Pyrophosphate Phosphor Solid Solution with High Quantum Efficiency and Thermal Stability for Efficient LED Lighting.

Phosphors with high quantum efficiency and thermal stability are greatly desired for lighting industries. Based on the design strategy of solid solution, a series of deep-blue-emitting phosphors (Sr0.99-xBax)2P2O7:0.02Eu2+ (SBxPE x = 0-0.5) are developed. Upon excitation at 350 nm, the optimized SB0.3PE phosphor shows a relatively narrow full width at half maximum (FWHM = 32.7 nm) peaking at 420 nm, which matches well with the plant absorption in blue region. Moreover, this phosphor exhibits obvious enhancement of internal quantum efficiency (IQE) (from 74% to 100%) and thermal stability (from 88% to 108% of peak intensity and from 99% to 124% of integrated area intensity at 150°C) compared with the pristine one. The white LED devices using SB0.3PE as deep-blue-emitting component show good electronic properties, indicating that SB0.3PE is promising to be used in plant growth lighting, white LEDs, and other photoelectric applications.

Introduction

Nowadays, artificial light has greatly lengthened daytime and changed our lifestyle (Smith, 2000, Pust et al., 2015). The advanced lighting technology and mission of saving energy require scientists to explore highly efficient luminescent materials. Phosphors, as a widely used inorganic luminescent material, play a key role in all phosphor-converted light-emitting diodes (pc-LEDs) (Ye et al., 2010). The luminescent properties of phosphors determine the performances of LED devices such as color rendering index (CRI), correlated color temperature (CCT), luminance efficiency, and service life, directly. Thus, in order to further develop the LED lighting technology, phosphors possessing better luminescent properties are highly required. Among the numerous luminescent parameters, quantum efficiency (QE) and thermal stability are recognized as the two most important indexes in most LED lighting applications (Wang et al., 2018, Shang et al., 2014, Zhao et al., 2019, Zhong et al., 2019, Song et al., 2019). On the other hand, the deep-blue-emitting phosphors, which is a significant component to improve the color rendering index of white LEDs, are rarely reported (Wang et al., 2019). Therefore, it is urgent to develop deep-blue-emitting phosphors with both higher QE and thermal stability.

Up to now, several famous blue emitters including mer-Ir(III) NHC complexes, 2,4,6-triphenylpyrimidine, CsPbCl3 nanocrystals, and (C6H5CH2NH3)2Pb·Br4 have become the research hotspots because of their low cost, high quantum efficiency, as well as super narrow emission (Nakao et al., 2017, Pal et al., 2018, Xuan et al., 2018, Gong et al., 2018, Im et al., 2011, Wang et al., 2017). However, from the viewpoint of their structure, the wet chemical approach endows serious thermal quenching property. Moreover, the usage of oleic acid (OA) and oleylamine (OLA) in the synthetic process brings intrinsic traps on their surface, making them easily decomposable even in the atmosphere. On the contrary, the inorganic phosphors generally own good QE and thermal behavior owing to their intrinsic nature of high chemical and physical stability. As for blue-emitting phosphors, BaMgAl10O17:Eu2+ (BAM) is a commercialized blue-emitting phosphor. The internal quantum efficiency (IQE) and external quantum efficiency (EQE) of BAM is about 91% and 65%, respectively. To further improve the QE and thermal stability of BAM, carbon coating had been adopted by Yin et al. via chemical vapor deposition (CVD) (Yin et al., 2016). The IQE and EQE were demonstrated to increase up to 95% and 76%, and the thermal degradation decreased from 20% to 4.6%. In addition, Lin and co-workers had reported another blue phosphor BaAl12O19:Eu2+ (BAO), which showed much greater thermal stability and IQE than BAM (Wei et al., 2018). However, most of commercialized blue-emitting phosphors still face serious thermal quenching. This problem should be solved for the goal of developing thermally stable blue phosphors. Recently, Kim et al. had reported a zero-thermal-quenching blue phosphor Na3Sc2(PO4)3:Eu2+ synthesized at a relatively low temperature (Kim et al., 2017). However, the IQE of this phosphor is 74%, which needs to be further improved. For the sake of greater light quality, blue phosphors with higher QE and better thermal stability must be further developed.

In order to explore more outstanding blue phosphors, many strategies had been designed and employed by scientists. In 2014, Xie et al. developed a new technology called “Single-Particle-Diagnosis Approach” to select phosphors in a large amount of randomly synthesized bulk. Many excellent blue phosphors such as BaSi4Al3N9:Eu2+ and Sr3Si8-xAlxO7+xN8-x:Eu2+ had been found in a short time, indicating the high efficiency of this approach (Hirosaki et al., 2014). Xia et al. used “mineral learning” to design phosphor based on the existing prototype compounds (Liao et al., 2018a, Zhao et al., 2018). Recently, blue-emitting phosphor RbNa3(Li3SiO4)4:Eu2+ was found by them. The intrinsic super narrow emission bands of these phosphors open a new avenue for high color gamut backlighting. However, their chemical stability and QE need to be further improved. “Machine learning” also has emerged as another feasible pathway to seek highly efficient phosphors. The machine-selected blue-emitting phosphor NaBaB9O15:Eu2+ was demonstrated to possess high IQE up to 95% and good thermal stability. Nevertheless, the excitation band is centered at 275 nm and not well matched with n-UV chips (Zhuo et al., 2018, Bos, 2006). Thus, further optimizations are necessary for this phosphor and this novel intelligent method.

For a long time, solid solution design has been considered as an efficient strategy to obtain outstanding phosphors with color tunability, high QE, as well as good thermal stability (Hermus et al., 2017, Bai et al., 2015, Wang et al., 2016, Im et al., 2011). The most widely used approach involves the substitution among Ca, Sr, and Ba because of their similar alkaline earth properties (Ji et al., 2018, Lin et al., 2017). Pyrophosphate phosphor (α-Sr2P2O7:Eu2+) has been recognized as an excellent blue-emitting phosphor. Previous works mainly focused on investigating the thermoluminescence of Sr2P2O7:Cu2+, Ln3+ (Ln = Pr, Nd, Dy, Ho, Er and Tm); long-lasting luminescence by co-doping M2P2O7:Eu2+ (M = Ca, Sr) with other rare earth ions; the energy transfer by introducing Eu2+-Mn2+, Ce3+-Mn2+, or Tb3+-Eu3+ into the crystal structure of Sr2P2O7; and the luminescent properties of Sr2P2O7 single-doped with rare earth ions (such as Eu2+, Bi2+, Dy3+, Tb3+, Sm3+) (Yazici et al., 2010, Pang et al., 2009a, Pang et al., 2009b, Yu et al., 2017, Xu et al., 2013, Xu et al., 2015, Li et al., 2015, Han et al., 2015, Cao et al., 2015). Among these researches, it has proved that the traps are easily formed in α-Sr2P2O7 host, providing a theoretical possibility to achieve superior thermal stability. The photoluminescence properties of Sr2−xCaxP2O7:Eu2+ phosphors have been investigated by Zhang et al. (Yu et al., 2012). However, the influence of Ca doping on IQE and thermal stability have not been solved. Additionally, Ba-incorporated α-Sr2P2O7:Eu2+ phosphor has not been reported so far. It is probably due to the absence of efficient emission of Eu2+ in the end member of σ-Ba2P2O7. Inspired by this, in this study, solid solution pyrophosphate phosphors (Sr0.99-xBax)2P2O7:0.02Eu2+ (SBxPE x = 0–0.5) have been prepared and investigated. The finally optimized SB0.3PE phosphor can be efficiently excited by the n-UV light and shows narrow deep blue emission (λem = 420 nm, FWHM = 32.7 nm). Moreover, a record of 100% IQE and an unexpected thermal stability (108% for peak intensity and 124% for integrated intensity at 150°C) were detected as well. These results indicate that SB0.3PE may be a suitable inorganic luminescent material for efficient white LED lighting.

Results and Discussion

The morphology of as-prepared Sr1.98P2O7:0.02Eu2+ (SPE) phosphor is shown in Figure 1A. The crystal edges can be clearly distinguished, demonstrating a typical agglomeration phenomenon of crystal grains in the solid-state reaction process. In order to further confirm the existence and relative content of each element in SPE, we conducted element mapping and energy dispersive X-ray spectroscopy (EDS) on this sample. Figure 1B presents the targeted particle that we selected for element mapping analysis. As can be seen, the component elements of Sr, P, O, and Eu are homogeneously dispersed in the crystal of SPE. As shown in Figure 1C, SB0.3PE phosphor exhibits bright blue emission while illuminated by 365 nm light, indicating that this phosphor can be efficiently excited by commercial n-UV chips. To demonstrate the promising applications in various LED lighting applications, we have systematically investigated the luminescent properties of SBxPE x = 0–0.5 solid solution phosphors. The corresponding photoluminescence excitation (PLE) and photoluminescence (PL) spectra of SB0.3PE phosphor are shown in Figure 1D. The excitation region of this series of phosphors spanned from 230 to 420 nm and was centered at 350 nm. Under the excitation at 350 nm, a characteristic 4f65d1 → 4f7 emission band of Eu2+ was obtained in the range of 375–475 nm and peaked at 420 nm. The FWHM of this band is as narrow as 32.7 nm. In addition, this blue emission band overlapped greatly with the absorption of Chlorophyll A, B in plants in the wavelength range of 400–460 nm (Figure S1), implying that this blue light can be used as a plant growth LED lighting source (Zhou et al., 2018). Among numerous optical properties of phosphor, quantum efficiency and thermal stability are recognized as the two most important indicators for an excellent phosphor that could be put into practical use. Accordingly, we first measured the variation of quantum efficiencies of SBxPE (x = 0–0.5) phosphors. As shown in Figure 1E, the absorption efficiencies were kept nearly unchanged (69% in average) when the content of Ba increased from x = 0 to 0.5. However, the IQE of these phosphors was enhanced tremendously. The IQE of pristine SPE phosphor was 74%, and it was enhanced with Ba doping. The highest IQE reached up to 100% when the doping level of Ba is x = 0.3, and after that, it decreased significantly (Figure S2 and Table S1). To the best of our knowledge, such a unitive IQE has been found in blue-emitting phosphors without precedent, which is much higher than that of the commercialized ones such as BaAl12O19:Eu2+ (92%), BaMgAl10O17:Eu2+ (95%), and Ba0.6Sr1.4SiO4:Ce3+ (97%) (Table S2). Nevertheless, owing to the comparatively lower absorption efficiency of SB0.3PE, the EQE (70%) is not comparable enough with these blue phosphors. The Ba doping-dependent PL spectra are presented in Figure S3A. As shown, the emission intensity of SBxPE (x = 0–0.5) solid solution phosphors reached a maximum when the Ba doping level was x = 0.3. In this situation, the emission intensity came to 157.4% of its initial sample (SPE). With a continuing increase of the Ba content, the emission intensities dropped down quickly, matching well with the tendency observed in the IQE. However, from the viewpoint of the emission position, doping Ba into SPE's crystal structure did not alter the emission peak obviously. All the emission peaks were located at around 420 nm, and the maximal deviation of 2 nm happened in SB0.2PE phosphor whose emission peak is 421.5 nm (Figure S3B and Table S3). It is widely accepted that the luminescence properties of Eu2+-based phosphors are easily affected by the crystal environment around it. Therefore, the nearly unchanged emission position in SBxPE x = 0–0.5 phosphor implies that the crystal environment around the luminescence centers of Eu2+ would not be influenced by Ba doping obviously, which may originate from the low doping level of Ba.

Figure 1

Micromorphology and Optical Property Characteristics

(A) SEM image of SPE phosphor.

(B) Element mapping images of selected particle, Sr, P, O, Eu, and total elements.

(C) The corresponding photographs of SB0.3PE phosphor under daylight and radiated by 365 nm light.

(D) PLE and PL spectra of SB0.3PE excited at 350 nm and monitored at 420 nm.

(E) The variations of IQE, EQE, and absorption efficiency in SBxPE x = 0–0.5 solid solution phosphors.

Also see Figures S1–S3 and Tables S1–S3

As we know, the structure of phosphors decides the luminescent properties directly. Thus, as a proof of concept, we first measured the X-ray diffraction (XRD) of SBxPE (x = 0–0.5) solid solution phosphors and the corresponding patterns are shown in Figure 2A. At the first look of the XRD results we can find that the diffraction pattern of SPE sample is in accordance with its standard diffraction data (JCPDS#24-1011, α-Sr2P2O7) (Lagos, 1970). There is no impurity or secondary phase detected implying that pure α-Sr2P2O7 had been formed. By increasing the Ba content continuously, the characteristic diffraction peaks of (210) and (013) shifted toward lower diffraction angles, which agrees well with the Bragg's law. Moreover, we can find that another phase of σ-Ba2P2O7 (JCPDS#30-0144) started to show up in the x = 0.4 case and dominated in x = 0.5 (Elbelghitti et al., 1995). So the solubility of Ba in α-Sr2P2O7 is about 30 mol%. The simulated crystal structures of α-Sr2P2O7 and σ-Ba2P2O7 are shown in Figures S4A and S4B, respectively. Typically, α-Sr2P2O7 belongs to the orthorhombic crystal system and crystallized in the Pnma space group. The lattice parameters of α-Sr2P2O7 are a = 8.9104(6), b = 5.4035(4), c = 13.1054(14) Å, α = β = γ = 90°, and V = 630.99 Å3. Each P atom in α-Sr2P2O7 is coordinated by four oxygen atoms to form [PO4] tetrahedron. In addition, Sr atoms mainly exist in the form of nine and seven oxygen atoms coordinated [Sr1O9] and [Sr2O7] polyhedrons. The crystal structure of the other phase σ-Ba2P2O7 is in a hexagonal system and crystallized in the P-62m space group. The lattice parameters of σ-Ba2P2O7 are a = 9.415(1), b = 9.415(1), c = 7.078(1) Å, α = β = 90°, γ = 120°, and V = 543.35 Å3. All P atoms are located in [PO4] tetrahedrons. However, differing from Sr in α-Sr2P2O7, Ba1 in σ-Ba2P2O7 is coordinated by twelve oxygen atoms, whereas Ba2 is coordinated by ten oxygen atoms, forming [Ba1O12] and [Ba2O10] polyhedrons, respectively. Based on the similar valence states and ionic radius between Sr2+ (rCN=9 = 1.36, rCN=7 = 1.21 Å) and Eu2+ (rCN=9 = 1.30, rCN=7 = 1.20 Å), we deduce that the incorporated Eu2+ prefers to occupy the crystallographic sites of Sr1 in [Sr1O7] polyhedrons in α-Sr2P2O7 phase-dominated samples (SBxPE x = 0–0.4). To have deeper insights into the band structure of SBxPE (x = 0–0.5) solid solution, the density functional theory with Perdew-Burke-Ernzerhof (DFT-PBE) simulation was conducted using the VASP package and the related results are shown in Figures 2B and S5. The electronic band gaps for SPE, SB0.3PE, and SB0.5PE were calculated to be 5.06, 5.11, and 5.06 eV, respectively. At the same time, the band structure also indicates all band gaps belong to direct structure. The optical band gap of SBxPE x = 0–0.5 was calculated based on the absorption data (Figure S6) as well. The values of the calculated band gap increased from 5.05 to 5.09, 5.08, 5.10, 5.08, and 5.07 eV. This variation basically agrees well with the electronic band gap calculated from DFT-PBE. In addition, the calculated density of states (DOS) of the three samples indicates that all the valence bands are dominated by the orbits of O atoms, whereas the conduction bands are composed of the orbits of O, P, Sr, and (Ba) atoms together. Therefore, the band structures are modified with the gradual doping of Ba, which would influence their optical properties.

Figure 2

Crystal Phase, Band Structure, and Photoluminescence Dynamic Characteristics of SBxPE x = 0–0.5

(A) XRD patterns of SBxPE x = 0–0.5 solid solution (left) and magnified XRD patterns in the 2θ range of 25.0°–26.5° (right).

(B) DFT-PBE simulated electronic band structure and partial density of states (PDOS) for SPE phosphor.

(C) Resonance Raman spectra of SBxPE x = 0–0.5 solid solution (left) and magnified Raman spectra between 1,000 and 1,090 cm−1 (right).

(D and E) Decay lifetimes of SBxPE x = 0–0.5 upon the excitation of 350 nm and monitoring at 420 nm.

Also see Figures S4–S7.

In order to obtain more information about the molecular bonds, the measurements of Resonance Raman spectra were carried out for SBxPE x = 0–0.5 phosphors. As shown in Figure 2C, two characteristic Raman peaks locating at 1,056 and 759 cm−1 are observed in the Raman spectra of SPE, which are assigned to the anti-symmetric stretching mode of PO3 and POP in P2O4- 7. In addition, there are three shoulder peaks located at 566, 526, and 491 cm−1, corresponding to the three different deformations of δOPO, δPO3, and δPOP, respectively. The other peaks below 400 cm−1 should be caused by external vibrational modes (Edhokkar et al., 2012, Seyyidoğlu et al., 2007). By increasing the Ba content in SBxPE (x = 0–0.5) solid solution, we can observe that the peak intensities of PO3 and POP increased as well. The peak intensity meets the maximum in SB0.3PE sample. However, once the Ba content exceeded 0.3, the Raman peak at 1,056 and 759 cm−1 suddenly decreased to the extreme low level. Astoundingly, two additional Raman peaks locating at 1,023 and 714 cm−1 should be attributed to the symmetric stretching mode of PO3 and POP (Figure 2C). The mutational Raman spectra should associate with the phase transition from α-Sr2P2O7 to σ-Ba2P2O7, which has been confirmed by XRD measurement. It is commonly believed that the increased vibrational intensity of P-O band reflects that the connective framework of this material was increasing with Ba doping. Therefore, the intrinsic non-radiation between luminescent centers of Eu2+ and killer centers should be reduced, and the corresponding luminescence properties were improved. In order to further check this, the corresponding lifetimes of SBxPE x = 0–0.5 were measured and shown in Figures 2D and 2E. Upon 350 nm excitation and 420 nm monitoring, the obtained decay data are close to a typical single exponential decay model. As can be seen, the lifetimes of SBxPE x = 0–0.5 was lengthened with the gradual increase in the doping concentration of Ba. The longest lifetime was measured to be 575.6 ns when x was equal to 0.3, and after that, it decreased dramatically. Generally, we believe that the anisotropic substitution of Ba for Sr in SPE host would generate some defects. Furthermore, the lengthened lifetimes observed here can be assigned to the occurrence of energy transfer from these defect levels to Eu2+ luminescent centers. However, once the Ba content exceeds a certain content (30 mol%), the maintained energy in Eu2+ would further flow to the killer centers, which is the main reason accounting for the shortening of the corresponding lifetimes for SB0.4PE and 0.5 samples. A visual schematic was given in Figure S7 to explain the mechanism of the much improved IQE in SB0.3PE phosphor. Our conclusions for the improved IQE in SB0.3PE phosphor were subdivided into three aspects: (1) the increased Eu2+ concentration in each crystallographic site of Sr1. Owing to the similar valence state and effective ionic radius, the incorporated Eu2+ seems to still substitute Sr1. Thus, more Eu2+ gathered at the crystallographic sites of Sr1 in SB0.3PE. (2) The energy transfer from the formed defects to Eu2+ luminescent centers. (3) The enhanced connective framework induced less energy transfer from Eu2+ to killer centers. Therefore, by the co-working of the above three effects, the emission intensity and QE in SB0.3PE reached the maximum.

Another key factor affecting the electronic luminescence (EL) of LED devices is the thermal stability of phosphors. Generally, the working temperature of pc-LEDs could reach up to 150°C or higher. Herein, the impact of Ba doping on the thermal stability of SPE was investigated as well. As shown in Figures 3A and 3B, the emission intensity of SPE declined remarkably as the environmental temperature rose from the natural condition to 200°C. At 150°C, SPE phosphor can maintain about 88% (for peak intensity) and 99% (for integrated area intensity) of its initial emission intensity, implying a comparable thermal behavior of commercialized BaMgAl10O17:Eu2+ phosphor (Wei et al., 2018; Figure 3C). Although the thermal stability of SPE is almost satisfied for practical applications, the 30 mol% Ba-substituted phosphor shows a greater thermal behavior, which reached up to 108% (for peak intensity) and 124% (for integrated area intensity) at 150°C, respectively. More interestingly, the emission intensity of this phosphor experienced an up-and-down process, which is totally different from the pristine SPE phosphor. Figure S8 displays the corresponding temperature-dependent excitation spectrum of SB0.3PE phosphor. Same as the temperature-dependent emission of this phosphor, the excitation intensity enhanced until the temperature reached 125°C and then dropped rapidly. However, the rise in temperature would not cause obvious shift in the excitation spectra, indicating the stable crystal environment of the luminescent centers of Eu2+ in SB0.3PE (Figure S8B). According to the literature, the amazing improved thermal stability may be associated with the formation of some trap levels created by Ba doping and can be evidenced by thermal luminescence (TL) measurement, which would be discussed below (Kim et al., 2017). The FWHM increased from 32.5 to 39.2 nm for SPE and from 32.8 to 39.3 nm for SB0.3PE. In addition, we have checked the temperature-dependent decay times of SPBE x = 0.3. As shown in Figure S9A, the lifetimes started to extend from 25°C (337.3 ns) and met the maximum at 125°C (352.1 ns), very close to the law that temperature-dependent PL shows. The prolonged lifetime demonstrates that the energy transferred from the trap levels that were formed by Ba doping to the luminescent centers, and this effect should benefit the thermal stability of this phosphor because Eu2+ can transfer more energy into efficient radiations. However, the lifetime declined unusually at relatively higher temperature. To further check the origin of this result, the temperature-dependent PLE and PL spectra and the corresponding magnified plot were given in Figures S10A and S10B, respectively. Intriguingly, the emission band become more and more broader, causing the increase of FWHM, which has been illustrated in Figure 3C. The low-energy tail in temperature-dependent PLE showed a red shift, whereas the high-energy tail in temperature-dependent PL spectra located more in the blue region. The overlap area got larger, demonstrating a more serious reabsorption in this phosphor. Therefore, the decline of lifetime should be assigned to the enhanced reabsorption in SB0.3PE at higher temperature (Liao et al., 2019). However, the IQE and EQE decreased monotonously. At 150°C, the IQE and EQE is 85% and 60%, respectively (Figures S9B and S9C). The observed decreases of IQE and EQE are linked with the energy storage of these traps rather than direct emission. The thermal degradation of SB0.3PE was also checked by heating and cooling the sample for several times. As shown in Figure S11A, after cooling the sample back to room temperature, the emission intensity abnormally increased up to 120%, 138%, and 141% for cycles 1, 2, and 3. This unnatural discovery needs to be resolved in the future works. Moreover, this phosphor possesses good irradiant stability at high temperature. The emission intensity varied less than 0.7% at 150°C even after continuously being irradiated for 4 h (Figure S11B).

Figure 3

Improved Thermal Stability Performance of SB0.3PE and the Related Mechanism

(A and B) Temperature-dependent emission spectra of SPE and SB0.3PE.

(C) The relationship of peak and integrated intensity, FWHM of SPE, and SB0.3PE versus different temperatures.

(D) TL spectra of SB0.3PE.

(E) Proposed mechanism for improved thermal stability in SB0.3PE phosphor.

Also see Figures S8–S11.

To further confirm the existence of traps levels in SB0.3PE phosphor, the TL spectrum was measured. As shown in Figure 3D, the TL curve exhibited three obvious peaks in the temperature range of 20°C –300°C. By using Gaussian fitting, three individual peaks locating at 15.6, 71.5, and 119.9°C emerged. Thus, the corresponding trap depths (ET) were calculated to be 0.58, 0.69, and 0.79 eV according to the empirical equation ET = T/500 eV (Zhuo et al., 2018, Ding and Wang, 2017, Liao et al., 2018b, Qiao et al., 2019). Moreover, Dorenbos also proposed a classical model to explain the thermal quenching phenomenon of Eu2+-activated phosphors, which illustrated that the excited electrons in the 5d level could be thermally excited into the conduction band (CB) and finally quenched in it. The thermal stability of phosphor depends on the energy gap between 5d level and CB (E5d-CB) (Qiao et al., 2019). The smaller the value of this energy is, the more serious the thermal quenching would be. However, both the integrated emission intensities of SPE and SB0.3PE did not drop by 50% even at 200°C, and this reflects that this model should not be the main working mechanism for the thermal quenching in these phosphors.

It is essential to comprehensively realize the mechanism of the greatly improved thermal stability in SB0.3PE phosphor. Here, we give our explanation and put it in Figure 3E. First, in SPE phosphor, the electrons in Eu2+ can be excited by n-UV light from 4f to 5d energy level. The excited electrons directly relax to the lowest 5d level and then return to 4f ground state through a classical 5d → 4f transition. However, in SB0.3PE phosphor, the excited electrons in 5d level will go through an additional ionization approach to be further excited into the conduction band (CB). Then, these electrons will be charged into three kinds of traps. These traps are located at different energy positions. The larger the ET is, the deeper the position these traps are located at. When the environmental temperature increased, the charged electrons could be thermally re-excited by recombining with the inherent holes in traps. Through CB and the non-radiative process, these thermally excited electrons reach the lowest 5d level, followed by 4f65d1 → 4f7 transition to return to the ground state. In this process, the thermally quenched emission could be compensated by the additional 5d → 4f transitions of Eu2+. Therefore, the thermal stability of SB0.3PE phosphor has been improved by this thermal activated process.

The unprecedented IQE, thermal stability, relatively narrow FWHM, as well as the plant absorption-matched emission band indicate that SB0.3PE phosphor has promising applications in plant growth lighting and white LEDs. To demonstrate this, we have fabricated a series of plant growth LED and white LED. The plant growth LED combined SB0.3PE blue-emitting phosphor with the commercial 365-nm chip directly, whereas other white LEDs were fabricated by blending SB0.3PE deep-blue phosphor, green-emitting (Sr,Ba)2SiO4:Eu2+ phosphor (λem = 524 nm), and red-emitting (Ca,Sr)AlSiN3 phosphor (λem = 610 nm) as trichromatic emitters. Figure 4A shows the emission spectra of as-fabricated plant growth LED and white LED under different current driven conditions. By increasing the driving current, the emission intensities of two LED devices increased simultaneously. As for white LED, the intensities of blue, green, and red light emitted from trichromatic phosphors were enhanced constantly, indicating a comparable stability of SB0.3PE with these commercial phosphors (Figure 4B). Figure 4C shows the CIE coordinates of plant growth LED and white LED driven by different currents. The CIE coordinates of plant growth LED remained stable, whereas the white LED shifted from (0.3958, 0.4065) to (0.3958, 0.3984) (Table S4). Figure 4D presents the photographs of our fabricated plant growth LED and white LED. Driven under a current of 25 mA, the plant growth LED shows deep blue emission and the CIE coordinates are located at (0.1660, 0.0155). For white LED, the CIE coordinates are (0.3958, 0.4065). Moreover, a relatively high CRI of 86.8, low CCT of 3831 K as well as a high luminance efficiency of 45.9 lm·W−1 were obtained as well.

Figure 4

Performance of as-fabricated pc-LED Devices

(A) The emission spectra of fabricated plant growth LED (up) and white LED (down) driven at various currents.

(B) The emission intensities of blue, green, and red phosphors as a function of driving current.

(C) CIE chromaticity coordinate of plant growth LED and white LED under different current driving conditions.

(D) The corresponding photographs of plant growth LED (driven), white LED, and driven white LED.

Also see Table S4.

Conclusions

In summary, we have developed a deep-blue-emitting phosphor of SB0.3PE from SBxPE x = 0–0.5 solid solution with high IQE and thermal stability. This phosphor possesses a narrow FWHM of 32.7 nm, and its blue emission at 420 nm matches well with plant's absorption. The IQE of SB0.3PE phosphor is as high as 100%, which is record to the best of our knowledge. In addition, SB0.3PE possesses fascinating thermal stability. It can maintain 108% of the initial peak intensity and 124% of the integrated area intensity at 150°C, which is much better than most of other commercial blue phosphors. Thus, the suitable emission range, narrow FWHM, unprecedented IQE, as well as superior thermal stability allow SB0.3PE to be promisingly applied in plant growth lighting, white LED, and other related photoelectric fields.

Limitations of the Study

The absorption efficiency of phosphors is another key parameter to affect its EQE. Although in this study the best IQE reached up to 100%, the absorption efficiency is still limited at 71% and did not change much with the co-doping of Ba. Therefore, the value of EQE needs to be further optimized by improving the absorption efficiency.

Methods

All methods can be found in the accompanying Transparent Methods supplemental file.