A novel extra-broadband visible-emitting garnet phosphor for efficient single-component pc-WLEDs

Abstract

The development of extra-broadband visible emission phosphors is crucial to achieve next-generation illumination with better color experience. Herein, a defect engineering strategy mediated by the structural cationic substitution is proposed and experimentally demonstrated for specific ultra-broadband emission in a garnet phosphor. The induced oxygen vacancies and interstitial cation through lattice distortion break the periodic potential field of the crystal and provide electronic levels in the band gap. As a result, excited by blue-light-emitting diodes, the novel Y₃Sc₂Al₃O₁₂:B³⁺ shows an ultra-broad emission with a full width at half maximum (FWHM) of ∼170 nm. Compared to general defect-emitting phosphors, the unique Y₃Sc₂Al₃O₁₂:B³⁺ exhibits excellent thermal quenching resistance and superior internal quantum efficiency of up to 95%. These findings not only show great promise of Y₃Sc₂Al₃O₁₂:B³⁺ as an extra-broadband emitter but also provide a new design strategy to achieve a full-visible-spectrum phosphor in a single-component material for white-light applications.

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1. Introduction

Broadband visible lighting underpins a plethora of applications in optics and photonics, including optical communication, visible spectroscopy^1,2 and, particularly, white-light-emitting diodes (WLED).^3,4 The typical commercial phosphor-converted WLEDs (pc-WLEDs) were achieved by the combination of a blue InGaN chip with a yellow-emitting Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce) phosphor.⁵ However, such WLEDs often suffer from deficiency of the red-light component and “cyan cavity”, leading to the severe problem of poor colour rendition. After the discovery of red-emitting phosphors^6–8 (e.g. CaAlSiN₃:Eu²⁺, BaAl₄Sb₂O₁₂:Eu²⁺ or SrLiAl₃N₄:Eu²⁺) and cyan phosphors^9–12 (e.g. NaMgBO₃:Ce³⁺, Ca₂LaHf₂Al₃O₁₂:Ce³⁺, Ba_3.92Ca_0.9La_0.1(PO₄)₃Cl:Eu²⁺ and CaLuGaO₄:Eu²⁺), an alternative approach of combining tricolour phosphors with a near-ultraviolet (n-UV) LED chip has achieved a high colour rendering index (CRI) and low correlated colour temperature (CCT). Unfortunately, it has been observed that such pc-WLEDs based on a multi-phase phosphor system inevitably faces the problems of reabsorption energy loss and dissonance deterioration rates during long-term operation.¹³ Thus, it is essential to develop an ultra-broad emission phosphor with full-spectrum emission in a single-component material.

One option is to co-dope several rare-earth ions into a single system to introduce multiple luminescence,^14–16 but this also results in reabsorption and poor luminescence efficiency. Recently, defect engineering is effectively applied for design and modulation of novel luminescent materials.¹⁷ In principle, controlling the intrinsic defects in the lattice can affect the energy level distribution of the electrons, which could act as unusual activators. A feasible route to investigate broadband emissions is to modulate the oxygen vacancy (V_O) and interstitial atoms to introduce defect levels that can trap electrons from the conduction band. In this respect, it is preferred to choose wide bandgap oxides (energy gap > 3.5 eV) because their range of native defects provides an electronic level in the bandgap that is capable of generating an emission band in the visible range (Fig. 1). Guided by this design strategy, a few binary oxides, such as ZnO¹⁸ and TiO₂,¹⁹ with full-visible spectrum have been explored. However, these materials are very sensitive to the surroundings and possess relatively poor thermal quenching owing to the low activation energy of the thermally activated defects,^20,21 which may hinder the practical applications of pc-WLED devices. Therefore, it is essential to develop a defect-stabilized luminescent phosphor that broadens the choices for broadband visible emission.

Fig. 1 Strategies for visible broadband luminescence utilizing defect engineering.

Interestingly, we successfully adopted defect engineering in the Y₃Sc₂Al₃O₁₂ (YSAG) phosphor without a lanthanide dopant, which presents a highly reproducible and intense broadband emission that spans the cyan to orange-red region. YSAG is an outstanding matrix with high structural rigidity, which has been widely applied in the field of solid-state lasers, displays, and illumination. However, it is still difficult for YSAG phosphors based on rare-earth ion or transition metal ion doping to meet the requirements of high-performance single-component WLEDs due to their narrow emission bandwidth.^22–24 Herein, we demonstrate novel emitting through the consecutive substitution of matrix cations in this garnet material. The crystal local field and electronic microstructures are purposefully modulated, thereby controlling the broadband performance through variation in the defect concentration. The finding provides new insight into the garnet structure design and oxygen vacancy tailoring for luminescent applications.

2. Results and discussion

To experimentally elaborate the defect engineering strategy, we synthesized a series of YSAG powders doped with varying B³⁺ concentrations using the traditional high-temperature solid-state reaction. The diffuse reflectance (DR) spectra of the novel YSAG:xB³⁺ (x = 0.00 and 0.15) are exhibited in Fig. 2a. For the garnet phosphor, the solid optical absorption is observed in the UV and visible region from 250 to 500 nm. Notably, the Y₃Sc₂Al₃O₁₂ phosphor shows one prominent band, which is centered at around 258 nm. After B³⁺-doping, there is also a prominent band in the region of 400–500 nm. According to the Kubelka–Munk function, the direct optical bandgap value can be obtained from a fit to the linear part of the DRS spectrum as follows:²⁵

[hνF(R)]² = A(hν − E_g) (1)

F(R) = (1 − R)²/2R (2)

where hν represents the photon energy, R is the reflectance coefficient (%), F(R) is the absorption coefficient, A is the absorption constant and E_g represents the optical bandgap value. The calculated E_g value of YSAG and YSAG:0.15B³⁺ are 4.407 eV and 4.329 eV, respectively (inset in Fig. 2a). Fig. 2b presents the photoluminescence excitation (PLE) and photoluminescence emission (PL) spectra of the YSAG:0.15B³⁺ sample at 298 K and 83 K, respectively. The PLE spectrum of the YSAG:0.15B³⁺ shows a wide excitation band from 400–500 nm with a peak at 445 nm, thereby demonstrating that the as-made YSAG:0.15B³⁺ can be adequately excited by a blue LED chip. This result is consistent with the results from the DR spectra. In response to the 445 nm excitation, YSAG:0.15B³⁺ exhibits a surprising cyan-red emission from 450–800 nm. The influence of B³⁺ on the luminescence was subsequently investigated, as shown in Fig. S1 (ESI†). The PLE and PL intensity reach a maximum value of x = 0.15 with the increased B³⁺ concentration, indicating a higher density of defects at this doping level. The results of the X-ray fluorescence spectrometer (XRF) in Table S1† further exclude the presence of other possible lanthanide activator ions in YSAG:B³⁺. In addition, from the low-temperature spectrum, it can be found that this broadband emission consists of two peaks with central wavelengths of 530 nm and 548 nm, respectively. The IQE value of YSAG:0.15B³⁺ can reach 95.16%. Such a high IQE is a crucial reformation for the blue phosphor series since the quantum efficiency is sufficiently high for potential applications in lighting devices. It is known that the low thermal quenching (TQ) behavior is also expected for phosphors to realize high-quality lighting in pc-WLED devices.^26,27 Unfortunately, such phosphors based on defect designs often suffer from poor TQ. For example, the luminous intensity of Zn₂GeO₄ at 300 K is only about 10% of the intensity at 10 K.²⁸ In ZnO, the intensity at 300 K is only ∼40% of the original.²⁹Fig. 2c demonstrates the temperature-dependent PL spectra of YSAG:0.15B³⁺ at 445 nm excitation. There is no significant shift in the wavelength of the luminous center. Surprisingly, the sample exhibits exceptionally high TQ at low temperature, and the intensity at 298 K is up to 73.23% of the original intensity at 83 K (Fig. 2d). The above results highlight the thermal stability of the sample for the practical application of pc-WLED.

Fig. 2 Luminescent properties of YSAG:B³⁺. (a) Diffuse reflectance spectrum of YSAG and YSAG:0.15B³⁺ phosphor (inset: optical band gap calculated from the UV-Vis absorption spectrum). (b) PLE and PL spectra of YSAG:0.15B³⁺. (c) Temperature-dependent PL spectra in the temperature range of 83 K–373 K. (d) Integrated emission intensity of YSAG:0.15B³⁺.

To explore the origin of the unique cyan-red emission of the B³⁺ ions in YSAG, the crystal structure configuration and composition of YSAG:B³⁺ were investigated. YSAG is a typical garnet with a highly rigid framework structure composed of a [YO₈] dodecahedron, [ScO₆] octahedrons and [AlO₄] tetrahedrons. In YSAG:B³⁺, because B³⁺ and Al³⁺ have more similar ionic radii than Sc³⁺ or Y³⁺, this smaller size allows B³⁺ to fit more comfortably into the crystal lattice where Al³⁺ is present, facilitating substitution. Moreover, B³⁺ typically prefers a lower coordination number and a tetrahedral geometry, and might better match the electronegativity of the surrounding anions when replacing Al³⁺. Therefore, when B³⁺ enters the lattice, it prefers to replace Al³⁺ sites rather than Sc³⁺ or Y³⁺.

The XRD pattern of YSAG:xB³⁺ (x = 0.00–0.17) could be indexed with the standard Y₃Sc₂Al₃O₁₂ (PDF No. 79-1846), thereby demonstrating the formation of the targeted phase (see Fig. S2†). Fig. 3a, Fig. S3 and Table S2† display the Rietveld refinement XRD results of YSAG:xB³⁺. All of the as-prepared samples crystalized in a cubic structure with a space group of Ia3d. In terms of defect formation, the difference of the ion radii between the doped ions and host cations can be evaluated by the parameter D_r, which is obtained from the empirical formula:³⁰

where R_r and R_d represent the radii of the host cations and the doped ions, respectively. The D_r value for B³⁺ (r = 0.20 Å) substituting Al³⁺ (r = 0.50 Å) is about 60%. The substitutions of two ions with a large difference in radius are expected to cause a significant lattice distortion, which is favorable to introduce defects for the realization of defect engineering. As illustrated in Fig. 3b, the Rietveld refinement results show that upon increasing the B³⁺ doping concentration, the lattice volumes appear to gradually expand from 1839.456 ų to 1859.435 ų. To explore this phenomenon, transmission electron microscopy (TEM) experiments were performed. TEM images of a single YSAG:0.15B³⁺ particle is shown in Fig. S4.† The component elemental maps of the particle clearly show that the elements O, Al, Sc, Y and B are homogeneously distributed into the YSAG:0.15B³⁺ sample. The HRTEM images (Fig. 3c and d) of the single particle exhibit the crystal lattice striped with spacings of 0.275 nm, which is a significant enlargement of the lattice spacing compared to the strongest (4 2 0) diffraction of the YSAG. A high crystalline quality with no second phase is observed from the selected area electron diffraction (SAED) pattern represented in Fig. 3e. The combined XRD and TEM results suggest that the coexistence of some interstitial atoms besides the B³⁺ substitution in the host Y₃Sc₂Al₃O₁₂ lattice leads to an expansion of the lattice parameters.

Fig. 3 Morphology and composition of the YSAG:B³⁺ phosphor. (a) Rietveld refinement XRD patterns of the YSAG:0.15B³⁺ samples. (b) Lattice volumes of YSAG:xB³⁺ (x = 0.00–0.17), which is obtained by Rietveld refinements. (c) HRTEM image of a YSAG:0.15B³⁺ single particle. (d and e) The corresponding selected area electron diffraction (SAED) pattern of YSAG:0.15B³⁺. (f) Raman spectra of YSAG and YSAG:0.15B³⁺. (g) EPR spectra of the YSAG and YSAG:0.15B³⁺ phosphor at 293 K. (h) XPS analysis of the O 1s orbital for the YSAG and YSAG:0.15B³⁺ phosphor.

The Raman spectra are shown in Fig. 3f, reflecting the evolution of the microscopic structure with and without the B³⁺ dopant. The bands around 251, 375 and 766 cm⁻¹ are induced by the characteristic vibrations of the YSAG garnet.²² Additionally, the bands of YSAG:0.15B³⁺ shift toward lower frequency, suggesting that the local lattice coordination environment in the matrix slightly changes with B³⁺ incorporation. EPR measurements were performed to analyze the valence state in the YSAG:0.15B³⁺ samples. The valence state via the g-factor value can be calculated by the following formula:³¹

g = hv/βH (4)

where h is the Planck constant, ν is the electromagnetic wave frequency, β is the Bohr magneton (9.27410 × 10⁻²¹ erg G⁻¹), and H is the resonance magnetic field strength. As shown in Fig. 3g, the ratio of the EPR signal (g = 1.9733) is assigned to Vo. This phenomenon stems from the non-negligible local structural distortion caused by the substitution of B³⁺ for Al³⁺. The g value is slightly smaller than 2.004 due to the different binding capacities of the atoms around the local electrons in each material.³² Fig. S5a and b† displays the XPS survey scan of the matrix YSAG and YSAG:0.15B³⁺. The high-resolution Sc 2p XPS scan (Fig. S5c and d†) demonstrates two binding energy peaks corresponding to 2p_1/2 and 2p_3/2, and there is no significant difference between the two spectra. As shown in Fig. 3h, to determine the existing forms of oxygen in the structure, O 1s XPS spectra of the YSAG and YSAG:0.15B³⁺ samples were specifically investigated. The O 1s orbital was fitted by three Gaussian peaks that were centered at 530.46 eV, 531.69 eV and 533.18 eV in both samples. The XPS intensity of the oxygen vacancy³³ of YSAG:0.15B³⁺ was remarkably increased in comparison with that of the sample without B³⁺ doping, thereby indicating the generation of Vo after the substitution of B³⁺ for Al³⁺.

To reveal the crystal local field-related defects in YSAG, the effect of oxygen deficiency and the local electron structure variation around the B³⁺ ions on the luminescence behavior of YSAG:B³⁺ was firstly explored by density functional theory (DFT) calculation. The calculated energy band structure and electron densities of state (DOS) in the YSAG matrix and YSAG:B³⁺ are shown in Fig. S6 and S7.† The calculated bandgap (E_g) value (4.39 eV) is close to the experimental value (4.407 eV). DFT calculations in Fig. 4a reveal that no electron transition energy levels ions are observed in the band gap for the dopant of B³⁺ in the absence of V_O, which implies that this model cannot provide effective energy levels for electron transitions. To actualize defective engineering, we purposefully introduced Vo randomly around the B ion. We calculated the electron structures of YSAG:B³⁺ with three types of Vo defects, which are exhibited in Fig. 4b. The whole electron distribution shifts in the low-energy direction. DFT results show that the electron transition energy levels of the B 2p orbital appear between the conduction band and the valence band of the YSAG matrix.

Fig. 4 DFT calculations for the YSAG:B³⁺ phosphor. (a and b) PDOSs obtained via DFT calculations, where (a) is the YSAG matrix and YSAG:B³⁺ without oxygen vacancies and (b) is YSAG:B³⁺ with oxygen vacancies. (c) Crystal structure of B³⁺-doping into the Al site and the electron localization function (ELF) maps without and with oxygen vacancies in the B site, where the green, sky-blue, purple, sea-green, and red spheres represent the B, Al, Sc, Y, and O atoms, respectively. (d and e) Band structure and PDOSs of the YSAG matrix with Al_i, respectively. (f) Schematic of the transition energy levels of B replacing Al without and with oxygen vacancies (1–3) between the valence band and the conduction band.

To further investigate the influence of oxygen vacancies on the B³⁺ luminescence behavior in the YSAG matrix, the electron localization function (ELF) maps around the B atoms (Fig. 4c) without and with oxygen vacancies are analyzed. Thus, when B³⁺ ions occupy Al³⁺ ions without generating oxygen vacancies, the B 2s energy level is embedded in the valence band, while the 2p energy level contributes to the valence band and the conduction band. The separation energy between the 2p energy levels is almost equal to the band gap value. There are no electron transitions on B³⁺ ions for generating luminescence. In the presence of V_O, the B 2p excited-state energy level appears between the conduction band and the valence band, and the 2s energy level mainly embeds the maximum of the valence band. In addition, DFT calculations show that B³⁺ substitution of Al³⁺ results in the formation of interstitial aluminum Al_i with an energy level (2.30 eV) in the bandgap, as depicted in Fig. 4d and e. The proposed electron transition, which is based on DFT calculations, is summarized schematically in Fig. 4f. Our experimental results agree well with this theoretical calculation. The Gaussian fitting of the broadband PL spectra (Fig. S8†) supports the existence of two luminescence centers with peak positions at 2.30 eV (535 nm) and 2.26 eV (548 nm).

To utilize the advantages of this newly designed phosphor, we firstly fabricated two prototype pc-WLED devices, where the YSAG:0.15B³⁺ and commercial YAG:Ce³⁺ phosphors acting as light converters are each pumped by a blue LED chip (445 nm). Fig. 5a and b demonstrate the electroluminescence (EL) spectra of the LED devices in the 400–800 nm region at a driving current of 30 mA. It exhibits a broad emission peak around 460 nm–700 nm with a surprising FWHM of approximately 170 nm. The FWHM of other visible broadband emission phosphors include YSAG:Ce³⁺ with 80–90 nm,³⁴ GdYAG:Ce³⁺ with 100–110 nm,³⁵ and SrAl₁₂O₁₉:Eu²⁺ with 80–90 nm,³⁶ which highlight the application potential of YSAG:B³⁺ in advanced lighting. In addition, compared to commercial YAG:Ce³⁺, the YSAG:0.15B³⁺ phosphor exhibits stronger luminescence in the cyan-blue region. Notably, the blue peak of YSAG:0.15B³⁺ located at ∼460 nm is ascribed to the electronic transition of V_O2, which agrees well with the DFT calculations. The emission intensity increases with increasing currents, while demonstrating stable output efficiency (Fig. 5c). With 3 V and 30 mA driving current, the lumens efficiency of the as-prepared pc-WLED is 42.1 lm W⁻¹. As shown in Fig. 5d, the Commission Internationale de L'Eclairage (CIE) chromaticity coordinate of the WLED with a correlated colour temperature (CCT) of 7245 K is (0.296, 0.337). These results demonstrate that the developed Y₃Sc₂Al₃O₁₂:0.15B³⁺ is a promising broadband emitter for application in blue-based pc-WLED lighting.

Fig. 5 Performance of the as-fabricated white LED devices. (a) Emission spectrum of the as-fabricated pc-WLED combining a 445 nm LED chip with the YSAG:0.15B³⁺ phosphor and (b) commercial YAG:Ce³⁺ phosphor at 30 mA driven currents. (c) Integral intensity of the EL spectra and external quantum efficiency (EQE) under 30–210 mA driven current. (d) CIE chromaticity diagram of the fabricated YSAG:0.15B³⁺ WLED.

3. Experimental

3.1 Computational methods

First-principles density functional theory (DFT) calculations were performed with the Vienna ab initio simulation package (VASP) code.^37,38 The electron-ion interaction was treated with the projector augmented wave (PAW) method. Al (3s²3p¹), B (2s²2p¹), O (2s²2p⁴), Sc (3d¹4s²), and Y (4d¹5s²) electrons were treated as the valence electrons. The generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functional was employed as the exchange–correlation potential. A plane-wave cutoff energy of 450 eV was applied in our calculations, and 3 × 3 × 3 Monkhorst–Pack k grids were used during the optimization. The crystal lattice was fully relaxed until the atomic force was less than 0.03 eV Å⁻¹. The energy convergence criterion for self-consistent electronic calculation was set to 10⁻⁵ eV per atom.

3.2 Materials preparation

A series of Y₃Sc₂Al₃O₁₂:xB³⁺ (x = 0.00–0.17) samples were produced via conventional high-temperature solid-state reaction. The raw materials, including Y₂O₃ (99.99% purity, Aladdin), Sc₂O₃ (99.99%, Aladdin), Al₂O₃ (99.9%, Macklin), and H₃BO₃ (A.R. Aladdin), were weighed based on their stoichiometric proportions and ground together for 0.5 h in agate mortars. The mixture was placed in alumina crucibles and sintered in a muffle furnace at 1550 °C for 6 h in air. After being naturally cooled to room temperature, the synthesized samples were ground again for further characterization.

3.3 Materials characterizations

Powder X-ray diffraction (XRD) was collected on an X'Pert PRO X-ray diffractometer (PANalytical, Netherland) using Cu Kα (λ = 1.5418 Å) radiation. Rietveld refinements were performed with General Structure Analysis System (GSAS) software based on XRD data. The morphology and elemental mapping were studied via TEM (JEOL-JEM 2100 F) operated at 200 kV and equipped with an energy dispersive X-ray spectrometer (EDS). The DR spectra were measured by a UV-Vis-NIR spectrophotometer (UH4150, Hitachi) over the spectral range of 200–800 nm. The PLE and PL spectra (the excitation light is blocked off by the 450 nm long-pass filter) and lifetime measurements of YSAG:xB³⁺ were obtained by an Edinburgh FLS-920 spectrometer. The Raman spectra were characterized by a Micro-Raman spectrometer (Renishaw inVia) under the excitation of a 532 nm laser. To analyse the value state of the defect, X-ray photoelectron spectroscopy (XPS, Axis Supra+) measurements were carried out. Accurate elemental quantification of the samples was performed by an X-ray fluorescence spectrometer (Zetium, Malvern Panalytical). The internal quantum efficiency (IQE) of the broadband visible emission phosphor was measured by a fluorescence spectrophotometer (FLS 1000, Edinburgh Instruments) with an additional integrating sphere. All of the above measurements were performed at room temperature. Temperature-dependent PL spectra (83 K–373 K) were recorded on a fluorescence spectrophotometer (FLSP-920, Edinburgh Instruments) with a temperature controller.

3.4 LED fabrication and characterizations

A pc-WLED device was fabricated using the as-prepared YSAG:0.15B³⁺ phosphor and a 445 nm LED chip. Particularly, the YSAG:0.15B³⁺ phosphor was evenly blended with ultraviolet photosensitive resins (Shenzhen Looking Long Technology Co., LTD, A/B epoxy resin mixed in a 1 : 4 ratio) and the resulting mixture was coated on the LED chip. The epoxy-encapsulated LED chips were cured in an oven at 110 °C for 4 h. The white light emission spectra and performance parameters of the home-made WLED were analyzed by a UV-vis-NIR spectrophotometer with an integrating sphere (PMS-50 PLUS, Everfine).

4. Conclusions

In summary, based on the DFT structure calculation and defect design, we have discovered a new strategy to achieve ultra-broadband emission by systematically tailoring vacancy defects and cationic defect in the Y₃Sc₂Al₃O₁₂:xB³⁺ (x = 0.10–0.17) phosphor. At 445 nm excitation, the emission covers the region of 450 nm to 800 nm with a satisfactory FWHM of ∼170 nm and has a high internal quantum efficiency of up to 95%. Importantly, in comparison with most other defective light-emitting phosphors, YSAG:B³⁺ exhibits promising thermal quenching performance, retaining 73.23% of the original intensity after the test temperature was increased by ∼230 °C. These results reveal the mechanism of defect formation in garnet, and pave the way toward the design of efficient broadband lighting emitters.

Author contributions

Qianyi Chen: conceptualization, data curation, formal analysis, investigation, methodology, and writing – original draft; Zhenjie Lun: data curation and software; Dongdan Chen: conceptualization, funding acquisition, project administration, resources, supervision, and writing – review & editing; Yongsheng Sun: data curation; Puxian Xiong: data curation; Siyun Li: data curation; Shanhui Xu: investigation; Zhongmin Yang: investigation.

Data availability

The data supporting this article have been included as part of the ESI.† More raw data are available from the corresponding author on reasonable request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was funded by the National Natural Science Foundation of China (No. 62235014) and the Fundamental Research Funds for the Central Universities (No. 2023ZYGXZR002).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi01824a

‡ Co-first author.

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