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Synthesis and down-conversion luminescence of Ba4Y3F17:Yb:Pr solid solutions for photonics
S.V. Kuznetsov1, A. S. Nizamutdinov2, M.N. Mayakova1, V. V. Voronov1, E.I. Madirov2, A. R. Khadiev2, D. A. Spassky3, I. A. Kamenskikh4, A. D. Yapryntsev5, V. K. Ivanov5, M. A. Marisov2, V.V. Semashko2, P.P. Fedorov1
1Prokhorov General Physics Institute of the Russian Academy of Sciences, 38 Vavilov str., Moscow, 119991 Russia
2 Kazan Federal University, 18 Kremljovskaya, Kazan, 420008 Russia
3Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University, Leninskie Gory, Moscow 119991, Russia
4Physics Faculty of Lomonosov Moscow State University, Leninskie Gory, Moscow 119991, Russia
5Kurnakov Institute of General and Inorganic Chemistry of the Russian Academy of Sciences,
31 Leninsky pr., Moscow,119991 Russia
PACS 42.70.-a, 81.20.Fw DOI 10.17586/2220-8054-2019-10-2-190-198
Single-phase powders of Ba4Y3Fi7:Yb:Pr solid solutions with an average agglomerate size of 400 nm were synthesized by co-precipitation from aqueous solutions. It was shown that the down-conversion mechanism in the investigated samples was quantum cutting, with one photon absorbed by Pr3+ ions resulting in two photons emitted by
Yb3+
ions. At first, overall the external quantum yield of down-conversion luminescence measured appeared to be relatively high, with a maximum value of 2.9 % for the Ba4Y3Fi7:Pr(0.1 %):Yb(10 %) sample. It makes this compound promising for Si-based solar cells efficiency enhancement. Keywords: synthesis, down-conversion, luminescence, solid solutions, photonics, fluorides.
Received: 2 March 2019 Revised: 8 April 2019
1. Introduction
The forecast for the development of renewable energy [1], conducted by Fraunhofer ISE, showed that by 2030, humanity will reach the terawatt power generation capacity through photovoltaic devices. About 95 percent of this power will be generated by silicon solar panels. The cost of generating 1 kilowatt hour (kWh) of energy will be no higher than the cost of generating energy from fossils and nuclear power. One of the most significant drawbacks of Si-based solar cells is the low efficiency of power generation due to the limited range of high spectral susceptibility of crystalline silicon to sunlight. There are various options to increase the efficiency [2-8], including additional layers based on up-conversion (UC) [6,7] and down-conversion [8] luminophores. These methods result in energy transfer from non-sensitive regions of the spectrum to crystalline silicon photosensitive range. The most efficient UC material excited at 980 nm is 3-NaYF4:21.4%Yb:2.2%Er with a reported photoluminescence quantum yield (PLQY) 10.5 % at pump power density P=35 W/cm2 [9]. Other efficient UC materials described in the literature include BaY2ZnO5:7%Yb:3%Er with PLQY=5 % at P=2.2 W/cm2 [10], La2O2S:9%Yb:1%Er with PLQY=5.8 % at P=13 W/cm2 [11] and SrF2:Yb(2 mol.%):Er(2 mol%) with PLQY=2.8 % at 10 W/cm2 [7]. The phenomenon of quantum cutting is one of the down-conversion mechanisms, which allows one to transform the blue pump radiation to near infrared with an efficiency of more than 100 %. It was previously shown, that one of the most promising from the point of view of high quantum energy transfer efficiency is the ytterbium-praseodymium doping pair [12-17]. Previously, we studied solid solutions based on calcium fluoride and strontium fluoride doped with praseodymium and ytterbium [18,19]. The best result was achieved for a solid solution based on SrF2, which demonstrated an energy transfer coefficient of more than 100 % and a quantum yield of 1.1 % [18]. Usually, the luminescence efficiency increases with the transition to heavier matrices and with reduced symmetry. In this connection, it was logical to proceed to the study of fluorite solid solutions based on barium fluoride. It was previously shown that it was impossible to synthesize single-phase solid solutions Ba1-xRxF2+x (R - rare earth elements) by co-precipitation technique [20-22], since two-phase samples were synthesized. It is possible to synthesize Ba1-xRxF2+x solid solutions by high temperature melting technique, while fluorite-related trigonal
distorted Ba4Y3Fi7 single-phases are synthesized from aqueous solutions [20-23]. The aim of this work was to study the synthesis and spectral-luminescent characteristics of Ba4Y3F17 solid solutions.
2. Experimental
Ba4(Y,Yb,Pr)3F17 samples were synthesized by co-precipitation from aqueous solutions as reported elsewhere [21,24]. We used 99.99 wt% pure ytterbium, yttrium and praseodymium nitrate hexahydrates, barium nitrate (all reagents were manufactured by LANHIT, Russia), 99.99 wt% pure dihydrate potassium fluoride (REACHEM, Russia) and double distilled water as starting materials without further purification. Preliminary, the potassium fluoride was dried at 350oC for 3 hours. 0.08 M aqueous solutions of barium nitrate and rare earth nitrate were added dropwise to potassium fluoride (0.16 M) with intense stirring. Potassium fluoride was taken with a 50 % excess from stoichiometry. The process was carried out according to the following reaction:
4Ba(NO3)2 + 3R(NO3)3 + 17KF = Ba4R3Fi7 I +17KNO3, R = Y, Yb, Pr.
The resulting precipitates were dried at 45oC and annealed at 600oC. As a result, single-phase powders of Ba4(Y,Yb,Pr)3F17 solid solutions were synthesized.
The samples were analyzed by X-ray powder diffraction on a Bruker D8 Advance (CuKa radiation) diffrac-tometer. The unit cell parameters were calculated by TOPAS software (Rwp <10). Particle size, morphology and composition of the samples were analyzed by a Carl Zeiss NVision 40 scanning electron microscope equipped with an EDX detector.
Diffuse reflection spectra were recorded by a Thorlabs IS200 integrating sphere and a StellarNet EPP2000 spectrometer equipped with deuterium and halogen lamps. Luminescence and luminescence excitation spectra were measured using specialized setup for luminescence spectroscopy. A 150 W Xe lamp combined with monochromator MDR-206 was used as the excitation source. Luminescence was detected using an Oriel MS257 spectrograph equipped with Marconi 30-11 CCD detector. Also the luminescence was recorded by a StellarNet spectrometer with a spectral resolution of 0.5 nm and excited with 445 nm continuous wave laser diode. Luminescence kinetics were recorded with the use of MDR-23 equipped with FEU-100 and FEU-62 photomultipliers as detectors for UV-visible and IR spectral ranges, respectively. The time scanning for luminescence kinetics registration was carried out by two digital oscilloscopes: a BORDO oscilloscope with a bandwidth of 200 MHz, dynamic range of 10 bits, and a Tektronix DPO7354 oscilloscope with a bandwidth of 3.5 GHz, dynamic range of 8 bits. Pulsed excitation was arranged from OPO system Lotis TII LT2211 with 7 ns pulse duration and 10 Hz pulse repetition rate. The quantum yield of down-conversion luminescence was measured directly using a Thorlabs IS200 integrating sphere with previously-reported methods [25]. The radiation from the integrating sphere was transferred to a StellarNet spectrometer by optical fiber. The spectral characteristics of the recording system were calibrated with the use of TRSh-2850 and DRGS-12 lamps. All measurements were performed at 300 K.
3. Samples characterization
X-ray powder diffraction patterns of 45oC-dried Ba4(Y,Yb,Pr)3F17 solid solutions are presented in Fig. 1a. The synthesis was carried out by the co-precipitation from aqueous solutions, and as a result, the particles have physically and chemically adsorbed water on their surfaces. It leads to the quenching of luminescence. The effect of annealing on the increase in luminescence intensity was previously demonstrated for both up-conversion and down-conversion phosphors in [19]. Thermal treatment was performed in the platinum crucible under air at 600oC for 1 hour. Annealing has resulted in a significant narrowing of the XRD peaks in comparison to the samples dried at 45oC (Fig. 1b).
Sobolev and Tkachenko [26] constructed phase diagrams of the BaF2-RF3 systems from melting points to 800oC for R=Sm-Lu and up to 900oC for R=La-Nd. In all systems, extensive regions of Ba1-xRxF2+x solid solutions with fluorite structure (space group Fm3m) are formed. The maximum is x=0.50±0.02 for R=La-Nd and decreases with decreasing ionic radius of R3+. Fluorite-related phases of variable composition Ba4±xR3±xF17±x with a structure derived from fluorite (hexagonal crystal symmetry, space group R-3), formed in a concentration range of 40-45 mol.% RF3, detected in systems with R=Sm-Lu. These phases melt incongruently for R=Tb-Lu, Y and decay in the solid state for R=Sm-Gd. A detailed consideration of the crystal structure of such phases for R=Y, Yb was carried out in [27], where a hexagonal structure with the ideal formula Ba4Y3F17 was confirmed for them. It is shown that this hexagonal structure is a distortion of the cubic lattice of barium fluoride. However, the degree of this distortion is small, and on the X-ray powder patterns, the corresponding cleavage of the main reflexes is very weak. In accordance with this, the Table 1 presents only the data for the cubic sub-cell. The unit cell parameters (a) and coherent scattering range (D) have been calculated for Ba4(Y,Yb,Pr)3F17 solid solutions (Table 1). The size of the coherent scattering region of samples dried at 45oC was about 20 nm. After annealing
ô
100 -, 30-eo-
4020-
] l i| , ! || _—J w'1 W—
100 n
20
30
40
50
60
70
- b
- Il 1 L h ^ - i WV . .
20 30 40 50
2 thêta, degree
60
70
Fig. 1. X-ray powder diffraction patterns of Bao.5r14Yo.3282Ybo.1Pro.ooo4F2,4286 upon drying at 45°C (a) and annealing at 600°C (b)
at 600° C, this value was increased several-fold. It should be noted that there is a regular decrease in the unit cell parameters with increasing ytterbium content in the crystal lattice. It is due to the fact that the ionic radius of ytterbium is smaller than that of yttrium according to the Shannon system [28].
Table 1. Unit cell parameters of Ba4(Y,Yb,Pr)3Fi7 solid solutions
Compositions of the initial aqueous solution After drying at 45 °C After annealing at 600° C
a, A D, nm a, A D, nm
Bao.5714 Yo.3982 Yb0.03Pr0.0004F2,4286 5.9175(5) 20±1 5.8978(4) >100
Ba0.5714Y0.3282Yb0.1Pr0.0004F2,4286 5.9021(7) 14±1 5.8851(2) 70±6
Ba0.5714 Y0.2782 Yb0.15Pr0.0004F2,4286 5.8818(6) 25±1 5.8710(3) 90±8
Bao.5714 Yo.3976 Yb0.03Pr0.00lF2,4286 5.9118(6) 19±1 5.8905(2) 71±6
Bao. 5714 Yo. 3276 Yb0.1Pr0.001F2,4286 5.8997(6) 23±1 5.8800(3) >100
Bao.5714 Yo. 2776 Ybo.15Pro.001F2,4286 5.8843(8) 21 ± 1 5.8684(6) >100
The particles of Ba4(Y,Yb,Pr)3F17 solid solutions are agglomerates with an average particle size of 400 nm (Fig. 2). The change in size depending on the content of ytterbium and praseodymium is not substantial.
The composition of the samples has been determined by energy-dispersive analysis (Table 2). Yb content is higher than in the initial aqueous solutions. The yttrium content is the same as in the initial aqueous solutions. The barium content decreases with increasing ytterbium content, which is confirmed by the corresponding distribution coefficient. The appearance of potassium in the crystal lattice of this solid solution arises from the fluorinating agent. The absence of potassium in a number of samples indicates that its amount is less than the detection limit.
4. Spectral-kinetic characteristics
The diffusion reflection spectrum of Ba4Y3F17:Pr(0.1 %):Yb(1.00 %) sample is shown in Fig. 3. The characteristic Pr3+ ions transitions from 3H4 manifold to 3Pj and 1D2 states appear in the blue and red spectral regions correspondingly. However, the reflection spectrum is dominated by the transition from 2F7/2 to 2F5/2 states of Yb3+ ions at -980 nm.
Fig. 2. SEM image of Ba4(Y,Yb,Pr)3Fn powder annealed at 600°C
Table 2. The energy-dispersive analysis of Ba4(Y,Yb,Pr)3F17 solid solutions
Compositions of the initial aqueous solution Composition of the solid solutions as determined by EDX* Barium, yttrium and ytterbium distribution coefficients (EDX composition / Initial composition)
Ba0.5714 Y0.3982 Yb0.03Pr0.0004 F2,4286 Ba0.5624 Y0.4026 Yb0.0350F2,4376 0.98/1.01/1.16
Bao.5714 Yo.3282 Ybo.iPro.ooo4F2,4286 Bao.5380Yo.3440 Ybo.118oF2,4620 0.94/1.05/1.18
Ba0.5714 Y0.2782 Yb0.15Pr0.0004 F2,4286 Bao.5100K0.0131 Yo. 2814 Ybo. 1961 F2,4644 0.89/1.01/1.30
Ba0.5714 Y0.3976 Yb0.03Pr0.001F2,4286 Ba0.5490 Y0.4140 Yb0.0370F2,4510 0.96/1.04/1.23
Bao.5714 Yo.3276 Yb0.1Pr0.001F2,4286 Bao.5340K0.0110 Yo.3300Ybo. 1250 F2,4440 0.93/1.00/1.25
Bao.5714 Yo. 2776 Ybo.15Pro.001F2,4286 Bao.5140K0.0140 Yo. 2830 Ybo. 1890F2,45800 0.90/1.02/1.26
*The praseodymium content is below the EDX detection limit
The luminescence spectra of the samples were investigated under excitation by 445 nm light, which corresponds to the excitation of Pr3+ ions and lies within efficient solar spectrum. All samples have exhibited luminescence of both Pr3+ and Yb3+ ions. In Fig. 4, the luminescence spectra are shown for Ba4Y3F17:Pr(0.1 %) samples co-doped with 10 % and 15 % Yb.
The ions pair Pr3+ and Yb3+ often exhibit a quantum cutting effect, which consists in emission of two photons of Yb3+ luminescence as a result of absorption of one photon to 3Pj manifold of Pr3+ ions. It is clearly seen from the comparison in the spectral range 400-650 nm of excitation spectrum monitored at 980 nm and diffusion reflection spectrum of the sample (see Fig. 5).
Excitation to 3Pj manifold of Pr3+ ions may lead to two excited Yb3+ ions whereas energy of excitation to 1D2 manifold of Pr3+ is not enough to achieve quantum cutting effect. From Fig. 5, we see qualitatively that the ratio of areas under lines 3Pj/1D2 is higher for excitation spectrum than that for reflectance of those which is the evidence of quantum cutting effect [29].
It is worth noting that the excitation spectrum was not corrected for the spectral sensitivity function because of the absence of a reference sample with constant quantum yield in the region 500-600 nm. However, the intensity
BaaYsF1i:Pr(0,1 %)+Yb(1 %)
400 500 600 700 800 900 1000 1100
Wavelength, rim
Fig. 3. Diffusion reflection spectrum of Ba4Y3Fi7:Pr(0.1 %):Yb(1.0 %) powder sample. inset shows the magnified visible spectral range reflection
The
Fig. 4. Luminescence spectra of Ba4Y3Fn:Pr(0.1 %):Yb(10.00 %) (1) and Ba4Y3F17:Pr(0.1 %):Yb(15.00 %) (2) powder samples excited by 445 nm CW laser light. The sign (*) indicates the second order observation of excitation light
of the excitation source at 590 nm is several times higher than that at 450 nm. Therefore, we expect even higher relative intensity for the group of lines in the region 440-500 nm after the correction. The presented spectra allow one to make a qualitative conclusion concerning the quantum cutting effect in the studied samples without any quantitative estimations on the efficiency of the process.
As a result, we clearly see the down-conversion luminescence of Yb3+ ions when samples are excited to 3H4-3P2 transition of Pr3+ ions. The energy transfer features will inevitably appear in luminescence kinetics curves. The Pr3+ luminescence decays at 605 nm are shown in Fig. 6.
Curves in Fig. 6 indicate that the luminescence decay from 3Pj manifold of Pr3+ ions is non-exponential, especially for the early stage of decay. We see that an increase in the Yb3+ ion concentration leads to quenching of Pr3+ luminescence, which speaks for non-radiative energy transfer. Due to the non-exponential character of decay, the average luminescence lifetimes were calculated with the use of formula (1):
ft* I (t)dt 11(t)dt '
where I(t) is the intensity of the decay curve, and t is time.
The results of calculation are presented in Table 3.
The luminescence lifetime values presented in Table 3 indicate the energy transfer from Pr3+ ions to Yb3+ ions. But also we see that there is some increase for Pr3+ luminescence lifetime with the increase of Pr3+ content. As this is seen in double doped samples this can be the evidence of certain cross-relaxation process resulting in energy back transfer from Yb3+ to Pr3+ as transitions 2F7/2 to 2F5/2 of Yb3+ ions and 1G4-3P0 of Pr3+ ions are almost equal in energy. These peculiarities are also shown in Yb3+ luminescence kinetics.
tavg p ri-t\ j-t ' (1)
400 450 500 550 600 650 Wavelength, nm
Fig. 5. Excitation spectrum monitored and 980 nm and diffusion reflection spectrum for Ba4Y3F17:Pr(0.1 %):Yb(10.0 %) powder sample both normalized to the maximum of 3H4-1 D2 transition. Higher values of intensity of excitation lines correspondent to 3H4-3Pj transitions compared to normalized values of reflectance of those illustrate the quantum cutting effect
0 ' 20 40 60
Time, us
Fig. 6. Luminescence decay curves of Ba4Y3F17:Pr(X %):Yb(Y %) samples excited at 445 nm registered at 605 nm. Here X/Y corresponds to 0.04/3.00 (1), 0.04/10.00 (2), 0.04/15.00 (3), 0.1/3.00 (4), 0.1/10.00 (5), 0.1/15.00 (6)
Table 3. Average luminescence lifetime of Pr3+ detected at 605 nm in Ba4Y3F17:Pr:Yb powder under 445 nm excitation, ^s
Yb and Pr content, mol.% Yb (3.0 %) Yb (10.0 %) Yb (15.0 %)
Pr (0.04 %) 9.0 2.8 2.5
Pr (0.1 %) 15.9 3.0 2.3
i — I ■ - - - |J ! t | i I " I ' I
0,000 0,001 0,002 0,003 0,004 0,000 0,001 0,002 0.003 0,004
Time, s Time, s
Fig. 7. Luminescence decay curves of Ba4Y3F17:Pr(X %):Yb(Y %) samples registered at 980 nm excited at 445 nm (a) and 930 nm (b). Here X/Y corresponds to 0.04/3.00 (1), 0.04/10.00 (2), 0.04/15.00 (3), 0.1/3.00 (4), 0.1/10.00 (5), 0.1/15.00 (6)
The luminescence decays of Yb3+ ions both under 445 nm excitation (Fig. 7a) and 980 nm excitation (Fig. 7b) appear to be non-exponential due to non-radiative quenching and also due to sensitized character of excitation for the latter. The luminescence lifetime was estimated as the average lifetime by formula (1) also, the results are presented in Table 4.
Table 4. Average luminescence lifetime of Yb3+ detected at 980 nm in Ba4Y3F17:Pr:Yb powder under 445 and 930 nm excitation
Avg. time, ms Yb 3.0 % Yb 10.0 % Yb 15.0 %
Excitation, nm 445 930 445 930 445 930
Pr 0.04 % 1.47 1.48 0.345 0.320 0.159 0.124
Pr 0.10 % 0.729 0.756 0.266 0.248 0.184 0.167
Results of calculation in Table 4 show that Yb3+ ions exhibit strong concentration quenching. Also, it is important to note that the increase of Pr3+ ions also leads to a decrease in Yb3+ luminescence lifetime, which is seen for high concentrations of Yb3+ ions. It is a known feature of Pr/Yb ions pair when Yb3+ ions transfer their excitation to 1G4 manifold of Pr3+ ions [19,29]. At the same time, the down-conversion mechanism results in a bit longer luminescence lifetime when excited at 445 nm. From Fig. 7a, we can see that the luminescence of Yb3+ under 445 nm pulse excitation exhibits some build-up which was not observed in kinetics with direct Yb3+ excitation (Fig. 7b). This build-up time is on the same order of magnitude with the 3P0 manifold of Pr3+ lifetime, which speaks for a rate of energy transfer to Yb3+ as high as on the order of 106 s-1.
The luminescence spectrum corrected for the spectral sensitivity of our detection system allows estimation of energy transfer efficiency [19]. Ratios of integral luminescence intensity of Yb3+ ions to the integral luminescence intensity of the whole emission spectrum of the sample can be the measure of energy transfer coefficient between sensitizer (Pr3+) and activator (Yb3+) ions together with their concentration quenching processes (2):
qE = _J 1 Y6(A)dA__(2)
q /(IYb(X) + I(X))^ K>
where IYb(X) is the Yb3+ ion luminescence intensity, IPr (X) is the Pr3+ ion luminescence intensity. The corresponding calculation results are presented in Table 5.
From Table 5, we see that the energy transfer efficiency from Pr3+ ions to Yb3+ ions reaches 74 % for Ba4Y3F17:Pr(0.1 %):Yb(15.0 %) powder sample which is relatively large and speaks for high efficiency of the down-conversion system, since Yb3+ luminescence is not quenched at a high rate at this concentration.
Table 5. Energy transfer from Pr3+ to Yb3+ ions in Ba4Y3F17 powder estimated from luminescence spectra, %
Yb and Pr content, mol.% Yb (10.0 %) Yb (15.0 %)
Pr (0.04 %) 54 58
Pr (0.10 %) 58 74
The external efficiency of down conversion was investigated for studied samples by means of measurement of quantum yield of Yb3+ ions luminescence excited at 445 nm in the integrating sphere attached to the spectrometer by the technique described in [25]. Results are presented in the Table 6.
Table 6. Estimated external quantum yield of down-conversion luminescence of Yb3+ ions of Ba4Y3F17:Pr:Yb powder samples measured in integrating sphere, %
Yb and Pr content, mol.% Yb (10.0 %) Yb (15.0 %)
Pr (0.04 %) 0.8 0.8
Pr (0.1 %) 2.9 2.6
The results of external quantum yield measurement of down conversion luminescence in Ba4Y3F17:Pr:Yb powder samples speak for the potential of the compound. Values of quantum yield above 2 % are relatively high compared to CaF2:Pr:Yb and SrF2:Pr:Yb solid solutions with its values lower than 1 [18,19], or upconverter materials for solar cells (about 1 %) [30].
5. Conclusions
Single-phase powders of Ba4Y3F17:Yb:Pr solid solutions with an average agglomerate size of 400 nm were synthesized by co-precipitation from aqueous solutions. The down-conversion luminescence was investigated in Ba4Y3F17 co-doped with Pr3+ and Yb3+ ions. When excited to 3Pj manifold of Pr3+ ions, the samples exhibited luminescence of both Pr3+ and Yb3+ ions. The energy transfer efficiency appeared to be 74 % for Pr3+ 0.1 % and Yb3+ 15.0 %, which appears to be efficient, since Yb3+ luminescence is still not strongly quenched at this concentration. It was shown that the down-conversion mechanism in the investigated samples is quantum cutting, when one photon absorbed by Pr3+ ions results in two photons emitted by Yb3+ ions. At the same time, the energy back transfer from Yb3+ to Pr3+ was observed apparently through resonant energy transfer involving 2F7/2-2F5/2 of Yb3+ ions and 1G4-3P0 of Pr3+ ions transitions. Overall, the external quantum yield of down-conversion luminescence measured in our study appeared to be relatively high, with a maximum value of 2.9 % for Ba4Y3F17:Pr(0.1 %):Yb(10.0 %) sample, which makes this compound promising for Si-based solar cell efficiency enhancement.
Acknowledgements
The study was funded by the Russian Science Foundation (project # 17-73-20352). Authors thank E. V. Cher-nova for her assistance with the preparation of this manuscript.
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