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.

Keywords: high color rendering; high-power lighting; transparent phosphor ceramics; white LD; white LED.

Figure 1

The crystal structure (left) of Y₃Al₅O₁₂ and the coordination environment of Y³⁺ and Al³⁺ cations (right) in the lattice. Reprinted with permission from ref. [47]. Copyright 2014, American Chemical Society.

Figure 2

(a) PL and PL excitation (PLE) spectra of the (Gd_γY_1−γ)₃Al₅O₁₂:Ce³⁺ (γ = 0–0.75) ceramics. (b) Chromaticity color coordinates of the samples under 465 nm LED excitation. (c) Gd content dependence of CRI and correlated color temperature (CCT) of the Ce:GdYAG ceramics. Reprinted with permission from ref. [54]. Copyright 2011, IOP Publishing.

Figure 3

(a–h,a’) Photographs of GAGG:xCe³⁺ ceramics on InGaN-based blue chips. (b’–h’) Electroluminescent spectra, CCT, and CRI of the obtained WLEDs. (i) Images of a 445 nm blue LD with 2 W (left) and a prototype wLD combining the blue LD and the GAGG:0.75%Ce³⁺ ceramic (thickness(d) = 1.05 mm) in operation (right). Reprinted with permission from ref. [56]. Copyright 2019, American Chemical Society.

Figure 4

(a) Image of LMAS:xCe³⁺ ceramic plates under daylight. (b) PL spectra (λ_ex = 455 nm) of LMAS:xCe³⁺ ceramic plates. The inset shows the variation of PL peak intensities with different x values. (c) Temperature-dependent PL peak intensities of the ceramic plate and the phosphor of LMAS:0.08Ce³⁺. (d) Sketch map of wLD lighting with a reflection mode. (e) Chromaticity coordinates of the LMAS:0.08Ce³⁺ ceramic plate excited by different laser powers. Inset, a wLD device that has a maximum CRI of 77.7. (f) CRI of the LMAS:0.08Ce³⁺ ceramic plate driven by different laser powers. Reprinted with permission from ref. [58]. Copyright 2020, The Royal Society of Chemistry.

Figure 5

(a) The electroluminescent spectra of Ce/Cr co-doped Al₂O₃/YAG eutectic ceramics with different doping concentrations of Cr³⁺. (b) The CRI, CCT, and LE data of ceramics under a 175 mA driving current. Reprinted with permission from ref. [61]. Copyright 2022, Elsevier.

Figure 6

(a) PLE and PL spectra of YAG:Ce³⁺ (yellow), YAG:Cr³⁺ (blue), and YAG:Ce³⁺,Cr³⁺ (magenta). (b) CRI change trend graph of Y_2.94Al_5−xCr_xO₁₂:0.06Ce³⁺ (x = 0.004, 0.029, 0.054, 0.079, 0.104 mol). Reprinted with permission from ref. [62]. Copyright 2023, Elsevier.

Figure 7

(a) Photograph of the YAG:Ce³⁺,xMn²⁺,xSi⁴⁺ (x = 0, 0.01, 0.02, 0.04, 0.08, 0.16) phosphor ceramics. (b) Electroluminescent spectra of YAG:Ce³⁺,xMn²⁺,xSi⁴⁺ coupled with 450 nm GaN blue chips. (c) The color coordinates and CRI of the packaged LEDs. (d) The CIE chromaticity diagram of YAG:Ce³⁺,xMn²⁺,xSi⁴⁺ samples. Reprinted with permission from ref. [63]. Copyright 2019, Elsevier.

Figure 8

(a) Picture of as-prepared transparent ceramics 1–8. (b) PL and PLE spectra of YAG:Ce³⁺,Pr³⁺,Cr³⁺. (c) Ingredients, color coordinates, and CRI of all the transparent ceramic packaged LEDs. Reprinted with permission from ref. [64]. Copyright 2017, Elsevier.

Figure 9

(a) Normalized PL spectra and (b) the FWHM evolution of the PL spectra of the prepared ceramics. (c) Ingredients, color coordinates, and CRI of all the ceramic packaged LEDs. Reprinted with permission from ref. [65]. Copyright 2020, The Royal Society of Chemistry.

Figure 10

(a) Ingredients, color coordinates, CCT, and CRI of the YTAMG:Ce³⁺ ceramics. (b) The variation of lattice parameters and volume of Tb00Mn00–Tb20Mn00 with Tb³⁺ concentration. (c) The Y/Tb/Ce1-O and Y/Tb/Ce2-O bond lengths of YTAMG:Ce³⁺ ceramics as a function of Tb³⁺ concentration. (d) Normalized PL spectra of all the YTAMG:Ce³⁺ ceramics (λ_ex = 460 nm). Reprinted with permission from ref. [52]. Copyright 2021, Elsevier.

Figure 11

(a) Compositions, color coordinates, CCT, and CRI of the S0–S4 ceramics. (b) Normalized PL spectra of S0–S4 (λ_ex = 460 nm). (c) Schematic diagram of wLEDs based on the double-layer ceramic structure. Reprinted with permission from ref. [66]. Copyright 2021, Elsevier.

Figure 12

(a) PL spectra (λ_ex = 450 nm), (b) photos taken under natural light (left) and ultraviolet light (right), and (c) color coordinates, CCT, and CRI of the Lu_2.98Al_5−2xSi_xO₁₂:0.02Ce³⁺,xMn²⁺ (x = 0–0.12) ceramics. Reprinted with permission from ref. [72]. Copyright 2020, The Royal Society of Chemistry.

Figure 13

(a) Photograph of as-prepared LuAG:Ce,Mn phosphor ceramics. (b) Schematic diagram of high-CRI wLED encapsulation and actual chip-on-board lighting source. (c) The CIE chromaticity coordination diagram of these fabricated LEDs. Reprinted with permission from ref. [76]. Copyright 2022, The American Ceramic Society.

Figure 14

(a) PLE spectra (λ_em = 618 nm) of LuAG:0.04Sm³⁺ ceramics (blue line) and PL spectrum (λ_ex = 450 nm) of LuAG:0.02Ce³⁺ ceramics (red dashed line). (b) Relative intensity of Ce³⁺ (i) and Sm³⁺ (ii) of LuAG:0.02Ce³⁺,xSm³⁺ as a function of Sm³⁺ concentration (x). (c) PL spectra of LuAG:0.02Ce³⁺, LuAG:0.04Sm³⁺, LuAG:0.02Ce³⁺,0.04Sm³⁺, and LuAG:0.02Ce³⁺,0.04Sm³⁺,0.04Mn²⁺ ceramics. (d) Schematic energy level diagram of Ce³⁺, Sm³⁺, and Mn²⁺. Reprinted with permission from ref. [77]. Copyright 2021, The Royal Society of Chemistry.

Figure 15

(a) Ingredients, color coordinates, and CRI of the prepared ceramics with different chemical components. (b) Normalized PL spectra of the corresponding ceramics under excitation at 460 nm. (c) Schematic diagram of composite structure ceramic-based wLED device (inset) and electroluminescence spectra of the corresponding ceramics. Reprinted with permission from ref. [78]. Copyright 2021, Elsevier.

Figure 16

(a) Schematic diagrams of 2D PCL-assisted CPP/free-standing red film phosphor (SiN_X-PCL CPP/f-film)-based LED. (b) LE (lm/W), and (c) CRIs of the LuAG:Ce³⁺ CPP/free-standing red film phosphor (c-flat CPP/f-film)-based LED, the SiN_X-PCL CPP/f-film -based LED, and the thickness-increased CPP (0.15 mm)/free-standing red film phosphor (thick-flat CPP-0.15/f-film)-based LED as functions of the red phosphor concentration at equal current (350 mA). Reprinted with permission from ref. [80]. Copyright 2015, American Chemical Society.

Figure 17

(a) EDS spectra of red phosphor layer before (up) and after (below) laser ablation. (b) PLE and PL spectra of Lu_0.1Y_2.84Al₅O₁₂:0.06Ce³⁺ ceramic, SrAlSiN₃:Eu²⁺ phosphor, and composite ceramics before and after laser ablation. (c) PL spectra of the composite ceramics with different times of SrAlSiN₃:Eu²⁺ screen-printing under 460 nm excitation. (d) Schematic drawing of the packaged W-LEDs with the red phosphor layer facing (up) and facing away from (below) the blue LED chip. (e) Color coordinates, CCT, and CRI of the wLED packaged with 1 W blue LED chip and composite ceramics with different numbers of SrAlSiN₃:Eu²⁺ screen printings. Reprinted with permission from ref. [81]. Copyright 2022, Elsevier.

Figure 18

(a) Schematic diagrams of the single-layer converter (above) and double-layer converter (below). (b) Photograph and (c) cross-sectional SEM image of the double-layer white-light converter. (d) PL spectra of the (Sm_xY_1−x)₃Al₅O₁₂ (x = 0.005, 0.01, 0.02, and 0.03) ceramics under 405 nm excitation wavelength. (e) PL spectrum of the LED-driven tricolor converter under 405 nm excitation. Reprinted with permission from ref. [82]. Copyright 2019, American Chemical Society.