Ultrafast Self-Crystallization of High-External-Quantum-Efficient

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Ultrafast Self-Crystallization of High-External-Quantum-Efficient Fluoride Phosphors for Warm White Light-Emitting Diodes Wenli Zhou, Mu-Huai Fang, Shixun Lian, and Ru-Shi Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03525 • Publication Date (Web): 11 May 2018 Downloaded from http://pubs.acs.org on May 13, 2018

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ACS Applied Materials & Interfaces

Ultrafast Self-Crystallization of High-ExternalQuantum-Efficient Fluoride Phosphors for Warm White Light-Emitting Diodes Wenli Zhou†, ‡, Mu-Huai Fang†, Shixun Lian‡ and Ru-Shi Liu,*,†, †

∥

Department of Chemistry, National Taiwan University, Taipei 106, Taiwan

‡

Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research, Ministry of

Education, College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, China ∥

Department of Mechanical Engineering and Graduate Institute of Manufacturing Technology,

National Taipei University of Technology, Taipei 106, Taiwan KEYWORDS: self-crystallization, fluoride, red phosphor, quantum efficiency, Mn4+

ABSTRACT: In this study, we used HF (as good solvent) to dissolve K2GeF6 and K2MnF6 and added ethanol (as poor solvent) to cause ultrafast self-crystallization of K2GeF6:Mn4+ crystals, which had an unprecedentedly high external quantum efficiency that reached 73%. By using the red phosphor, we achieved a high-quality warm white light-emitting diode with color-rendering index of Ra = 94, R9 = 95, luminous efficacy of 150 lm W−1, and correlated color temperature at

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3652 K. Furthermore, the good–poor solvent strategy can be used to fast synthesize other fluorides.

Currently, great efforts have been made to exploit Mn4+-activated red fluoride phosphors.1-6 Synthesis of Mn4+-doped fluorides is unpractical by solid-state reaction and can be alternatively performed in solutions. At present, many strategies, including chemical etching7, coprecipitation8-9, hydrothermal processes10, and cation exchange1,11, have been developed to fabricate them. Generally, the PL efficiency of fluorides synthesized by chemical etching is low, possibly because the activator Mn4+ does not evenly distribute into the six-coordinated cation lattice of hosts. Through co-precipitation, fluorides can exhibit relatively highly efficient red-line emissions; however, fluorides would undergo some complex synthesis processes, for instance, ice bath and dropwise addition. Hydrothermal methods can produce good morphology, but quantum efficiency (QE) would still be low. Wang’s group utilized a H3PO4–KHF2 couple as the fluorine source to synthesize K2SiF6:Mn4+ (KSFM) micro-crystals during hydrothermal process11. No HF was used in their method and the QE was too low at only 28%. Such low QE was possibly due to the lack of HF to stabilize the [MnF6]2− group in the reaction. For cation exchange reaction, the benefits were obvious: short reaction time (minutes), high internal QE (IQE = 93%), suitable particle size, and uncomplicated process13. Whereas, when increasing the Mn4+ concentration (5.5% to 6.5%) in K2TiF6:Mn4+ (KTFM) to increase the absorption efficiency (54% to 60%), the IQE suffered a serious decrease (93% to 78%), thereby decreasing the external QE (EQE = 50% to 46%). The decrease in IQE may be related to concentration quenching effects and the increase in surface states (defects) of KTFM crystals when increasing HF volume to dissolve more K2MnF6. High-EQE phosphor is distinctly a key to producing high


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luminous efficacy (LE) warm white LEDs. To the best of our knowledge, the IQE of Mn4+doped red phosphors can be optimized to over 90%, but the EQE over 60% is difficult to be achieved before. Related data collected from published literatures are compiled in Table S1. Additionally, fabrication of the fluoride involves in usage of HF that is harmful to environment and health, therefore simple and fast production process could contribute to decreasing volatilization of HF. In this work, we report an ultrafast self-crystallization of red-emitting fluorides in a two solutions mixture at room temperature (RT). The idea source and possible mechanism of goodpoor solvent (GPS, Figure S2) strategy are stated in the supporting information for the readership. We first dissolved the fluoride resources [K2GeF6 (KGF) and K2MnF6] in 48% HF solution (good solvent) and then added ethanol (poor solvent); bright yellow precipitates instantly appeared (Figure 1). Under the 460 nm light irradiation, the yellow solution emitted harsh red lights (movie S1); this result indicated that Mn4+ was incorporated into the octahedron sites of KGF crystals. Morphology and optical properties of the KGFM crystals were affected by the volume ratio (R) of ethanol and HF. Therefore, we fixed the volume of HF solvent at 5 mL and varied the volume of ethanol at 5, 10, and 25 mL, and the as-prepared products were named R1, R2, and R5, respectively. The X-ray diffraction (XRD) results showed that the phases of these products changed from ܲ3ത݉1 to P63mc with the increasing volume of ethanol (Figure 2a). R1 is a pure phase of ܲ3ത݉1, which was determined by refining its synchrotron XRD (Figure 2b). R5 was in pure P63mc phase when using Bruker D2 measurement, whereas the same R5 sample measured by synchrotron XRD partially was transferred to ܲ3ത݉1 (Figure S3); this result indicated that the ܲ3ത݉1 is more stable relative to P63mc phase in terms of energy. R2 is a mixture of both. From


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SEM images, R1 appeared octahedral at around 20 µm in size (Figures 2c and S4) and had a clear and smooth surface. The particle size of R5 was approximately 0.6 µm (Figures 2b and S4). R2 consisted of big and small particles (Figure S4), which should be ܲ3ത݉1 and P63mc KGFM according to the XRD result (Figure 2a). KGFM particles were not stable enough under the electron beam, but we still clearly observed the crystal lattices of R1 and R5 (Figure S5). Additionally, all of the elements of KGFM were uniformly distributed across the crystals (Figure S6). These results indicated that the fluorides were well crystallized by the GPS method. The IQE, AE, and EQE of ܲ3ത݉1 KGFM with different doping Mn4+ concentrations (x, which were determined using inductively coupled plasma technique) are shown in Table S3. As x increased, the AE consistently increased, whereas the IQE peaked (96%) when x = 6.75%. Additionally, the AE increase was also supported by diffuse reflectance spectra (Figure S7). Therefore, the cross variation of both causes the best EQE value (73%) at x = 11.74%, which was higher than that (54%) synthesized by co-precipitation9. Such high EQE value of KGFM has never been recorded in published literature; moreover, this value is higher than that of commercial KSFM (61%) and KTFM (65%) phosphors. In addition to ethanol, methanol, acetone and propanol can also drive the ultrafast selfcrystallization of the KGFM in fluoride/HF solution. The phases of KGFM can be controlled by varying their volumes (Figure S8–S10). From the above results, the GPS strategy has at least three advantages for the synthesizing Mn4+-doped fluorides: (1) simple operation; (2) extremely short reaction time (in seconds); and (3) activator Mn4+ would not be of non-equilibrium segregation or concentration gradient in KGFM crystals, therefore causing high EQE. These advantages are crucial for its scale production. The strategy can be further applied to fabricate KSFM (Figure S11) and KTFM (Figure S12). Similarly, the morphology can be controlled by


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varying the amount of ethanol. Especially, the method can only produce one pure phase for KSFM (‫݉ܨ‬3ത݉) and KTFM (ܲ3ത݉1), even if we decreased the R to 0.5 or increased it to 10. Their PL spectra showed characteristic transitions of Mn4+; however, no zero-phonon-line (ZPL) was observed. Both ܲ3ത݉1 and P63mc KGFM gave characteristic excitation and emission spectra,12-13 as shown in Figures 3a and S13. The 4A2 → 4T2 band of Mn4+ was not beyond 520 nm, indicating no significant re-absorption between KGFM and Y3Al5O12:Ce3+ (YAG) would occur for producing warm white-LEDs. At RT, the PL spectrum of P63mc gains an intensive ZPL at 620 nm, which we also observed in our previous reports.14-15 As the x increased, the PL intensity of ܲ3ത݉1 KGFM exhibited a maximum at x = 11.74% (Figure 3b); the trend was similar to that of EQE. Such small concentration quenching effect could be related to (1) even distribution of Mn4+ at KGFM crystal lattice, (2) smooth surface of KGFM micro-crystals, (3) high crystallinity due to the ultrafast self-crystallization. Furthermore, we measured the decay curves of the eight samples at room temperature. The single exponential fitting results show that the lifetimes (τ) of Mn4+ in KGFM were in the range of 6.65–6.56 ms as x ≤ 11.74%. τ does evidently decrease (to 6.45 ms) when x = 12.76% (Figure S14) due to the enhanced nonradiative transition possibility among Mn4+ ions. For the highest EQE sample (R1-11), its integrated PL intensity kept 99% of its initial value (at 298 K) when we increased the temperature to 473 K (Figure 3c). Above 498 K, the relative intensity decreased sharply. The activation energy (Ea) of R1-11 was estimated to be 1.08 eV, which was four times than that of nitride phosphors (~0.25 eV)16 and higher than those of other fluorides (~0.70 eV for KTFM)1. However, the thermal stability of R1-11 was weaker than that of commercial KSFM phosphor in the high temperature area (Figure 3c).


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To estimate the potentials of the KGFM phosphor as a red component for warm WLEDs, we employed blue chip, commercial YAG, and the best KGFM powder together to encapsulate LED packages. At 3.0 V and 20 mA, these LEDs showed high-brightness warm white-light (Figure 3e). The chromaticity coordinates of the three typical LEDs were close to the black body radiation locus (BBR), as marked in Figure 3d. As the amount of KGFM increased, the electroluminescence (EL) spectra of the WLEDs show an increased red component (Figure 3f), and the warmer white-light can be observed. The important photoelectric performance parameters of the LEDs are listed in Table S4. LED2 and LED3 exhibited Ra > 90 and CCT < 4,000 K, and the R9 (rendition of the red color) were 95 and 84, respectively. A cooler LED1 with CCT = 4,221 K generated high Ra (= 94) and R9 (= 94). The LE of LED1 and LED2 reached 158 and 150 lm/W at 20 mA, respectively, which were higher than that using nitrides or KTFM red phosphors. The good photoelectric performance parameters of the LEDs suggested that the highly efficient K2GeF6:Mn4+ red phosphor can significantly improve the EL performance of WLEDs. In summary, we developed a simple good–poor solvent strategy for fast fabrication of Mn4+activated fluorides. The self-crystallization reaction can be achieved in seconds. The particle size and phase-control of KGFM depend on the volume ratio of HF (good solvent) and ethanol (poor solvent). The fabricated KGFM crystals (~20 µm) showed high IQE (93%), high AE (78 %), and an unprecedentedly high EQE of up to 73%, which was higher than that of the commercial KSFM and KTFM. Additionally, the successful synthesis of KSFM and KTFM demonstrates the universality of this approach. Finally, by using the best KGFM phosphor, we encapsulated a warm white LED with CCT = 3652 K, Ra = 94, R9 = 95, and LE = 150 lm/W. Therefore, we


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believe such a simple and ultrafast synthesis strategy can be adopted for mass production of fluoride materials.

ASSOCIATED CONTENT Supporting Information The supporting Information is available free of charge via the Internet at http://pubs.acs.org. Experimental details, idea source, possible mechanism, XRD, SEM, TEM, elements mapping and PL spectra of K2GeF6:Mn4+ crystals, and tables. (PDF) Movie S1, showing the ultrafast fabrication of K2GeF6:Mn4+. (AVI) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (R. S. Liu) Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology of Taiwan (contract no. MOST 104-2113-M-002-012-MY3). W. Zhou appreciates the support from the National Natural


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Decay Rate of K2SiF6:Mn4+ Phosphors. J. Mater. Chem. C 2015, 3, 5484-5489. (3)

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Figure 1. (a) Schematic good–poor solvent strategy for the fabrication of the K2GeF6:Mn4+ phosphor. (b) Pictures of three key states during the fabrication process. (1) K2GeF6 and K2MnF6 powders are dissolved in HF and form a yellow transparent solution; (2) adding ethanol causes ultrafast self-crystallization of K2GeF6:Mn4+, which emits intensive red light under 460 nm-light radiation.


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Figure 2. (a) XRD patterns of KGFM products, (b) Refined synchrotron XRD of R1, SEM images of R1 (c) and R5(d) samples, the scale bars are 10 µm in (c) and 1 µm in (d), respectively.


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Figure 3. (a) Excitation and emission spectra of R1 sample. (b) PL intensity of ܲ3ത݉1 KGFM as a function of concentration (x) of Mn4+ ions. (c) Integrated PL intensity as a function of temperature for the R1-11 and commercial KSFM samples. (d) CIE coordination. (e) Photographs of the lighted LEDs, and (f) EL spectra of three LED packages.


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SYNOPSIS


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Ultrafast Self-Crystallization of High-External-Quantum-Efficient

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