Site preference and defect engineering of a highly efficient blue-emitting phosphor Sr₂SiO₄:Ce³⁺/K⁺ toward thermally enhanced luminescence

Abstract

The blue-emitting phosphor is one of the trichromatic phosphors essential for the development of near-ultraviolet (nUV) pumped phosphor-converted white light-emitting diodes (pc-WLEDs). Efficiency and thermal stability are two critical factors for the potential application of phosphors in solid-state lighting. In this work, we report a highly efficient blue-emitting phosphor, α-Sr₂SiO₄:0.02Ce³⁺/0.02K⁺ (emission peak at 425 nm) with superior thermally enhanced photoluminescence properties. The Rietveld refinement results suggest that Ce³⁺ preferentially occupies the nine-fold coordinated Sr2 site only whereas K⁺ occupies both Sr1 and Sr2 sites in the host structure. Its emission intensity gradually increases with increasing temperature and retains 120% of its room-temperature peak intensity at 250 °C. Besides, its luminescence spectral peak does not shift at elevated temperatures. Under nUV irradiation, the phosphor is highly luminous with an excellent internal quantum efficiency (IQE) of 91.7%. The abnormal thermal quenching phenomenon can be ascribed to the defect energy levels introduced by defect engineering using an aliovalent doping strategy of Ce³⁺ and K⁺ in the α-Sr₂SiO₄ host structure. The as-fabricated WLED prototypes demonstrated the great potential of the highly efficient blue-emitting phosphor with superior thermal stability for solid-state lighting applications.

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1 Introduction

Phosphor-converted white light-emitting diodes (pc-WLEDs) have attracted extensive attention due to their numerous advantages such as small size, long life span, fast response, high luminous efficacy, and low energy consumption.^1,2 The traditional approach for making white LEDs is to combine a blue-emitting chip with a yellow garnet phosphor (YAG:Ce³⁺).^3,4 However, this approach lacks a red emission component, thus leading to a poor color rendering index (CRI, Ra < 80) and a relatively high correlated color temperature (CCT > 4500 K).⁵ An alternative strategy has been attracting significant attention in recent years, namely combining near ultraviolet (nUV) pumped red/green/blue (RGB) emitting phosphors.⁶ This new approach enables the full visible spectrum, thereby leading to the feasibility of a high CRI (Ra ≥ 95), a tunable CCT and a wide color gamut of white LED devices for different application scenarios. To meet the new development trend, highly efficient blue phosphors with superior thermal stability are of particular interest and importance.

Recent efforts have been devoted to the exploration of new blue-emitting phosphors with better photoluminescence properties.^7,8 The properties of phosphors largely determine the performance of solid-state lighting devices.^1,2 Among them, the thermal quenching effect plays a vital role in the luminescence stability of phosphors at increased temperatures, further affecting the photoelectric properties of the pc-WLED devices whose working temperature can reach ca. 150 °C.⁹ The luminescence thermal stability of phosphors is particularly crucial for high-power lighting applications in which the working temperature can be even higher (over 200 °C).¹⁰ To date, several blue phosphors have been reported as showing excellent quantum efficiencies and thermal stability.⁷ Most of them are Eu²⁺-activated phosphors, such as Ca₂PO₄Cl:Eu²⁺,¹¹ RbBaPO₄:Eu²⁺,¹² Na₃Sc₂(PO₄)₃:Eu²⁺,¹³ BaLi₂[Be₄O₆]:Eu²⁺,¹⁴ BaAl₁₂O₁₉:Eu²⁺,¹⁵ (Sr_0.69Ba_0.3)₂P₂O₇:Eu²⁺¹⁶ and the commercially available BAM phosphor (BaMgAl₁₁O₁₇:Eu²⁺).¹¹ These phosphors have an external quantum efficiency (EQE) of 40–70% and a thermal stability of 83–110%@200 °C (the percentage of its integrated emission intensity at 200 °C relative to that at room temperature).⁷ Our recent work developed a blue phosphor, K₂Sr_xBa_2−x(PO₄)₂:Eu²⁺, with a thermal stability of 93%@200 °C, an IQE of 96.4% and an EQE of 76.4%.^7,8 Nevertheless, blue-emitting phosphors activated by Ce³⁺ are less studied. Ce³⁺-activated boride phosphors such as Ba₃Y₂B₆O₁₅:Ce³⁺ (ref. 17) and NaSrBO₃:Ce³⁺ (ref. 18) have poor thermal stability, retaining only 20%@200 °C and 58%@200 °C respectively. Some other phosphors exhibit relatively better thermal stability at the level of 70–80%@200 °C, such as Ca₃ZrSi₂O₉:Ce³⁺ (ref. 19) and SrLu₂O₄:Ce³⁺.²⁰ Therefore, it is of great interest to explore Ce³⁺-activated blue-emitting phosphors with excellent thermal stability and quantum efficiencies.

Silicate phosphors are an important family that has been extensively investigated due to its advantages of chemical and structural diversity, abundance of raw materials, and simple and cost-effective synthesis methods.^21–25 Alkaline earth metal orthosilicates, M₂SiO₄ (M = Ca, Sr, and Ba), have shown their suitability for application as a host in commercial phosphors such as (Sr,Ba)₂SiO₄:Eu²⁺. These phosphors are tunable for green/yellow emission, but their thermal stability remains poor as they retain merely 50% at 200 °C.²⁶ Although Sr₂SiO₄-based phosphors have recently been rekindling research interest, those activated by Ce³⁺ are rarely studied. In addition, when divalent alkaline metal ions are substituted by trivalent ions such as Eu³⁺, charge compensation with monovalent ions is essential for the bright emission of phosphors.^27,28

Herein, we report the preparation and characterization of a blue-emitting phosphor, Sr₂SiO₄:Ce³⁺/K⁺, featuring an abnormal thermal quenching effect and excellent quantum efficiency, where Ce³⁺ acts as the activator and K⁺ as the charge compensator. The effect of the doping concentration on the phase structure and luminescence properties has been investigated. The quantum efficiencies and thermal stability of phosphors as well as the mechanism involved have been characterized and discussed. To demonstrate their potential application, white LEDs have been prototyped and their photoelectric properties have been studied.

2 Experimental section

2.1 Material synthesis

Sr₂SiO₄:xCe³⁺/xK⁺ phosphor powders were synthesized via a high-temperature solid-state reaction. The raw chemicals, including K₂CO₃, SrCO₃, SiO₂, and CeO₂, were weighed in stoichiometric amounts. K⁺ ions were introduced to compensate for the excess charge of Ce³⁺ at the Sr²⁺ site. Then the mixtures were thoroughly ground in an agate mortar for half an hour, and transferred to alumina crucibles for calcination in air at 700 °C for 8 h. After that, the calcined samples were re-ground and transferred into alumina crucibles with lids for synthesis at 1250 °C for 4 h under a reducing atmosphere of 95% Ar/5% H₂ forming gas. After synthesis, the samples were ground gently into powders for further analysis.

2.2 Characterization methods

The phase structure of the Sr₂SiO₄-based phosphor powders was characterized by X-ray diffraction (XRD) using a diffractometer (D2 Phaser, AXS Bruker, Germany) with Cu Kα radiation (λ = 1.5418 Å) under operating conditions of 30 kV and 10 mA. The structure refinement upon X-ray diffraction was conducted using the Fullprof Suite program.²⁹ The morphology and elemental distribution of phosphors were examined with a scanning electron microscope (SEM, JEOL JSM-6480, Japan), operating at an acceleration voltage of 20 kV. Transmission electron microscopy (TEM, Tecnai G2 F30 S-TWIN) was performed to investigate the morphology, lattice structure and elemental distribution of the prepared phosphor samples.

Photoluminescence excitation (PLE) and emission (PL) spectra were acquired using a spectrofluorometer (Fluoromax PLUS, Horiba, Japan) equipped with a 150 W xenon lamp at room temperature (20 °C) and elevated temperatures (25–250 °C). Each temperature-dependent spectrum was collected after the target temperature had been maintained for no less than 5 min so that the set temperature could be attained. Quantum efficiencies (QE) were measured with an integral sphere and BaSO₄ as the reference material. The spectral bands used for QE calculation are 380–500 nm (for integrating the emission intensity of the phosphor) and 335–365 nm (for integrating the excitation light source). The internal quantum efficiency (IQE) refers to the ratio of the number of photons emitted to that absorbed, which is calculated using eqn (1):

where ε is the number of photons emitted by the phosphor sample and α represents the number of photons absorbed. L_S is the luminescence emission spectrum of the phosphor; E_S is the spectrum of the light used for exciting the sample; and E_R is the spectrum of the excitation light without the sample in the sphere. All the spectra were collected using an integral sphere. Subsequently, the external quantum efficiency (EQE) was determined, which is the product of the internal quantum efficiency (IQE) and the absorption coefficient (Abs), as EQE = IQE × Abs.

The thermoluminescence (TL) curves were measured on an SL08 system (Guangzhou RuiDi Aisheng Technology Co. Ltd, China) in a range of 20–500 °C with a ramp rate of 1 K s⁻¹. Each sample was irradiated under 254 nm UV light for 5 min before TL testing.

2.3 Fabrication of white LED prototypes

To demonstrate its potential application in solid-state lighting, we used the as-prepared phosphor Sr₂SiO₄:0.02Ce³⁺/0.02K⁺ to fabricate prototype white LED devices with a near-ultraviolet (n-UV) InGaN LED chip (λ = 365 nm) and commercial green phosphor (Sr,Ba)₂SiO₄:Eu²⁺ and red phosphor (Sr,Ba)AlSiN₃:Eu²⁺. The phosphors were mixed with epoxy resin, and the obtained mixture was dropped on top of the LED chips. The photoelectric properties, including the electroluminescence (EL) spectrum, correlated color temperature (CCT), color rendering index (Ra), CIE chromaticity coordinates and luminous efficacy of the WLED prototypes, were collected using an OHSP-350M spectrophotometer system (Hangzhou Hopoo Light & Color Co. Ltd, China) equipped with a 0.5 m integrating sphere.

3 Results and discussion

3.1 Phase and structure analysis

Alkaline earth metal orthosilicates, M₂SiO₄ (M = Ca, Sr, and Ba), have two polymorphs, namely the orthorhombic α′ phase (isostructural to β-K₂SO₄ type structure, space group Pmnb) and the monoclinic β phase (space group P21/n). The α′ phase of pure Sr₂SiO₄ transforms to the β phase at 358 K (85 °C) while cooling during the synthesis process.³⁰ Studies show that elemental doping can stabilize the α′ phase structure of Sr₂SiO₄ at room temperature.³¹

The XRD patterns of the Sr₂SiO₄:xCe³⁺/xK⁺ phosphor samples synthesized at 1250 °C for 4 h are shown in Fig. 1. As clearly shown in Fig. 1b, a characteristic peak at 2θ of ∼32.40° indicates the existence of monoclinic β-Sr₂SiO₄ in the samples with an x value of 0.02 or smaller. Thus, biphasic powders were received if the x value was 0.02 or lower whereas the orthorhombic α′-Sr₂SiO₄ was the main phase and the monoclinic β-Sr₂SiO₄ was diminishing gradually with the increase in Ce³⁺ concentration (x). Pure α′-Sr₂SiO₄ powders were obtained for samples with x ≥ 0.3.

Fig. 1 (a and b) XRD patterns of Sr₂SiO₄:xCe³⁺/xK⁺ phosphors where (b) is a zoomed-in graph of (a), with arrows showing some characteristic peaks of β-Sr₂SiO₄ in the samples of x ≤ 0.02; SEM (c), TEM (d) and HRTEM (e) images and elemental mapping results (f) of Sr₂SiO₄:0.02Ce³⁺/0.02K⁺.

The SEM image of the Sr₂SiO₄:0.02Ce³⁺/0.02K⁺ phosphor powders calcined at 1250 °C is shown in Fig. 1c. The phosphor powders are spherical or round in shape with a relatively homogeneous grain size of 2–5 μm, and some grains are clustered together as a result of solid-state synthesis. We also used TEM to investigate the microstructure of the prepared phosphor powder. Fig. 1d shows a typical high-resolution TEM image (HRTEM) of a single grain, and EDS mapping was conducted to examine the elemental distribution in the grain (Fig. 1f). As seen from the HRTEM image, the grain reveals a well-ordered lattice infringed structure with a lattice spacing of 0.282 Å, which corresponds to the (200) lattice plane of the orthorhombic α′-Sr₂SiO₄ phase. The mapping data further demonstrate the homogeneous elemental distribution of Ce, K, Sr, Si and O.

In order to understand the crystallographic sites of Ce³⁺ and K⁺ in Sr₂SiO₄, Rietveld refinements were performed for the Sr₂SiO₄:0.05Ce³⁺/0.05K⁺ compound upon its X-ray powder diffraction patterns using the FullProf Suite. The refined diffraction pattern is plotted in Fig. 2a. Details about the crystallographic data, refinement parameters, atomic positions and site occupancies are given in Tables 1 and 2. Further details of the crystal structure investigation may be obtained from an online deposition service of the joint CCDC/FIZ Karlsruhe: https://www.ccdc.cam.ac.uk/structures/ by quoting the deposition number CCDC 2225570.†

Fig. 2 (a) The refined XRD pattern and (b) crystal structure of Ce³⁺/K⁺ co-doped α′-Sr₂SiO₄ and the coordination of Sr sites (Sr1O₁₀ and Sr2O₉).

Table 1 Crystallographic data for the phosphor Sr₂SiO₄:0.05Ce³⁺/0.05K⁺ as refined by Rietveld method

Composition from the refinement	Ce0.0225K0.1275Sr1.85SiO4
Radiation source	X-ray (Cu Kα)
Source wavelength (Å)	1.5418
Crystal system	Orthorhombic
Space group	Pmnb (no. 62)
No. of refined parameters	38
R exp	6.2956
R wp	12.0198
Bragg R-factor	5.7439
χ ^2	3.6452
Z	4
a (Å)	5.6624(1)
b (Å)	7.0790(1)
c (Å)	9.7392(2)
α, β, γ (°)	90
V (Å^3)	390.39(1)
2θ range (°)	10–110

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Table 2 Fractional atomic coordinates, isotropic displacement factor B_iso and site occupancy fraction (SOF, Ų) of Ce_0.0225K_0.1275Sr_1.85SiO₄

Atom	x	y	z	B iso	Occ.	Wyckoff pos.
Si1	0.25	0.7765(7)	0.5837(8)	0.0430(19)	1	4c
Sr1	0.2697(13)	0.3400(2)	0.5795(2)	0.0336(7)	0.452(7)	8d
K1	0.2697(13)	0.3400(2)	0.5795(2)	0.0336(7)	0.048(7)	8d
Sr2	0.2734(9)	−0.0013(2)	0.3025(2)	0.0336(7)	0.473(7)	8d
Ce2	0.2734(9)	−0.0013(2)	0.3025(2)	0.0336(7)	0.011(7)	8d
K2	0.2734(9)	−0.0013(2)	0.3025(2)	0.0336(7)	0.0157	8d
O1	0.312(2)	1.0050(12)	0.5697(10)	0.021(2)	0.5	8d
O2	0.302(2)	0.6661(17)	0.4261(10)	0.021(2)	0.5	8d
O3	0.030(2)	0.7312(15)	0.6863(17)	0.021(2)	0.5	8d
O4	0.0108	0.6882	0.6471	0.021(2)	0.5	8d

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The phosphor crystallized in the structure of α′-Sr₂SiO₄ (space group Pnma).³² It is interesting to find out that any attempt at putting Ce at the Sr1 site led to negative values for its occupancy, whereas the refinement of Ce at Sr2 went nicely. This suggests that the activator Ce³⁺ ions prefer not to reside at the Sr1 site but only at the Sr2 site. The refinements demonstrated that the actual amount of Ce³⁺ was ∼0.0225 per formula unit, which was less than the nominal amount of 0.05. The remaining Ce³⁺ forms as CeO₂ impurity in a trace amount. On the other hand, K⁺ ions tend to reside at the Sr1 (dominantly) and Sr2 sites in the structure of the host α′-Sr₂SiO₄. The content of K⁺ ions largely increased to compensate for the unbalanced charge in the system due to the aliovalent doping of Ce³⁺. As per the refinement, the chemical formula of the phosphor is Ce_0.0225K_0.1275Sr_1.85SiO₄, and its crystal structure is shown in Fig. 2b.

3.2 Luminescence properties and the mechanism involved

Our preliminary studies showed that charge compensation (adding K⁺ ions) can improve the emission intensity of the phosphor, as demonstrated in Fig. S2.† Thus, we further investigated the concentration quenching effect of Sr₂SiO₄:xCe³⁺/xK⁺. Their photoluminescence (PL) spectra are shown in Fig. 3a. The peak wavelength and full width at half maximum (FWHM) of the emission spectra are fixed at around 425 nm and 65.5 nm respectively while varying the doping concentration of Ce³⁺/K⁺. The optimal Ce³⁺/K⁺ concentration was around 0.02, beyond which the phosphors demonstrated a concentration quenching effect. The concentration corresponding to its critical distance is the optimal doping concentration. Thus, the critical distance between the activator Ce³⁺ can be calculated using the Blasse equation:³³where V is the volume of a single unit cell (V = 393.71 ų), χ_c is the critical concentration (χ_c = 0.02), and N is the number of cations in a single cell (N = 8).

Fig. 3 (a) The PL spectra of Sr₂SiO₄:xCe³⁺/xK⁺ under excitation at 342 nm and the inset is the normalized intensity as a function of x, (b) excitation and emission spectra, (c) Gaussian fitting diagram of emission spectra of Sr₂SiO₄:0.02Ce³⁺/0.02K⁺, and (d) fluorescence decay curves of Sr₂SiO₄:xCe³⁺/xK⁺ at room temperature.

Through calculation, the critical distance for concentration quenching of the Sr₂SiO₄:xCe³⁺/xK⁺ phosphor was found to be 16.75 Å. Since the calculated value is greater than 5 Å, the energy transfer between Ce³⁺ ions is achieved by multipole interaction. According to the literature, there are three types of multipole interactions: electric dipole/electric dipole interaction, electric dipole/electric quadrupole interaction, and electric quadrupole/electric quadrupole interaction.³⁴ The type of multipole interaction can be determined according to Dexter's energy transfer theory using formula (3):³⁵

where I represents the intensity corresponding to the emission peak, x is the concentration of activator ions (Ce³⁺), and β is a constant.

According to this formula, log(I/x) can be used as the ordinate and log (x) as the abscissa to fit data and derive the slope and θ values. The fitting data are shown in Fig. S1,† and the slope is −1.005. Thus θ equals 3.015, indicating that the energy transfer in the Sr₂SiO₄:Ce³⁺ phosphor is mainly attributed to the electric dipole interaction.

Fig. 3b plots the PL and PLE spectra of the Sr₂SiO₄:0.01Ce³⁺ phosphor. This phosphor shows a blue emission band peaking at 425 nm with a FWHM of 65.8 nm at room temperature, which can be ascribed to the typical 5d → 4f transition. When monitored at 425 nm, the excitation spectrum demonstrates a band from 300 to 390 nm, peaking at around 342 nm. By applying Gaussian deconvolution to the PL spectrum, two peaks at 23 368 cm⁻¹ (428 nm) and 25 037 cm⁻¹ (399 nm) can be obtained (Fig. 3c). The energy difference between the two as-fitted Gaussian peaks is 1669 cm⁻¹. Such a difference is close to the energy difference of 2000 cm⁻¹ between the ²F_5/2 and ²F_7/2 transitions of Ce³⁺.³⁶ We further measured the emission spectra of Sr₂SiO₄:0.02Ce³⁺/0.02K⁺ at different excitation wavelengths, and the normalized spectra are shown in Fig. S3.† When the excitation wavelength gradually increases from 282 to 365 nm, a subtle change can be observed in the emission spectrum. This confirms the single emission center in the phosphor.

Fig. 3d shows the fluorescence decay plots of the Sr₂SiO₄:xCe³⁺/xK⁺ samples monitored at 425 nm under a 365 nm excitation. All decay curves are fitted using the following formula:³⁷

where I is the emission intensity, t is the time, A_i is the fitting constant, and τ_i is the lifetime. The decay curves of all samples with various Ce³⁺ doping concentrations can be well fitted by a single exponential function, and the fitting results are listed in Table S1.†

When the doping concentration of Ce³⁺ is 0.03 or smaller, the fluorescence lifetime gradually increases from 27.84 ns to 28.83 ns, and when it is higher than 0.03, the fluorescence lifetime gradually decreases. The phenomenon of increasing fluorescence lifetime for 0.005 ≤ x ≤ 0.03 can be ascribed to the biphasic composition. According to the analysis of XRD, when the Ce³⁺ doping concentration is lower than 0.03, the phosphors consist of α′ and β phases where β-Sr₂SiO₄ is gradually diminished. Accordingly, the fluorescence lifetime of Ce³⁺ in the β phase is likely shorter than that in the α′ phase structure. As the Ce³⁺ doping concentration reaches 0.03 or higher (x ≥ 0.03), the phosphor is composed of a single α′ phase. The fluorescence lifetime decreases because of the concentration quenching effect.

3.3 Luminescence thermal stability and the associated mechanism

The temperature-dependent emission spectra of Sr₂SiO₄:xCe³⁺/xK⁺ (x = 0.01 and 0.02) were collected from 25 to 250 °C under the excitation at 342 nm and are shown in Fig. 4a and c. The as-synthesized Sr₂SiO₄:Ce³⁺/K⁺ phosphor samples exhibit excellent thermal quenching resistance. Fig. 4b and d show the normalized intensities versus temperature for samples with x = 0.01 and 0.02, respectively. The thermal stabilities of the two samples are compared in Fig. S4† with respect to their peak intensity and integrated intensity.

Fig. 4 Temperature-dependent PL spectra and of Sr₂SiO₄:xCe³⁺/xK⁺ under the excitation at 342 nm, (a) x = 0.01 and (c) x = 0.02; and the evolution of emission intensity as a function of temperature, (b) x = 0.01 and (d) x = 0.02.

When the Ce³⁺ doping concentration is 0.01, the peak intensity and integrated intensity of the sample Sr₂SiO₄:0.01Ce³⁺/0.01K⁺ increase gradually, then they decrease when the temperature is higher than 150 °C. Specifically, the sample with x = 0.02 (Sr₂SiO₄:0.02Ce³⁺/0.02K⁺) has a superior thermal enhancement effect such that the luminescence intensities (peak and integrated) continuously increase with the rise in temperature. At 250 °C, the peak intensity and integrated intensity can retain 119.7% and 122.9% of their initial intensity (at 25 °C), respectively. The temperature-dependent PL spectra do not reveal a shift in the peak position but only show subtle changes in the spectral shape (Fig. S5†). This result suggests that there is no color drift while the working temperature changes. The sample with x = 0.01 shows zero thermal quenching up to 200 °C. Therefore, the phosphor with an optimal doping concentration, namely Sr₂SiO₄:0.02Ce³⁺/0.02K⁺, has both the brightest emission and superior thermal enhancement features, which are very promising for high-power nUV-pumped white LED devices.

Recent studies suggest that zero (or abnormal) thermal quenching may be related to the interaction between activator ions and crystal defects acting as electron trapping centers.^38,39 In order to investigate the mechanism involved, we further investigated the thermoluminescence (TL) performance of Sr₂SiO₄:xCe³⁺/xK⁺ samples with x values of 0.01 and 0.02. The TL curves in the temperature range of 20–500 °C are shown in Fig. 5a. Three individual peaks are evidently observed at 122, 282 and 415 °C for the sample with x = 0.02, associated with corresponding electron traps due to the existence of defect energy levels. Using the relation E_T = T/500 eV where T is the absolute temperature in K,⁴⁰ the characteristic trap depths (E_T) are estimated to be 0.79, 1.11 and 1.38 eV, respectively.

Fig. 5 (a) TL curves of Sr₂SiO₄:xCe³⁺/xK⁺ with x = 0.01 and x = 0.02, and (b) simplified model for the mechanism of the thermally activated emission of Sr₂SiO₄:0.02Ce³⁺/0.02K⁺.

In comparison with x = 0.02, the sample with x = 0.01 has one trap less (trap C), and the peak intensity associated with traps A and B is much weaker, indicating that the population of defects is much smaller. From both the temperature-dependent PL spectra and the TL data of the two samples, the thermal stability of Sr₂SiO₄:0.01Ce³⁺/0.01K⁺ is thermally enhanced in a certain temperature range (namely 25–150 °C) before quenching again, attributed to the thermally activated emission of the electron traps A and B, whereas that of Sr₂SiO₄:0.02Ce³⁺/0.02K⁺ is activated for the additional trap C.

Thus, the abnormal thermal quenching effect of Sr₂SiO₄:0.02Ce³⁺/0.02K⁺ can be explained by the simplified model schematically shown in Fig. 5b. For steady-state luminescence at a given temperature, some 5d electrons of Ce³⁺ are thermally ionized into the conduction band of the ground state with a trapping process at the defect levels (processes 3 and 4). Then these electrons are released into the conduction band and transferred to the 4f level by a radiative transition (procedures 5, 6 and 2).³⁹ The relationship between emission intensity (I) and temperature (T) is derived by modeling the process using a probability equation:³⁹

where I₀ is the emission intensity at T = ∞ K, A is a constant, ΔE = E_trap − E_5d, E_trap is the energy difference between the defect level and the bottom of the conduction band of the matrix, and E_5d is the energy difference between the bottom of the matrix conduction band and the 5d level of Ce³⁺. The value of ΔE obtained by fitting is 0.055 eV (Fig. S6†). The thermal energy enables the detrapping of electrons, which contributes to the observed enhancement of emission as a function of temperature (thermal energy).

3.4 Quantum efficiency

Quantum efficiency is an important parameter of phosphors for solid-state lighting applications. Therefore, the quantum efficiencies of Sr₂SiO₄:0.02Ce³⁺/0.02K⁺ were measured (Fig. S7†). It is seen that the phosphor has an excellent internal quantum efficiency (IQE) of 91.7% when excited at 350 nm. The absorption rate (Abs) is 39.5%, and the external quantum efficiency (EQE) is 36.2%. The EQE of the phosphor could be further boosted by optimizing the synthesis process of phosphors.

Table 3 compares the internal quantum efficiencies and thermal stability of blue-emitting phosphors reported in recent years. As the number of reports of Ce³⁺-doped blue-emitting phosphors is limited, some Eu²⁺-doped blue-emitting phosphors are also compared. The IQE of Sr₂SiO₄:Ce³⁺/K⁺ phosphors is higher than that of most blue-emitting Ce³⁺-doped phosphors. Moreover, the luminescence thermal stability of Sr₂SiO₄:Ce³⁺/K⁺ is superior to most of the blue-emitting phosphors. In view of its excellent PL properties, Sr₂SiO₄:Ce³⁺/K⁺ is very promising for solid-state lighting applications.

Table 3 Comparison of quantum efficiencies and thermal stability of some blue phosphors

Phosphor	λ Ex (nm)	λ Em (nm)	TQ	IQE (%)	EQE (%)	Ref.
a Percentage estimated from the published plot (relative to the room temperature intensity).
Sr2SiO4:0.02Ce^3+/0.02K^+	342	425	120%@250 °C	91.7	36.2	This work
Ca6BaP4O17:Ce^3+,Si^4+	400	480	80%@150 °C	70	—	41
Ba9Y2Si6O24:0.03Ce^3+	394	480	85%@200 °C ^a	57	—	42
NaSrBO3:Ce^3+	365	422	58%@200 °C	74.7	61.6	18
BAM:Eu^2+	365	462	70%@250 °C	89	—	43
K2SrxBa2−x(PO4)2:Eu^2+	330	425	93%@200 °C	96.4	76.4	7 and 8
Sr5(PO4)3Cl:Eu^2+	395	444	∼78%@200 °C	80.5	—	44

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3.5 Fabrication and performance of WLED prototypes

In order to evaluate the potential applications of Sr₂SiO₄:Ce³⁺/K⁺ blue phosphors in improving the color rendering index of white LEDs and the coordinating color temperature, we combined near-ultraviolet InGaN chips (λ = 365 nm) with Sr₂SiO₄:0.02Ce³⁺/0.02K⁺ and commercial (Sr,Ba)₂SiO₄:Eu²⁺ (green emitting) and (Sr,Ba)AlSiN₃:Eu²⁺ (red emitting) to fabricate four white LED prototypes. Fig. 6 shows the EL emission spectra of the WLED prototypes at 20 mA. Four main emission peaks (365 nm, 425 nm, 525 nm, 610 nm) can be clearly seen in the emission spectra of these devices, corresponding to the emission peaks of the chip and blue, green and red phosphors, respectively.

Fig. 6 (a–d) The EL spectra and key performances of the assembled white LED devices using Sr₂SiO₄:Ce³⁺/K⁺ with commercial green and red phosphors pumped by an nUV chip.

Table 4 shows the relevant properties measured by the device, such as CRI, including Ra and R9, CCT, CIE coordinates and luminous efficacy. Prototype WLED devices with adjustable CRI (87–92) and CCT (3200–6700 K) are achieved by controlling the addition of phosphors. Explained in detail, the CCT changed from 4765 to 6663 K when we fixed the ratio of green and yellow phosphors and varied the content of blue phosphor (Fig. 6b–d), which further decreased to 3257 K if an additional amount of red phosphor was added (Fig. 6a). These results demonstrate that the Sr₂SiO₄:Ce³⁺/K⁺ phosphors have great potential for high-power solid-state lighting applications.

Table 4 The correlated color temperature, color rendering index (Ra and R9), CIE chromaticity coordinates and luminous efficacy of WLED prototypes

No.	CCT (K)	Ra	R9	CIE (x, y)	Luminous efficacy (lm W^−1)
1	3257	91.3	69	(0.4045, 0.3614)	24.3
2	4765	91.1	69	(0.3558, 0.3865)	26.5
3	5772	89.0	50	(0.3260, 0.3516)	22.9
4	6663	87.7	60	(0.3080, 0.3403)	27.8

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4 Conclusions

In summary, a highly efficient blue-emitting phosphor α-Sr₂SiO₄:0.02Ce³⁺/0.02K⁺ with an abnormal thermal quenching effect was successfully developed in this work by a defect engineering strategy. It exhibits superior luminescence thermal stability, retaining 120% of its room-temperature peak intensity at 250 °C. The site preference of aliovalent dopants Ce³⁺ and K⁺ in the structure of α-Sr₂SiO₄ was investigated by structural refinement via the Rietveld method, which suggests that Ce³⁺ preferentially occupies the nine-fold coordinated Sr2 site only, while K⁺ occupies both Sr1 and Sr2 sites. Under nUV irradiation (350 nm), the phosphor shows a blue light emission peak at 425 nm and has an excellent internal quantum efficiency (IQE) of 91.7%. The mechanism for the abnormal thermal quenching phenomenon was investigated by thermoluminescence measurements. Several defect energy levels are formed by defect engineering using an aliovalent doping strategy of Ce³⁺ and K⁺ in the α-Sr₂SiO₄ host structure. The defect energy levels serve as electron traps that can be thermally activated, which continuously boost the emission intensity of the phosphor while being heated up to 250 °C. The as-fabricated WLED prototypes demonstrated the great potential of the high-performance blue-emitting phosphor Sr₂SiO₄:Ce³⁺/K⁺ for solid-state lighting applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the Jiangsu Specially-Appointed Professorship Foundation (Grant No. 1064902003) and the Postgraduate Research & Practice Innovation Program of Jiangsu Province (Grant No. SJCX21_1772).

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2225570. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3qi00180f

‡ These authors contributed equally.

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