂) ceramic with LuAG:Ce³⁺,Cr³⁺ ((Lu_0.009Ce_0.001)₃(Al_0.996Cr_0.004)₅O₁₂) ceramic, the ceramic-based wLED exhibited a CRI up to 88. Therefore, the “ceramic combination strategy” is highly promising for constructing high-quality wLEDs/wLDs.

5.2. Combining a Phosphor Ceramic Layer with a Free-Standing Phosphor Film

Because the combination of a color conversion layer with green emission and a color conversion layer with red emission can produce high CRI, Park et al. [80] selected a green-emitting LuAG:Ce³⁺ ceramic phosphor plate (CPP) and a red-emissive (Sr,Ca)AlSiN₃:Eu²⁺-silicone resin film to form composite structural ceramics. The red film was designed and prepared as a free-standing film (f-film) because it can make the phosphor less affected by the high temperature of the LED chip. In addition, a two-dimensional (2D) SiN_X photonic crystal layer (PCL) was introduced to improve the LE. All the fabricated wLEDs were color-converter-on-cup types, and different types of color converters, namely, the LuAG:Ce³⁺ CPP (c-flat CPP), the (Sr,Ca)AlSiN₃:Eu²⁺ f-film, the 2D SiN_X PCL-assisted CPP (SiN_X-PCL CPP), and the SiN_X-PCL CPP/f-film (Figure 16a), were attached to the top of the LED cups with silicone resin. The comparative influence of the f-films with different concentrations of (Sr,Ca)AlSiN₃:Eu²⁺ on the EL properties of LEDs capped with c-flat CPP, SiN_X-PCL CPP, and thickness-increased CPP/f-film (thick-flat CPP-0.15/f-film) was investigated. The LE of SiN_X-PCL CPP/f-film-based LEDs were always higher than those of the c-flat CPP/f-film and thick-flat CPP-0.15/f-film-based LEDs (Figure 16b), indicating that the 2D SiN_X PCL significantly increased the intensity of green emission. As shown in Figure 16c, the introduction of the f-films between the LuAG:Ce³⁺ CPP and the blue LED cup effectively improved the CRI. When the concentration of (Sr,Ca)AlSiN₃:Eu²⁺ in the red film was over 7.5 wt%, the CRIs of both the SiN_X-PCL CPP/f-film-based LED and the thick-flat CPP-0.15/f-film-based LED were over 90. Comprehensively, the SiN_X-PCL CPP/f-film-based LED with a red phosphors concentration of 7.5 wt % exhibited an excellent CRI of 94 and an acceptable LE of 71.1 lm/W. Therefore, the combination of green-light-emitting phosphor ceramics, 2D PCL, and red phosphor f-film is effective to achieve high-power wLED with high CRI.

5.3. Coating a Phosphor/QDs Layer onto a Phosphor Ceramic Layer

To obtain high-quality white lighting with a high CRI, Zhou et al. [81] proposed a method to coat red-emitting SrAlSiN₃:Eu²⁺ slurry composed of SrAlSiN₃:Eu²⁺ phosphor and transparent organic ink on green-emitting Lu_0.1Y_2.84Al₅O₁₂:0.06Ce³⁺ (LuYAG:Ce³⁺) ceramic substrate. Since SrAlSiN₃:Eu²⁺ slurries contain organic matter which suffers poor thermal conductivity, they firstly coated the slurry on the LuYAG:Ce³⁺ ceramic using a screen-printing technique, and then removed the organic matter by laser ablation drying. According to the EDS spectra of the red phosphor layer (Figure 17a), the C element in the red phosphor was greatly reduced after laser ablation, confirming the removal of organic components by the laser ablation. Under 460 nm excitation, the composite ceramic exhibited a wide emission band of 500–675 nm (Figure 17b), which is due to the 5d₁ → 4f transitions of Ce³⁺ and Eu²⁺ ions, respectively. It is worth noting that the emission intensity of the composite ceramic before laser ablation is much lower than that after laser ablation, which can be explained by the difference in refractive index between the organic matter and the SrAlSiN₃:Eu²⁺ fluorescent powder. In addition, the emission spectrum can be modulated by adjusting the number of screen prints (Figure 17c). When the composite ceramics were combined with blue LED chips, it was found that the color coordinates, the CRI, and the CCT of the wLEDs were influenced by both the package pattern (Figure 17d) and the thickness of the red-emitting film (Figure 17e). When the red phosphor film was facing away from the blue LED chip, the yellow-green radiation emitted by the LuYAG:Ce³⁺ ceramic layer under excitation of the LED chip was absorbed by the red phosphor, resulting in decreases in LE and CRI. When the red phosphor layer was facing the 1 W blue LED chip, a wLED with CRI of 89 was obtained with a printing number of 6.

In another work described by Zheng et al. [82], a tricolor converter system combined with a 405 nm near ultraviolet chip was employed. In traditional tricolor converter systems, the nitride/fluoride red phosphors always show low thermal stability under high-power excitation and exhibit broad emission bands exceeding 650 nm that are difficult to perceived with human eyes, leading to a decrease in LE. To address the issue, the authors used vacuum sintering to prepare red-color-converter (Sm_xY_1−x)₃Al₅O₁₂ (x = 0.005, 0.01, 0.02, and 0.03) (YAG:Sm³⁺) ceramics with a narrow emission band and excellent thermal conductivity. A double-layer tricolor converter system was fabricated by coating a phosphor in silicone (PIS) film consisting of green-emitting (Sr,Ba)₂SiO₄:Eu²⁺ phosphor powders, blue-emitting BaMgAl₁₀O₁₇:Eu²⁺ phosphor powders, and organic silicon resin on the red-emitting YAG:Sm³⁺ ceramic. The blue/green-emitting PIS (PIS(GB))-coated YAG:Sm³⁺ samples were then hardened at 140 °C for 4 h to form the double-layer YAG(R)-PIS(GB) converter (Figure 18a,b). The cross-sectional SEM image (Figure 18c) shows that the PIS(GB) film is tightly bonded to the YAG:Sm³⁺ ceramic, effectively avoiding light-transmission loss and reducing interfacial thermal resistance. By modulating the concentration of Sm³⁺ in YAG:Sm³⁺ ceramics and fixing the thickness of the PIS(GB) film to 100 μm, the optical properties of the YAG(R)-PIS(GB) converter were optimized, and a CRI of 92.6 was achieved when x = 0.01 (Figure 18d,e). Compared with single-layer PIS(RGB) composed of red-emitting YAG:Sm³⁺, green-emitting (Sr,Ba)₂SiO₄:Eu²⁺ and blue-emitting BaMgAl₁₀O₁₇:Eu²⁺ phosphor powders, and organic silicon resin (Figure 18a), the YAG(R)-PIS(GB) converter exhibited higher heat transfer efficiency and avoided the blue emission from being self-absorbed by the red converter. Therefore, such a double-layer tricolor converter has broad application prospects in high-power lighting devices.

Xu et al. [83] coated a red-emitting quantum dot (QD) (λ_em = 634 nm) layer (CaSiAlN₃:Eu²⁺) instead of red-emitting PIS on an Al₂O₃-YAG:Ce³⁺ ceramic plate to construct composite structural ceramics. The red-emitting QD was employed to overcome the luminescence saturation behavior of red-emitting phosphors at low power densities. Due to the excellent thermal conductivity of CPP, the coating of QD onto CPP can reduce the thermal erosion of the QD layer caused by laser irradiation. Under the irradiation of a blue laser (445 nm, 5 W), the surface temperature of the QD layer was as low as 68 °C, and no luminescence saturation occurred. When the CPP-QD was combined with a blue LD, the wLD device exhibited a CRI of up to 80, indicating the feasibility of applying CPP-QD in high-power wLD lighting.

6. Conclusions and Perspectives

In this review, we have provided a comprehensive overview of the preparation strategies for TPCs for the construction of high-power and high color rendering wLED/wLD lighting sources. These strategies can be primarily classified into four categories: host regulation, red-emitter doping, host regulation/red-emitter doping combination, and composite structure design. All the lighting systems exhibit high CRI (>75) with high stability and long lifespan. Additionally, their design principles, PL characteristics, and mechanisms for PL modulation were thoroughly discussed. This review addresses the previous lack of a summary of this research topic.

At present, the results of research in this area are still limited. On the one hand, we believe that the appropriate combination of the strategies summarized in this review can further improve device performance. On the other hand, although high-power and high-CRI wLED/wLD devices based on various TPCs have been successfully built, achieving the delicate balance between high chrominance and high LE remains a huge challenge. We believe that improving CRI while maintaining the high LE of wLED/wLD is a key goal for the next generation of high-power lighting. Moreover, in practical applications, in addition to LE and CRI, some other factors need to be considered. For instance, in multi-phase ceramics, taking Al₂O₃-YAG:Ce³⁺ as an example, it is usually difficult to achieve uniform distribution, which has a negative impact on the heat dissipation and uniformity of the luminescent centers of the ceramics. To address the issue, “unique” microstructures of these phosphor converters were developed to control light scattering and achieve color uniformity [84]. Homogeneous initial powder materials were obtained by regulating the excess of Al³⁺ of YAG:Ce³⁺ in the process of co-precipitation, which provided a possible way to optimize the composition uniformity of Al₂O₃-YAG:Ce³⁺ TPCs [85]. Many of these techniques to achieve high-quality TPCs are yet to be explored.