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Enhanced Quantum Cutting via Li+ Doping from a Bi3+/Yb3+-Codoped Gadolinium Tungstate Phosphor Ran Vijay Yadav,† Ram Sagar Yadav,† Amresh Bahadur,† Akhilesh Kumar Singh,‡ and Shyam Bahadur Rai*,† †

Department of Physics, Institute of Science, Banaras Hindu University, Varanasi 221005, India School of Materials Science and Technology, Indian Institute of Technology (Banaras Hindu University), Varanasi 221005, India

‡

ABSTRACT: The Bi3+/Yb3+-codoped gadolinium tungstate phosphor has been synthesized through a solid-state reaction method. The structural characterization reveals the crystalline nature of the phosphor. The Bi3+-doped phosphor emits visible radiation from the blue to red regions upon excitation with 330 and 355 nm. The addition of Yb3+ to the Bi3+-doped phosphor reduces the emission intensity in the visible region and emits an intense near-infrared (NIR) photon centered at 976 nm through a quantum-cutting (QC) phenomenon. This is due to cooperative energy transfer (CET) from the 3P1 level of Bi3+ to the 2F5/2 level of Yb3+. The presence of Li+ ions in the Bi3+/ Yb3+-codoped phosphor enhances the emission intensity in the NIR region up to by 3 times, whereas the emission intensity in the visible region is significantly reduced. The energy transfer (ET) from the Bi3+ ions to the Yb3+ ions is confirmed by lifetime measurements, and the lifetime for the 3P1 level of Bi3+ decreases continuously with increasing Yb3+ concentration. The ET efficiency (ηETE) and corresponding QC efficiency (ηQE) are calculated and found to be 29% and 129%, respectively. The presence of Li+ enhances the QC efficiency of the phosphor up to 43%. Thus, the Bi3+/Yb3+/Li+-codoped phosphor is a promising candidate to enhance the efficiency of a crystalline-silicon-based solar cell through spectral conversion.

1. INTRODUCTION Solar energy is an important source of renewable energy, because of its environmentally friendly nature for future worldwide energy demand, that could reduce the dependency on fossil fuel sources. Considerable research has been focused on improving the efficiency of solar cells through the absorption of solar radiation via a photon conversion process.1−6 The theoretical photon conversion efficiency is 30% for a crystalline silicon (c-Si) solar cell with a band gap of ∼1.12 eV and is restricted by reflection losses, charge-carrier separation, conduction efficiency, etc. These factors are responsible for the poor utilization of the solar energy, set the theoretical upper efficiency limit, and can be enhanced by overcoming these parameters. There are two types of spectral losses found for incoming solar radiation. One is the transparency of semiconducting devices having band gaps below the incident photons, which results in unutilization of the incident photons for energy conversion. On the other hand, when the semiconducting devices contain band gaps above the incident radiation, heating is created because of the excited carriers present in the conduction band as a thermalization, which degrades the efficiency of the solar cell. Thermalization can be reduced significantly by coating a layer of quantumcutting material on the semiconducting devices. If the one UV or vacuum-UV photon can be converted into two low-energy photons, the quantum efficiency (QE) is more than 100%. This © XXXX American Chemical Society

theory was predicted by Dexter in 1957 and is called as quantum cutting (QC). These QC materials are highly sensitive to the UV and/or visible light and result in two near-infrared (NIR) photons due to solar spectral conversion. Finally, these NIR photons can be absorbed by the coated semiconducting device, which leads to the generation of electron−hole (e−h) pairs for each incident photon.7−11 When sunlight is applied to a solar cell, electrons are generated from the cell and are promoted to the conduction band. These electrons are then used to generate an electric current for the solar cells.12 The development of QC phosphorbased solar cells is the subject of investigation because of the matching of the band gap of c-Si (∼1100 nm) because sunlight is a free and abundant source of energy for versatile use in different fields. On the other hand, the rare-earth (RE) ions have abundant energy levels; many of them are long-lived (metastable) and play a crucial role in energy transfer (ET), which gives rise to the QC process. Moreover, the QC phenomenon has been observed in RE-codoped systems such as Tb3+/Yb3+, Tm3+/Yb3+, etc., upon excitation with UV−vis photons, which yields two or more NIR photons.3 Generally, the QC phosphor contains two ions in which one ion acts as a donor and the other one as acceptor. Among RE ions, the Received: June 14, 2016

A

DOI: 10.1021/acs.inorgchem.6b01439 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

agate mortar for 30 min for homogeneous mixing. Thus, the obtained homogeneous product was kept in an alumina crucible and placed in a closed programmable furnace maintained at 1300 °C for 4 h. These synthesized materials were then structurally and optically characterized using different techniques. 2.2. Instrumentation. The phosphor samples were characterized by X-ray diffraction (XRD) using a Rigaku Benchtop X-ray diffractometer having Cu Kα radiation with a nickel filter for identification of the phase, crystallinity, and average crystallite size calculation. Rietveld structural analysis of XRD data was carried out using the Fullprof suite programs.27 The surface morphology of the phosphor samples was examined by scanning electron microscopy (SEM; QUANTA 200). The Fourier transform infrared (FTIR) spectra of the phosphors were monitored using a PerkinElmer IR spectrometer (FT-IR/FIR spectrometer Frontier) to see the presence of different vibrational features in the host matrix. The UV−vis−NIR spectra of the different phosphors were recorded by a diffusereflectance method using a PerkinElmer UV−vis−NIR spectrometer (Lambda 750). The PL excitation (PLE) and PL spectra of the phosphor samples were monitored using a Fluorolog-3 450 W fluorescence spectrofluorometer (Horiba Jobin Yvon model FL3-11). A Nd:YAG laser with an excitation wavelength of 355 nm and a charged-coupled-device (CCD) detector (Ocean Optics, QE 65000) were used to record the visible−NIR PL spectra. The PL decay curves were monitored using a spectrofluorometer (Edinburgh Instrument Ltd.). All of the experiments were performed under identical conditions at room temperature.

trivalent ytterbium ion acts as a QC emitter for silicon-based photovoltaic fields because its excited state, ∼10000 cm−1 (1.24 eV), matches the band gap of the silicon and it absorbs NIR light efficiently without any significant thermalization losses. The choice of a suitable donor ion such as Nd3+,7,13,14 Pr3+,8,15 Tm3+,16 Ho3+,17 Tb3+,18 etc., and an acceptor ion such as Yb3+ can easily generate QC emission due to cooperative ET (CET). Bi3+ is one of the good activators, and the 6s2 → 6s6p transition of Bi3+ can easily harvest the UV part of the solar energy. It gives a broad continuum in the visible region with its peak in the green region due to the 3P1 → 1S0 transition, and the position of this transition varies from one host to another.19 It acts as sensitizer and enhances the emission intensity of the doped materials. The QC mechanism in Bi3+, Yb3+ has been reported by Huang and Zhang in the Gd2O3 host.3 They have explained the mechanisms to be CET from Bi3+ to Yb3+. Similarly, the QC mechanism was reported by several workers in different host materials such as Y2O3, YNbO4, and CaTiO3.20−22 However, QC from these ions codoped in the gadolinium tungstate host is not reported in the literature. The host material plays an important role in the emission process. The host with lower phonon frequency reduces the nonradiative transition, thereby resulting in a larger emission intensity. Gadolinium tungstate has been proven to be an excellent host material because it has a large mechanical strength and chemical and thermal stability. It also shows good optical transparency with low phonon frequency and emits better photoluminescence (PL) intensity. The QE of these QC materials can be significantly enhanced by codoping an impurity element. Alkali ion, viz., Li+, has been proven to be a potential sensitizer and enhances the emission intensity of the up- and downconverted emissions from different materials to a great extent. Herein, we have studied the effect of Li+ ions on QC emission. We observe an enhancement in the NIR emission intensity by up to 3 times. This is due to the fact that the Li+ ions with low ionic radii can easily occupy the position in the host lattice and modify the local crystal field, which is favorable for the optical properties. It is also reported that Li+ ions create charge imbalance and charge compensation depending not only on the activator and donor ions but also on the host materials.23−25 In some cases, it has been noticed that they replace ions having ionic radii higher than them. These parameters make Li+ ions suitable candidates to improve the efficiency of the doped materials. In this Article, we have studied the NIR QC emission from Bi3+/Yb3+-codoped gadolinium tungstate phosphor upon excitation with 355 nm radiation. The structural and optical properties, along with the mechanism involved in ET, are investigated in detail. It is interesting to note that the presence of Li+ ions in the codoped phosphor enhances the QE of the phosphor by up to 43% for the first time. Thus, the Bi3+/Yb3+/ Li+-codoped gadolinium tungstate phosphor can be a potential candidate for enhancing the efficiency of c-Si solar cells.

3. RESULTS AND DISCUSSION 3.1. Structural Characterization. Parts a and b of Figure 1 show the XRD patterns of the 1 mol % Bi3+, 5 mol % Yb3+ and

Figure 1. XRD patterns of the 1 mol % Bi3+, 5 mol % Yb3+ codoped gadolinium tungstate phosphors in the (a) absence and (b) presence of Li+ ions.

1 mol % Bi3+, 5 mol % Yb3+, 3 mol % Li+ codoped gadolinium tungstate phosphor. The diffraction peaks of the phosphor match well with the monoclinic phases having JCPDS 23-1074 and 23-1076 of Gd2WO6 and Gd2(WO4)3 with space groups I2/a and C2/c, respectively. The XRD pattern clearly suggests that the codoped ions are completely dissolved in the gadolinium tungstate host lattice. The average crystallite size of the synthesized phosphor sample was calculated by the Debye−Scherrer equation and found to be 120 nm. In Figure 1a, space group I2/a dominates over space group C2/c and is indexed well. The presence of Li+ in the codoped phosphor dominates space group C2/c over space group I2/a (see Figure 1b), which is favorable for a larger emission intensity.

2. EXPERIMENTAL SECTION 2.1. Materials and Methods. The 1 mol % Bi3+; 5 mol % Yb3+; 1 mol % Bi3+, x mol % Yb3+ (i.e., x = 3, 5, 7, 10); and 1 mol % Bi3+, 5 mol % Yb3+, y mol % Li+ (i.e., y = 3, 5, 7) codoped gadolinium tungstate phosphor samples were prepared by a solid-state reaction method.26 The Gd2O3 (99.99%), WO3 (99%), Bi2O3 (99.99%), Yb2O3 (99.99%), and Li2CO3 (98.5%) were used as starting materials. After precise weighing, the starting materials were crushed rigorously in an B

DOI: 10.1021/acs.inorgchem.6b01439 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry The presence of the two coexisting monoclinic phases with space groups I2/a and C2/c is also confirmed by Rietveld analysis of the XRD patterns. The Rietveld refinement fits for 1 mol % Bi3+, 5 mol % Yb3+ codoped gadolinium tungstate phosphors in the absence and presence of Li+ ions are shown in Figure 2a,b. A very good fit between the experimentally

Figure 3. SEM micrographs of the 1 mol % Bi3+, 5 mol % Yb3+ codoped gadolinium tungstate phosphors in the (a) absence and (b) presence of Li+ ions.

Figure 2. Experimentally observed (red dots), Rietveld calculated (black line), and difference (continuous bottom line) profiles obtained after Rietveld analysis using coexisting monoclinic space groups I2/a and C2/c for 1 mol % Bi3+, 5 mol % Yb3+ codoped gadolinium tungstate phosphors in the (a) absence and (b) presence of Li+ ions.

observed and theoretically calculated XRD profiles is seen for both compositions. The insets in Figure 2a,b show enlarged XRD patterns in the 27.5−30.5° region, which clearly indicates variation in the phase fractions between the two space groups. The lattice parameters and phase fractions of the two phases thus obtained from Rietveld structure refinement are summarized in Table 1. Parts a and b of Figure 3 show the SEM micrographs of the gadolinium tungstate phosphor sample in the absence and presence of Li+ ions. It is clear from the figure that the particles are spherical and agglomerated with each other with the particle size in the submicron range. The particle size of the synthesized phosphor sample increases with the addition of Li+ ions (see Figure 3b). 3.2. Optical Characterization. 3.2.1. FTIR Studies. FTIR measurements of the samples have been carried out to see the different vibrational bands present in the sample. The FTIR spectra of 1 mol % Bi3+; 1 mol % Bi3+, 5 mol % Yb3+; 5 mol % Yb3+; and 1 mol % Bi3+, 5 mol % Yb3+, 3 mol % Li+ codoped gadolinium tungstate phosphor samples recorded in the range of 400−4000 cm−1 are shown in Figure 4. The inset figure is shown for clear visibility of the bands. The spectra show different vibrational bands centered at 468, 684, 760, and 878 cm−1, which are assigned as the stretching vibrations of W−O

Figure 4. FTIR spectra of 1 mol % Bi3+; 1 mol % Bi3+, 5 mol % Yb3+; 5 mol % Yb3+; and 1 mol % Bi3+, 5 mol % Yb3+, 3 mol % Li+ codoped gadolinium tungstate phosphor samples.

bands.28 The spectra also show a band at 521 cm−1, which is attributed to the stretching vibration of Gd−O.29 It is clear from the figure that when Yb3+ ions are added to a Bi3+-doped sample, the intensity of the bands lies between 1 mol % Bi3+ and 5 mol % Yb3+. Furthermore, upon the addition of Li+ ions in the Bi3+/Yb3+-codoped sample, the intensity of the different vibrational bands is reduced considerably, which improves the crystallinity of the sample and enhances the QC intensity of the synthesized samples. 3.2.2. Absorption and PLE Spectra of Bi3+/Yb3+-Codoped Gadolinium Tungstate Phosphor. The absorption spectra of 1 mol % Bi3+; 1 mol % Bi3+, 5 mol % Yb3+; 5 mol % Yb3+; and 1 mol % Bi3+, 5 mol % Yb3+, 3 mol % Li+ codoped gadolinium tungstate phosphor samples were recorded in the 200−1200 nm region and are shown in Figure 5a. The spectra show a band centered at 330 nm, which is assigned to the 1S0 → 3P1 transition of the Bi3+ ion.30 The pure Bi3+-doped sample does not show any absorption band in the NIR region. However, when Yb3+ is added to the Bi3+-doped sample, the spectra show

Table 1. Rietveld Refined Lattice Parameters and Phase Fractions for 1 mol % Bi3+, 5 mol % Yb3+ Codoped Gadolinium Tungstate Phosphors in the Absence and Presence of Li+ Ions Using Monoclinic Phases with Space Groups I2/a and C2/c space group I2/a (No. 15)

C2/c (No. 15)

sample

a

b

c

β

phase %

a

b

c

β

phase %

0 mol % Li+ 3 mol % Li+

15.558(1) 15.5958(8)

11.1598(7) 11.1664(4)

5.4071(4) 5.4161(3)

91.411(4) 91.568(3)

63.26 47.18

7.6198(9) 7.6495(5)

11.418(1) 11.4085(5)

11.405(1) 11.4367(7)

108.894(8) 109.033(5)

36.73 52.81

C

DOI: 10.1021/acs.inorgchem.6b01439 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 6. PL spectra of 1 mol % Bi3+ doped with different concentrations of Yb3+ ions, i.e., 0, 3, 5, 7, and 10 mol %, in the gadolinium tungstate phosphors upon excitation with (a) 330 and (b) 355 nm.

mechanisms involved in ET for QC can be easily understood with the help of a schematic energy-level diagram. Figure 7 shows the schematic energy-level diagram for the Bi3+ and Yb3+ ions. The 6s2 electronic configuration of Bi3+ has Figure 5. (a) Absorption spectra of 1 mol % Bi3+; 1 mol % Bi3+, 5 mol % Yb3+; 5 mol % Yb3+; and 1 mol % Bi3+, 5 mol % Yb3+, 3 mol % Li+ codoped gadolinium tungstate phosphor samples. (b) PLE spectra of 1 mol % Bi3+ doped with different concentrations of Yb3+ ions, i.e., 0, 3, 5, 7, and 10 mol %, in the gadolinium tungstate phosphors at λemi = 552 nm.

a band at 976 nm, which is due to the 2F7/2 → 2F5/2 transition of the Yb3+ ion31 and is shown as an inset in the figure. The presence of Li+ ion in the sample significantly enhances the intensity of the absorption bands, which results in better emission intensity of the codoped sample. Figure 5b shows the PLE spectra of 1 mol % Bi3+ doped with different concentrations of Yb3+ ions, i.e., 0, 3, 5, 7, and 10 mol %, in the gadolinium tungstate phosphor sample recorded in the 200−400 nm region at λemi = 552 nm. The excitation spectra contain a broad band in the 260−380 nm region centered at 330 nm, which is ascribed to the 1S0 → 3P1 transition of Bi3+ ions. The spectra show a regular decrease in the emission intensity of the band with increasing concentration of the Yb3+ ion.3 The spectra in the NIR region could not be monitored because of the instrument limitation (up to 850 nm). However, it may be due to ET from Bi3+ to Yb3+. 3.2.3. PL of the Bi3+/Yb3+-Codoped Gadolinium Tungstate Phosphor. The PL spectra of 1 mol % Bi3+ doped with different concentration of Yb3+ ions, i.e., 0, 3, 5, 7, and 10 mol %, in the gadolinium tungstate phosphor upon excitation with 330 and 355 nm from a xenon lamp are shown in Figure 6a,b. The spectra contain a broad band in the 450−750 nm region centered at 552 nm due to the 3P1 → 1S0 transition of Bi3+ ions.3 A large Stokes shift is observed for the 3P1 → 1S0 emission due to vibrational relaxation of the host. The emission intensity of the Bi3+-doped sample decreases regularly with increasing concentration of the Yb3+ ion, which is similar to the excitation spectra of 1 mol % Bi3+ doped with different concentrations of Yb3+ ions. This is due to CET from Bi3+ to Yb3+ and leads to QC emission. However, it could not be recorded because of the instrument limitation. Similarly, QC emission from Bi3+ and Yb3+ was also reported by Huang and Zhang3 in the Gd2O3 host, whereas Tao et al.20 reported a similar QC process in the Y2O3 host. They explained that QC emission occurs because of CET from Bi3+ to Yb3+. The

Figure 7. Schematic energy-level diagram of Bi3+/Yb3+-codoped gadolinium tungstate phosphors showing the CET mechanism for the NIR QC emission from the 3P1 level of Bi3+ to Yb3+ in the ground state upon excitation with a 355 nm laser.

a ground state (1S0), whereas the 6s6p excited state is split into four sublevels of 3P0, 3P1, 3P2, and 1P1 in which transitions from 1 P1 and 3P1 to 1S0 are allowed. When Bi3+ ions are excited with 355 nm, the ions are promoted to the 3P1 level. The excitedstate 3P1 level of the Bi3+ ion has twice the energy of the Yb3+ ion (i.e., 10000 cm−1).3,6 Thus, there is a CET from the 3P1 level of Bi3+ to the 2F5/2 level of Yb3+. The energy is transferred from the cooperative level of Bi3+ to two Yb3+ ions in the 2F5/2 level. This gives rise to QC emission at 976 nm. Thus, the single UV photon is split into two NIR photons corresponding to the 2F5/2 → 2F7/2 transition of the Yb3+ ion.23 The 1 mol % Bi3+ doped sample emits a reddish-white emission in the range of 450−850 nm centered at 611 nm due to the 3P1 → 1S0 transition of Bi3+ ions upon excitation with a 355 nm wavelength from a Nd:YAG laser source. On the other hand, the 5 mol % Yb3+ doped sample emits NIR emission in the 900−1030 nm region centered at 976 nm due to the 2F5/2 → 2F7/2 transition of Yb3+ ions upon 355 nm excitation.32 When the Bi3+ and Yb3+ ions are codoped together, the emission intensity of the Bi3+ band decreases whereas the emission intensity of the Yb3+ band increases many times in the D

DOI: 10.1021/acs.inorgchem.6b01439 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry NIR region. This is clear evidence of CET from Bi3+ to Yb3+, and the spectra thus obtained are shown in Figure 8.

Figure 10. Effect of the Li+ concentration (i.e., 0, 3, 5, and 7 mol %) on the emission intensity of 1 mol % Bi3+, 5 mol % Yb3+ codoped gadolinium tungstate phosphors. Figure 8. Vis−NIR emission spectra of 1 mol % Bi3+; 1 mol % Bi3+, 5 mol % Yb3+; and 5 mol % Yb3+ doped and codoped gadolinium tungstate phosphor samples upon excitation with a 355 nm laser.

sample is optimized with different concentrations of Li+ ions, and it is optimum for its 3 mol % concentration. Beyond this, the emission intensity further decreases because of the concentration quenching. This is due to the fact that Li+ ions modify the local crystal field around the Bi3+ and Yb3+ ions, which is favorable to enhance the emission intensity.33 Moreover, the Li+ ion also increases the crystallinity in the sample and affects the lattice vibrations in the crystal, which reduces the nonradiative relaxation to a great extent. It is also reported that the Li+ ions in the phosphor sample create charge imbalance.24 In our case, it creates charge imbalance as 2W4+ → Bi3+ + Yb3+ + Li+. The presence of Li+ in the codoped phosphor dominates space group C2/c over space group I2/a as discussed earlier. These parameters are favorable for a large enhancement in the emission intensity. Thus, the Bi3+/Yb3+/Li+-codoped gadolinium tungstate phosphor can be a potential candidate for enhancing the efficiency of the c-Si solar cells. 3.2.5. Excitation Power Dependence Properties. In order to establish QC, we have monitored the visible and NIR power dependence emission intensities upon excitation with a 355 nm laser source for 3P1 → 1S0 (Bi3+) and 2F5/2 → 2F7/2 (Yb3+) transitions. The dual-logarithmic plots for the integrated emission intensity versus pump power for 3P1 → 1S0 (611 nm) and 2F5/2 → 2F7/2 (976 nm) transitions in the Bi3+/Yb3+codoped gadolinium tungstate phosphor upon excitation with a 355 nm laser in the absence and presence of Li+ ions are shown in Figure 11. The slope values for visible and NIR emissions are found to be 1.08 and 0.53, respectively. This suggests that the visible emission is a linear process, whereas the NIR emission is

Further, we have studied the effect of the Yb3+ concentration on the emission intensity of the Bi3+-doped sample from the visible to NIR regions upon excitation with a 355 nm laser, and the spectra are shown in Figure 9. When the concentration of

Figure 9. Effect of the Yb3+ concentration (i.e., 0, 3, 5, 7, and 10 mol %) on the vis−NIR emission in the Bi3+ codoped gadolinium tungstate phosphor samples upon excitation with a 355 nm laser.

Yb3+ ions increases from 3 to 5 mol %, the emission intensity increases in the NIR region simultaneously and is optimum at 5 mol %. A further increase in the concentration of Yb3+ decreases the emission intensity because of the concentration quenching effect.32 Actually, at higher concentration, the distance between the ions becomes smaller than the critical distance and the excitation energy migrates to the quenching centers. On the other hand, the emission intensity in the visible region continuously decreases because of CET from Bi3+ to Yb3+. This energy is transferred to Yb3+ ions to give emission in the NIR region, which increases rapidly. This clearly indicates an efficient ET from Bi3+ to Yb3+. 3.2.4. Effect of Li+ Ions on the PL and QC Emission upon Excitation with a 355 nm Laser. The efficiency of emission in the NIR can be significantly enhanced by codoping Li+ ions, and we found 3 times enhancement in the NIR region. The emission spectra thus obtained are shown in Figure 10. It is interesting to note that the emission intensity in the NIR region increases, whereas it decreases continuously in the visible region, which is similar to the previous case. The emission intensity of the 1 mol % Bi3+, 5 mol % Yb3+ codoped phosphor

Figure 11. Dual-logarithmic plots of the integrated emission intensity versus pump power for (a) 611 nm (3P1 → 1S0) and (b) 976 nm (2F5/2 → 2F7/2) transitions in the Bi3+/Yb3+-codoped gadolinium tungstate phosphor upon excitation with a 355 nm laser in the absence and presence of Li+ ions. E

DOI: 10.1021/acs.inorgchem.6b01439 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry a nonlinear downconversion process.34,35 The emission intensity (I) varies with nth order of the incident pump power (P) as I ∝ Pn. In the case of upconversion, the value of the slope (n) is found to be 2. However, in our case the slope value for the QC process is obtained as 0.53. This is clear evidence for conversion of one-UV photon into two NIR photons. Thus, NIR emission occurs because of downconversion CET. A similar type of observation has been reported by Shestakov et al. for QC. 35 They have experimentally obtained this process in a Li/Yb-codoped ZnO system and calculated the slope values of 1 and 0.5 for visible and NIR emissions, respectively. In our case, we also experimentally achieved slope values of 1.08 and 0.53 for visible and NIR emissions, respectively. Moreover, in the presence of a Li+ ion with the gadolinium tungstate phosphor sample, the slope values slightly increase for these emissions as 1.15 and 0.59. Thus, the visible emission arises linearly, whereas the NIR emission is nonlinear because of a dowconversion CET process. 3.3. Lifetime Measurements. ET from Bi3+ to Yb3+ can be verified with the help of lifetime measurements. The lifetime of the 3P1 level of the Bi3+ ion is recorded for different concentrations of Yb3+ at λemi = 611 nm upon excitation with a 303 nm light-emitting-diode (LED) source, and the decay curves thus obtained are plotted in Figure 12a−e. The decay

The lifetimes thus obtained are summarized in Table 2. The singly-doped Bi3+ sample gives a lifetime of 915 ns with a Table 2. Variation in the Lifetime (τ) for the 3P1 → 1S0 Transition of Bi3+ and the ET Efficiency (ηET) and QE (ηQE) of Bi3+ (1 mol %) with Different Concentrations of Yb3+ Ions in the Gadolinium Tungstate Phosphors upon Excitation with a 303 nm LED Source λexc = 303 nm Yb3+ concn (mol %)

τ (ns)

ηET (%)

ηQE (%)

0 3 5 7 10

915.4576 855.3452 806.1340 703.3637 641.2649

0 6.55 11.91 23.16 29.94

100 106.55 111.91 123.16 129.94

monoexponential luminescence decay profile. However, the lifetime decreases regularly with increasing Yb3+ concentration from 0 to 10 mol %, and the corresponding lifetime varies from 915 to 641 ns, respectively. We have further studied the effect of Li+ ions on the lifetime for the 3P1 level of the Bi3+ ion, and the curves are shown in Figure 13a−d. The lifetime of the 3P1 level decreases further as

Figure 13. (a−d) Decay curves for the 3P1 → 1S0 transition of Bi3+ ions in the Bi3+ (1 mol %), Yb3+ (5 mol %) codoped gadolinium tungstate phosphors with different concentrations of Li+ (i.e., 0, 3, 5, and 7 mol %) upon excitation with a 303 nm LED excitation source. Figure 12. (a−e) Decay curves for the 3P1 → 1S0 transition of Bi3+ ions for different concentrations of Yb3+ ions (i.e., 0, 3, 5, 7, and 10 mol %) in the 1 mol % Bi3+ codoped gadolinium tungstate phosphor samples upon excitation with a 303 nm LED.

the concentration of Li+ increases. The decay curves become nonexponential for higher concentrations of Li+ ions. The faster decline of the decay is attributed to the involvement of extra decay pathways for Li+ doping. The nonexponential character of the decay may be due to different ET rates from Bi3+ to Yb3+ ions. Therefore, analyses of PLE, emission spectra, and lifetime measurements of the phosphor samples suggest that it is possible to achieve two NIR photons from one incident UV photon through a CET mechanism from a Bi3+/Yb3+-codoped phosphor sample. The lifetimes of the 3P1 level of Bi3+ ions for different concentrations of Li+ ions upon excitation with a 303 nm LED source are summarized in Table 3. The ET efficiency (ηETE) and total theoretical QE (ηQE) were calculated by the equations

curves matched well with monoexponential fittings with the following equation: ⎛ t⎞ I = I0 exp⎜ − ⎟ ⎝ τ⎠

(1)

where I0 and I are the intensities at time 0 and t (s), respectively, and τ is the lifetime of the 3P1 level of the Bi3+ ion. When the concentration of the Yb3+ ions increases, the lifetime of the 3P1 level of Bi3+ ions decreases regularly. This is due to ET from the 3P1 level of Bi3+ to the 2F5/2 level of Yb3+. F

DOI: 10.1021/acs.inorgchem.6b01439 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 3. Variation in Lifetime (τ) for the 3P1 → 1S0 Transition of Bi3+ and ET Efficiency (ηET) and QE (ηQE) of Bi3+ (1 mol %), Yb3+ (5 mol %) Codoped Gadolinium Tungstate Phosphors for Different Concentrations of Li+ at a λexc = 303 nm LED Source

200%, but we have achieved its value as 154% because of the involvement of nonradiative relaxation.

4. CONCLUSIONS The Bi3+/Yb3+-codoped gadolinium tungstate phosphor has been synthesized through a solid-state reaction method. The structural measurements of the synthesized samples reveal the crystalline nature with the coexistence of two monoclinic phases. The 1 mol % Bi3+ doped phosphor sample gives emission in the blue to red regions centered at 611 nm, whereas the 5 mol % Yb3+ doped phosphor sample emits NIR emission centered at 976 nm upon excitation with a 355 nm laser. When Yb3+ is doped in the 1 mol % Bi3+ doped sample, the sample gives visible and NIR emissions simultaneously. The emission intensity of the band in the visible region decreases continuously with increasing concentration of Yb3+ ions. The emission intensity in the NIR region is optimum for 5 mol % Yb3+. This is due to CET from Bi3+ to Yb3+. The emission intensity in the NIR region is found to enhance by up to 3 times further in the presence of Li+ ions, and the corresponding QE (ηQE) of the codoped phosphor is enhanced by up to 43%. Thus, the Bi3+/Yb3+/Li+-codoped gadolinium tungstate phosphor is a potential candidate to enhance the efficiency of c-Si solar cells through spectral conversion.

λexc = 303 nm Li+ concn (mol %)

τ (ns)

ηET (%)

ηQE (%)

0 3 5 7

806.1340 415.2736 379.8885 370.5552

11.91 48.51 52.97 54.09

111.91 148.51 152.97 154.09

ηETE = 1 −

τx τ0

(2)

and ηQE = ηBi(1 − ηETE) + 2ηYbηETE

(3)

where τ x denotes the lifetime, x stands for the Yb 3+ concentration, and ηBi and ηYb stand for the luminescent QEs of the Bi3+ and Yb3+ ions, respectively.3,5 It is assumed that if all excited Yb3+ and Bi3+ ions decay radiatively, then ηBi = ηYb = 1. ηETE was calculated for the 3P1 level using eq 2, and its values were found to be 6%, 11%, 23%, and 29% in the 1 mol % Bi3+ doped phosphor sample for 3, 5, 7, and 10 mol % concentrations of Yb3+ ions, respectively. Similarly, ηETE values for different concentrations of Li+ ions, i.e., 3, 5, and 7 mol %, for a fixed concentration of 1 mol % Bi3+, 5 mol % Yb3+ were found to be 48%, 52%, and 54%, respectively. The ηETE values thus found are summarized in Tables 2 and 3. On the other hand, the ηQE values were calculated using eq 3 for 3, 5, 7, and 10 mol % concentrations of Yb3+ ions, and its values were found to be 106%, 111%, 123%, and 129%, respectively. However, the ηQE values for different concentrations of Li+ ions, i.e., 3, 5, and 7 mol %, were found to be 148%, 152%, and 154%, respectively. Thus, it is interesting to notice that Li+ doping enhances the value of ηQE by up to 43% in the 1 mol % Bi3+, 5 mol % Yb3+ codoped gadolinium tungstate phosphor sample. This is the first report of the effect of Li+ ions on QC emission for enhancing its efficiency by up to 43%, to our knowledge. The lifetime (τ) and QE (ηQE) are plotted as a function of the Yb3+ and Li+ doping concentrations and are shown Figure 14a,b. It is noticed that ηQE increases monotonically and reaches maximum values of 129% for 10 mol % Yb3+ and 154% for 7 mol % Li+. This gives a maximum QE, which are summarized in Tables 2 and 3. The maximum QE should be

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +91 542 230 7308. Fax: +91 542 236 9889. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.V.Y. is grateful to the Department of Science and Technology, New Delhi, India, for providing a Senior Research Fellowship (Grant SR/S2/LOP-023/2012). The authors acknowledge the Biophysics Laboratory, Department of Physics, Banaras Hindu University, Varanasi, India, for providing the lifetime measurements.

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Figure 14. Plots of lifetime (τ) versus QE (ηQE) as a function of the different concentrations of (a) Yb3+ and (b) Li+ ions in the gadolinium tungstate phosphor samples. G

DOI: 10.1021/acs.inorgchem.6b01439 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b01439 Inorg. Chem. XXXX, XXX, XXX−XXX