Yb3+ Doped La2O2S

Article pubs.acs.org/JPCC

Near-Infrared Quantum Cutting Material Er3+/Yb3+ Doped La2O2S with an External Quantum Yield Higher than 100% Bo Fan,*,† Christophe Chlique,† Odile Merdrignac-Conanec,† Xianghua Zhang,† and Xianping Fan‡ †

Laboratoire Verres & Céramiques, UMR CNRS 6226, Université de Rennes 1, Campus Beaulieu, 35042 Rennes Cedex, France Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China

‡

ABSTRACT: Near-infrared quantum cutting is widely studied for its potential application in photovoltaic solar cells. By using a fluorometer equipped with an integrating sphere, we measured the external quantum yield of near-infrared quantum cutting in Er3+/Yb3+ doped La2O2S. It is found that by optimizing the synthesis process and the dopant concentration, the external quantum yield higher than 100% can be achieved. Additionally, by comprehensively considering the emission efficiency of the dopants as well as the energy transfer efficiency between them, the estimated quantum yield could be in good accordance with the measured one.

1. INTRODUCTION The term quantum cutting (QC) describes the phenomenon where the energy of one absorbed photon is transformed into two or more emitted photons. This phenomenon, which theoretically results in a quantum yield (QY) larger than 100%, was predicted by Dexter as early as 1957.1 The experimental demonstration was focused on QC from vacuum ultraviolet (VUV) to the visible range. It is motivated by the development of efficient visible phosphors to improve the energy efficiency of VUV-excited mercury-free fluorescence tubes and highperformance plasma display panels.2 Visible QC has been studied in many rare-earth-based phosphors, and an impressive calculated QY close to 200% has been achieved in Gd3+-Eu3+ systems.3 Recently, the research of QC extends to the near-infrared (NIR) range for applications on third-generation photovoltaic solar cells. The photovoltaic (PV) solar cell market is currently predominated by crystalline Si solar cells with a conversion efficiency of around 15%.4 To improve the Watt/cost ratio, the development of third generation solar cells with a significantly higher conversion efficiency is in progress. One method to achieve this goal is to cover these solar cells with a NIR downconversion layer.5,6 The layer can split the energy of a UV− visible solar photon into two NIR photons which can be efficiently responded by the solar cell. Thus, the excess energy of UV−visible photons, which is initially lost as heat, can be used to double the number of electron−hole pairs generated in the solar cell, and the theoretical efficiency of 30.9% can be greatly improved to 39.63%.5 Under the guide of Dieke energy level diagram, NIR QC has been realized with RE3+-Yb3+ couples (RE = Pr, Er, Tb, Tm) based on first- or secondorder energy transfer.7−18 Usually, the existence of QC is indirectly verified by comparing the photoluminescence spectra of RE3+-singly doped sample and RE3+-Yb3+-co-doped sample, © 2012 American Chemical Society

and QY is calculated by the decrease of lifetime of the emission of RE3+ ions induced by addition of Yb3+, under the assumption that no nonradiative energy loss occurs. Even though high calculated QY approaching 200% has been observed in several systems,2,12,14 a directly measured QY higher than 100% has not been reported. Recent studies of the QY on Pr3+-Yb3+ 19,20 and Tb3+-Yb3+ 21 couples revealed that the measured QY was much smaller than the calculated QY. A directly measured QY higher than unity is consequently necessary. Scientifically, it provides direct evidence of the existence of QC, and practically, it brings NIR QC phosphors closer to application. The possibility of NIR QC involving Er3+-Yb3+ couple in the host lattice with a phonon energy higher than 200 cm−1 has been discussed in previous studies.11−13 It was pointed out that the theoretical QY of emission before 1700 nm may be achieved by QC. This QC is potentially useful to improve the conversion efficiency of germanium solar cells.22 However, direct measurements of QY are seldom found in literature. Our previous study on the Er3+-Yb3+ couple in chloro-chalcogenide glasses indicated that the QY directly measured by integrating a sphere is only half of the theoretical value.23 Therefore, further efforts are needed to achieve efficient QC processes and measured QYs (or external QYs) higher than 100%. In this paper, NIR QC with measured QY higher than 100% is demonstrated in the carefully synthesized Er3+/Yb3+ doped La2O2S. Several important factors to achieve an efficient QC process are discussed. Received: February 20, 2012 Revised: April 12, 2012 Published: May 14, 2012 11652

dx.doi.org/10.1021/jp3016744 | J. Phys. Chem. C 2012, 116, 11652−11657

The Journal of Physical Chemistry C

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2. EXPERIMENTAL DETAILS To study the NIR QC based on energy transfer between Er3+ and Yb3+, a series of La2O2S codoped with 0.25 mol % Er3+ and 0, 0.25, 0.5, 0.75, and 1.0 mol % Yb3+ was prepared by the ethanol-assistant solution combustion method. The starting raw materials La(NO3)3·6H2O (Alfa Aesar, 99.9%, 5.000 g), Er(NO3)3·5H2O (Aldrich, 99.9%), Yb(NO3)3·5H2O (Aldrich, 99.9%), and thioacetamide CH3CSNH2 (Aldrich ≥99.9%, 3.345 g) were dissolved with absolute ethanol (Prolabo, Normapur, 20 mL) contained in a beaker (100 mL). The solution was heated below 80 °C to allow the dissolution of thioacetamide. The preparation was then introduced into a muffle furnace (Thermolyne 48000) preheated to 500 °C. The beaker was kept at 500 °C into the furnace after the reaction for few minutes to obtain a homogeneous white powder. The asprepared powder was then ground and treated in a tubular furnace at 1000 °C under N2/H2S (90:10) flow for 5.5 h to eliminate the impurities in the powder. X-ray diffraction (XRD) patterns were recorded at room temperature in the 2θ range 10−70° using a Philips PW3710 diffractometer operating with Cu Kα radiation (λ = 1.5418 Ǻ ). X'PERT softwareData Collector and Graphics and Identifywere used, respectively, for recording, analysis, and phase matching of the patterns. Fourier transform infrared (FTIR) patterns were recorded at room temperature with a Nicolet 380 (FTIR, Thermo Electron Corporation) using atmospheric corrections between 400 and 4000 cm−1. The samples were dissolved in KBr pellets for FTIR measurements. The measurements of QY were performed with an FLS920 fluorescence spectrometer (Edinburgh Instruments Ltd., UK) equipped with a barium sulfate coated integrating sphere (150 mm in diameter). The measurements were carried out at room temperature. A Xe lamp Xe900 (450W) and a microsecond xenon flashlamp μF900H were used as light sources for steady state and decay-curve measurements, respectively. Visible emissions were detected by a Hamamatsu R928 photomultiplier tube, while near-infrared emissions were detected by a liquid-nitrogen-cooled Hamamatsu R5509-72 photomultiplier tube. To remove the influence from the instrument, all of the measured spectra were corrected by the correction curves provided by the manufacturer. These correction curves contain excitation correction curves and emission correction curves. The excitation correction curves were obtained by an excitation reference detector equipped in the sample chamber, which can eliminate the influence from the lamp spectrum and the spectral response of the excitation grating. The emission correction curves were obtained by the detection of the spectrum of a calibrated tungsten lamp (370−2000 nm) placed inside the integrating sphere. This lamp has been calibrated by the National Physical Laboratory (London, UK). The emission correction curves can eliminate the influence from the spectral response of the detectors and the emission gratings, as well as the spectral dependence of the integrating sphere reflectivity. Moreover, because the sensitivities of the two detectors used are different, a correction of the relative sensitivity is needed to combine the spectra measured by the two detectors together. This correction was carried out by comparing the integral intensity of the 663 nm emission peak which was respectively measured by the two detectors. To obtain the QY, measurements were carried out following a similar procedure proposed in ref 24. Four steps have been done for each sample: (1) The scattering intensity of the

excitation light La was recorded with a white standard (a BaSO4-coated disk with the same size of the sample holder) placed inside the sphere. The excitation light was directed onto the white standard. (2) The scattering intensity of the excitation light La′ was recorded with a white standard placed, while the excitation light was directed onto the sphere wall. (3) The scattering intensity of the excitation light Lb and the emission intensity Pb were recorded with the sample placed, while the excitation light was directed onto the sphere wall. (4) The scattering intensity of excitation light Lc and the emission intensity Pc were recorded with the sample placed, while the excitation light was directed onto the sample. The manipulation of steps 2 and 3 can eliminate the contribution of the emission excited by the reflected light from the sphere wall. Finally, the QY was calculated by the following equation: ηQY = (Pc − (1 − A)Pb)/ALa A = 1 − PcLa′/PbLa

(1)

where A is the absorption coefficient. The relative error of the visible QY is 2%, which comes from the accuracy of sample replacement after changing the sample. The relative error of the NIR QY primarily arises from the determination of the relative sensitivity of the two detectors. Thus the deviation of the relative sensitivity of the two detectors among the measurements of different samples is calculated. The relative error of the NIR QY is less than 10%. To verify the validity of this approach, a commercial yellow phosphor powder YAG:Ce (Hongda Co. Ltd.) was used as a standard. YAG:Ce is widely used in the white LED. After optimization, its QY under blue light excitation is around 90%.25,26 Using the approach described above, the QY of YAG:Ce turns out to be 98 ± 2%, which is slightly higher than the recognized value. The difference is about 10%.

3. RESULTS AND DISCUSSION According to XRD patterns (Figure 1), the as-prepared powders still contain a small amount of lanthanum oxide phase. After sulfurization, the oxides can be eliminated, and a pure La2O2S phase can be obtained. FTIR spectra were also used to assess the purity of the materials (Figure 2). The as-

Figure 1. XRD patterns of La2O2S codoped with 0.25 mol % Er3+ and 0.5 mol % Yb3+ before and after sulfurization (1000 °C, 5.5 h). 11653



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concentration is higher than 0.75 mol %. On the contrary, the QY of the visible emission of Er3+ decreases monotonically after the codoping of Yb3+. After exciting an Er3+ ion into the 4F7/2/2H11/2 levels, the ion decays rapidly to the 4S3/2 level by multiphonon relaxation (process ① in Figure 3b). Subsequently, two different crossrelaxation processes may realize the QC. The first crossrelaxation occurs from Er3+ (4S3/2 → 4I13/2) to Yb3+ (2F7/2 → 2 F5/2; process ② in Figure 3b). Finally, two photons are emitted by the 4I13/2 → 4I15/2 transition of Er3+ and the 2F5/2 → 2F7/2 transition of Yb3+, respectively (process ③ in Figure 3b).11 The second process is the cross-relaxation between Er3+ ions, Er3+ (4S3/2) + Er3+ (4I15/2) → Er3+ (4I9/2) + Er3+ (4I13/2); then the energy is redistributed between Er3+ and Yb3+ and emitted radiatively.12 The two processes transform a blue/green photon into two photons at 1000 and 1550 nm. In the second process, the addition of Yb3+ only changes the distribution of the NIR energy but does not create a new QC path, so the energy conversion from visible to NIR demonstrated in Figures 3a and 4a at the beginning of adding Yb3+ is caused by the first crossrelaxation process. When the concentration of Yb3+ is high, the slight decrease of NIR emissions should be ascribed to the effect of concentration quenching.14 The QYs of the total emissions for the samples before and after sulfurization are present in Figure 4b. Before sulfurization, the QYs behave irregularly, which is attributed to the uncontrollability of the fast combustion reaction. A regular variation of the QYs can be observed after 5.5 h of sulfurization. The QYs are enhanced by sulfurization except for the sample with 1.0 mol % Yb3+. Interestingly, the QYs of the total emissions of the sulfurized samples with Yb3+ concentrations of 0.25, 0.5, and 0.75 mol % are 117 ± 7%, 110 ± 7%, and 106 ± 7%, respectively. Even considering that our measurement may have 10% offset, the sample with 0.25 mol % Yb3+ still has a QY higher than 100%, while other two are close to 100%. It demonstrates that a measured QY higher than unity can be achieved in purified La2O2S:Er3+,Yb3+ powders, proving experimentally the existence of NIR QC in this system. Equation 2 has been widely used for estimating theoretic QYs in previous studies.14 Here, we use this equation to help us understand how the high measured QY is achieved.

Figure 2. FTIR of La2O2S codoped with 0.25 mol % Er3+ and 0.5 mol % Yb3+ before and after sulfurization (1000 °C, 5.5 h).

prepared powders contain several absorption peaks between 1000 cm−1 and 4000 cm−1: The absorption peak at 1350−1580 cm−1 can be assigned to carbonates caused by CO2 on the surface of La2O2S:Er3+,Yb3+ particles.27 Relatively weak absorption peaks located at 1630 cm−1 and 3300−3610 cm−1 correspond to the water ν2 bend, ν3 asymmetric stretching, and ν1 symmetric stretching.28,29 There exist two absorption bands at 1050−1200 and 580−640 cm−1 which are attributed to ν1, ν3, and ν4 characteristic vibrations of sulfate group caused by the presence of the La2O2SO4 phase.30 All of these peaks decrease significantly after sulfurization, except the La−O and La−S vibrational bands situated at 400−550 cm−1.31 Figure 3a presents the typical emission spectra after exciting Er3+ ions at 523 nm (2H11/2 level of Er3+) in the purified samples. It can be seen that the visible emission peaks decrease while the infrared emission peaks increase after codoping of Yb3+. Moreover, compared with the Er-singly doped sample, the Er−Yb codoped samples have a broader and more intense 1000-nm emission peak, which is in accordance with the typical emission peak of Yb3+ (observed in Yb-singly doped sample). All of these indicate that the energy transfer between Er3+ and Yb3+ occurs. Figure 4a presents the QY of the visible and infrared emission of the purified samples, which is directly measured by using an integrating sphere. The QY of the emission at 1000 and 1550 nm first increases with the Yb3+ concentration and then decreases slightly when the Yb3+

Figure 3. (a) Emission spectra for La2O2S codoped with 0.25 mol % Er3+ and 0 or 0.5 mol % Yb3+. The spectra were recorded under excitation at 523 nm. (b) Energy level diagram of Er3+ and Yb3+ scheming the mechanism of NIR quantum cutting based on energy transfer between the two ions. This process consumes one blue/green photon to generate two NIR photons. The solid, dotted, and curved arrows represent radiative transition, nonradiative energy transfer, and nonradiative relaxation, respectively. 11654



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Figure 4. Quantum yields of La2O2S doped with Er3+ (0.25 mol %) and Yb3+ (0, 0.25, 0.5, 0.75, and 1.0 mol %), which were measured directly by using an integrating sphere. The excitation was realized with a 523-nm light (2H11/2 level of Er3+) at room temperature. (a) Quantum yields of the visible and NIR emission peaks in the sulfurized samples. (b) Quantum yields of total emissions in the samples before and after sulfurization, as while as the estimated quantum yields by eq 1.

Figure 5. Decay curves of (a) 4S3/2 (Er3+) emission at 553 nm and (b) 2F5/2 (Yb3+) emission at 958 nm in La2O2S doped with Er3+ (0.25 mol %) and Yb3+ (0, 0.25, 0.5, 0.75, and 1.0 mol %).

ηQY = ηRE(1 − ηET) + 2ηNIR ̅ ηET

the Yb3+ concentration, the shortening of buildup time indicates a faster decay rate due to the energy transfer from Er3+ to Yb3+. By fitting the decay curve with a combination of a buildup part and a direct relaxation decay part, the average decay lifetime can be obtained. Then ηET can be estimated by the following equation:14 τ ̅ − Yb ηET = 1 − Er τEr (3) ̅

(2)

According to eq 2, the QY ηQY is determined by three parameters: ηET the energy transfer efficiency from RE3+ (here it is Er3+) to Yb3+, ηRE the emission efficiency of RE3+, and ηN̅ IR the emission efficiency of the generated NIR photons. The first term denotes the contribution of residual excited RE3+ ions after QC, and the second term denotes the contribution of the photons generated by QC. Usually, theoretic QY is estimated with the assumption ηRE = ηN̅ IR = 1. However, the existence of several nonradiative processes such as concentration quenching8 always makes this assumption more or less false, and consequently it leads to an overestimation of the QY. Here, all three parameters are carefully checked. To determine ηET, luminescence decay curves were recorded for the 4S3/2 level of Er3+ (Figure 5a) after excitation to the 2 H11/2 level. A buildup is clearly observed at the beginning of all of the decay curves, which is attributed to the fast multiphonon relaxation from the 2H11/2 to 4S3/2 level. The decay curves of both codoped and singly doped samples have an exponential final portion with the same decay rate. It indicates that the energy migration among Er3+ ions plays insignificant role in the process of energy transfer between Er3+ and Yb3+ and the decay behavior can be described by the direct relaxation model proposed in ref 32. It can be also seen that the addition of Yb3+ results in a shorter buildup time. Reasonably assuming that the relaxation rate from the 2H11/2 to 4S3/2 level is independent with

From the decay curves in Figure 5a, the ηET from the Er3+ S3/2 level can be determined as 20%, 27%, 21%, and 27% for the samples with 0.25, 0.5, 0.75, and 1.0 mol % Yb3+, respectively. The energy transfer efficiency is weak. It is attributed to the fact that the phonon-assisted cross-relaxation process ② (Figure 3b) should emit 3−4 phonons to compensate for a large energy mismatch (∼1500 cm−1). It makes the energy transfer between Er3+ and Yb3+ inefficient. The emission efficiency of the generated NIR photons ηN̅ IR, in this case, is the average efficiency of the 1000-nm and 1550nm emissions. Reference 12 pointed out that equilibrium of population between the 4I13/2 (Er3+) and 2F5/2 (Yb3+) level can be achieved due to the high energy transfer rate between the 4 I11/2 (Er3+) and 2F5/2 (Yb3+) level. Therefore, ηN̅ IR can be approximately determined by the emission efficiency of Yb3+ ions. Figure 5b depicts the decay curves for the 2F5/2 level of Yb3+ after excited by the charge transfer band of Yb3+ at 310 nm. With the increase of the Yb3+ concentration, the

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4. CONCLUSION In summary, we have demonstrated, by direct measurement, that a QY higher than 100% can be achieved by NIR QC in La2O2S:Er3+,Yb3+. The key point to achieve such a high QY is to improve the emission efficiency of Yb3+ and Er3+. The former can be achieved by optimizing the Yb3+ concentration to avoid concentration quenching, while the latter can be achieved by preparing high-purity and well crystallized samples and by minimizing energy diffusion among Er3+ ions. This study gives the direct evidence of the existence of NIR QC. It also indicates that to have a precise estimation of the QY, besides the energy transfer efficiency, the emission efficiency of the involving ions should be taken into account.

luminescence decays slightly faster. Thus, concentration quenching occurs to a certain extent even in such low Yb3+ concentrations. The radiative decay rate of Yb3+ is estimated by the sample singly doped with a low concentration of Yb3+ (0.1 mol %). It turns out to be 0.00223 μs−1. Then η̅NIR can be estimated by the ratio of the radiative decay rate to the measured total decay rate. η̅NIR of the samples with 0.25, 0.5, 0.75, and 1.0 mol % Yb3+ are 99%, 99%, 92%, and 91%, respectively. The last parameter ηRE is assumed to be the measured QY of the Er3+-singly doped sample, which is 95%. Substituting the three parameters into eq 2, QY can be finally estimated. These results are also plotted in Figure 4b. It can be seen that the estimated values are in very good agreement with the measured QYs except the last one with 1.0 mol % Yb3+. This may be caused by the overestimation of ηRE for this sample. Even though the energy transfer efficiency is weak in the La2O2S:Er3+,Yb3+ powders, a measured QY higher than 100% is still achieved. Here, the high emission efficiency of Er3+ and Yb3+ plays a crucial role. Usually, the concentration quenching of Yb3+ is not considered to be important because of its lack of the possibility of self-quenching by cross-relaxation. It leads to the assumption that the emission efficiency of Yb3+ is close to unity. However, this seems not to be the real case. This comment is supported by the low concentration quenching threshold observed in various host lattices2,10,19 and is also directly demonstrated in the study of the concentration quenching effects of rare-earth ions in Y2O3.33 The problem is the energy diffusion among Yb3+ ions. By migrating quickly the energy to an Yb3+ ion just close to a luminescence quenching center, energy diffusion promotes the quenching process. The energy diffusion of Yb3+ is important, especially in a lattice with a short RE3+−RE3+ site distance, such as La2O2S (RE3+−RE3+ distance is 3.36 Å). To improve the emission efficiency of Yb3+, the energy diffusion should be restrained. This can be achieved by decreasing the Yb3+ concentration (like in our case) or by choosing a host lattice with a long RE3+−RE3+ site distance, for example, YAG, which can contain more than 30 mol % Yb3+ without significant concentration quenching.34 However, these measures also weaken the energy transfer between RE3+ and Yb3+. Thus a compromise should be found to optimize the final QY. A high ηRE is also vital for achieving a QY higher than 100%. This study demonstrates two important factors to obtain a high ηRE. First, the material synthesis should be optimized to have a host lattice with less defects and impurities. In this study, the sulfurization improves the crystallinity of the host lattice. It also eliminates the impurities with a high phonon energy, such as OH, CO32−, and SO42−. Thus, the extrinsic luminescence quenching centers are greatly decreased. The measured QY of the Er3+ singly doped sample increases from 87% to 95% after sulfurization. Second, the energy diffusion among RE3+ ions should be controlled. Even in a high-purity sample, quenching centers still cannot be avoided since they can be even formed by the cluster of RE3+ ions35 Then, controlling the energy diffusion rate is a good way to prevent the energy migrating to both extrinsic and intrinsic quenching centers. The low concentration of Er3+ in our samples results in a negligible diffusion rate which is confirmed by the analysis of its decay curves. Consequently, the nonradiative decay is inhibited.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +33 223235611. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the by the French Agence National de la Recherche with the project reference ANR-09HABISOL-009-03.

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REFERENCES

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