ARTICLE pubs.acs.org/JPCC
Efficient Near-Infrared Down-Conversion in Pr3þYb3þ Codoped Glasses and Glass Ceramics Containing LaF3 Nanocrystals Yinsheng Xu,†,‡,§ Xianghua Zhang,*,‡ Shixun Dai,|| Bo Fan,‡ Hongli Ma,‡ Jean-luc Adam,‡ Jing Ren,† and Guorong Chen† †
)
Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China ‡ Laboratoire des Verres et Ceramiques, UMR-CNRS 6226, Sciences chimiques de Rennes, Universite de Rennes 1, Rennes, 35042, France § Faculty of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, China Faculty of Information Science and Engineering, Ningbo University, Ningbo 315211, China
bS Supporting Information ABSTRACT: The Pr3þYb3þ ion codoped 40SiO230Al2O318Na2O 12LaF3 glasses were synthesized by a conventional meltingquenching method. Near-infrared downconversion emission with 12% quantum yield has been realized. Transparent glass ceramics containing LaF3 nanocrystals were prepared by heat treatment. X-ray diffraction and scanning electron microscopy results show that fluorides favor the formation of small crystals. Comparing the absorption and emission spectra of the glasses with that of the glass ceramics indicates that the Pr3þ ions are incorporated in priority into the LaF3 crystals, due to the similar crystal lattice. Thus, energy transfer efficiency cannot be enhanced in the present glass by crystallization. It is suggested that fluorides from TbF3 to LuF3 (including YF3) having the orthorhombic structure in aluminosilicate glass are suitable for incorporating YbF3 during the crystallization process.
1. INTRODUCTION Among the green energies, solar power is one of the most sustainable energies due to its abundance and renewability. Using the photovoltaic (PV) effect, sunlight can be converted directly to electricity. Among all the PV materials, Si has many advantages including abundance, nontoxicity, and a relatively high performance/cost ratio. Commercially, crystalline Si (c-Si) solar cells dominate the solar cell market with a conversion efficiency of around 15%;1 even the efficiency obtained in laboratories is significantly higher. The way for PV energy to be cost-effective compared to fossil energy is to lower its production cost or/and to increase the conversion efficiency. Due to the discrete band structure of semiconductors, only photons with energies equal to or greater than the band gap energy Eg will be absorbed and may contribute to an electrical output of a photovoltaic (PV) device. Photons of higher energy, although absorbed, rapidly thermalize to the conduction band edge. The excess photon energy is therefore lost as heat within the lattice of the semiconductor. Photons of lower energy are transmitted through the solar cell and do not contribute to the electrical output. As a result, a compromise between thermalization and sub-band gap losses must be found when selecting a semiconducting material for PV applications. Fortunately, this drawback can be overcome by modifying the solar spectrum.2,3 The objective is to convert r 2011 American Chemical Society
the solar light as much as possible to lights efficiently absorbed by solar cells. Shalav et al. suggested that light with energy lower than 1.25 eV would be suited for upconversion (UC), whereas lights with energy higher than 1.25 eV would be better suited for downconversion (DC) applications for an ideal semiconductor with a band gap of 1.35 eV.3 According to the calculated Shockley Queisser limit, the best absorption band is around 8001100 nm for a SiPV with 1.05 eV bandgap.4 However, the external quantum efficiency of mc-Si devices decreases at wavelengths less than 500 nm due to higher reflection and absorption by the antireflection coatings optimized for longer wavelengths and due to increased emitter recombination caused by the high P-doping levels.5 Hence, the wavelength between 0.5 and 1.10 μm can be effectively utilized by SiPV. For wavelengths less than 500 nm or more than 1100 nm, the DC and UC can be available to convert the photon to suitable for the solar cell, respectively.6 Recently, DC has been considered for increasing the conversion efficiency of Si-based solar cells. DC has been realized with Er3þYb3þ, Tb3þYb3þ, Pr3þYb3þ, Eu2þYb3þ, Ce3þYb3þ, Received: February 15, 2011 Revised: May 31, 2011 Published: June 03, 2011 13056
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The Journal of Physical Chemistry C Ho3þYb3þ, and Nd3þYb3þ couples to convert visible photons into near-infrared (NIR) photons.722 In this work, we focus on the Pr3þYb3þ couple because the transition of Pr3þ:3H4f 3P0,1,2,1I6 has relative strong absorption at wavelengths less than 500 nm and the Yb3þ:2F5/2f2F7/2 emission is around the operating wavelength of Si-PV. Recently, Meijerink et al. elucidated the energy transfer mechanism for the downconversion in the Pr3þYb3þ couple.23,24 It is proposed that the first-order energy transfer by cross relaxation is the dominant process, as compared to cooperative transfer. Among the numerous luminescent materials, rare earth (RE) doped oxyfluoride glasses and glass ceramics have the advantages of lower phonon energies compared with oxide glasses as well as higher chemical durability and mechanical strength compared with fluoride glasses.13,2528 In this paper, we report the DC efficiency in the Pr3þYb3þ codoped SiO2Al2O3Na2OLaF3 glasses. The absorption spectra, excitation and emission spectra, fluorescence decay curves, and quantum yields (QY) have been measured. LaF3 crystals have been precipitated in the transparent glass ceramic by heat treatment (HT) to serve as the rare earth ions host to achieve efficient NIR emission by downconversion.
2. EXPERIMENTAL SECTION 2.1. Sample Preparation. The glasses were prepared with the following composition (in mol %): 40SiO230Al2O3 18Na2O12LaF3:0.5PrF3xYbF3 (x = 0, 0.5, 1.0, 5) by a conventional meltingquenching method. Na2O was introduced with sodium carbonate. The well-mixed stoichiometric chemicals were put into a covered platinum crucible and melted at 1450 C for 2 h. These glasses were synthesized by pouring the melt into a preheated stainless steel mold and annealed at 550 C for 2 h. After cutting and polishing, the samples with 9 9 2 mm3 were prepared for measurement. To further enhance the emission intensity, heat treatment (HT) was carried out on all the glass compositions with different time or temperature. 2.2. Physicochemical and Surface Properties. Differential thermal analysis (DTA) experiments (SDT-2960, TA Instruments) were performed in air at a heating rate of 10 K/min in the range from 25 to 800 C. To verify the amorphous state or crystallization of the samples, X-ray diffraction measurement was performed on a Phillips PW3020 diffractometer (Voltage 40 kV, current 30 mA, Cu Ka) with a step width of 0.02. The XRD patterns of the samples were collected in the range of 10 < 2θ < 70. The glass compositions were checked with a field emission scanning electron microscope (JSM 6400, JEOL) equipped with X-ray energy disperse spectroscopy (EDS), and the precision is within 0.5 mol %. A scanning electron microscope (JSM 6301F, JEOL) was used to observe the interior of the glass ceramics. 2.3. Spectroscopic Property. Optical absorption spectra were measured by using a spectrophotometer (Lambda 1050 UV/vis/NIR, Perkin-Elmer) at room temperature. The excitation and emission spectra as well as the fluorescence decay curves were recorded by using a FLS920 fluorescence (Edinburgh Instruments Ltd., UK) spectrophotometer. The emission spectra were collected by Hamamatsu R928 and liquid-nitrogen-cooled Hamamatsu R5509-72 photomultiplier tubes in the range of 475750 nm and 6001150 nm, respectively. The excitation source is a 450 W Xe lamp. To eliminate the noise from the excitation lamp, different filters were put before the detector. All
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the spectra were corrected by the correction files supplied by the manufacturer. 2.4. Quantum Yield. The fluorescence quantum yield (QY, defined as η) is the probability for a sample to emit a photon through fluorescence when it is excited by a single photon. It is therefore also the percentage of photons emitted by a bulk sample when a given number of photons are absorbed by the sample, i.e., the number of photons emitted divided by the number of photons absorbed by a sample. The number of photons absorbed by a bulk sample is equal to the number of photons incident on the sample minus the photons passing through and not being absorbed by it. The QY can thus be calculated by the following equation Z LS ε Z ð1Þ η ¼ ¼Z R ER ES where ε is the number of photons emitted by the sample, and R is the number of photons absorbed by the sample. LS is the luminescence emission spectrum of the sample; ES is the spectrum of the light used for exciting the sample; ER is the spectrum of the excitation light without the sample in the sphere; and all the spectra were collected using the sphere. To obtain the QY, a barium sulfate coated integrating sphere (150 mm in diameter) was attached to the FLS920. The blank laser excitation line from 475 to 485 nm was measured first. Then, the sample was placed on the sample holder in the integrating sphere for measurements of the emission spectra with Hamamatsu R928 and liquid-nitrogen-cooled Hamamatsu R5509-72 photomultiplier tubes in the visible (475750 nm) and NIR (6001150 nm) ranges, respectively. To eliminate the noise, every spectrum was scanned at least 5 times. After correction with the correction files supplied by the manufacturer, the NIR spectra were carefully normalized to the visible emission spectrum with the overlapped emission band in the 660750 nm range. Then, the quantum yields (QY) were calculated according to eq 1, with the software supplied by the manufacturer. However, due to the poor sensitivity of the Hamamatsu R5509-72 photomultiplier tube in the 600750 nm range, large error would be introduced during the normalization. In this paper, we have proposed a new way to calculate the QY of the NIR emission. Because all the emission spectra are measured by using the time-correlated single photon counting technique, the two emission spectra collected directly and indirectly (through the integrating sphere) are proportional. After normalization (using integrated intensity from 660 to 750 nm), the ratio of the integrated intensity of the NIR (9501150 nm) emission to that of the visible (575675 nm) emission for the directly collected is equal to that of the indirectly collected. Defining this ratio as Φ, the η for the NIR can thus be calculated by the following equation Z Z Φ LSNIR LSNIR εNIR Z Z ¼Z ¼Z ηNIR ¼ R ER ES ER ES ¼ Φ ηVIS
ð2Þ
where the subscripts “NIR” and “VIS” represent NIR emission in the range of 9501150 nm and visible emission in the range of 13057
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575675 nm, respectively. The ηVIS can be obtained from eq 1 with the detailed calculations provided in the Supporting Information. All the measurements were carried out at room temperature.
where A is the absorbance and d is the thickness of the sample. Strong absorption centered at 980 nm can be seen, due to the Yb3þ:2F7/2f2F5/2 transition. The other absorption bands in the
visible region are all from the energy transition of the ground state 3H4 to the excited state of the Pr3þ ions as marked in Figure 2. Three relatively strong absorption bands from 430 to 490 nm of Pr3þ ions are helpful to absorb photons, which are not effectively used by Si-based PV solar cells. Figure 3 shows the excitation spectra of the PrYb5.0 glass sample monitored at 610 or 980 nm. Three excitation bands centered at 440, 470, and 480 nm are assigned to the transitions of 3H4f3Pj (j = 0, 1, and 2) of Pr3þ ions, respectively. Under the 440 nm excitation, six visible emission bands centered at 530, 608, 640, 684, 702, and 726 nm can be observed in Figure 4, which are ascribed to the 3P0f3H5, 3P0f3H6, 3P0f3F2, 3P1f3F3, 3 P0f3F3, and 3P0f3F4 transitions of the Pr3þ ions, respectively. All the visible emissions decreased remarkably with the increasing YbF3 contents and almost quenched when the Yb3þ ion concentration reached 5.0 mol %. It is an obvious indication of ET from Pr3þ to Yb3þ ions. In Figure 4, the spectra in the NIR range were normalized with integrated intensity of the visible emission from 660 to 750 nm. The emission at 880 nm is a second-order peak. For the glass doped only with Pr3þ ions, a NIR emission centered at 1040 nm along with a shoulder at 1000 nm can be observed and assigned to the 1G4 f3 H4 electronic transition of Pr3þ ions. Another NIR emission band from 800 to 950 nm is assigned to the 1 3 I6, Pj(j=0,1,2)f1G4 electronic transition. Under the same excitation wavelength, the emission bands around 980 and 1020 nm in the NIR region are emerged in the spectra of the Pr3þYb3þ ions codoped samples, which are assigned to the 2F5/2f2F7/2 transition of Yb3þ ions. However, the emission at 1040 nm has vanished, indicating that the ET from Pr3þ ions to Yb3þ ions is
Figure 1. DTA curves of the PrYb5.0 glass sample.
Figure 2. Absorption spectra of Pr3þYb3þ ions codoped oxyfluoride glasses.
3. RESULTS AND DISCUSSION The DTA curve of the PrYb5.0 glass sample is presented in Figure 1. Two exothermic peaks are found around 602 and 690 C. The first one is ascribed to the precipitation of LaF3 crystals, and the second one is related to the precipitation of oxides. The glass transition temperature Tg and the first crystallization temperature Tp1 values are listed in Table 1 with the glass compositions and other properties. Clearly, both Tg and Tp1 decrease with the increase of the YbF3 contents, whereas the density increases. Due to the high melt temperature, the glass melt was partially evaporated. To make sure that fluorides are still present in the samples, the glass compositions were determined by EDS. The results show that the content of fluorides increases with the increasing YbF3 content. Figure 2 shows the absorption spectra of the Pr3þYb3þ codoped 40SiO230Al2O318Na2O12LaF3 glasses. To eliminate the thickness influence, the y axis is converted to the absorption coefficient R by the following equation from the absorbance spectra δ ¼ 2:303A=d
ð3Þ
Table 1. Compositions (mol %) and Properties of Pr3þYb3þ Ions Codoped 40SiO230Al2O318Na2O12LaF3 Glasses sample Pr PrYb0.5 PrYb1.0 PrYb5.0
PrF3 0.5 0.5 0.5 0.5
YbF3 0 0.5 1 5
Tg (C)
Tp1 (C)
density (g/cm3)
EDS
O
568
610
3.005
nominal
50.3
10.0
9.6
16.1
measured
43.4
11.2
9.6
17.1
nominal
50
10.4
9.6
16
measured
43.8
11
9.7
3.058
nominal
49.7
10.7
3.245
measured nominal
42.3 47.7
measured
40.8
563 550 542
613 603 602
3.040
13058
F
Na
Al
Si
La
Pr
Yb
10.7
3.2
0.1
14.2
4.3
0.1
10.6
3.2
0.1
0.1
17
14
4.3
0.1
0.2
9.5
15.9
10.6
3.2
0.1
0.3
12.9 13.3
9.3 9.1
17.1 15.2
13.7 10.2
4.2 3.1
0.1 0.1
0.4 1.3
15.4
8.9
16
13.2
3.9
0.1
1.6
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Figure 3. Excitation spectra of the PrYb5.0 glass sample monitored at 610 and 980 nm.
Figure 4. Visible and NIR emission spectra of Pr3þYb3þ ions codoped glass samples under 440 nm excitation. The NIR emission spectra were normalized to the visible emission spectra with the integrated intensity from 660 to 750 nm.
complete at high concentrations. With the increasing YbF3 contents, the emission intensity that arose from Yb3þ ions was reduced as a result of concentration quenching. Figure 5 presents the decay curves of the 608 nm emission bands of all the studied glasses under 440 nm excitation. Clearly, the lifetimes of all 608 nm emission bands decrease with the increasing Yb3þ ions, indicating the occurrence of ET from Pr3þ ions to Yb3þ ions. The mean decay lifetime (τm) is calculated by Z ¥ IðtÞdt τm ¼ ð4Þ I0 t0 where I(t) is the luminescence intensity as a function of time t, and I0 is the maximum of I(t) that occurs at the initial time t0. The energy transfer efficiency (ETE), ηET, is defined as the ratio of donors (Pr3þ) that are depopulated by ET to the acceptors (Yb3þ) over the total number of donors being excited. By dividing the mean lifetime of the Pr3þ ions in the Pr3þ and Yb3þ ions codoped glasses by the lifetime of the Pr3þ ions in the Yb3þ ions
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Figure 5. Time decay curves of the Pr3þYb3þ ions codoped glass samples monitored at 608 nm under 440 nm excitation.
Figure 6. Excitation line and emission spectra of the PrYb0.5 glass sample collected by using an integrating sphere. The inset figure shows the magnification of excitation line and emission spectra. The inset table is the quantum yield of the sample in the visible and NIR.
free glass, the ETE is obtained as a function of the Yb3þ ions concentration. ηET ¼ 1 τmx Yb=τm0 Yb
ð5Þ
where x represents the Yb3þ ions concentration. From the data of Figure 5, the lifetimes of Pr3þ ions are determined, and from them the corresponding ETE was also calculated as shown in the inset table of Figure 5. Clearly, the ETE increases with increasing YbF3 contents. The total quantum efficiency (QE) can be defined as the ratio of the photons emitted to the photons that are absorbed. Some literature assumed that all excited Yb3þ ions decay radiatively and thus that the upper limit of the QE can be obtained.8 In this paper, the real QY was measured by using an integrating sphere. As an illustration, the excitation line and the visible emission spectra of the PrYb0.5 glass sample are shown in Figure 6. The measured QYs are listed in the inset table of Figure 6. With the increasing of YbF3 content, ηVIS (575675 nm) decreases, while ηNIR (9501150 nm) increases first and then decreases. 13059
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The maximum ηNIR for the PrYb0.5 glass sample is only 12%, which is much lower than the values calculated by the method from the literature.8 To further enhance the NIR emissions, heat treatment (HT) was carried out on the glass samples to precipitate the hexagonal LaF3 nanocrystals in glasses. Figure 7 shows the XRD patterns of the PrYb5.0 glass sample under different HT, and the inset is the corresponding images of these samples. All the glass ceramics are transparent except the sample heat treated at 700 C for 20 h. According to the Scherrer equation,29 the size of the crystals is from 6 to 12 nm in these transparent glass ceramics, whereas the size increases to 33 nm for the sample heat treated at 700 C for 20 h. The calculated size of the crystals is in good agreement with observation under SEM (Figure 8). In these SEM images, nanocrystals can be seen in the sample treated at 650 C for 40 h, and
the higher the temperature, the larger the crystals. At 700 C for 20 h, the size of the crystals is less than 50 nm. Figure 9 shows the absorption spectra of the PrYb5.0 glass sample under different HT conditions. With increasing time or temperature, the beginning of the transmission is shifted toward longer wavelength. The structuring of the absorption peaks assigned to Pr3þ is evident after HT, indicating that Pr3þ ions are included in these crystals. No significant change is observed for the intense absorption peak around 1000 nm, which is assigned to Yb3þ ions. Meanwhile, the emission spectra in the visible become more intense and sharper with the increase of HT temperature or time as shown in Figure 10. However, the NIR emission reduces first and increases afterward with the increase of the crystal size. The variations of the absorption and emission spectra suggest the incorporation of Pr3þ into the low-photon-energy LaF3 crystalline environment after crystallization, while the environment for Yb3þ ions seems unchanged. This phenomenon is in disagreement with the reported work on the Tm3þYb3þ or Tb3þYb3þ codoped oxyfluoride glass ceramics,13,14,16,30 in which the NIR emission increased monotonously with the HT time or temperature. In aluminosilicate glass, the crystallization of lanthanide fluorides (LnF3) occurring in the interstices of the glass framework would be affected by the structure of the glass. The fluorides
Figure 7. XRD patterns of the PrYb5.0 glass sample under different HT conditions.
Figure 9. Absorption spectra of the PrYb5.0 glass sample under different HT conditions.
Figure 8. SEM pictures of the PrYb5.0 glass sample under different HT conditions, 600 C, 10 h (left); 650 C, 40 h (middle); and 700 C, 20 h (right). 13060
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Figure 10. Visible and NIR emission spectra of the PrYb5.0 glass sample under different HT conditions, excited at 442 nm.
Figure 11. Visible and NIR emission spectra of the PrYb0.5 glass sample under different HT conditions, excited at 442 nm.
from LaF3 to GdF3 in the glass matrix exhibited a hexagonal structure, while those from TbF3 to LuF3 (including YF3) exhibited an orthorhombic one.31 As the hexagonal phase requires smaller interstitial space for its dense packing structure, the hexagonal LaF3 crystals are precipitated more easily from the glass matrix. Thus, the Pr3þ ion (which has the same hexagonal structure in glass) has the priority to enter the LaF3 crystals compared with the Yb3þ ion (in orthorhombic YbF3 crystals). Moreover, fluorides in oxyfluoride glass act as a nucleating agent and suppress crystal growth by increasing nucleus quantity.3234 Therefore, the size of the precipitated crystals is constrained in the PrYb5.0 glass sample due to the highest fluoride contents. As a result, the Yb3þ ions are very difficult to be incorporated into the small LaF3 crystals. The increasing lifetime (from 4.7 to 10.9 μs; Figure S1, Supporting Information) of emission from Pr3þ ions at 608 nm supports the above analysis. As a result, both the NIR emission and the ETE decreased until the Yb3þ ions enter the crystals. This may be why the ETE cannot be effectively increased by the crystallization process in the present work.
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For the PrYb0.5 glass sample, a similar result can be predicted, that the visible emission increases more than the NIR emission due to the preferential incorporation of Pr3þ ions into the LaF3 crystals. Actually, both the visible and the NIR emission intensity increase with the increasing HT time or temperature, especially the visible emission intensity, as shown in Figure 11. However, for the PrYb0.5 glass sample, the size of the precipitated crystals cannot be controlled due to the lower content of fluorides. In fact, it has been demonstrated that at low concentration of ytterbium (between PrYb0.5 and PrYb5.0 glass samples) the precipitation of larger and fewer crystals is favorised (Figure S2, Supporting Information). As a result, less RE ions are incorporated into the crystals, and no clear Stark splitting can be seen in the emission and absorption spectra. Consequently, the Pr3þ and Yb3þ ions are included into the LaF3 crystals nearly at the same time, and no decrease of the NIR emission can be seen. Another evidence is that the lifetime of the 3P0 level (emission at 608 nm) did not change too much (from 11.3 to 12.5 μs, Figure S3, Supporting Information) for different glass ceramics. Hence, the visible emission increases more than the NIR emission, and the ETE decreases slightly. Anyway, to obtain higher NIR emission efficiency of Yb3þ ions, the hosts which can precipitate orthorhombic fluoride crystals have to be considered. In other words, the precursor which has similar crystal lattice with activator ions is propitious to incorporate the activator ions into the nanocrystals, and consequently the emission can be enhanced. From this point of view, we can predict that fluoride crystals from TbF3 to LuF3 (including YF3) with orthorhombic structure are suitable to incorporate the Yb3þ ions.
4. CONCLUSION In this paper, near-IR downconversion emission with 12% quantum yield has been measured in the Pr3þYb3þ ions codoped 40SiO230Al2O318Na2O12LaF3 glasses and glass ceramics. The transparent glass ceramics containing LaF3 nanocrystals were prepared by heat treatment. Comparing the absorption and emission spectra of the glasses with that of the glass ceramics, Pr3þ ions are preferentially incorporated into the LaF3 crystals due to the similar crystal lattice. Consequently, energy transfer efficiency between Pr3þ and Yb3þ ions cannot be enhanced in the present glass by crystallization. Fluorides from TbF3 to LuF3 (including YF3) having the orthorhombic structure in aluminosilicate glass are suitable for incorporating YbF3 during the crystallization process. ’ ASSOCIATED CONTENT
bS
Supporting Information. Calculation for quantum yield, time decay curves of the PrYb5.0 glass sample with different heat treatment conditions (Figure S1) and the SEM images and time decay curves of the PrYb0.5 glass sample with different heat treatment conditions (Figures S2 and S3). This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This study is supported by Shanghai Leading Academic Discipline Project, Project B502, Shanghai Key Laboratory Project 13061
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(08DZ2230500), and National Natural Science Foundation of China, No. NSFC 51072052. The first author wishes to express thanks to China Scholarship Council for providing the scholarship to study in France.
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