Crystal structure, Bi³⁺ yellow luminescence, and high quantum efficiency of Ba₃SbAl₃Ge₂O₁₄:Bi³⁺ phosphor for white light-emitting diodes

Abstract

Bi³⁺-activated luminescent materials have attracted increasing attention owing to their strong excitation in the near-ultraviolet (NUV) range instead of the visible range. Such a unique feature allows them to avoid reabsorption among phosphors, resulting in their growing popularity in research and applications. However, the majority of Bi³⁺-doped phosphors suffer from low quantum efficiency, imposing limitations on their practical applications. We hereby present a newly developed phosphor, Ba₃SbAl₃Ge₂O₁₄:Bi³⁺ (BSAG:Bi³⁺), which emits a vibrant yellow light when excited by NUV light. Importantly, this phosphor exhibits a high internal quantum efficiency (IQE) of 95.3%, marking a significant advancement in the field. Through charge compensation, BSAG:Bi³⁺, K⁺ phosphor achieves a remarkable IQE of 97.2%. The photoluminescence (PL) spectroscopy analysis reveals that this phosphor contains only one Bi³⁺ luminescent center, which is consistent with the trigonal structure of BSAG. This is supported by the fact that only one Ba site in the structure can accept Bi³⁺ ions. The critical distance was estimated to be 9.71 Å. The energy transfer mechanism between Bi³⁺ ions was determined as a dipole–dipole interaction. To explore the application of BSAG:Bi³⁺ phosphor, pc-WLED devices were fabricated by depositing a blend of this phosphor and one or two commercial phosphors on a 365 nm chip. The final warm pc-WLED device exhibits ideal photoelectric performance with a low CCT of 4229 K and a high Ra of 91.5.

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1. Introduction

Phosphor-converted white light-emitting diodes (pc-WLEDs) have gained popularity in recent years in terms of their high efficiency, environmental sustainability, energy-saving capability, and long service life. As a result, they are progressively supplanting traditional lighting sources such as fluorescent lamps and incandescent bulbs.^1–4 With the rapid expansion of pc-WLEDs in the field of lighting, the demand for high-quality pc-WLEDs has increased, particularly for indoor lighting, and the focus of pc-WLEDs has changed from the original goal of achieving high brightness to the current emphasis on achieving high quality, including low correlated color temperature (CCT) and high color rendering index (Ra).^5–7 Nowadays, there are two main ways to obtain pc-WLEDs: blue LED chip-based technology (blue LED chip and multiple phosphors) and NUV LED chip-based technology (NUV LED chip and multiple phosphors). In blue LED chip-based technology, the typical combination of a blue LED chip and a commercial YAG:Ce³⁺ phosphor, a high correlated color temperature (CCT > 5400 K) and a low color rendering index (Ra < 75) are encountered owing to the inadequacy of red light components. This makes it challenging to fulfill the requirements for high quality pc-WLEDs.⁸ With the increasing maturity of NUV LED chip manufacturing technology, the combination of a NUV LED chip and multiple phosphors has become the preferred strategy for the package of pc-WLEDs.⁹

Phosphors are essential components in pc-WLEDs because they help convert the blue or NUV light emitted by LED chips into a wider range of colors, such as green, yellow, and red.^10,11 While many phosphors efficiently respond to blue light, most of them do not respond well to NUV light. Green, yellow, and red phosphors that can be excited by NUV light have been studied more extensively, but they are concentrated in rare earth doped systems. These systems often suffer from reabsorption among phosphors, which affects their suitability for manufacturing high-quality pc-WLEDs.^12,13

Recently, Bi³⁺ doped phosphors have become increasingly attractive. Similar to Ce³⁺ or Eu²⁺ ions, Bi³⁺ ions are highly influenced by the crystal field environment in the host lattice, enabling them to emit light ranging from UV to red in various host compounds.^14–16 More importantly, Bi³⁺ doped phosphors exhibit strong absorption in the NUV range and weak or no absorption in the visible range, effectively minimizing reabsorption among phosphors. Unfortunately, few Bi³⁺ doped phosphors have high quantum efficiency. For example, La₃BWO₉:Bi³⁺ (19.2%),¹⁷ KLaTa₂O₇:Bi³⁺ (42.3%),¹⁸ Ca₃Lu₂Ge₃O₁₂:Bi³⁺ (52%),¹⁹ and BaSc₂O₄:Bi³⁺ (23.4%)²⁰ phosphors suffer from relatively low quantum efficiency, which hampers their practical applications in pc-WLEDs.

In this work, we have successfully developed a phosphor, Ba₃SbAl₃Ge₂O₁₄:Bi³⁺ (BSAG:Bi³⁺), which exhibits an impressive quantum efficiency of up to 95.3%. This phosphor exhibits strong excitation in the UV range and emits broadband yellow luminescence with a peak at 545 nm and a full width at half maximum (FWHM) of approximately 129 nm. To gain a comprehensive understanding of BSAG:Bi³⁺ phosphor, we meticulously analyzed its crystal structure and PL behavior. This analysis aims to establish the relationship between the crystal structure and the luminescence properties. Furthermore, we explored the potential applications of BSAG:Bi³⁺ phosphor by incorporating it into pc-WLED devices. To achieve this, we coated a blend of BSAG:Bi³⁺ phosphor and one or even two commercial phosphors on a 365 nm LED chip. By undertaking these investigations, we expect to contribute valuable insights into the development of highly efficient Bi³⁺ doped phosphors for various practical applications.

2. Experimental

2.1. Preparation

A series of Ba₃SbAl₃Ge₂O₁₄:Bi³⁺ (BSAG:Bi³⁺) samples were synthesized using the high-temperature solid-state method. All the raw materials involved in this work, namely BaCO₃ (99.95%), Li₂CO₃ (99.99%), Na₂CO₃ (99.99%), K₂CO₃ (99.99%), Al(OH)₃ (99.9%), Sb₂O₃ (99.5%), GeO₂ (99.99%), and Bi₂O₃ (99.99%), were used as received without any additional purification steps. Considering the preferential occupation of Ba sites by Bi³⁺ ions, a target compound with a nominal chemical composition of Ba_3(1−x)SbAl₃Ge₂O₁₄:3xBi³⁺ (0 ≤ x ≤ 4.0%) was designed. The raw materials were precisely weighed according to the stoichiometric ratio, and then thoroughly ground and mixed in an agate mortar. The resulting mixture was pretreated at 873 K for 2 h, followed by calcination at 1673 K for 6 h in air, and naturally cooled to room temperature (RT). Finally, the obtained products were reground for testing.

2.2. Characterization

The phase composition and purity were identified by X-ray diffraction (XRD) using a Rigaku Ultima IV powder diffractometer. GSAS 2.0 academic program was used for Rietveld refinement.²¹ The valence state of the Bi element was verified by X-ray photoelectron spectroscopy (XPS) using a Kratos Axis Supra spectrometer. Scanning electron microscopy (SEM, TESCAN MIRA LMS) was used to determine the morphology and elemental composition. Photoluminescence excitation (PLE) and photoluminescence (PL) spectra (RT-450 K), decay curves, and quantum efficiency were obtained using an Edinburgh FLS1000 fluorescence spectrometer. The excitation photons were provided by a 450 W Xe lamp and a 150 W nF900 flash lamp. Diffuse reflectance spectra were measured on a Cary 5000 UV-vis-NIR spectrophotometer. Photoelectric data of LED devices were recorded using an Everfine HAAS-2000 high-accuracy array spectroradiometer.

2.3. Packaging

The LED devices were fabricated by integrating a mixture of commercially available BaMgAl₁₀O₁₇:Eu²⁺ (BAM), (Sr,Ca)AlSiN₃:Eu²⁺, and homemade BSAG:1.0%Bi³⁺ phosphors on a 365 nm LED chip. Initially, the thermally curable silicone resin A was mixed with the hardener B (A : B = 10 : 1), and the phosphors were added to the mixture. Then, the mixture was dropped on the 365 nm LED chip after removing air bubbles by vacuum treatment. Finally, the chip was hardened using a two-stage heat treatment. The first stage was carried out in a vacuum oven for 1 h at 373 K, followed by 4 h at 423 K in a regular oven.

3. Results and discussion

Fig. 1a shows the XRD patterns of BSAG:xBi³⁺ samples and the standard BSAG card (JCPDS no. 52-1678). All diffraction peaks for each sample can be exactly indexed to the trigonal phase of BSAG, with no additional peaks observed after Bi³⁺ doping. This strongly suggests that each sample is the pure phase, and the incorporation of Bi³⁺ ions does not introduce any undesired phases or impurities. Furthermore, a slight shift of the primary diffraction peak towards the higher 2θ direction can be observed as x increases from 0 to 4.0% due to the smaller ionic radius of Bi³⁺ than that of Ba²⁺ (Fig. 1a).²² BSAG has a trigonal crystal structure with the space group P321 (no. 150). As visualized in Fig. 1b, four independent crystallographic cation sites can be identified in this structure, namely Ba sites (3e), Sb sites (1a), Al sites (3f), and Ge sites (2d). Each cation site is occupied by a single atom. Each Ba atom coordinates with eight oxygen atoms to form a polyhedron [BaO₈], and each Sb atom coordinates with six oxygen atoms to form an octahedron [SbO₆]. These two polyhedra interconnect by edge-sharing, resulting in a Ba–Sb layer. Meanwhile, each Al atom and Ge atom coordinate with four oxygen atoms to form a tetrahedron [AlO₄] and an ortho-tetragonal pyramid [GeO₄], respectively. These two different tetrahedra connect with each other by corner-sharing, resulting in an Al–Ge layer. The Ba–Sb layer is bound to the Al–Ge layer by co-edges or co-corners, and the two layers are alternately stacked along the c-axis to form a stabilized and orderly three-dimensional framework. It is essential to evaluate the possible site occupancy by Bi³⁺ ions by reason of several cation sites in the trigonal structure. In general, when a foreign ion replaces a local ion, the radius percentage discrepancy (D_r) between the two should be less than 30%. The value of D_r can be calculated using the following equation:²³where R_d(CN) and R_m(CN) refer to the radii of the foreign cation and local cation, respectively, and CN refers to the coordination number. Table 1 presents the coordination number, the ionic radii, and the corresponding D_r values, respectively. Upon comparing the ionic radii of Ba²⁺, Sb⁵⁺, Al³⁺, and Ge⁴⁺ with that of Bi³⁺, the calculated D_r values are 17.6%, −86.67, −138.30%, and −138.30%, respectively. Consequently, it can be inferred that Bi³⁺ ions may potentially occupy Ba site rather than other cation sites. To further confirm the site occupied by Bi³⁺ ions, Rietveld refinement was performed. Fig. 1c describes the observed and calculated XRD patterns of BSAG:1.0%Bi³⁺ along with the difference profile for structure refinement. The residual factors converge to R_wp = 8.26%, R_p = 6.10%, R_b = 6.41% and χ² = 1.69, implying that the refined results are acceptable. The refined structural data of BSAG:1.0%Bi³⁺ are summarized in Table 2. These results indicate that there are no impurity phases that can provide alternative sites for Bi³⁺ ions. Instead, Bi³⁺ ions preferentially occupy Ba site to create the only Bi³⁺ luminescent center.

Fig. 1 (a) XRD patterns of BSAG:xBi³⁺; (b) crystal structure of BSAG; and (c) observed (black dots) and calculated (red line) XRD patterns of BSAG:1.0%Bi³⁺ along with the difference profile (blue line).

Table 1 Coordination number, ionic radii, and the corresponding D_r values

Local ion	Bismuth ion	R m (Å)	R d (Å)	D r (%)
Ba^2+ (CN = 8)	Bi^3+ (CN = 8)	1.42	1.17	17.61
Sb^5+ (CN = 6)	Bi^3+ (CN = 6)	0.60	1.12	–86.67
Al^3+ (CN = 4)	Bi^3+ (CN = 6)	0.47	1.12	–138.30
Ge^4+ (CN = 4)	Bi^3+ (CN = 6)	0.47	1.12	–138.30

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Table 2 Refined structural data of BSAG:1.0%Bi³⁺

Atom	Site	x	y	z	Occ.	Uiso
O1	6g	0.10503	0.21385	0.24721	1.000	0.007
O2	6g	0.45329	0.16516	0.35387	1.000	0.019
Al1	3f	0.24941	0.00000	0.50000	1.000	0.004
Ba1	3e	0.57193	0.00000	0.00000	0.990	0.014
O3	2d	0.33333	0.66667	0.18610	1.000	0.003
Ge1	2d	0.33333	0.66667	0.51267	1.000	0.013
Sb	1a	0.00000	0.00000	0.00000	1.000	0.010
Bi	3e	0.57193	0.00000	0.00000	0.010	0.014
Cell parameters	a = b = 8.539 Å, c = 5.125 Å, V = 323.730 Å^3
Refined factors	R wp = 8.26%, Rp = 6.10%, Rb = 6.41%, χ^2 = 1.69

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Furthermore, the cell volume of BSAG:1.0%Bi³⁺ decreases from 323.825 to 323.730 ų compared to that of blank BSAG, resulting from the substitution of smaller Bi³⁺ ions for Ba²⁺ ions. This leads to cell shrinkage after Bi³⁺ doping into the host lattice.

The microstructure and chemical composition of BSAG:1.0%Bi³⁺ phosphor were analyzed using SEM. Fig. 2a and b show that this phosphor has an irregular shape, smooth surface, and narrow size distribution with diameters ranging from 1 to 5 μm. This uniformity in size is beneficial for LED device packaging, although some slight agglomeration was observed due to the solid-state reaction at high temperature.^24,25 EDX analysis confirms that Ba, Sb, Al, Ge, O, and Bi elements survive, as displayed in Fig. 2d. Meanwhile, elemental mapping reveals that Ba, Sb, Al, Ge, O, and Bi elements are uniformly distributed (Fig. 2c). Bismuth is known to be unstable at high temperature and can easily generate various valence states. Therefore, XPS analysis was carried out on the representative BSAG:1.0%Bi³⁺ phosphor, and the high-purity α-Bi₂O₃ (99.999%) was used as a control. As Fig. 2e shows, the full XPS spectrum of the phosphor further confirms the presence of Ba, Sb, Al, Ge, O, and Bi elements, supporting the result of EDX analysis (Fig. 2d). In the fine XPS spectrum of the Bi element (Fig. 2f), there are two peaks at 159.6 and 164.6 eV, namely ⁴f_5/2 and ⁴f_7/2, derived from spin–orbit splitting of f orbitals.²⁶ These peaks closely match those observed in α-Bi₂O₃, indicating that the Bi element exists in the +3 valence state and trivalent bismuth ions (Bi³⁺) are dominant.

Fig. 2 (a and b) SEM images of BSAG:1.0%Bi³⁺; (c) elemental mapping of BSAG:1.0%Bi³⁺; (d) EDX spectra of BSAG:1.0%Bi³⁺; (e) full XPS spectra of BSAG:1.0%Bi³⁺; and (f) fine XPS spectra of the Bi element in BSAG:1.0%Bi³⁺ and α-Bi₂O₃.

Fig. 3 displays the DRS of BSAG:xBi³⁺ (x = 0–4.0%). For the undoped sample (x = 0), the absorption ranging from 200 to 340 nm originates from the intrinsic absorption of BSAG. Upon introducing Bi³⁺, a new absorption band emerges at 340–400 nm, arising from the ¹S₀ → ³P₁ transitions of Bi³⁺. Notably, the absorption edge extends from 400 to 420 nm as Bi³⁺ content rises, which aligns well with 350–410 nm NUV LED chip. The optical band gap (E_g) for BSAG can be estimated using the following equation:²⁷

where F(R_∞) is the Kubelka–Munk function, hν is the energy per photon, E_g is the optical band gap, and C is the proportionality constant. The Kubelka–Munk function can be represented as follows:²⁷where R, S, and K correspond to the reflection, scattering, and absorption parameters, respectively. The absorption spectra come from the DRS, as depicted in the inset of Fig. 3. The value of E_g for BSAG determined using [F(R_∞)hν]² = 0 is 5.35 eV.

Fig. 3 DRS of BSAG:xBi³⁺. The inset shows the absorption spectra of BSAG.

Fig. 4a illustrates the excitation and emission spectra of BSAG:1.0%Bi³⁺ phosphor at RT. Upon excitation with UV light, the phosphor exhibits yellow emission, and the corresponding peak position and FWHM are 545 and 129 nm, respectively. This luminescence is attributed to the ³P₁ → ¹S₀ transitions of Bi³⁺ given that the host material does not emit at RT. When the phosphor is monitored at 545 nm, a strong excitation band appears in the range of 280–400 nm, originating from the ¹S₀ → ³P₁ transitions of Bi³⁺. To further understand the PL behavior of Bi³⁺, a detailed analysis of the PLE and PL spectra at RT was conducted in BSAG:1.0%Bi³⁺ phosphor. The normalized PLE and PL spectra are given in Fig. 4b and c, respectively. On the one hand, the profiles of excitation spectra remain unchanged as the monitoring wavelength increases from 460 to 660 nm (Fig. 4b). On the other hand, the peak position and FWHM of emission spectra remain nearly fixed with the increase in the excitation wavelength from 320 to 370 nm (Fig. 4c). These findings indicate that there is only one Bi³⁺ luminescent center in the host lattice. The PL behavior of BSAG:1.0%Bi³⁺ phosphor agrees with the trigonal structure of BSAG in which only one crystallographic Ba site is suitable for the occupation of Bi³⁺ ions. Fig. 4d depicts the concentration dependence of the PL property of BSAG:xBi³⁺ phosphors. Despite changes in Bi³⁺ concentration, both the peak position and the FWHM remain unchanged, which further supports the presence of a single Bi³⁺ luminescent center. By comparison, as the Bi³⁺ concentration increases, the PL intensity gradually rises until x reaches 1.0%, after which it falls due to concentration quenching, as depicted in the inset of Fig. 4d. This phenomenon might be caused through the nonradiative energy transfer from Bi³⁺ to Bi³⁺ ions.²⁸ To confirm the fact of energy transfer, the decay behaviors of BSAG:xBi³⁺ phosphors are examined. As shown in Fig. 4e, the decay curves are non-exponential and challenging to fit accurately. Consequently, only average decay lifetimes τ were integrated using the following formula:²⁹

where I is the emission intensity and t is the time. The values of τ are marked in Fig. 4e. With the increase of Bi³⁺ content from 0.1 to 3.0%, the decay lifetimes decrease from 1.081 to 0.993 μs, indicating the facticity of energy transfer between Bi³⁺ ions. Importantly, the decay lifetimes show a negligible variation as the Bi³⁺ content rises, suggesting the absence of extra nonradiative pathways for Bi³⁺ luminescence, which is an indicator of a phosphor with high quantum efficiency.

Fig. 4 (a) Excitation and emission spectra of BSAG:1.0%Bi³⁺; (b) normalized PLE spectra of BSAG:1.0%Bi³⁺; (c) normalized PL spectra of BSAG:1.0%Bi³⁺; (d) normalized PL spectra of BSAG:xBi³⁺ (the inset shows the integrated PL intensity as a function of Bi³⁺ concentration); (e) decay curves of BSAG:xBi³⁺; and (f) relationship between log(I/x) and log(x).

Generally, nonradiative energy transfer occurs through two mechanisms: electric multipolar interaction and exchange interaction, which are distinguished by the critical distance R_c. The following equation proposed by Blasse and Grabmaier can be used to determine the R_c value:³⁰

where V denotes the cell volume; x_c denotes the critical concentration; and N denotes the number of cations per unit cell. In the particular case, V = 323.825 ų, x_c = 0.03, and N = 1. The resulting value of R_c is 9.71 Å. Based on this result, the exchange interaction has nothing to do with the energy transfer between Bi³⁺ ions, as it only works at very short distances (<5 Å). Consequently, the electric multipolar interaction primarily contributes to the energy transfer. Following Dexter's theory, the following equation can be used to evaluate the type of electric multipolar interaction:³¹

where x refers to the Bi³⁺ concentration (x ≥ x_c); I refers to the emission intensity; β and K refer to constants; and θ is a parameter which is related to the type of electric multipolar interaction. Concretely, θ is assigned the values of 10, 8, and 6, responsible for quadrupole–quadrupole interactions (q–q), dipole–quadrupole interactions (d–q), and dipole–dipole interactions (d–d), respectively. By plotting log(I/x) versus log(x), as shown in Fig. 4f, all data points can be fitted well as a linear function with a slope −θ/3 of −1.94. Obviously, the value of θ is 5.82, which is in proximity to 6, indicating that the energy transfer between Bi³⁺ ions dominates via dipole–dipole interaction.

The quantum efficiency of a phosphor is a crucial factor to consider when choosing one for application. The internal quantum efficiency (IQE), absorption efficiency (AE), and external quantum efficiency (EQE) of the optimal BSAG:1.0%Bi³⁺ phosphor were measured using a BaSO₄-coated integrating sphere with a diameter of 150 mm. The IQE, AE, and EQE can be calculated using the following equations:³²

where denotes the integral area for the emission spectrum of BSAG:1.0%Bi³⁺ phosphor (number of emitted photons); denotes the integral area for the excitation spectrum of BaSO₄ reference (number of incident photons); and denotes the integral area for the excitation spectrum of BSAG:1.0%Bi³⁺ phosphor (number of reflected photons). As Fig. 5a shows, the IQE, AE, and EQE were determined to be 95.3%, 26.6% and 25.4%, respectively. Such a high quantum efficiency tallies with the low probability of nonradiative energy migration mentioned above. Fig. 5b provides an overview of the quantum efficiency of Bi³⁺-doped phosphors reported recently.^{17–20,32–37} In comparison with other Bi³⁺-doped phosphors, BSAG:1.0%Bi³⁺ phosphor exhibits a promising application in pc-WLEDs due to its high quantum efficiency.

Fig. 5 (a) Excitation line of BaSO₄ and the excitation line and emission spectrum of BSAG:1.0%Bi³⁺ and (b) quantum efficiency of some Bi³⁺-doped phosphors.

The thermal stability of a phosphor is an important parameter for its application in high-power LED devices. The temperature-dependent PL spectra of BSAG:1.0%Bi³⁺ were recorded in the range from 300 to 450 K. In Fig. 6a, it can be clearly observed that the intensity of emission peaks sharply decreases with the increase in temperature due to thermal quenching. Always, the configurational coordinate diagram can be used to explain thermal quenching, as shown in Fig. 6b. Thermal quenching occurs due to the thermally activated cross-over from the excited state ³P₁ to the ground state ¹S₀.³⁸ Additionally, the emission peak position exhibits a slight blue shift as temperature increases (Fig. 6a). This can be explained by thermally active phonon-assisted excitation from lower energy sublevels to higher-energy sublevels in the excited states of Bi³⁺.³⁹

Fig. 6 (a) Temperature-dependent PL spectra of BSAG:1.0%Bi³⁺. (b) Configurational coordinate diagram of Bi³⁺.

To clarify the effect of charge compensation on Bi³⁺ luminescence, Li⁺, Na⁺, or K⁺ ions were added to maintain charge balance. As shown in Fig. 7a, the emission intensity of Bi³⁺ increases after adding K⁺ ions, and it decreases when Li⁺ or Na⁺ ions were added. However, the emission peak position and FWHM remain unchanged regardless of the addition of Li⁺, Na⁺, or K⁺ ions (Fig. 7b). Fig. 7c shows the IQEs of BSAG:Bi³⁺, BSAG:Bi³⁺, Li⁺, BSAG:Bi³⁺, Na⁺, and BSAG:Bi³⁺, K⁺ phosphors. By charge compensation, BSAG:Bi³⁺, K⁺ phosphor achieves a remarkable IQE of 97.2%. Furthermore, the thermal stability of BSAG:Bi³⁺ phosphor can be improved by the addition of Li⁺, Na⁺ or K⁺ ions, as shown in Fig. 7d.

Fig. 7 (a) Emission spectra, (b) normalized emission spectra, (c) internal quantum efficiency, and (d) thermal stability of BSAG:Bi³⁺, BSAG:Bi³⁺, Li⁺, BSAG:Bi³⁺, Na⁺, and BSAG:Bi³⁺, K⁺ phosphors.

As proof-of-concept devices, pc-WLEDs were fabricated by combining the homemade BSAG:1.0%Bi³⁺ phosphor and one or two commercial phosphors on a 365 nm LED chip. Fig. 8a presents the CIE color coordinates of these phosphors when excited at 365 nm. In Fig. 8b (III), the pc-WLED device, combining our phosphor and BAM phosphor, distinctly displays three emission bands centered at 365, 450, and 545 nm, corresponding to the LED chip, BAM phosphor, and BSAG:1.0%Bi³⁺ phosphor, respectively. This device emits cool white light with CIE color coordinates of (0.27, 0.31), a CCT of 9978 K, a Ra of 80.9, and a luminous efficiency of 0.58 lm W⁻¹. To achieve warm white light, the red phosphor (Sr,Ca)AlSiN₃:Eu²⁺ was incorporated. The optimized device exhibits outstanding photoelectric performance with CIE color coordinates of (0.37, 0.35), a CCT of 4229 K, a Ra of 91.5, and a luminous efficiency of 0.65 lm W⁻¹, as illustrated in Fig. 8b (IV). For comparative purposes, the emission spectra of the naked LED chip and the yellow LED device are presented in Fig. 8b (I) and (II), respectively. Each inset in Fig. 8b (I–IV) showcases a physical image of the respective device driven by 350 mA current. These findings suggest that BSAG:1.0%Bi³⁺ phosphor holds significant promise for applications in NUV pc-WLEDs.

Fig. 8 (a) CIE color coordinates of BAM (B), (Sr,Ca)AlSiN₃:Eu²⁺ (R), and BSAG:1.0%Bi³⁺ (Y) phosphors excites at 365 nm; and (b) emission spectra of the (I) naked LED chip, (II) yellow pc-LED device based on single BSAG:1.0%Bi³⁺ phosphor, (III) cool pc-WLED device based on a blend of BSAG:1.0%Bi³⁺ and BAM phosphors, and (IV) warm pc-WLED device based on a blend of BAM, (Sr,Ca)AlSiN₃:Eu²⁺, and BSAG:1.0%Bi³⁺ phosphors driven by a 350 mA current; each inset shows the physical image of the lighted LED devices.

4. Conclusion

In summary, we prepared a series of BSAG:Bi³⁺ phosphors by a solid state reaction. These phosphors can be effectively excited by NUV light, and emit yellow luminescence with a peak at 545 nm and a FWHM of ∼129 nm. XPS analysis confirms that the trivalent Bi³⁺ dominates in these phosphors. The optical band gap of BSAG is ∼5.35 eV. PL spectroscopy analysis reveals that there is only one Bi³⁺ luminescent center in the host lattice, which is in agreement with the trigonal structure of BSAG because only one Ba site in this structure can accommodate Bi³⁺ ions. The optimal doping concentration of Bi³⁺ was determined to be 1.0%, and the related IQE reached 95.3%. By charge compensation, BSAG:Bi³⁺, K⁺ phosphor achieves a remarkable IQE of 97.2%. The critical distance was calculated to be 9.71 Å, and the energy transfer mechanism between Bi³⁺ was demonstrated as a d–d interaction. As proof-of-concept devices, pc-WLEDs were fabricated by combining the homemade phosphor and one or two commercial phosphors on a 365 nm LED chip, of which the warm pc-WLED device exhibits ideal photoelectric performance with a low CCT of 4229 K and a high Ra of 91.5.

Author contributions

Huihui Fei: conceptualization, data curation, formal analysis, investigation, software, methodology, and writing – original draft; Baolong Jing: data curation and formal analysis; Jin Han: conceptualization, funding acquisition, project administration, resources, supervision, and writing – review & editing; Kaian Shan: data curation, formal analysis; Dongming Cheng: investigation and software; Xueqing Xu: resources, and writing – review & editing; Xinmin Zhang: resources; Jing Wang: resources, and writing – review & editing.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the Hunan Provincial Natural Science Foundation (Grant No. 2020JJ5983) and the Scientific Research Fund of Hunan Provincial Education Department (Grant No. 23C0106).

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