English

• 1. INTRODUCTION

  Solar cells have become more and more important in recent years because they could convert sunlight into electricity. And the photovoltaic market is dominated by crystalline silicon (c-Si) solar cells^[1]. However, there is a mismatch between the solar spectrum (300~2500 nm) and silicon spectral response^[2]. The strongest absorption of silicon solar cells is in a range of 800~1100 nm, but in the short wavelength region (especially in 300~500 nm), the response becomes weak because of higher reflection^{[3, 4]}. Spectral mismatch leads to the theoretical conversion efficiency of solar cells only about 30%^[5]. One feasible way to solve this problem is spectral modification by using down-conversion (DC) luminescent materials, which can convert high-energy photons to NIR photons^[6].

  At present, many DC phenomena from visible to NIR have been reported in RE/Yb³⁺ (RE = Tb³⁺, Pr³⁺, Tm³⁺, Ho³⁺) co-doped system^[7-11]. Yb³⁺ ion is used as an acceptor which has an excited state at approximately 10000 cm^-‍1, matching well with the band gap of crystalline silicon. However, the absorption of these RE sensitizer ions arises from the parity forbidden 4f-4f transitions, which is naturally small in cross section (merely 10^-21 cm²) and narrow in bandwidth^[12]. Unlike above RE ions, the absorption of Ce³⁺ is originated from the parity allowed 4f-5d electric-dipole transitions with a large cross section (about 10^-18 cm²)^{[13, 14]}. Besides, the excitation and emission wavelength of Ce³⁺ can be turned by the host materials, which means that choosing appropriate host materials can enhance the efficiency of DC^[15]. The DC of Ce³⁺/Yb³⁺ co-doped has been reported in various hosts, such as YAG, phosphates, silicates, and borates^[16-23].

  La₃Ga₅SiO₁₄ (LGS) has a trigonal structure with space group P321^[24]. It belongs to the langasite family with the general formula A₃BC₃D₂O₁₄, in which La³⁺ occupies decahedral site (A), and Ga³⁺ occupies the octahedral (B) and tetrahedral (C) sites. The D site is shared equally by Si⁴⁺ and Ga^3+[25]. Meanwhile, the material exhibits high thermal stability^[26]. The LGS has been reported as a promising host for achieving red and NIR emitting in these years, such as LGS: Eu³⁺ and LGS: Er³⁺/Ce^{3+[25, 27]}. However, Ce³⁺/Yb³⁺ co-doped LGS phosphor was rarely mentioned. Moreover, ions substitution is a useful means for modifying the properties of materials^[28]. Reports of Al³⁺ replacing Ga³⁺ in LGS have focused on crystal growth and properties such as La₃Ga_3.5Al_1.5SiO₁₄, but not on La₃Ga_5-zAl_zSiO₁₄: Ce³⁺ phosphors^{[29, 30]}.

  In this work, a NIR emission phosphor of Ce³⁺/Yb³⁺ co-doped LGS was synthesized by high-temperature solid-state reaction. The visible and NIR emission, decay lifetime, energy transfer efficiency, and energy transfer mechanism were studied. Besides, La₃Ga_5-zAl_zSiO₁₄ (z = 0, 1, 2, 3): Ce³⁺/Yb³⁺ phosphors were synthesized for the first time, and the influence of Al³⁺ ions on the luminescent properties was analyzed.

  2. EXPERIMENTAL

  2.1 Syntheses of the materials

  A series of (La_1-xCe_x)₃Ga₅SiO₁₄ (LGS: xCe³⁺, x = 0.5%, 0.8%, 1%, 1.5%, 2%), (La_0.99-yCe_0.01Yb_y)₃Ga₅SiO₁₄ (LGS: 1%Ce³⁺/yYb³⁺, y = 1%, 3%, 5%, 7%, 10%), La₃Ga_5-zAl_zSiO₁₄ (z = 1, 2, 3, 4, 5): 1%Ce³⁺ and La₃Ga_5-zAl_zSiO₁₄: 1%Ce³⁺/5%Yb³⁺ (z = 1, 2, 3, 4, 5) phosphors were synthesized by high-temperature solid-state reaction with raw materials of high-purity La₂O₃, Ga₂O₃, Al₂O₃, SiO₂, CeO₂ and Yb₂O₃. The starting materials were weighted according to the stoichiometric ratio. Then the materials were mixed and ground in an agate mortar thoroughly. After that, the homogenous raw materials were transferred to an alumina crucible and sintered in air at 1450 ℃ for 20 h in a muffle furnace. After cooling to room temperature naturally, the products were ground to powders for further characterization.

  2.2 Materials characterization

  X-ray diffraction (XRD) measurements were carried out on a MiniFlex 600 powder diffractometer with CuKα radiation (λ = 1.5405 Å) operating at 40 KV/15 mA. The scanning speed for phase determination was 0.2 °/min. The morphology and elemental analysis of the samples were recorded using a field emission scanning electron microscope (FESEM, SU-8010) equipped with an energy-dispersive spectrometer (EDS). The photoluminescence (PL) and excitation (PLE) spectra as well as decay curves were recorded by a spectrometer (Edinburgh Instruments, FLS 980) equipped with xenon lamps (450W) as exciting sources. Diffuse reflection spectra (DRS) of the power samples were measured by a spectrophotometer (Lambda 900) and using BaSO₄ as a standard reference.

  3. RESULTS AND DISCUSSION

  3.1 Structure determination of LGS: xCe³⁺ and LGS: 1%Ce³⁺/yYb³⁺

  The recorded XRD patterns of LGS: xCe³⁺ (x = 0.5%, 0.8%, 1%, 1.5%, 2%) and LGS: 1%Ce³⁺/yYb³⁺ (y = 1%, 3%, 5%, 7%, 10%) phosphors are similar, so only several of them are shown in Fig. 1a for the sake of brevity. The XRD patterns of LGS: xCe³⁺ (x = 0.5%, 0.8%, 1%, 1.5%, 2%) and LGS: 1%Ce³⁺/yYb³⁺ (y = 1%, 3%, 5%) samples match well with the standard card of LGS (PDF #41-0155) and no impurity phases are observed. But for LGS: 1%Ce³⁺/yYb³⁺ (y = 7%, 10%), there is a little impurity identified as Yb₃Ga₅O₁₂, indicating the end of the solubility range. Besides, it is obvious that the positions of two strong diffraction peaks shift slightly to higher angle at 2θ = 28.2° and 31° with the doping of Ce³⁺ and Yb³⁺ in Fig. 1b. According to the Bragg equation λ = 2dsinθ, the shift can be attributed to the smaller radii of Ce³⁺ (R_Ce³⁺ = 1.28 Å for CN = 8) and Yb³⁺ (R_Yb³⁺ = 1.12 Å for CN = 8) than that of La³⁺ (R_La³⁺ = 1.30 Å for CN = 8). These results indicate that single-phase phosphors are obtained and Ce³⁺/Yb³⁺ ions enter the host lattice.

  Figure 1

  

  Figure 1.  (a) XRD patterns of Ce³⁺/Yb³⁺ doped LGS; (b) Main peaks of Ce³⁺/Yb³⁺ doped LGS in the range of 27.5°~32°

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  3.2 Luminescence properties of LGS: xCe³⁺ and LGS: 1%Ce³⁺/yYb³⁺

  The PL spectra of Ce³⁺ singly doped LGS under excitation at 345 nm are shown in Fig. 2a. The emission intensity increases initially and then decreases with the Ce³⁺ concentration from 0.5% to 2%, and it reaches a maximum value when the concentration is 1%. Therefore, 1% was chosen as the concentration of Ce³⁺ in Ce³⁺/Yb³⁺ co-doped LGS. Fig. 2b shows the PLE and PL spectra of LGS: 1%Ce³⁺. The PLE spectrum monitored at 415 nm exhibits strong excitation band peaking at 345 nm, which is assigned to the 4f-5d transition of Ce³⁺. Under excitation at 345 nm, the emission band peaking at 415 nm can be decomposed to two Gaussian bands centered at about 413 nm (24213 cm^-‍1) and 450 nm (22222 cm^-1), which are attributed to the transitions from the lowest 5d level to the double 4f ground levels (²F_5/2 and ²F_7/2) of Ce³⁺. The energy difference is 1991 cm^-1, which is very close to the splitting of 4f ground state of Ce³⁺ (≈ 2000 cm^-1)^[31].

  Figure 2

  

  Figure 2.  (a) PL spectra of LGS: xCe³⁺ (x = 0.5%, 0.8%, 1%, 1.5%, 2%); (b) PLE and PL spectra of LGS: 1%Ce³⁺

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  The PL spectrum of LGS: 1%Ce³⁺/yYb³⁺ (y = 0, 1%, 3%, 5%, 7%, 10%) under excitation at 345 nm is shown in Fig. 3. With the increase of Yb³⁺ concentration, the emission in visible region of Ce³⁺ decreases monotonously, and the NIR emission of Yb³⁺ occurs, which illustrates the energy transfer from Ce³⁺ to Yb³⁺. The inset of Fig. 3 shows integrated PL intensities of visible and NIR emissions versus Yb³⁺ concentrations. The NIR emission intensity of Yb³⁺ reaches a maximum value when y = 5%. The intensity starts to decrease when y is higher than 5% due to the concentration dependent fluorescence quenching among Yb³⁺ ions. Therefore, the optimal Yb³⁺ concentration is 5%.

  Figure 3

  

  Figure 3.  Visible and NIR emission spectra of LGS: 1%Ce³⁺/yYb³⁺ (y = 0, 1%, 3%, 5%, 7%, 10%) under excitation at 345 nm

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  3.3 Energy transfer mechanism

  In order to determine the energy transfer mechanism of Ce³⁺→Yb³⁺ in LGS, the energy level diagrams with the possible energy transfer processes are shown in Fig. 4. Under ultraviolet excitation at 345 nm, the 4f ground state electron of Ce³⁺ is excited to higher 5d level, and then relaxes to the lowest 5d level without radiation. The transition from excited 5d level to 4f ground state can emit blue light or transfer its energy to Yb³⁺ ion. The emission of Ce³⁺: 5d→4f is located at approximately twice the energy of Yb³⁺: ²F_5/2→²F_7/2 emission, and there are no middle resonance energy levels between Ce³⁺ and Yb³⁺. Therefore, the energy transfer from Ce³⁺ to Yb³⁺ has two possible ways^{[6, 14]}, i.e. quantum cutting (QC) and single photon processes. In the former process, one excited Ce³⁺ ion transfers its energy to two Yb³⁺ ions nearby simultaneously, resulting in two NIR emitting photons (978 nm) of Yb³⁺ with a theoretical quantum efficiency up to 200%. While in the latter process, one excited Ce³⁺ ion only transfers its energy to one neighboring Yb³⁺ ion, and theoretical quantum efficiency is below 100%^{[16, 20]}.

  Figure 4

  

  Figure 4.  Energy level diagrams of Ce³⁺/Yb³⁺ and the possible energy transfer process

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  For a single photon process, one Ce³⁺ ion transfers its energy to only one Yb³⁺ ion, so the energy transfer rate W should be proportional to Yb³⁺ concentration and written as W_single = Cy^[32], where C represents the energy transfer coefficient and y is the concentration of Yb³⁺. For a quantum cutting process, however, one Ce³⁺ ion transfers its energy simultaneously to two nearby Yb³⁺ ions, so the energy transfer rate W_QC = C'y^2[33]. W can be obtained by^[23]

  $ \mathrm{W}=\frac{1}{\tau }-\frac{1}{{\tau }_{0}} $

  (1)

  Where τ₀ is the fluorescence lifetime of Ce³⁺ 5d level in Ce³⁺ singly doped LGS, and τ is that of Ce³⁺ in LGS: Ce³⁺/Yb³⁺. Fig. 5 shows the decay curves of Ce³⁺: 5d→4f emission at 415 nm in LGS: 1%Ce³⁺/yYb³⁺ (y = 0, 1%, 3%, 5%, 7% and 10%). It can be seen that the decay of Ce³⁺ becomes faster with the increase of Yb³⁺ concentration, and the lifetimes can be obtained by^[6]

  $ \tau =\frac{{\int }_{0}^{\infty }tI\left(t\right)dt}{{\int }_{0}^{\infty }I\left(t\right)dt} $

  (2)

  Figure 5

  

  Figure 5.  Decay curves of Ce³⁺: 5d level in LGS: 1%Ce³⁺/yYb³⁺ (y = 0, 1%, 3%, 5%, 7%, 10%)

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  Where I(t) is the luminescence intensity of Ce³⁺ at time t. The results are shown in Fig. 5. It can be found that the fluorescence lifetime decreases from 3.95 to 1.47 ns with the increase of Yb³⁺ concentration from 0 to 10%, which also confirms the existence of energy transfer from Ce³⁺ to Yb³⁺.

  To judge the energy transfer mechanism from Ce³⁺ to Yb³⁺, the dependence of energy transfer rate W upon Yb³⁺ concentration was analyzed. The calculated energy transfer rate as a function of Yb³⁺ concentration is plotted in a double-logarithmic coordinate in Fig. 6. With the concentration of Yb³⁺ increasing from 1% to 10%, the slope is fitted to be 0.85 (close to 1), which indicates that the energy transfer from Ce³⁺ to Yb³⁺ is dominated by a single photon process rather than a quantum cutting process.

  Figure 6

  

  Figure 6.  Fitting line (log-log) of the energy transfer rate versus the Yb³⁺ concentration in LGS: 1%Ce³⁺/yYb³⁺ (y = 1%, 3%, 5%, 7%, 10%)

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  3.4 Energy transfer efficiency

  Based on the fluorescence lifetimes, the energy transfer efficiency η from Ce³⁺ to Yb³⁺ in LGS can be calculated by^[23]

  $ \eta =1-\frac{\tau }{{\tau }_{0}} $

  (3)

  Where τ and τ₀ are the fluorescence lifetime of Ce³⁺ in the presence and absence of Yb³⁺, respectively. The calculated η values of the samples are exhibited in Table 1. With the increase of Yb³⁺ concentration from 0 to 10%, the energy transfer efficiency increases from 0 to 63%. At the optimal doping concentration (y = 5%), η reaches 51%. Table 2 shows the energy transfer efficiency of some reported materials under the optimal Ce³⁺/Yb³⁺ doping concentration. It can be seen that compared with other Ce³⁺/Yb³⁺ phosphors, the LGS has higher η under low Ce³⁺/Yb³⁺ doping concentrations. The result shows that LGS: 1%Ce³⁺/5%Yb³⁺ may have good application prospect in NIR down-conversion.

  Table 1

  

  Table 1.  Energy Transfer Efficiency (η) of LGS: 1%Ce³⁺/yYb³⁺ (y = 0, 1%, 3%, 5%, 7%, 10%)

  DownLoad: CSV

  Yb3+ concentration (%)	η (%)
  0 1 3 5 7 10	0 19 42 51 56 63

  Table 2

  

  Table 2.  Comparison of Energy Transfer Efficiency (η) of Several Ce³⁺/Yb³⁺ Co-doped Phosphors

  DownLoad: CSV

  Host material	Concentration (1020 ions/cm3)		η (%)	Ref.
  	Ce3+	Yb3+
  CaSc2O4	1.21	6.05	38	14
  YBO3	1.84	5.52	25	22
  Lu3Al5O12	1.42	14.2	51	33
  La3Ga5SiO14	1.02	5.10	51	This work

  3.5 Structure determination of La₃Ga_5-zAl_zSiO₁₄: Ce³⁺/Yb³⁺

  In order to investigate the effect of ion substitution on the luminescence properties of LGS: Ce³⁺/Yb³⁺, a series of La₃Ga_5-zAl_zSiO₁₄ (z = 1, 2, 3, 4, 5): 1%Ce³⁺ and La₃Ga_5-zAl_zSiO₁₄ (z = 1, 2, 3, 4, 5): 1%Ce³⁺/5%Yb³⁺ phosphors were synthesized. The XRD patterns of La₃Ga_5-zAl_zSiO₁₄: 1%Ce³⁺ and La₃Ga_5-zAl_zSiO₁₄: 1%Ce³⁺/5%Yb³⁺ are similar, so only those of La₃Ga_5-zAl_zSiO₁₄: 1%Ce³⁺ are shown in Fig. 7. When z = 1, 2 and 3, all XRD patterns of the samples match well with the standard card of LGS (PDF #41-0155). When z > 3, LaAlO₃ phase and little impurities appear, indicating that the solid solubility limit of Al in La₃Ga_5-zAl_zSiO₁₄ (z = 1, 2, 3, 4, 5) is about 3. Since the phases are not pure at z = 4 and 5, they are not considered in the following research.

  Figure 7

  

  Figure 7.  XRD patterns of La₃Ga_5-zAl_zSiO₁₄: 1%Ce³⁺ (z = 1, 2, 3, 4, 5)

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  3.6 Luminescence properties of La₃Ga_5-zAl_zSiO₁₄: 1%Ce³⁺

  The PL spectra (λ_ex = 345 nm) of La₃Ga_5-zAl_zSiO₁₄ (z = 0, 1, 2, 3): 1%Ce³⁺ are shown in Fig. 8. And the integral emission intensities for various compositions of Al³⁺ are compared in the inset. It can be seen that the emission intensity increases with the Al³⁺ ions in the host. In order to explore the reason for the increase of emission intensity with composition of Al³⁺ in La₃Ga_5-zAl_zSiO₁₄ (z = 0, 1, 2, 3): 1%Ce³⁺, their thermal stabilities were studied. The emission intensities versus temperature from 100 to 500 K for all samples (z = 0, 1, 2, 3) are shown in Fig. 9. The emission intensity is normalized by the one at 100 K. It can be seen that the thermal stability of the phosphors is better with higher composition of Al³⁺ in the host, which indicates that the energy threshold of the thermal nonradiative process increases with the composition of Al^3+[34].

  Figure 8

  

  Figure 8.  PL spectra of La₃Ga_5-zAl_zSiO₁₄: 1%Ce³⁺ (z = 0, 1, 2, 3)

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  Figure 9

  

  Figure 9.  Normalized temperature-dependent integrated emission intensity of La₃Ga_5-zAl_zSiO₁₄ (z = 0, 1, 2, 3): 1%Ce³⁺ between 100 and 500 K (λ_ex = 345 nm)

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  3.7 Thermal quenching mechanism

  In general, there are two possible mechanisms to explain the thermal quenching of 5d₁→4f luminescence in Ce³⁺ doped materials. The first mechanism is that Ce³⁺ ion is thermally excited from its 5d₁ level to the host conduction band^[35-38]. In this case, the relative position of the 5d₁ level and the host conduction band are important. Hence the DRS of undoped La₃Ga_5-zAl_zSiO₁₄ (z = 0, 1, 2, 3) is shown in Fig. 10. In La₃Ga_5-zAl_zSiO₁₄ phosphors, the top of the valance band is formed by the O 2p states and the bottom by the La 5d states, and the upper is built up of the Ga 4p and Si 3p states^[25]. In Fig. 10, the broad absorption band around 280 nm is attributed to the ligand O^2- to La³⁺, and the absorption band around 210 nm to O^2-→Ga³⁺/Si^4+[39]. The band gap E_g can be estimated by^[40]

  $ {\left(\alpha h\nu \right)}^{n}=A(h\nu -Eg) $

  (4)

  Figure 10

  

  Figure 10.  Diffuse reflectance spectra (DRS) of (a) La₃Ga₅SiO₁₄; (b) La₃Ga₄AlSiO₁₄; (c) La₃Ga₃Al₂SiO₁₄; (d) La₃Ga₂Al₃SiO₁₄

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  Where α is the absorption coefficient, hυ the incident photon energy (hυ = hc/λ), and A a proportional constant. The value of n depends on the type of inter-band transition: n = 1/2 for an indirect transition and n = 2 for a direct transition (n = 2 for LGS)^[25]. The absorption coefficient α can be obtained via the Kubelka-Munk function^[41]

  $ \alpha =\frac{{(1-R)}^{2}}{2R} $

  (5)

  Where R is the observed reflectance in DRS. Using the data of DRS to figure out (αhυ)² and hυ, the graph is drawn in the inset of Fig. 10. The straight part of the graph to the horizontal axis was extrapolated, and the intersection point is E_g^[42]. It can be seen that E_g barely changes with the composition of Al³⁺ ions from z = 0 to z = 3 (3.93 eV for La₃Ga₅SiO₁₄, 3.98 eV for La₃Ga₄AlSiO₁₄, 4.0 eV for La₃Ga₃Al₂SiO₁₄ and 4.02 eV for La₃Ga₂Al₃SiO₁₄). Thus, the distance between the 5d₁ level and the host conduction band is almost constant, indicating that thermal ionization is unlikely to be the reason for thermal quenching^[43].

  The second and more probably quenching mechanism is thermal relaxation from 5d₁ level to the 4f ground level through the crossing point^{[35, 44]}, as shown in Fig. 11. ΔE is the energy separation from the bottom of 5d₁ to the crossing point, and ΔR is the shift of the excited state potential energy surface occurring along the R axis upon excitation. The larger ΔR is, the smaller ΔE is, and the more easily the thermal quenching occurs^[37]. And ΔR is associated with the rigidity of crystal structure. Normally, the more rigid the material is, the smaller ΔR is^{[45, 46]}. In La₃Ga_5-zAl_zSiO₁₄ (z = 0, 1, 2, 3): 1%Ce³⁺ phosphors, replacing the larger Ga³⁺ with smaller Al³⁺ ion will increase the rigidity of the structure, thus reducing ΔR and making ΔE larger^[43]. Therefore, the thermal quenching is more difficult to occur with the increase of Al³⁺ ion, and the emission intensity becomes stronger.

  Figure 11

  

  Figure 11.  Simplified configurational coordinate diagram for the thermal relaxation

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  3.8 Phase composition

  After the reason for the increase of emission from the Ce³⁺ doped La₃Ga_5-zAl_zSiO₁₄ (z = 0, 1, 2, 3) was clarified, La₃Ga_5-zAl_zSiO₁₄ (z = 0, 1, 2, 3): 1%Ce³⁺/5%Yb³⁺ phosphors were synthesized. The SEM morphology and EDS elemental analysis results of La₃Ga₂Al₃SiO₁₄: 1%Ce³⁺/5%Yb³⁺ phosphor are shown in Figs. 12 and 13. The SEM morphology in Fig. 12 shows that the sizes of the irregular particles are mainly ranging from 4 to 7 μm. The EDS measurement result in Fig. 13 confirms that the presence of La, Ga, Al, Si, O, Ce and Yb, and there is no other impurity peaks. These results demonstrate that well-crystallized La₃Ga₂Al₃SiO₁₄: 1%Ce³⁺/5%Yb³⁺ powders were obtained.

  Figure 12

  

  Figure 12.  SEM morphology of La₃Ga₂Al₃SiO₁₄: 1%Ce³⁺/5%Yb³⁺ phosphor

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  Figure 13

  

  Figure 13.  EDS data of La₃Ga₂Al₃SiO₁₄: 1%Ce³⁺/5%Yb³⁺ phosphor

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  3.9 NIR PL spectra of La₃Ga_5-zAl_zSiO₁₄: 1%Ce³⁺/5%Yb³⁺

  Fig. 14 shows the NIR emission spectra (λ_ex = 345 nm) of Yb³⁺ in La₃Ga_5-zAl_zSiO₁₄ (z = 0, 1, 2, 3): 1%Ce³⁺/5%Yb³⁺. The inset is the integrated emission intensity. With the increase of Al³⁺ ions, NIR emission is enhanced. The NIR emission intensity of La₃Ga₂Al₃SiO₁₄: 1%Ce³⁺/5%Yb³⁺ is 4.6 times that of La₃Ga₅SiO₁₄: 1%Ce³⁺/5%Yb³⁺. Therefore, replacing Ga³⁺ by Al³⁺ has a positive effect on the LGS: Ce³⁺/Yb³⁺ phosphor applied in solar cells.

  Figure 14

  

  Figure 14.  NIR emission spectra of La₃Ga_5-zAl_zSiO₁₄: 1%Ce³⁺/5%Yb³⁺ (z = 0, 1, 2, 3)

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  4. CONCLUSION

  A series of down-conversion materials LGS: xCe³⁺ (x = 0.5%, 0.8%, 1%, 1.5%, 2%), LGS: 1%Ce³⁺/yYb³⁺ (y = 1%, 3%, 5%, 7%, 10%), La₃Ga_5-zAl_zSiO₁₄ (z = 0, 1, 2, 3): 1%Ce³⁺ and La₃Ga_5-zAl_zSiO₁₄ (z = 0, 1, 2, 3): 1%Ce³⁺/5%Yb³⁺ have been successfully synthesized. The optimal doping concentrations of Ce³⁺ and Yb³⁺ are 1% and 5%, respectively, and the main emission of Yb³⁺ is around 978 nm, which matches well with the spectral response of c-Si solar cells. The photoluminescence properties and two possible energy transfer mechanisms were investigated. Through analyzing the dependence of energy transfer rate on Yb³⁺ concentration, the energy transfer from Ce³⁺ to Yb³⁺ occurs mainly by a single photon process rather than a quantum cutting. And the energy transfer efficiency reaches 51% in LGS: 1%Ce³⁺/5%Yb³⁺. In addition, the NIR emission of LGS: 1%Ce³⁺/5%Yb³⁺ was enhanced by replacing Ga³⁺ by Al³⁺, and the NIR emission intensity of the La₃Ga₂Al₃SiO₁₄: 1%Ce³⁺/5%Yb³⁺ is 4.6 times that of LGS: 1%Ce³⁺/5%Yb³⁺. The enhancement of emission can be explained by the thermal relaxation. These results indicate that La₃Ga_5-zAl_zSiO₁₄ (z = 0, 1, 2, 3): 1%Ce³⁺/5%Yb³⁺ phosphors are promising down-conversion materials for application in solar cells.

  
  
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  Figure 1  (a) XRD patterns of Ce³⁺/Yb³⁺ doped LGS; (b) Main peaks of Ce³⁺/Yb³⁺ doped LGS in the range of 27.5°~32°

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  Figure 2  (a) PL spectra of LGS: xCe³⁺ (x = 0.5%, 0.8%, 1%, 1.5%, 2%); (b) PLE and PL spectra of LGS: 1%Ce³⁺

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  Figure 3  Visible and NIR emission spectra of LGS: 1%Ce³⁺/yYb³⁺ (y = 0, 1%, 3%, 5%, 7%, 10%) under excitation at 345 nm

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  Figure 4  Energy level diagrams of Ce³⁺/Yb³⁺ and the possible energy transfer process

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  Figure 5  Decay curves of Ce³⁺: 5d level in LGS: 1%Ce³⁺/yYb³⁺ (y = 0, 1%, 3%, 5%, 7%, 10%)

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  Figure 6  Fitting line (log-log) of the energy transfer rate versus the Yb³⁺ concentration in LGS: 1%Ce³⁺/yYb³⁺ (y = 1%, 3%, 5%, 7%, 10%)

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  Figure 7  XRD patterns of La₃Ga_5-zAl_zSiO₁₄: 1%Ce³⁺ (z = 1, 2, 3, 4, 5)

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  Figure 8  PL spectra of La₃Ga_5-zAl_zSiO₁₄: 1%Ce³⁺ (z = 0, 1, 2, 3)

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  Figure 9  Normalized temperature-dependent integrated emission intensity of La₃Ga_5-zAl_zSiO₁₄ (z = 0, 1, 2, 3): 1%Ce³⁺ between 100 and 500 K (λ_ex = 345 nm)

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  Figure 10  Diffuse reflectance spectra (DRS) of (a) La₃Ga₅SiO₁₄; (b) La₃Ga₄AlSiO₁₄; (c) La₃Ga₃Al₂SiO₁₄; (d) La₃Ga₂Al₃SiO₁₄

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  Figure 11  Simplified configurational coordinate diagram for the thermal relaxation

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  Figure 12  SEM morphology of La₃Ga₂Al₃SiO₁₄: 1%Ce³⁺/5%Yb³⁺ phosphor

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  Figure 13  EDS data of La₃Ga₂Al₃SiO₁₄: 1%Ce³⁺/5%Yb³⁺ phosphor

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  Figure 14  NIR emission spectra of La₃Ga_5-zAl_zSiO₁₄: 1%Ce³⁺/5%Yb³⁺ (z = 0, 1, 2, 3)

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  Table 1.  Energy Transfer Efficiency (η) of LGS: 1%Ce³⁺/yYb³⁺ (y = 0, 1%, 3%, 5%, 7%, 10%)

  Yb3+ concentration (%)	η (%)
  0 1 3 5 7 10	0 19 42 51 56 63

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  Table 2.  Comparison of Energy Transfer Efficiency (η) of Several Ce³⁺/Yb³⁺ Co-doped Phosphors

  Host material	Concentration (1020 ions/cm3)		η (%)	Ref.
  	Ce3+	Yb3+
  CaSc2O4	1.21	6.05	38	14
  YBO3	1.84	5.52	25	22
  Lu3Al5O12	1.42	14.2	51	33
  La3Ga5SiO14	1.02	5.10	51	This work

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