Yb3+

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

NIR Downconversion and Energy Transfer Mechanisms in Tb3+/Yb3+ Codoped Na5Lu9F32 Single Crystals Jianxu Hu,†,‡,# Yuanpeng Zhang,§,# Haiping Xia,*,† Huanqing Ye,*,‡ Baojiu Chen,∥ and Yongsheng Zhu⊥ †

Key Laboratory of Photo-electronic Materials, Ningbo University, Ningbo, Zhejiang 315211, China School of Physics and Astronomy, Queen Mary University of London, London E1 4NS, United Kingdom § Oak Ridge National Laboratory, 1 Bethel Valley Rd, Oak Ridge, Tennessee 37830, United States ∥ Department of Physics, Dalian Maritime University, Dalian, Liaoning Province 116026, China ⊥ College of Physics and Electronic Engineering, College of Chemistry and Charmaceutical Engineering, Nanyang Normal University, Nanyang 473061, China ‡

ABSTRACT: Tb3+/Yb3+ codoped Na5Lu9F32 single crystals with near-infrared (NIR) emission are achieved by an improved Bridgman approach. Energy transfer from Tb3+ to Yb3+ ions is affirmed by the photoluminescence (PL) emission spectra and decay curves characterization. On the basis of the analysis of energy transfer rate dependence on the Yb3+ concentration, the interaction between Tb3+ and Yb3+ ions in Na5Lu9F32 single crystals is confirmed through the one-to-one energy transfer process. Results demonstrate that the prepared Na5Lu9F32 single crystals might be promising candidates to convert sunlight to improve the performance of the silicon solar cells.

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INTRODUCTION Solar energy as sustainable green energy is a promising alternative to tackle energy crisis in future, which is predicted to account for 16% of the global electricity output by the year of 2050.1 The fulfillment of efficient application of solar energy has stimulated extensive research interests in the past decade.2,3 Photovoltaics (PV) directly converting solar energy into electricity render them attractive candidates for the capture of sunlight in large-scale and conversion of the sunlight to meet the growing global energy demand.4 Crystalline silicon (c-Si) solar cells with an efficiency of ∼18% dominate the commercial photovoltaic market.5 However, the current efficiency is still not comparable to the Shockley-Queisser limit with a theoretical maximum value of 30% due to the so-called spectral mismatch between solar spectrum and the band gap of c-Si.6 Photons with energy less than the band gap (Eg) will not be absorbed, and the ultraviolet−visible (UV−vis) photons with energy higher than the band gap are lost as heat by electronic recombination and thermal relaxation. A significant enhancement of conversion efficiency is expected if this part of the © XXXX American Chemical Society

incoming spectrum can be modified to the wavelength that is in the range of c-Si absorption.7 There are two approaches to overcome the intrinsic loss mechanisms in solar cells through the modification of sunlight before absorption. The first approach is to combine two lower energy photons to produce one higher energy photon that is within the absorption range of the solar cell, which is referred to as an upconversion (UC) process,8,9 while the reverse process of downconversion (DC) is to cut one higher energy photon into two photons with lower energy that can both be harvested by the solar cell.7,10 Since the first demonstration of downconversion of UV or visible photons into NIR photons in (Y, Yb)PO4:Tb3+, other RE3+/Yb3+ combinations, such as Pr3+/ Yb3+, Tm3+/Yb3+, Er3+/Yb3+, and Nd3+/Yb3+ have also been utilized to achieve downconversion.5,11−17 Yb3+ ion is an almost ideal acceptor for DC in solar spectrum conversion as the single excited state of Yb3+ matches closely to the maximum spectral Received: March 31, 2018

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DOI: 10.1021/acs.inorgchem.8b00867 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry efficiency of c-Si. For the Tb3+-Yb3+ couple, there is a dispute among researchers over the energy transfer process from Tb3+ to Yb3+.10 Some believe the DC emission intensity should have a linear correlation with excitation intensity. Hence, a slope value of 1 should be observed from power dependence curves of Yb3+ luminescence.2,18 Others suggest the slope value should be 0.5 if a nonlinear second-order cooperative process is responsible.19,20 Reports focused on the Tb3+/Yb3+ doped materials indicate the DC mechanism is significantly influenced by the host environments. For instance, it was experimentally found that the oxyfluoride glass, oxide powders, and oxide ceramics have a slope of 1 by fitting the Yb3+ luminescence power dependence curve, while a value of 0.5−0.8 is found in fluoride powder samples.21 It is worth mentioning that the majority of the reported hosts are glasses, phosphors, and ceramics.13,22,23 The significant scattering and low transmission for phosphors and poor chemical stability for glasses and ceramics prevent them from practical applications. In contrast, single crystals are optically transparent and chemically stable. Moreover, the DC energy transfer mechanism remains unknown for Tb3+/Yb3+ couples in single crystal hosts. In this paper, Na5Lu9F32 single crystal with low phonon energy (maximum phonon energy ∼440 cm−1) is selected to accommodate Tb3+ and Yb3+ ions to investigate the optical properties and DC energy transfer mechanism under visible light.24 Results show the current Tb3+/Yb3+ doped Na5Lu9F32 single crystal is a potential candidate to enhance the c-Si solar cell performance by the modification of solar spectrum.

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Figure 1. (a) XRD patterns of Tb3+/Yb3+ codoped Na5Lu9F32 single crystals. (b) Standard line pattern of cubic phase Na5Lu9F32 (Powder Diffraction File no. 27−0725, Joint Committee on Powder Diffraction Standards, 1965). (c) Photo of 0.5 mol % Tb3+/4 mol % Yb3+ codoped Na5Lu9F32 single crystal and the polished sample for optical measurements.

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RESULTS AND DISCUSSION Figure 1a illustrates the XRD pattern of Tb3+/Yb3+ codoped Na5Lu9F32 single crystals with various Yb3+ concentration. Compared to the standard line pattern of Na5Lu9F32 displayed in Figure 1b, all the diffraction peaks can be assigned to a pure cubic Na5Lu9F32 structure with lattice constants (a = b = c = 5.425 Å). The absence of diffraction peak relating to any impurity or allotropic phase implies that a pure Na5Lu9F32 is obtained. The ionic radii of Tb3+ (1.04 Å, CN = 8) and Yb3+ (0.99 Å, CN = 8) are close to that of Lu3+ (0.97 Å, CN = 8); hence, the Lu3+ sites are more possible to be substituted with little defect. Also, the increase of Yb3+ concentration does not lead to any significant peak shift or second phase indicating the current doping level does not affect the Na5Lu9F32 phase structure. Samples with a fixed Tb3+ concentration at 0.5 mol % with various Yb3+ concentration (0, 4, 8, and 12 mol %) have been prepared to study the downconversion mechanism and conversion efficiency in the Tb3+−Yb3+ pair. The visible photoluminescence spectra of Tb3+/Yb3+ codoped Na5Lu9F32 single crystals upon excitation at 473 nm corresponding to the 7 F6 → 5D4 transition of Tb3+ is shown in Figure 2a. The strongest emission band is positioned approximately at 545 nm, corresponding to the Tb3+ 5D4 → 7F5 transition; the other two peaks at 586 and 620 nm are ascribed to Tb3+ 5D4 → 7F4 and Tb3+ 5D4 → 7F3 transitions, respectively. Interestingly, a broad emission band peaked at 980 nm in the NIR region can be observed in Figure 2b under the same excitation condition, which corresponds to the 2F5/2 → 2F7/2 transition of Yb3+ ions. Since the direct excitation at 473 nm cannot resonate with the excitation wavelength of Yb3+ ions, the appearance of NIR emission indicates energy transfer from Tb3+ to Yb3+. The correlation of the visible and NIR emission intensities with the Yb3+ concentration of the Na5Lu9F32:Tb3+,Yb3+ under 473 nm excitation has been illustrated in Figure 2c. The optical setup is maintained as identical to compare the emission intensities for different samples. Integrated emission intensity is calculated for

EXPERIMENTAL SECTION

An improved Bridgman approach was employed to grow the Tb3+/ Yb3+ codoped Na5Lu9F32 single crystals. The materials of NaF, LuF3, TbF3 and YbF3 with 99.99% high purity were prepared to grow fluoride single crystals according to the formula NaF/LuF3/TbF3/ YbF3 = 40/(59.5 − x)/0.5/x (x = 0, 4, 8, and 12). The fluoride raw materials were exposed to anhydrous HF atmosphere to remove the residual moisture in the fluorides at 750−800 °C for 8−10 h. The treated materials were grounded for 1 h then transferred to platinum crucibles. A pure Na5Lu9F32 single crystal seed was used to initiate the growth process. The filled crucibles were sealed to obtain complete isolation from moisture and oxygen in the atmosphere and prevent the melt from evaporation in the growing process. A Bridgman furnace was used to synthesize the crystals. The temperature of the furnace was set at 860−880 °C and maintained at this temperature for 5−6 h to establish a stable solid−liquid interface. The temperature gradient near the solid−liquid interface was about 70−90 °C/cm. The crucibles were lowered by an electric motor at a rate of 0.5−0.6 mm/h during the growth process. The temperature of the furnace was slowly decreased to room temperature at a rate of 30−40 °C/h to avoid cracking induced by the thermal stress in the crystal when the growth process was finished. The obtained crystals by the improved Bridgman method with a dimension of 4 cm × 1 cm is shown in Figure. 1c. All the crystals are transparent without any grain boundary. The obtained crystals were cut into small pieces and polished by CeO2 to achieve high transmittance for optical characterizations. Sample structures were characterized by X-ray diffraction (XRD) using a Bruker D8 Advance (Germany) with a Cu tube and Kα radiation at 1.54056 Å. The measuring range was set from 20 to 90° with a 0.01° increment. The photoluminescence spectra were recorded by an FLSP 920 type spectrometer (Edinburgh Co., England). An optical parametric oscillator (OPO) was applied as the light source for the decay time measurements. A Tektronix digital oscilloscope (TDS 3052) was used to record the signal. All the measurements were conducted at room temperature in atmospheric condition. B

DOI: 10.1021/acs.inorgchem.8b00867 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. (a) Visible emission spectra; (b) NIR emission spectra of Tb3+/Yb3+ codoped Na5Lu9F32 single crystals under 473 nm excitation. (c) Integrated emission intensity of visible and NIR emission for Tb3+/ Yb3+ codoped Na5Lu9F32 single crystals.

Figure 3. (a) Luminescence decay curves of Tb3+ at 545 nm emission originated from the 5D4 → 7F5 transition under the excitation of 473 nm light. (b) Energy transfer efficiency dependence on the Yb3+ concentration.

comparison. Noticeably, the visible emission from Tb3+ ions has been dramatically reduced with an increase of Yb3+ doping concentration from 0 to 12 mol %. More than 50% reduction of integrated emission intensity is observed in the visible range from Figure 2c. Meanwhile, the NIR emission peaked at 980 nm undergoes a significant increase in the Yb3+ concentration ranging from 0 to 4 mol %. The integrated NIR emission intensity is enhanced by ∼32% when the Yb3+ doping content is further increased from 4 to 8 mol %. However, a reduction of NIR emission is observable with the Yb3+ doping content reaching to 12 mol %, which can be explained by the concentration quenching among Yb3+ ions. A reduced distance in high Yb3+ doping content sample is expected to have an enhanced quenching effect as the quenching rate is inversely proportional to the distance between Yb3+ ions. The decay curves of Tb3+ emission at 545 nm were recorded to gain a deeper understanding of the energy transfer mechanism. In Figure 3a, the decay curves are illustrated for Na5Lu9F32:Tb3+ (0.5 mol %) codoped with 0, 4, 8, and 12 mol % Yb3+. A single exponential decay with a lifetime of 6.91 ms is observed for the sample without Yb3+. Addition of Yb3+ ions leads to faster decay processes. Since the Tb3+ concentration is kept at 0.5 mol % for all samples but the Yb3+ concentration varies, the reduction of lifetimes can be ascribed to the energy transfer from Tb3+ ions to nearby Yb3+ ions rather than the concentration quenching of Tb3+ ions. It is worth mentioning that the decay curves have single exponential decay character for all samples. The variation in the distribution of acceptors around donors resulting in a variety of energy transfer rates for donor ions is believed to cause the nonexponential character of the donors, which indicates a uniform distribution of Yb3+ ions around Tb3+ in the Na5Lu9F32 host.5 The lifetimes at 545 nm are determined by using a single exponential decay function to fit the decay curves. The lifetime values are summarized in Figure 3a. A clear decrease of the 5D4 lifetime is observed as the Yb3+ concentration increases, indicating an enhanced energy transfer rate to Yb3+ ions to depopulate the ions from the 5D4 level as the shortened distance between donors and acceptors

facilitates energy transfer. The energy transfer efficiency (ETE) can also be estimated from the luminescent decay curves. The energy transfer efficiency (ηETE) is defined as the ratio of Tb3+ ions that depopulated by energy transfer to Yb3+ ions over the total number of Tb3+ ions excited. The ETE (ηETE) can be determined by dividing the integrated intensity of the decay curves of Tb3+/Yb3+ codoped samples to the Tb3+ single-doped sample using the following expression:11 ηETE = 1 −

∫ Ix %Yb dt ∫ I0%Yb dt

(1)

where I represents decay intensity and x% denotes the Yb3+ concentration. Figure 3b displays the evolution of energy transfer efficiency with Yb3+ concentration. The ETE increases monotonically with the Yb3+ concentration. The enhanced ETE from Tb3+ to Yb3+ leads to the reduced visible emission intensities and decreased lifetime values at 545 nm as observed in Figures 2a and 3a, respectively. The energy transfer mechanism from Tb3+ to Yb3+ in Na5Lu9F32 samples can be elucidated from the dependence of energy transfer rate (W) on the Yb3+ concentration (x). For the one-to-one energy transfer mechanism, the energy of Tb3+ ion is transferred to only one Yb3+ ion. The energy transfer rate (WET) from a Tb3+ ion to a Yb3+ ion is expressed as WET = Ax, in which A is the sum of the distance function multiplied by the radiative decay rate and x is Yb3+ doping content. For the cooperative energy transfer process (CET), the donor simultaneously transfers energy to two acceptors in a twophoton energy transfer process, the correlation is described as WET = Ax2.17,25 According to the obtained lifetime from Figure 3a, the energy transfer rate can be calculated as follows:17 1 1 WET = − τx τ0 (2) C

DOI: 10.1021/acs.inorgchem.8b00867 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry where τx is the decay time when Yb3+ content is x and τ0 is the decay time for the Yb3+ free sample. The double logarithmic plot of energy transfer rate against Yb3+ doping concentration is illustrated in Figure 4. A linear dependence with a slope of ∼1

Figure 5. Schematic energy level diagram of Tb3+/Yb3+ codoped Na5Lu9F32 samples. Process one is the cooperative energy transfer, and process two is the one-to-one energy transfer.

Figure 4. Plot (log−log) of the energy transfer rate dependence on Yb3+ concentration in 0.5 mol % Tb3+/x mol % Yb3+ Na5Lu9F32 single crystals (x = 4, 8, and 12). The red dotted line is the linear fit to the experimental data.

is observed with the Yb3+ concentration increasing from 4 to 12 mol %, indicating the one-to-one energy transfer process is dominant for Tb3+ and Yb3+ codoped samples instead of the second-order cooperative energy transfer process. In fact, the cooperative energy transfer process can only be observed under high Yb3+ doping concentration for Pr3+/Yb3+ codoped βNaLuF4 nanocrystals.25 Auzel et al. suggest that a CET process is typically 103 times less possible than a first-order process, which makes the CET process less likely to occur.26 Moreover, it is recognized that higher Yb3+ content is required to achieve a cooperative process in the Tb3+−Yb3+ couple.11 Hence, the probability is low to have a cooperative DC process in Tb3+/ Yb3+ codoped Na5Lu9F32 single crystal. Yu et al. revealed that the one-to-one energy transfer rather than the cooperative process occurs in the Ce3+/Yb3+ codoped YAG material by emitting a single NIR photon with the absorption of one visible photon,27 which is in accordance with the result from this work. Therefore, we conclude that the DC mechanism is through the one-to-one energy transfer route in the prepared samples as illustrated in the energy level diagram in Figure 5. Figure 6 illustrates the normalized PLE spectra and NIR PL spectra of 0.5 mol %Tb3+:Na5Lu9F32 single crystal with the optimal Yb3+ concentration, the AM 1.5G solar spectrum and the spectral response of c-Si solar cell to show the potential application of the synthesized material in the solar spectral modification. Evidently, the absorption of c-Si is most efficient from 900 to 1100 nm. However, the UV−vis range with much more significant energy than the band gap of c-Si is dominant in the incident solar spectrum. The Tb3+/Yb3+ codoped Na5Lu9F32 sample can effectively emit near-infrared light (red line) matching with the most sensitive range of c-Si solar cells under 473 nm excitation. Hence, Tb3+ /Yb 3+ codoped Na5Lu9F32 single crystals present great potential for promoting the performance of c-Si solar cells.

Figure 6. Normalized PLE spectra (blue) and NIR PL spectra (red) of 0.5 mol %Tb3+/8 mol %Yb3+ codoped Na5Lu9F32 single crystals. AM 1.5G solar spectrum (purple) and spectral response of c-Si (green) are the backgrounds.

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CONCLUSIONS In summary, Tb3+/Yb3+ codoped Na5Lu9F32 single crystals have been prepared by an improved Bridgman approach. At the excitation of Tb3+ ions with visible light at 473 nm, efficient NIR emission from Yb3+ ions is obtained. The energy transfer mechanism between Tb3+ ions and Yb3+ ions has been investigated in detail through the energy transfer rate dependence on the Yb3+ concentration. The one-to-one energy transfer mechanism is confirmed to dominate DC process in the prepared samples. This work enables researchers to gain a deeper insight of DC mechanism for a Tb3+-Yb3+ couple in the Na5Lu9F32 host, and the synthesized material may be a potential solar spectrum converter to harvest the solar energy and improve the efficiency of c-Si solar cells.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.X.). *E-mail: [email protected] (H.Y.). ORCID

Haiping Xia: 0000-0003-3372-514X Huanqing Ye: 0000-0002-7737-2676 D

DOI: 10.1021/acs.inorgchem.8b00867 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Author Contributions

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J.H. and Y.Z. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 51772159, 51472125, 11504188, and U1504626), the Natural Science Foundation of Zhejiang Province (Grant No. LZ17E020001), and K. C. Wong Magna Fund in Ningbo University.

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DOI: 10.1021/acs.inorgchem.8b00867 Inorg. Chem. XXXX, XXX, XXX−XXX