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Highly Efficient and Thermally Stable K3AlF6:Mn4+ as a Red Phosphor for Ultra-High-Performance Warm White Light-Emitting Diodes Enhai Song,† Jianqing Wang,‡ Jiahao Shi,† Tingting Deng,† Shi Ye,† Mingying Peng,† Jing Wang,§ Lothar Wondraczek,⊥,∥ and Qinyuan Zhang*,† †
State Key Laboratory of Luminescent Materials and Devices and Institute of Optical Communication Materials, South China University of Technology, Guangzhou 510641, China ‡ Guangzhou LEDteen Optoelectronics Co., Ltd. Guangzhou 510663, China § State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, P.R. China ⊥ Otto Schott Institute of Materials Research and ∥Center for Energy and Environmental Chemistry, University of Jena, 07743 Jena, Germany S Supporting Information *
ABSTRACT: Following pioneering work, solution-processable Mn4+-activated fluoride pigments, such as A2BF6 (A = Na, K, Rb, Cs; A2 = Ba, Zn; B = Si, Ge, Ti, Zr, Sn), have attracted considerable attention as highly promising red phosphors for warm white light-emitting diodes (W-LEDs). To date, these fluoride pigments have been synthesized via traditional chemical routes with HF solution. However, in addition to the possible dangers of hypertoxic HF, the uncontrolled precipitation of fluorides and the extensive processing steps produce large morphological variations, resulting in a wide variation in the LED performance of the resulting devices, which hampers their prospects for practical applications. Here, we demonstrate a prototype W-LED with K3AlF6:Mn4+ as the red light component via an efficient and water-processable cationexchange green route. The prototype already shows an efficient luminous efficacy (LE) beyond 190 lm/W, along with an excellent color rendering index (Ra = 84) and a lower correlated color temperature (CCT = 3665 K). We find that the Mn4+ ions at the distorted octahedral sites in K3AlF6:Mn4+ can produce a high photoluminescence thermal and color stability, and higher quantum efficiency (QE) (internal QE (IQE) of 88% and external QE (EQE) of 50.6%.) that are in turn responsible for the realization of a high LE by the warm W-LEDs. Our findings indicate that the water-processed K3AlF6 may be a highly suitable candidate for fabricating high-performance warm W-LEDs. KEYWORDS: warm white LEDs, red phosphor, green route, luminous efficiency, ultrahigh performance narrow band red light emission around ∼630 nm (2Eg → 4A2), as well as a strong and broad excitation band at ∼460 nm (4A2 → 4 T2) with a bandwidth of ∼50 nm. For these properties, various Mn4+ activated fluoride red-emitting phosphors such as A2BF6 (A = Na, K, Rb, Cs; A2 = Ba, Zn; B = Si, Ge, Ti, Zr, Sn) have been considered for application in warm W-LEDs.2,15,16,19−28 Such warm W-LEDs (Ra > 80, CCT < 4000 K) have been achieved with a luminous efficacy (LE) of approximately 120 lm/W, compared to the practical threshold for commercial applications of about 90 lm/W.2 However, synthesis of these fluoride materials is presently based on the use of toxic HF solutions, still associated with significant difficulties in controlling the formation of solid precipitates and extensive processing steps which produce large morphological variations, resulting in a wide
1. INTRODUCTION For illumination-grade white light-emitting diodes (W-LEDs), a low correlated color temperature (CCT = 2700−4500 K) and a high color rendering index (CRI, Ra > 80) are simultaneously required.1−3 However, to date, the most popular commercial WLEDs based on the combination of a blue-emitting InGaN chip and the yellow phosphor (Y,Gd)3(Al,Ga)5O12:Ce3+ (YAG:Ce3+) provide low CRI (Ra < 80) and high CCT (>4500 K), resulting from the notoriously insufficient red contribution in the spectral emission characteristic of YAG:Ce3+.4−7 To facilitate the next generation of high-power warm phosphor-converted W-LEDs, the discovery of more efficient red-emitting phosphors is essential.8−14 Recently, materials doped with the transitionmetal species Mn4+ have attracted considerable attention as promising red phosphors due to the broad band excitation scheme on the one hand and the sharp emission band associated with the 3d3 electron configuration of Mn4+ on the other hand.15−18 In particular, when Mn4+ ions are located in the octahedral sites of fluoride matrixes, they exhibit an intense © 2017 American Chemical Society
Received: January 16, 2017 Accepted: February 21, 2017 Published: February 21, 2017 8805
DOI: 10.1021/acsami.7b00749 ACS Appl. Mater. Interfaces 2017, 9, 8805−8812
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Figure 1. X-ray diffraction patterns of K3AlF6 and K3AlF6:Mn4+ (3.41 at. %) (a). Standard data of ICSD no. 60417 (K2MnF6) and JCPDS no. 03-0635 (K3AlF6) are included for comparison. (b) and (c) show the corresponding crystal structures of K3AlF6 and K2MnF6, respectively. prepared via a room-temperature coprecipitation method using water as the solvent and controlling the mole ratio of KF to Al(NO3)3·9H2O (Figure S1 of the Supporting Information). Specifically, 10 mmol of Al(NO3)3·9H2O was dissolved in 45 mL of H2O and the solution continuously stirred for 10 min at room temperature. KF (170 mmol) was added to this solution under stirring. After being stirred for 30 min, the solution was aged for 24 h at room temperature. The precipitate was collected and washed several times with absolute ethanol and deionized water to remove eventual soluble contaminants. Finally, the precipitate was dried at 80 °C for 4 h to obtain the white K3AlF6 powder (Figure S2). For a typical cation-exchange procedure, 0.123 g of K2MnF6 powder was first dissolved in 1.1 mL of HF solution (49 wt %). Then 2.583 g of K3AlF6 powder was gradually added to the solution while stirring for 20 min to form a uniform, yellowish mixture. This was then dried at 80 °C for 72 h to obtain K3AlF6:Mn4+ (Figure S2). Additional samples with different Mn4+ concentrations (Table S1) were prepared by following the same procedure. Structure and Optical Characterization. The crystal structure and morphology of the obtained samples were characterized with an Xray powder diffractometer (Philips PW1830, using Cu Kα radiation at λ = 1.5406 Å; tube voltage = 40 kV; tube current = 40 mA) and a scanning electron microscope (SEM) (NOVA NANOSEM 430), respectively. The doping concentration of Mn4+ ions in K3AlF6:Mn4+ was verified by inductively coupled plasma optical emission spectroscopy (ICPOES720, Varian). The room-temperature emission and excitation spectra as well as the fluorescence decay curves were recorded on a spectrofluorophotometer (FLS 920, Edinburgh Instruments Co. Ltd.) using a 450 W xenon lamp or a 150 W microsecond pulsed lamp as excitation sources, respectively. To investigate temperature-dependent
variation in the LED performance of the resulting material which inevitably limits their prospects for practical application.28,29 Here, we demonstrate an alternative solution, using an aluminum fluoride host to produce a prototype warm W-LED with ultrahigh emission power (LE > 190 lm/W, Ra = 84, CCT = 3665 K). We show that thermally stable K3AlF6:Mn4+ can be synthesized through a straightforward, efficient, and water-based (green) route to obtain a homogeneously doped material with minimum use of HF.2 The extraordinarily high emission efficiency of this phosphor arises from Mn4+ ions which precipitate on distorted octahedral sites with high-absorption cross section, low phonon energy, and consequently, high internal as well as external quantum efficiency (IQE and EQE, respectively). This provides a novel route toward ultra-highperformance W-LEDs.
2. EXPERIMENTAL SECTION Materials Synthesis. The reported materials were synthesized from the raw chemicals KF (99.5%), Al(NO3)3·9H2O (99.99%), KMnO4 (AR), acetone (AR), H2O2 (30%), and HF (49%). KMnO4 and acetone were supplied by Guangzhou Chemical Reagents Factory, China, while other chemicals were purchased from Aladdin Chemistry, China. All chemicals were used as received without further purification. Compounds of K3AlF6:Mn4+ were prepared via a cation-exchange method according to ref 2. Before cation exchange, we first synthesized the key intermediates K2MnF6 and K3AlF6. The tetravalent manganese source K2MnF6 was synthesized using Bode’s method.30 The K3AlF6 was 8806
DOI: 10.1021/acsami.7b00749 ACS Appl. Mater. Interfaces 2017, 9, 8805−8812
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Figure 2. (a) Excitation and emission spectra of K3AlF6:Mn4+ (3.41 at. %) (solid line) and commercial phosphor YAG:Ce3+ (dashed line). (b) Timeresolved emission spectra for K3AlF6:Mn4+ (3.41 at. %) upon 460 nm excitation. (c) Concentration-dependent emission spectra of K3AlF6:Mn4+ and (d) relative intensity and decay times of photoluminescence from K3AlF6:Mn4+ for varying doping concentrations. luminescence properties, the phosphor sample was placed on a hightemperature fluorescence instrument accessory (TAP-02, Tianjin Orient-KOJI Instrument), which was mounted on the spectrofluorophotometer. For photoluminescence quantum efficiency measurements, the samples were put into an integrating sphere coupled to the spectrofluorometer. The diffuse reflection spectra of the samples were measured with a spectrophotometer (UV2600, Shimadzu). LED Fabrication and Performance Measurement. Prototype high-power W-LEDs were fabricated with a commercial yellow phosphor YAG:Ce3+ (BM304A, Jiangsu Bree), the present red phosphor K3AlF6:Mn4+, and a blue InGaN chips (450−460 nm, 200 mW, Tsinghua Tongfang). The blue chips were fixed on a substrate with a diameter of 11.2 cm (Figure S3) using the chip-on-board (COB) technology. Here, each substrate contained 20 chips. The 20 blue chips were divided into two groups, using a parallel circuit. Each group consisted of 10 chips placed in series in a circuit; hence, the drive current and voltage for a single InGaN chip were 50% of the total drive current and 10% of the total drive voltage, respectively. The phosphors were thoroughly mixed with epoxy resin, and the obtained phosphor-epoxy resin mixture was coated on the LED chips. Three different CCT white LEDs (LED I−III) were fabricated with different mass ratios of the epoxy resin (A, B) to the phosphors (YAG:Ce3+, K3AlF6:Mn4+). Details are shown in Table S2. The photoelectric properties of the LEDs were measured by using an integrating sphere spectroradiometer system (HAAS-1200, Everfine).
indicating that the key pure-phase precursors K2MnF6 and K3AlF6 have been obtained as desired. K3AlF6, the basic host material, has a cubic structure (δ-phase) with space group Fm3̅m and a lattice constant of 8.46 Å (Figure 1b). In this structure, all Al3+ ions are located at the centers of the regular octahedron [AlF6], while K+ ions are found simultaneously at the centers of both the octahedron [KF6] and tetrakaidecahedron [KF12].31 The second precursor species, K2MnF6, adopts a hexagonal structure and with space group P63mc. Here, Mn4+ ions are located at the centers of the octahedron (Figure 1c). Although δK3AlF6 and K2MnF6 have different structure and there is a mismatch in the valence states between Al3+ and Mn4+, the Mn4+ ions can also be incorporated into the host lattice of δ-K3AlF6 due to the similar ionic radii of Mn4+ and Al3+ (Mn4+, r = 0.535 Å; Al3+, r = 0.540 Å), assumedly forming fluorine defects for charge compensation.32 The scanning electron microscope (SEM) image in Figure S4a reveals that the δ-K3AlF6 sample is composed of nanoparticles with an average size of ∼100 nm. As will be argued later, this is beneficial for cation exchange due to large surface area. As shown in Figure 1a, no traces of residual K2MnF6 or other impurities appear in the K3AlF6:Mn4+ sample; all the diffraction peaks are well-indexed to α-K3AlF6 with tetragonal superstructure. This means that Mn4+ ion doping into δ-K3AlF6 is accociated with a structural transition to the tetragonal superstructure (α-phase) (Figure S5).31,33,34 Different doping concentrations of Mn4+ are achieved by changing the mole ratio of K2MnF6 to δ-K3AlF6. Because all Al3+ ions are located at the centers of distorted octahedrons in α-K3AlF6, the Mn4+ ions substitute for Al3+ ions and an intense red emission
3. RESULTS AND DISCUSSION Figure 1a shows typical XRD patterns of as-synthesized K2MnF6, K3AlF6, and K3AlF6:Mn4+ (3.41 at. %). It is observed that all diffraction peaks of K2MnF6 and K3AlF6 match well with the ICSD card no. 60417 and JCPDS card no. 03-0635, respectively, 8807
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Figure 3. (a) Temperature-dependent emission spectra, (b) normalized emission spectra, and (c) luminescence decay curves of K3AlF6:Mn4+ (3.41 at. %). (d) Corresponding integrated emission intensity and lifetimes of K3AlF6:Mn4+ (3.41 at. %) as a function of temperature. (e) CIE chromaticity coordinates of K3AlF6:Mn4+ (3.41 at. %) at 298 and 473 K. (f) Drive current dependent chromaticity shift ΔE.
from Mn4+ ions can be expected from α-K3AlF6:Mn4+. In Figure S4b, it is shown that the α-K3AlF6:Mn4+ (3.41 at. %) sample also consists of nanoparticles, with an average size of ∼130 nm.
As expected, under 460 nm excitation, α-K3AlF6:Mn4+ (3.41 at. %) exhibits intense red photoemission. The emission spectrum consists of several sharp emission peaks around 8808
DOI: 10.1021/acsami.7b00749 ACS Appl. Mater. Interfaces 2017, 9, 8805−8812
Research Article
ACS Applied Materials & Interfaces ∼630 nm, corresponding to the spin-forbidden 2Eg → 4A2 transitions of Mn4+ (see Figure 2a). That is, emission peaks at 595, 605, 609, 618, 626, 630, and 643 nm are attributed to the anti-Stokes v3(t1u), v4(t1u), and v6(t2u), zero phonon line (ZPL), Stokes v6(t2u), v4(t1u), and v3(t1u) vibronic modes, respectively. In the present system, the ZPL peak is stronger than those in previously observed K 2 SiF 6 :Mn 4+ , K 2 GeF 6 :Mn 4+ , and K2TiF6:Mn4+15 because of the relatively lower symmetry of the substituted distorted octahedral Al3+ site in α-K3AlF6 (Figure S5b).33,35 On the basis of the above emission spectrum, the chromaticity coordinates of α-K3AlF6:Mn4+ are calculated as x = 0.6856 and y = 0.3143, that is, very close to the national television systems committee (NTSC) standard values for red color (x = 0.67, y = 0.33). The excitation spectrum (monitored at 626 nm) contains two broad excitation bands centered at ∼360 and ∼460 nm, originating from the spin allowed 4A2 → 4T1 and 4A2 → 4T2 transitions of Mn4+, respectively. This observation is in good agreement with the corresponding diffuse reflection spectrum (Figure S6). Noteworthy, the blue excitation band (∼460 nm) is much stronger than the ultraviolet (UV) (∼360 nm) excitation band, and almost no spectral overlap can be observed between the emission spectrum of the commercial yellow phosphor YAG:Ce3+ and the excitation spectrum of α-K3AlF6:Mn4+, indicating that the common problem of reabsorption can be resolved by using this present phosphor for the red light component. Furthermore, the blue excitation bands of YAG:Ce3+ and α-K3AlF6:Mn4+ have a large spectral overlap, meaning that it can be excited with the InGaN LED simultaneously to the YAG:Ce3+ yellow component so as to improve Ra and CCT in blue InGaN-excited W-LEDs. Figure 2b shows the time-resolved emission spectra (TRES) of α-K3AlF6:Mn4+ upon 460 nm excitation. It is observed that all characteristic peaks from Mn4+ in α-K3AlF6:Mn4+ decrease with increasing delay times. Meanwhile, the peak positions and band shapes of these emission peaks remain unchanged as the delay time increases. The constant relative emission intensity among the emission peaks suggests that all of them originate from the same active center with identical decay kinetics (Figure S7). By use of the same method, red phosphor K2AlF5·H2O:Mn4+ can also be obtained, which show similar emission and excitation spectra to those of αK3AlF6:Mn4+ (Figure S8). For further optimizing of the phosphor composition and, hence, enhancing of the luminescence intensity, Figure 2c shows the concentration-dependent emission of α-K3AlF6:Mn4+ upon 460 nm excitation. It is observed that the emission spectra of all samples consist of several sharp emission peaks located at 595, 605, 609, 618, 626, 630, and 643 nm, assigned to the transitions of anti-Stokes v3(t1u), v4(t1u), and v6(t2u), zero phonon line (ZPL), and Stokes v6(t2u), v4(t1u), and v3(t1u) vibronic modes, respectively, as already described above. As the dopant concentration of Mn4+ ions increases, the shape of the emission spectra remains practically unchanged. The emission intensity reaches its maximum value at a dopant concentration of ∼3.41 at. %. Further increase in the doping concentration causes the emission intensity to decrease gradually due to concentration quenching. Therefore, here, we chose a value of ∼3.41 at. % as the optimal dopant concentration (Figure 2d). For luminescent materials, quantum efficiency is a crucial parameter for practical application. 36 Under 463 nm blue light excitation, αK3AlF6:Mn4+ (3.41 at. %) has an IQE of 88% (Figure S9), which is a much higher value than that of commercial rare-earth (RE) doped (oxy)nitride red phosphors such as Ca-αSiAlON:Eu2+ (IQE ∼ 70%),37 CaAlSiN3:Ce3+ (IQE ∼ 80%),38
and fluorides K2SiF6:Mn4+ (IQE ∼ 74%).22 Moreover, the phosphor has an excellent EQE of ∼51%, which holds strong promise for warm W-LED practical application. The high EQE of this present phosphor is ascribed to the precipitation of Mn4+ ions on highly distorted octahedral Al3+ sites. The distorted crystal field leads to a higher transition probability for spinallowed transitions, such as the 4A2 → 4T2 transition of Mn4+. Besides, the QE of this phosphor can be further improved by controlling the particle size, doping concentration, morphology, and crystalline defects via optimization of the processing conditions. The room-temperature luminescence decay curves α-K3AlF6:Mn4+ with various dopant concentrations are shown in Figure S10. All decay curves fit well to a single exponential equation, which further confirms that the Mn4+ ion tends to distribute homogeneously in the α-K3AlF6 host lattice, on a single type of lattice site. With increasing doping concentration, the lifetime of photoemission decreases monotonously from ∼5.5 to 5.1 ms (Figure 2d) because of the gradual increase in nonradiative transition processes among the Mn4+ ions. The generally high value of lifetime is attributed to the spin-forbidden nature of the 2Eg → 4A2 transition. The design of luminescent converter components with excellent thermal stability is still a challenge in the field of WLED applications. Normally, with higher temperature, the QE of the luminescent materials decreases, reversibly or even irreversibly. For high-power W-LED applications, during encapsulation, the processing temperature of the LED chip can reach over ∼150 °C.29 Depending on cooling strategy, also the operating temperature may reach similar values. Hence, the temperature dependence of luminescence is crucial for practical applications as well as for fundamental research. Figure 3a shows the temperature dependence of photoemission from αK3AlF6:Mn4+ upon 460 nm excitation across the range of 298− 473 K. As seen here, all emission spectra are identical in terms of band shape within the limits of experimental observation: they all comprise the typical sharp emission peaks around ∼630 nm, assigned to 2Eg → 4A2 of Mn4+. With increasing temperature, on the other hand, the emission intensity varies significantly. In contrast to the monotonous decrease which is expected, for example, in the emission intensity of RE-doped materials, the emission intensity of α-K3AlF6:Mn4+ increases initially with rising temperature and reaches a maximum value at a temperature of ∼398 K. It then decreases gradually as the temperature increases further. Specifically, as the temperature rises to 398 K, the emission intensity is increasing to about twofold of its value at 298 K. When the temperature increases to 473 K, the intensity retains approximately the same value as it had at 298 K (Figure 3d). Upon decrease of the temperature again to 298 K, the corresponding luminescence lifetime fully recovers to its initial value before heating. These observations strongly indicate high thermal stability of the emission properties of αK3AlF6:Mn4+ versus common benchmarks. For better quantification, the normalized temperature-dependent emission spectra and luminescence decay curves of Mn4+ (monitored at 626 nm) are shown in Figure 3b,c, respectively. It is observed that all emission peaks show a slight red shift and become broader with increasing temperature as a result of the expansion of the host lattice and the enhancement of vibration modes with increasing thermal load. The anti-Stokes/Stokes intensity ratio follows a monotonously increasing trend, which is consistent with previous results. At the same time, the decay lifetime decreases monotonously with increasing temperature because of the increasing probability of nonradiative transition processes. As 8809
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Figure 4. (a) Electroluminescence spectra of prototype LEDs, employing K3AlF6:Mn4+ for the red converter components, commercial yellow YAG:Ce3+, and a blue InGaN chip at a 40 mA drive current. (b) Chromaticity coordinates of the three typical LEDs with correlated color temperatures of 3665, 3270, and 2706 K under a drive current of 40 mA. (c) Drive current dependent electroluminescence spectra of LED II. (d) Drive current dependent LE and CCT of LED II. (e) Drive current dependent color rendering index (Ra, R9). (f) CIE chromaticity coordinates of LED II at 10 mA (x = 0.4235, y = 0.4051) and 300 mA (x = 0.4127, y = 0.3832).
shown in Figure 3c, all luminescence decay curves fit well to a single exponential function, with lifetime gradually decreasing from ∼5.2 ms (298 K) to ∼0.6 ms (473 K). Unlike with RE-
doped materials, both the nonradiative and radiative transition probabilities of Mn4+ gradually increase with increasing temperature.39 Therefore, if in a given temperature range, the radiative 8810
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sensitive to moisture.19 Therefore, for practical application, a coating protective layer on the surface of this phosphor may be necessary.
transition probability increases faster than the nonradiative transition probability, a temperature-induced enhancement of emission intensity occurs. All emission peaks show a slight red shift and become broader with increasing temperature; however, the corresponding variation in CIE chromaticity coordinates is very small (Figure 3e). As shown in Figure 3f, when the temperature reaches 473 K, the chromaticity shift ΔE is ∼39 × 10−3, that is, within a single red-range MacAdam ellipse and smaller than the value of ∼53 × 10−3 which was observed for (Sr)CaAlSiN3:Eu2+.40 These observations qualitatively confirm the argued color stability of the present phosphor material. On the basis of the consideration of the luminescence characteristics, three different sets of warm W-LEDs were fabricated from a commercial yellow phosphor YAG:Ce3+, the present red phosphor α-K3AlF6:Mn4+ (3.41 at. %), and blue InGaN chips with differing phosphor ratios. Figure 4a shows the electroluminescence spectra of the three LEDs under a drive current of 40 mA and a drive voltage of 27 V. All emission spectra are composed of three parts: blue light from the InGaN chip, yellow light from YAG:Ce3+, and red light from α-K3AlF6:Mn4+ (3.41 mol %). As a result of the different emission ratios of the three emission parts, the LEDs show different CCTs; that is, the CCTs of LEDs I−III are 3665, 3270, and 2706 K, respectively, all in the warm light region (Figure 4b). The LEs of LEDs I−III are 191, 162, and 156 lm/W, respectively, much higher than corresponding data reported in the literature.41 In addition, the corresponding CRI (Ra) of LED I−III are at 84, 87, and 89, respectively, and also the R9 value (Table S3) is increased because of the significant enhancement of the red light emission from α-K3AlF6:Mn4+. These results demonstrate that αK3AlF6:Mn4+ is, in principle, suitable as a red light component for high-power warm W-LEDs. To further evaluate the dependence of LED performance on drive current, LE of LED II was recorded under drive currents between 10 and 300 mA (Figure 4c). It was observed that all emission spectra are similar in shape; that is, all of them contain the three contributions from the chip and the two converter materials. When the drive current increases from 10 to 300 mA, the LEDs produce only very small fluctuations in color (Table S3) and CCT (Figure 4d). Theoretically, a saturation of the emission intensity might be more likely in α-K3AlF6:Mn4+ than in Ce3+- or Eu2+-ions doped materials due to the much longer decay time of Mn4+ (5.2 ms) as compared to that of Ce3+ or Eu2+ (tens of nanoseconds). However, as shown in Figure 4c, no saturation of the emission intensity is observed even when the drive current rises to 300 mA in LED II, and thus the LED II exhibits only small color shift and color rendering index variation with drive current (Figure 4e,f). Similar results are obtained for LED I and LED III (Figures S11 and S12), again adding support to the potential use of the present phosphor as red component in high-power warm W-LEDs. With increasing drive current, LE of LED II follows a monotonically decreasing trend. This is ascribed to the lower EQE of the blue InGaN chip at higher drive current density. Meanwhile, LED II provides an LE of ∼113 lm/W even at a drive current as high as 240 mA. Under the same conditions, LED I and LED III reach a maximum LE of 134 and 112 lm/W, respectively. That is, LE of all three W-LEDs remains above 100 lm/W, even when the drive current reaches 300 mA. This excelling performance of the warm W-LEDs is primarily attributed to the higher EQE and good thermal and color stability of the red phosphor component α-K3AlF6:Mn4+. Similar to other Mn4+ doped fluoride phosphors, as partial of Mn4+ ions are inevitably exposed to the surface, the red phosphor α-K3AlF6:Mn4+ is also
4. CONCLUSION In summary, a highly efficient red phosphor K3AlF6:Mn4+ has been synthesized via an efficient and facile route. Different from previous fluorides A2BF6, the novel fluoride K3AlF6 can be synthesized via a green coprecipitation method at room temperature. Under 463 nm excitation, the optimal red phosphor K3AlF6:Mn4+ (3.41 at. %) exhibits a high photoluminescence IQE of 88% and an EQE of 50.6%. Moreover, the as-synthesized red phosphor K3AlF6:Mn4+ shows excellent thermal stability. This performance is attributed to the incorporation of Mn4+ ions on highly distorted octahedral lattice sites. Implementing the present phosphor with commercial YAG:Ce3+ and a blue InGaN chip, a high-power warm W-LED with LE beyond 190 lm/W, Ra ∼ 84, and CCT ∼ 3665 K under a 40 mA drive current was fabricated. This device exhibits excellent stability in its chromaticity and CCT across a drive current range from 10 to 300 mA, indicating that K3AlF6:Mn4+ may present a significant step toward the realization of high-performance, high-power warm W-LEDs for indoor illumination.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b00749. Additional XRD characterizations, synthesis diagram, used LED chip and substrate, SEM images, crystal structure of α-K3AlF6, diffuse reflection spectrum, luminescence decay curves, K2AlF5·H2O:Mn4+, quantum efficiency, performances of fabricated warm LEDs, and experimental details (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Shi Ye: 0000-0001-9625-8222 Jing Wang: 0000-0002-1246-991X Qinyuan Zhang: 0000-0001-6544-4735 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was financially supported by the National Natural Science Foundation of China (Grant Nos. 51472088, 51602104, and U1601205).
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REFERENCES
(1) Pust, P.; Weiler, V.; Hecht, C.; Tücks, A.; Wochnik, A. S.; Henß, A.; Wiechert, D.; Scheu, C.; Schmidt, P. J.; Schnick, W. Narrow-Band RedEmitting Sr[LiAl3N4]:Eu2+ As A Next-Generation LED-Phosphor Material. Nat. Mater. 2014, 13, 891−896. (2) Zhu, H. M.; Lin, C. C.; Luo, W. Q.; Shu, S. T.; Liu, Z. G.; Liu, Y. S.; Kong, J. T.; Ma, E.; Cao, Y. G.; Liu, R. S.; Chen, X. Y. Highly Efficient Non-Rare-Earth Red Emitting Phosphor for Warm White LightEmitting Diodes. Nat. Commun. 2014, 5, 4312. (3) Schubert, E. F.; Kim, J. K. Solid-State Light Sources Getting Smart. Science 2005, 308 (5726), 1274−1278.
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Research Article
ACS Applied Materials & Interfaces
(23) Wang, Z. L.; Zhou, Y. Y.; Yang, Z.; Liu, Y.; Yang, H.; Tan, H. Y.; Zhang, Q. H.; Zhou, Q. Synthesis of K2XF6:Mn 4+(X= Ti, Si and Ge) Red Phosphors For White LED Applications with Low-Concentration of HF. Opt. Mater. 2015, 49, 235−240. (24) Wang, Z. L.; Liu, Y.; Zhou, Y. Y.; Zhou, Q.; Tan, H. Y.; Zhang, Q.; Peng, J. Red-emitting phosphors Na2XF6:Mn4+(X= Si, Ge, Ti) with High Colour-purity For Warm White-Light-Emitting Diodes. RSC Adv. 2015, 5 (72), 58136−58140. (25) Arai, Y.; Adachi, S. Optical Properties of Mn4+-Activated Na2SnF6 and Cs2SnF6 Red Phosphors. J. Lumin. 2011, 131 (12), 2652−2660. (26) Kasa, R.; Adachi, S. Mn-Activated K 2ZrF6 and Na2 ZrF6 Phosphors: Sharp Red and Oscillatory Blue-Green Emissions. J. Appl. Phys. 2012, 112 (1), 013506. (27) Adachi, S.; Takahashi, T. Photoluminescent Properties of K2GeF6:Mn4+ Red Phosphor Synthesized from Aqueous HF/KMnO4 Solution. J. Appl. Phys. 2009, 106 (1), 3516. (28) Lin, C. C.; Meijerink, A.; Liu, R. Critical Red Components for Next-Generation White LEDs. J. Phys. Chem. Lett. 2016, 7 (3), 495− 503. (29) Huang, L.; Zhu, Y. W.; Zhang, X. J.; Zou, R.; Pan, F. J.; Wang, J.; Wu, M. M. HF-Free Hydrothermal Route for Synthesis of Highly Efficient Narrow-Band Red Emitting Phosphor K2Si1‑xF6:xMn4+ for Warm White Light-Emitting Diodes. Chem. Mater. 2016, 28 (5), 1495− 1502. (30) Bode, H.; Jenssen, H.; Bandte, F. Ü ber eine neue Darstellung des Kalium-Hexafluoromanganats (IV). Angew. Chem. 1953, 65 (11), 304− 304. (31) King, G.; Abakumov, A. M.; Woodward, P. M.; Llobet, A.; Tsirlin, A. A.; Batuk, D.; Antipov, E. V. The High-Temperature Polymorphs of K3AlF6. Inorg. Chem. 2011, 50 (16), 7792−7801. (32) Shannon, R. T. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1976, 32 (5), 751−767. (33) Abakumov, A. M.; King, G.; Laurinavichute, V. K.; Rozova, M. G.; Woodward, P. M.; Antipov, E. V. The Crystal Structure of α-K3AlF6: Elpasolites and Double Perovskites with Broken Corner-Sharing Connectivity of the Octahedral Framework. Inorg. Chem. 2009, 48 (19), 9336−9344. (34) Abakumov, A. M.; Rossell, M. D.; Alekseeva, A. M.; Vassiliev, S. Y.; Mudrezova, S. N.; Van Tendeloo, G.; Antipov, E. V. Phase Transitions in K3AlF6. J. Solid State Chem. 2006, 179 (2), 421−428. (35) Fang, M.; Nguyen, H.; Lin, C. C.; Liu, R. Preparation of A Novel Red Rb2SiF6:Mn4+ Phosphor with High Thermal Stability Through A Simple One-Step Approach. J. Mater. Chem. C 2015, 3 (28), 7277− 7280. (36) Wondraczek, L.; Batentschuk, M.; Schmidt, M. A.; Borchardt, R.; Scheiner, S.; Seemann, B.; Schweizer, P.; Brabec, C. J. Solar Spectral Conversion for Improving the Photosynthetic Activity in Algae Reactors. Nat. Commun. 2013, 4, 2407. (37) Yamada, S.; Emoto, H.; Ibukiyama, M.; Hirosaki, N. Properties of SiAlON Powder Phosphors for White LEDs. J. Eur. Ceram. Soc. 2012, 32 (7), 1355−1358. (38) Li, Y. Q.; Hirosaki, N.; Xie, R. J.; Takeda, T.; Mitomo, M. YellowOrange-Emitting CaAlSiN3:Ce3+ Phosphor: Structure, Photoluminescence, and Application in White LEDs. Chem. Mater. 2008, 20 (21), 6704−6714. (39) Song, E. H.; Wang, J. Q.; Ye, S.; Jiang, X. F.; Peng, M. Y.; Zhang, Q. Y. Room-Temperature Synthesis and Warm-White LEDs Applications of Mn4+ ions Doped Fluoroaluminate Red Phosphor Na3AlF6:Mn4+. J. Mater. Chem. C 2016, 4 (13), 2480−2487. (40) Zhu, Y. W.; Huang, L.; Zou, R.; Zhang, J.; Yu, J.; Wu, M. M.; Wang, J.; Su, Q. Hydrothermal Synthesis, Morphology and Photoluminescent Properties of An Mn4+-Doped Novel Red Fluoride Phosphor Elpasolite K2LiAlF6. J. Mater. Chem. C 2016, 4 (24), 5690− 5695. (41) Oh, J. H.; Eo, Y. J.; Yoon, H. C.; Huh, Y.; Do, Y. R. Evaluation of New Color Metrics: Guidelines for Developing Narrow-Band Red Phosphors for WLEDs. J. Mater. Chem. C 2016, 4 (36), 8326−8348.
(4) Li, X. F.; Budai, J. D.; Liu, F.; Howe, J. Y.; Zhang, J. H.; Wang, X.-J.; Gu, Z. J.; Sun, C. J.; Meltzer, R. S.; Pan, Z. W. New Yellow Ba0.93Eu0.07Al2O4 Phosphor for Warm-White Light-Emitting Diodes Through Single-Emitting-Center Conversion. Light: Sci. Appl. 2013, 2 (1), e50. (5) Dai, P. P.; Li, C.; Zhang, X. T.; Xu, J.; Chen, X.; Wang, X. L.; Jia, Y.; Wang, X. J.; Liu, Y. C. A Single Eu2+-Activated High-Color-Rendering Oxychloride White-light Phosphor for White-Light-Emitting Diodes. Light: Sci. Appl. 2016, 5 (2), e16024. (6) Daicho, H.; Iwasaki, T.; Enomoto, K.; Sasaki, Y.; Maeno, Y.; Shinomiya, Y.; Aoyagi, S.; Nishibori, E.; Sakata, M.; Sawa, H.; et al. A Novel Phosphor for Glareless White Llight-Emitting Diodes. Nat. Commun. 2012, 3, 1132. (7) Wei, L.-L.; Lin, C. C.; Wang, Y.-Y.; Fang, M.-H.; Jiao, H.; Liu, R. S. Photoluminescent Evolution Induced by Structural Transformation Through Thermal Treating in the Red Narrow-Band Phosphor K2GeF6:Mn4+. ACS Appl. Mater. Interfaces 2015, 7 (20), 10656−10659. (8) Piao, X. Q.; Machida, K.; Horikawa, T.; Hanzawa, H.; Shimomura, Y.; Kijima, N. Preparation of CaAlSiN3:Eu2+ Phosphors By the SelfPropagating High-Temperature Synthesis and Their Luminescent Properties. Chem. Mater. 2007, 19 (18), 4592−4599. (9) Pust, P.; Wochnik, A. S.; Baumann, E.; Schmidt, P. J.; Wiechert, D.; Scheu, C.; Schnick, W. Ca[LiAl3N4]: Eu2+· A Narrow-Band RedEmitting Nitridolithoaluminate. Chem. Mater. 2014, 26 (11), 3544− 3549. (10) Peng, M. Y.; Yin, X. W.; Tanner, P. A.; Brik, M. G.; Li, P. F. Site Occupancy Preference, Enhancement Mechanism, and Thermal Resistance of Mn4+ Red Luminescence in Sr4Al14O25:Mn4+ for Warm WLEDs. Chem. Mater. 2015, 27 (8), 2938−2945. (11) Zhong, J. S.; Chen, D. Q.; Zhao, W. G.; Zhou, Y.; Yu, H.; Chen, L. F.; Ji, Z. G. Garnet-Based Li6CaLa2Sb2O12:Eu 3+ Red phosphors: A Potential Color-Converting Material for Warm White Light-Emitting Diodes. J. Mater. Chem. C 2015, 3 (17), 4500−4510. (12) Wang, Z. B.; Chu, I. H.; Zhou, F.; Ong, S. P. Electronic Structure Descriptor for Discovery of Narrow-Band Red-Emitting Phosphors. Chem. Mater. 2016, 28 (11), 4024−4031. (13) Zhang, X. J.; Tsai, Y.; Wu, S. M.; Lin, Y. C.; Lee, J. F.; Sheu, H.; Cheng, B. M.; Liu, R. S. Facile Atmospheric Pressure Synthesis of High Thermal Stability and Narrow-Band Red-Emitting SrLiAl3N4:Eu2+ Phosphor for High Color Rendering Index White Light-Emitting Diodes. ACS Appl. Mater. Interfaces 2016, 8 (30), 19612−19617. (14) Ye, S.; Xiao, F.; Pan, Y. X.; Ma, Y. Y.; Zhang, Q. Y. Phosphors in Phosphor-converted White Llight-Emitting Diodes: Recent Advances in Materials, Techniques and Properties. Mater. Sci. Eng., R 2010, 71 (1), 1−34. (15) Paulusz, A. G. Efficient Mn (IV) Emission in Fluorine Coordination. J. Electrochem. Soc. 1973, 120 (7), 942−947. (16) Meyer, J.; Tappe, F. Photoluminescent Materials for Solid-State Lighting: State of the Art and Future Challenges. Adv. Opt. Mater. 2015, 3 (4), 424−430. (17) Wang, B.; Lin, H.; Xu, J.; Chen, H.; Wang, Y. S. CaMg2Al16O27: Mn4+-Based Red Phosphor: A Potential Color Converter for HighPowered Warm W-LED. ACS Appl. Mater. Interfaces 2014, 6 (24), 22905−22913. (18) Jin, Y.; Fang, M. H.; Grinberg, M.; Mahlik, S.; Lesniewski, T.; Brik, M. G.; Luo, G. Y.; Lin, J. G.; Liu, R. S. Narrow Red Emission Band Fluoride Phosphor KNaSiF6:Mn4+ for Warm White Light-Emitting Diodes. ACS Appl. Mater. Interfaces 2016, 8 (18), 11194−11203. (19) Nguyen, H. D.; Lin, C. C.; Liu, R. S. Waterproof Alkyl Phosphate Coated Fluoride Phosphors for Optoelectronic Materials. Angew. Chem., Int. Ed. 2015, 54 (37), 10862−10866. (20) Takahashi, T.; Adachi, S. Mn4+-Activated Red Photoluminescence in K2SiF6 Phosphor. J. Electrochem. Soc. 2008, 155 (12), E183−E188. (21) Xu, Y. K.; Adachi, S. Properties of Mn4+-Activated Hexafluorotitanate Phosphors. J. Electrochem. Soc. 2011, 158 (3), J58−J65. (22) Liao, C. X.; Cao, R. P.; Ma, Z. J.; Li, Y.; Dong, G. P.; Sharafudeen, K. N.; Qiu, J. R. Synthesis of K2SiF6: Mn4+ Phosphor from SiO2 Powders via Redox Reaction in HF/KMnO4 Solution and Their Application in Warm-White LED. J. Am. Ceram. Soc. 2013, 96 (11), 3552−3556. 8812
DOI: 10.1021/acsami.7b00749 ACS Appl. Mater. Interfaces 2017, 9, 8805−8812
