Enhanced near-infrared emission by co-doping Ce³⁺ in Ba₂Y(BO₃)₂Cl:Tb³⁺, Yb³⁺ phosphor

Abstract

Ce³⁺–Tb³⁺–Yb³⁺ tri-doped Ba₂Y(BO₃)₂Cl phosphors with intense near-infrared (NIR) emission and broad band absorption in the near ultraviolet region were synthesized by a convenient solid-state reaction in a reducing atmosphere, and the structure and the phase purity of all samples were characterized using X-ray diffraction (XRD). The visible and NIR emission spectra together with corresponding decay curves have also been measured to investigate the energy transfer (ET) processes and analysis ET mechanisms and efficiencies in Ce³⁺–Tb³⁺–Yb³⁺ tri-doped system. In addition, steady and dynamic luminescent properties of Tb³⁺/Ce³⁺–Yb³⁺ co-doped Ba₂Y(BO₃)₂Cl phosphors were also measured as a comparison. As an excellent sensitizer, Ce³⁺ ion could significantly enlarge the absorption cross-section and greatly enhance the NIR emission intensity of Yb³⁺ ion. It is believed that Ce³⁺–Tb³⁺–Yb³⁺ tri-doped Ba₂Y(BO₃)₂Cl phosphor is a promising down-conversion (DC) solar spectral convertor to enhance the efficiency of the silicon solar cells.

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1. Introduction

Due to the global energy crisis, much attention has been paid to green and renewable energy. Solar cells are recognized as the most promising devices to convert solar energy to electricity, while silicon solar cells occupy a dominant position in the solar market.^1–3 The major energy loss in crystalline Si (c-Si) solar cells is a spectral mismatch between the incident solar spectrum and the silicon spectral response, which is the main reason for the lower photoelectric conversion efficiency.⁴ The photons with lower energy than the band gap (E_g ≈ 1.12 eV, λ ≈ 1100 nm) of silicon are not absorbed, while the photons with much higher energy than the band gap lose their excess energy through thermalization of hot charge carriers.⁵ A promising method to reduce the thermalization effect is to turn one ultraviolet-visible (UV-Vis) high energy photon into one or more available near infrared (NIR) low energy photons via down-conversion (DC) luminescent materials, including down shift (DS, shift short-wavelength light to long-wavelength light) and quantum cutting (QC, split one incident high energy photon into two near infrared photons) processes. Since efforts are being made to enhance the efficiency of silicon solar cells, the highest efficiency is not limited to 30% (the Shockley–Queisser limit) via spectral modification with DC phosphors.^6,7

Recently, RE³⁺–Yb³⁺ (RE = Tb³⁺, Pr³⁺, Tm³⁺, Ho³⁺ or Er³⁺) ion pairs co-doped QC materials have been extensively investigated, in which Yb³⁺ ion is a suitable emitter and acceptor with its emission around 1000 nm NIR and matches well with the band gap of c-Si due to its simple energy levels of ²F_5/2 and ²F_7/2 (separated by ∼10 000 cm⁻¹).⁸ According to the quantum cutting concept, one blue photon absorbed by RE³⁺ due to the absorption of Tb³⁺: ⁷F₆ → ⁵D₄ around 486 nm, Pr³⁺: ³H₄ → ³P₂ around 445 nm, Ho³⁺: ⁵F₃ → ⁵I₈ around 447 nm and so on could be divided into two NIR photons of Yb³⁺ ion by the ²F_5/2 → ²F_7/2 transition, which may lower the adverse thermalization effect caused by excess energy.⁹ However, these RE³⁺ ions suffer from narrow absorption cross sections (in the order of 10⁻²¹ cm²) and low absorption intensity due to the forbidden 4f–4f transitions. Thus, the NIR emission from Yb³⁺ is so weak that application of RE³⁺–Yb³⁺ (RE = Tb³⁺, Pr³⁺, Tm³⁺, Ho³⁺ or Er³⁺) co-doped QC materials in silicon solar cells is limited.^10–13 Unlike other RE³⁺ ions, Ce³⁺ ions with allowed 4f–5d transitions generally have broad and strong absorption (in the order of 10⁻¹⁸ cm²) in the UV-Vis region.¹⁴ Ce³⁺ ion could be used as an efficient sensitizer and transfer its energy absorbed from the UV-Vis region to Tb³⁺ or Yb³⁺ effectively.¹⁵ In addition, cooperative energy transfer (CET) mechanisms of RE³⁺ → 2Yb³⁺ (e.g. RE = Ce³⁺, Tb³⁺, Pr³⁺, Tm³⁺) were theoretically realizable and had been investigated in detail, which is a possible way of energy transfer (ET) in RE³⁺–Yb³⁺ pair.^16,17 According to our previous result, phosphor Ba₂Y(BO₃)₂Cl:Ce³⁺, Tb³⁺ exhibited both a blue emission from Ce³⁺ and a green emission from Tb³⁺ under near ultraviolet light excitation in the range 320–390 nm, energy transfer from Ce³⁺ to Tb³⁺ was efficient.¹⁸ Therefore, the energy transfer from Ce³⁺ to Tb³⁺–Yb³⁺ ion pairs can occur.

In this paper, we investigated Ce³⁺–Tb³⁺–Yb³⁺ tri-doped Ba₂Y(BO₃)₂Cl phosphor. The visible and NIR emission spectra and corresponding decay curves were measured to prove the occurrence of ET in the present system, and the ET mechanisms were systematically studied and discussed. The results demonstrate that Ce³⁺ ion could efficiently sensitize the Tb³⁺–Yb³⁺ pair and enhance the NIR emission of Yb³⁺ ion greatly.

2. Experimental

All the Ba₂Y(BO₃)₂Cl:Tb³⁺/Ce³⁺, Yb³⁺ and Ba₂Y(BO₃)₂Cl:Ce³⁺, Tb³⁺, Yb³⁺ powder samples were prepared by a conventional solid-state reaction in a reducing atmosphere. Analytical grade BaCO₃, BaCl₂·2H₂O, H₃BO₃ (excess 5 mol% to compensate for the evaporation), and high purity (99.99%) rare earth oxides Y₂O₃, Yb₂O₃, Tb₄O₇, CeO₂ were used as raw materials. The composition for each sample was weighed in proper stoichiometric ratio and mixed thoroughly in an agate mortar. The mixture samples were preheated at 500 °C for 2 h in air. After being cooled to room temperature, the sample was thoroughly reground and heated for 4 h in a CO atmosphere at 900 °C to ensure the complete reduction of Ce³⁺ and Tb³⁺. It was then cooled to room temperature and crushed into a fine powder.

The X-ray diffraction (XRD) patterns were performed on a Rigaku-Dmax 3C powder diffractometer (Rigaku Corp, Tokyo, Japan) with Cu-Kα radiation (λ = 1.54065 Å) to identify the structure and phase purity. The photoluminescence emission (PL) and excitation (PLE) spectra and decay curves were recorded using an Edinburgh FLS920 spectrofluorometer with a 450 W Xe lamp as excitation source. Decay curve measurements of Ce³⁺ ion were obtained using a 375 nm nanosecond flash-lamp (nF900) as the excitation source. Photoluminescent spectra of all samples were tested three times to reduce the error and all of the measurements were performed at room temperature.

3. Results and discussions

3.1 Phase and structure

Fig. 1 shows the X-ray diffraction (XRD) patterns of Tb³⁺ solely doped, Tb³⁺–Yb³⁺ co-doped and Ce³⁺–Tb³⁺–Yb³⁺ tri-doped Ba₂Y(BO₃)₂Cl samples, respectively. The diffraction peaks of all samples agree well with those of the standard profile (JCPDS no. 79-0967) Ba₂Yb(BO₃)₂Cl and not any diffraction peaks from impurity. It is noted that all samples show similar XRD patterns in our experiments, which indicate that the obtained samples are single and pure phases, and RE³⁺ (Ce³⁺/Tb³⁺/Yb³⁺) ions completely enter the lattices even at high concentrations. Here, most of the rare earth ion dopants would prefer to occupy a Y³⁺ site in the Ba₂Y(BO₃)₂Cl compound according to the close effective ionic radius and valence of the cations. Compound Ba₂Y(BO₃)₂Cl shares the same structure with Ba₂Yb(BO₃)₂Cl, which crystallizes in a monoclinic structure with the space group P2₁/m, having the lattice parameters of a = 6.397 Å, b = 5.279 Å, c = 11.222 Å.¹⁹

Fig. 1 XRD patterns of phosphors Ba₂Y(BO₃)₂Cl:Tb³⁺, Ba₂Y(BO₃)₂Cl:Tb³⁺, Yb³⁺ and Ba₂Y(BO₃)₂Cl:Ce³⁺, Tb³⁺, Yb³⁺ along as well with the standard profile for Ba₂Yb(BO₃)₂Cl (JCPDS card no. 79-0967).

3.2 Analysis of photoluminescence spectra and energy transfer processes

3.2.1 Luminescence and ET in Tb³⁺–Yb³⁺ or Ce³⁺–Yb³⁺ codoped Ba₂Y(BO₃)₂Cl. Photoluminescence excitation (PLE) and emission (PL) spectra of the Ba₂Y(BO₃)₂Cl:0.03Tb³⁺, 0.20Yb³⁺ and Ba₂Y(BO₃)₂Cl:0.03Ce³⁺, 0.20Yb³⁺ samples are presented in Fig. 2a and b, respectively. Monitoring the characteristic green emission of Tb³⁺ at 542 nm, PLE spectrum shown in Fig. 2a consists of a weak band around 280 nm and several weak peaks in the 300–500 nm due to the lowest spin-forbidden 4f → 5d transitions and the forbidden 4f → 4f transitions of Tb³⁺ ion.²⁰ The amplifying excitation peak centered at 486 nm is assigned to the Tb³⁺: ⁷F₆ → ⁵D₄ transition, which is usually used as excitation peak in Tb³⁺–Yb³⁺ co-doped quantum cutting systems.²¹ The excitation spectrum profile monitoring at 972 nm from the ²F_5/2 → ²F_7/2 transition of Yb³⁺ is similar to that of monitoring at 542 nm, which indicates the occurrence of ET from Tb³⁺ to Yb³⁺, and the trivial difference might be from different detectors of the Vis and NIR spectrum measurements. Herein, both the emission and inset excitation intensities of Tb³⁺ ion are magnified 40 times for the sake of clarify. Under blue excitation at 486 nm, emissions peaking at 542, 583, 620 nm are observed, which can be assigned to the forbidden ⁵D₄ → ⁷F_J (J = 5, 4, 3) transitions of Tb³⁺.²² Meanwhile, the characteristic NIR emission band located at 900–1150 nm is also observed, due to the ²F_5/2 → ²F_7/2 transition of Yb³⁺. The NIR emission spectra separately excited by 355 nm and 486 nm are shown in the insert of Fig. 2a, and the former integrated intensity is 6 times that of the latter. With the increase of Yb³⁺concentration, it is clearly found that the emission intensities of Tb³⁺ rapidly decrease while the Yb³⁺ NIR emission intensities grow simultaneously, as shown in Fig. 2c, which further confirms the occurrence of ET from Tb³⁺ to Yb³⁺.

Fig. 2 (a) PLE spectra (λ_em = 972 and 542 nm) and Vis-NIR PL spectrum (λ_ex = 486 nm) of sample Ba₂Y(BO₃)₂Cl:0.03Tb³⁺, 0.20Yb³⁺; (b) PLE spectra (λ_em = 972 and 417 nm) and Vis-NIR PL spectrum (λ_ex = 355 nm) of sample Ba₂Y(BO₃)₂Cl:0.03Ce³⁺, 0.20Yb³⁺ and dependence of normalized integrated PL intensity in Vis and NIR region on Yb³⁺ concentration x in Ba₂Y(BO₃)₂Cl:0.03Tb³⁺, xYb³⁺ (c) and Ba₂Y(BO₃)₂Cl:0.03Ce³⁺, xYb³⁺ (d).

Fig. 2b exhibits the PLE and PL spectra of Ba₂Y(BO₃)₂Cl:0.03Ce³⁺, 0.20Yb³⁺ phosphor. Upon 355 nm excitation, the phosphor generates the blue emission from Ce³⁺ and NIR emission from Yb³⁺. For the PLE spectra monitoring both the 5d → 4f transition of Ce³⁺ at 417 nm and the ²F_5/2 → ²F_7/2 transition of Yb³⁺ at 972 nm, which are similar except for the intensity concluding an intense excitation band of Ce³⁺: 4f → 5d centered at 355 nm, and this indicates the energy transfer from Ce³⁺ to Yb³⁺.²³ It is observed that the emission of Ce³⁺ decreases and the emission of Yb³⁺ increases with increasing the contents of Yb³⁺, whereas the emission intensity of Yb³⁺ reaches the maximum at x = 0.2 due to concentration quenching effect (as shown in Fig. 2d). Regular changes in the emission intensities from Ce³⁺ or Yb³⁺ also provide powerful evidence for an ET process from Ce³⁺ to Yb³⁺.

In order to further prove for the occurrence of the energy transfer from Tb³⁺/Ce³⁺ to Yb³⁺ and gain understanding of the ET process, the decay curves of the Tb³⁺: ⁵D₄ → ⁷F₅ emission at 542 nm in Ba₂Y(BO₃)₂Cl:0.03Tb³⁺, xYb³⁺ and the Ce³⁺: 5d → 4f emission at 417 nm in Ba₂Y(BO₃)₂Cl:0.03Ce³⁺, xYb³⁺ were recorded in Fig. 3a and b. The Tb³⁺ 542 nm emission without Yb³⁺ shows a nearly single exponential decay, whereas the Ce³⁺ 417 nm emission without Yb³⁺ deviates from single exponential decay because of the energy transfer between possible distribution of Ce³⁺ in different cation sites (two Ba²⁺ cites and one Y³⁺ site).¹⁴ For a sample without any Yb³⁺, the decay time of 3.46 ms and 20.31 ns are obtained for Tb³⁺ and Ce³⁺ by fitting. With the introduction of Yb³⁺ and increasing its concentration, their decay curves became nonexponential and the lifetime of the sensitizer Tb³⁺ or Ce³⁺ decreases. The average lifetime (τ) of sensitizer Tb³⁺/Ce³⁺ in Tb³⁺/Ce³⁺–Yb³⁺ co-doped Ba₂Y(BO₃)₂Cl samples is given by:²⁴

where I_(t) is the luminescence intensity at time t. The decay life time of Tb³⁺/Ce³⁺ drops gradually with Yb³⁺ co-doping, and their lifetimes are about 3.40, 3.33, 3.32, 3.08, 2.73 ms and 10.44, 4.90, 2.83, 2.38, 1.38 ns for Tb³⁺ and Ce³⁺ in 0.03Tb³⁺–xYb³⁺ or 0.03Ce³⁺–xYb³⁺ co-doped Ba₂Y(BO₃)₂Cl, respectively.

Fig. 3 Decay lifetime of (a) Tb³⁺: ⁵D₄ → ⁷F₅ (542 nm) luminescence in Ba₂Y(BO₃)₂Cl:0.03Tb³⁺, xYb³⁺ under excitation of 486 nm and (b) Ce³⁺: 5d → 4f (417 nm) luminescence of Ba₂Y(BO₃)₂Cl:0.03Ce³⁺, xYb³⁺ under excitation of 375 nm; lifetime and energy transfer efficiency as function of Yb³⁺ concentrations in Tb³⁺–xYb³⁺ (c) and Ce³⁺–xYb³⁺ (d) co-doped Ba₂Y(BO₃)₂Cl.

Fig. 3c and d present the emission average lifetime and energy transfer efficiency of Tb³⁺/Ce³⁺–Yb³⁺ co-doped Ba₂Y(BO₃)₂Cl samples as function of Yb³⁺ concentrations. According to the decay times, the energy transfer efficiency (ETE, η_ETE) can be calculated by using the following equations:²⁵

η_ETE = 1 − τ_xYb/τ_0Yb (2)

where τ_xYb and τ_0Yb denote lifetime of the sensitizer with and without Yb³⁺ content. Accordingly, the ET efficiency (η_ETE) are 2.0, 3.8, 4.0, 11.0, 21.1% for Ba₂Y(BO₃)₂Cl:0.03Tb³⁺, xYb³⁺ and 48.6, 75.9, 86.1, 88.3 and 93.2% for Ba₂Y(BO₃)₂Cl:0.03Ce³⁺, xYb³⁺ with an increase in content of Yb³⁺ from 0.01 to 0.3, respectively. The η_ETE value rises evenly with increasing Yb³⁺ doping content, but the ET efficiency is much lower in Ba₂Y(BO₃)₂Cl:0.03Tb³⁺, xYb³⁺ compared to that of Ba₂Y(BO₃)₂Cl:0.03Ce³⁺, xYb³⁺, which may be due to the lower and inefficient forbidden 4f → 4f absorption cross-sections of Tb³⁺ in UV-visible region compared to that of the allowed 4f → 5d of Ce³⁺. It could be expected that the NIR emission intensity is stronger in Ce³⁺–Yb³⁺ co-doped Ba₂Y(BO₃)₂Cl compared with that of Tb³⁺–Yb³⁺ co-doped Ba₂Y(BO₃)₂Cl, which will be discussed in the following paragraph. In addition, it is found that the energy transfer efficiency of Ce³⁺ → Yb³⁺ is much higher than that of Ce³⁺ → Tb³⁺ in comparison with that of our previous results,¹⁸ which indicates that the energy absorbed by Ce³⁺ is more easily transfer to Yb³⁺.

3.2.2 Luminescence in Ce³⁺–Tb³⁺–Yb³⁺ tri-doped Ba₂Y(BO₃)₂Cl phosphor. According to the above-mentioned and previous results, the energy transfer Ce³⁺ → Tb³⁺ and Tb³⁺ → Yb³⁺ have occurred in the Ba₂Y(BO₃)₂Cl host, and it is possible for the energy transfer from Ce³⁺ to Tb³⁺ → Yb³⁺ ion pairs to take place. Fig. 4 displays the PLE and PL spectra of Ce³⁺–Tb³⁺–xYb³⁺ tri-doped Ba₂Y(BO₃)₂Cl phosphor. As the representative PLE spectra (shown in Fig. 4a), they include a broad excitation band at 320–390 nm with a maximum at 355 nm corresponding to the Ce³⁺: 4f → 5d transition. It is found that the profiles of PLE spectra obtained by monitoring at 542 nm (Tb³⁺: ⁵D₄ → ⁷F₅ emission), 417 nm (Ce³⁺: 5d → 4f emission) and 972 nm (Yb³⁺: ²F_5/2 → ²F₇₂ emission) are similar to that of Ce³⁺ solely doped Ba₂Y(BO₃)₂Cl except relative intensity, which illuminates that the possible appearance of energy transfer from Ce³⁺ to Tb³⁺/Yb³⁺/Tb³⁺–Yb³⁺. Under excitation of 355 nm light, the PL spectra of Ba₂Y(BO₃)₂Cl:0.03Ce³⁺, 0.03Tb³⁺, xYb³⁺ in visible and near infrared region as function of Yb³⁺ content are shown in Fig. 4b. The PL spectra are composed of a typical broad unsymmetrical doublet emission bands from the parity allowed transitions from the 5d excited state to the ²F_7/2 and ²F_5/2 ground states of Ce³⁺ (ref. 26) and a series of sharp lines emission in 480–680 nm region from the ⁵D₄ → ⁷F_J (J = 6, 5, 4 and 3) transitions of Tb³⁺ in visible region, as well as the NIR emission band centered at 972 nm accompanied by several weak shoulders in 900–1150 nm region due to transitions among the different stark levels of ²F_J (J = 5/2, 7/2) of Yb³⁺. Seen from the PL spectra, it is observed that the intensity of Tb³⁺ emission (integrated from 480 to 680 nm) and Ce³⁺ emission (integrated from 370 to 480 nm) decreases monotonously with the increase of Yb³⁺ concentration from 0 to 0.3, while the NIR emission intensity of Yb³⁺ (integrated from 900 to 1150 nm) firstly increases with Yb³⁺ content growth and then begins to decrease after reaching a maximum at x = 0.20 due to concentration quenching effect. In order to clearly visualise the variation, the inset of Fig. 4b displays the variable curves of normalized PL intensity from Ce³⁺, Tb³⁺ and Yb³⁺ on the Yb³⁺ concentration x, respectively.

Fig. 4 (a) PLE and (b) PL spectra of Ba₂Y(BO₃)₂Cl:0.03Ce³⁺, 0.03Tb³⁺, xYb³⁺ (x = 0, 0.01, 0.05, 0.10, 0.20, 0.30) samples as functions of NIR emission center Yb³⁺ concentration x. Inset of (b) shows the dependence of Vis and NIR normalized integrated emission intensity on Yb³⁺ concentration x.

The energy transfer process is complicated in Ce³⁺–Tb³⁺–Yb³⁺ tri-doped Ba₂Y(BO₃)₂Cl sample. To further understand the energy transfer in this system, the decay curves of the sensitizer Ce³⁺ and Tb³⁺ in sample Ba₂Y(BO₃)₂Cl:0.03Ce³⁺, 0.03Tb³⁺, xYb³⁺ (x = 0–0.3) are plotted against the concentration of Yb³⁺, and are shown in Fig. 5. With increasing Yb³⁺ concentration, the decay of Ce³⁺ and Tb³⁺(⁵D₄) in tri-doped Ba₂Y(BO₃)₂Cl:0.03Ce³⁺, 0.03Tb³⁺, xYb³⁺ sample speeds up (as shown in Fig. 5a and b), similar to those in co-doped Ba₂Y(BO₃)₂Cl:0.03Ce³⁺, xYb³⁺ and Ba₂Y(BO₃)₂Cl:0.03Tb³⁺, xYb³⁺. The effective decay times are calculated to be about 12.82, 9.63, 2.89, 2.60, 1.77, 1.28 ns for Ce³⁺ and 3.56, 3.26, 3.18, 3.06, 2.77, 2.48 ms for Tb³⁺, therefore the η_ETE values are 36.8, 52.6, 85.8, 87.2, 91.2, 93.7% (η_ETE-Ce) and 0, 8.6, 10.7, 14.0, 22.2, 30.3% (η_ETE-Tb) according to the decline of Ce³⁺ and Tb³⁺ lifetime, respectively. The corresponding lifetimes and energy transfer efficiency variable curves as function of Yb³⁺ contents are also shown clearly in Fig. 5c and d. The η_ETE-Ce values calculated on the basis of the decrease of Ce³⁺ lifetime do not start from 0 whereas the η_ETE-Tb values do start from 0, because η_ETE-Ce value is the sum of ET efficiency of Ce³⁺ → Tb³⁺ and Ce³⁺ → Yb³⁺, but η_ETE-Tb is the only ET efficiency value of Tb³⁺ → Yb³⁺. Comparison with the co-doped system, it is found that the ET efficiency was higher in a tri-doped sample than those of the corresponding co-doped samples. Thus we deduced that the NIR emission intensity in Ba₂Y(BO₃)₂Cl:0.03Ce³⁺, 0.03Tb³⁺, xYb³⁺ should be stronger than that of corresponding Ba₂Y(BO₃)₂Cl:0.03Ce³⁺, xYb³⁺ or Ba₂Y(BO₃)₂Cl:0.03Tb³⁺, xYb³⁺, which is strongly related to the energy transfer processes.

Fig. 5 Decay curves of (a) Ce³⁺: 5d → 4f (417 nm) emission in Ba₂Y(BO₃)₂Cl:0.03Ce³⁺, 0.03Tb³⁺, xYb³⁺ (x = 0, 0.01, 0.05, 0.10, 0.20, 0.30) under excitation of 375 nm and (b) Tb³⁺: ⁵D₄ → ⁷F₅ (542 nm) emission under excitation of 355 nm; the variation trend of lifetime (Ce³⁺ and Tb³⁺) and ET efficiency of (Ce³⁺ → Tb³⁺/Yb³⁺ and Tb³⁺ → Yb³⁺) are shown in (c) and (d).

3.2.3 Energy transfer processes in Ce³⁺–Tb³⁺–Yb³⁺ tri-doped Ba₂Y(BO₃)₂Cl. Based on the above-mentioned results, the possible ET processes were analyzed in detail by schematic energy level diagram with feasible transitions, which are shown in Fig. 6. In the present Ce³⁺–Tb³⁺–Yb³⁺ tri-doped system, the possible energy transfer processes include Ce³⁺ → Yb³⁺, Ce³⁺ → Tb³⁺, Tb³⁺ → Yb³⁺ and Ce³⁺ → Tb³⁺ → Yb³⁺ four procedures. For the first energy transfer process ① Ce³⁺ → Yb³⁺, it is usually considered that the cooperative down-conversion (or quantum cutting, QC) probably dominates the process because the energy of Ce³⁺: 5d → 4f transition is more than twice as high than the energy difference between the ²F_7/2 and ²F_5/2 levels of Yb³⁺.²⁷ Up to now, no direct experimental proof could support the existence or inexistence of QC, whereas many measured Yb³⁺ quantum yield (QY) in the samples with so-called “QC” are far below 100%.^28,29 The authors considered the ET Ce³⁺ to Yb³⁺ may include both Ce³⁺ → 2Yb³⁺ (1a) and Ce³⁺ → Yb³⁺ (1b) two processes: the former is a CET process and possible existence in theory, and the latter is possibly through a slow non-radiative relaxation from an intermediate Ce⁴⁺–Yb²⁺ charge transfer state companied with energy loss instead of directly via Ce³⁺: 5d → 2Yb³⁺: ²F_5/2.^29–31 The energy transfer processes ② Ce³⁺ → Tb³⁺ have been investigated in many hosts,^32,33 in which part of excited energy of Ce³⁺ could transfer to Tb³⁺ due to the Ce³⁺ excited 5d state energy level is close to that of the ⁵D₃ and ⁵D₄ of Tb³⁺, resonant energy transfer could take place and promote the electrons of Tb³⁺ from ⁷F₆ ground state to ⁵D₃ and other excited levels. Then non-radiative relaxation occurs from the high energy levels ⁵D₃ to the ⁵D₄ levels, resulting in several sharp line emissions from ⁵D₄ → ⁷F_J (J = 6, 5, 4 and 3) transitions of Tb³⁺ and peaking 486, 542, 583 and 620 nm, respectively (2).³⁴ For the energy transfer ③ Tb³⁺ → Yb³⁺, the energy of Tb³⁺(⁵D₄) level is approximately two times as high as the energy difference between the ²F_7/2 and ²F_5/2 levels of Yb³⁺, which probably makes CET or QC process Tb³⁺(⁵D₄) → 2Yb³⁺(²F_5/2) occur, the energy of the excited Tb³⁺ ion is transferred to two different Yb³⁺ ions (3a). A Tb³⁺–Yb³⁺ cross-relaxation mechanism is also possible to excite one Yb³⁺ with a subsequent energy loss through multi-phonon decay process in Tb³⁺ ion (3b).³⁵ On the basis of the above-mentioned three ET processes, the fourth energy transfer ④ Ce³⁺ → Tb³⁺ → Yb³⁺ is easy to understand. Upon excitation by 355 nm n-UV light, the Ce³⁺ ion in the ground state ²F_5/2 is excited to the higher excited state 5d level, part of the excited state electrons relax to the ground state ²F_5/2 and ²F_7/2 of Ce³⁺ in the form of broad band blue emissions, part of the excited state electrons energy transfer to Tb³⁺, then the part of the accepted energy of Tb³⁺ transfer to Yb³⁺ exhibiting the NIR emission of Yb³⁺: ²F_5/2 → ²F_7/2. Here, Tb³⁺ ions play the role of bridge to form a new energy transfer pathway. Yb³⁺ NIR emission could involve four energy transfer processes in Ce³⁺–Tb³⁺–Yb³⁺ tri-doped Ba₂Y(BO₃)₂Cl.

Fig. 6 Schematic energy level diagram and the possible ET progresses in Ce³⁺–Tb³⁺–Yb³⁺ tri-doped Ba₂Y(BO₃)₂Cl phosphor.

3.2.4 Comparison of NIR emission in the co-doped and tri-doped samples. The aim of the present paper is to enhance the NIR emission from Yb³⁺ transitions through the Ce³⁺–Tb³⁺–Yb³⁺ ion pairs tri-doped Ba₂Y(BO₃)₂Cl sample. In order to compare NIR emission intensity in a co-doped system with that of a tri-doped system, Fig. 7 shows the PL spectra in NIR region of 0.03Tb³⁺, 0.20Yb³⁺ and 0.03Ce³⁺, 0.20Yb³⁺ co-doped Ba₂Y(BO₃)₂Cl, as well as Ba₂Y(BO₃)₂Cl:0.03Ce³⁺, 0.03Tb³⁺, 0.20Yb³⁺ sample with optimal composition. It is observed that the NIR emission intensity of samples doped with Tb³⁺ under the excitation of 486 nm (Tb³⁺: ⁷F₆ → ⁵D₄) is lower than that of samples excited with 355 nm, which due to the weak absorption and narrow absorption cross-section of Tb³⁺ at 486 nm. Under excitation of 355 nm, the NIR emission intensity increases with the order I_Tb–Yb < I_Ce–Yb < I_Ce–Tb–Yb, which is consistent with the results of the energy transfer efficiency. Meanwhile, the NIR emission integrated intensity of Yb³⁺ ion in tri-doped Ce³⁺–Tb³⁺–Yb³⁺ system is 1.4 times of the co-doped Ce³⁺–Yb³⁺ sample and 23 times as much as that of the co-doped Tb³⁺–Yb³⁺ sample, which confirms the highly efficient excitation and subsequent Ce³⁺ → Yb³⁺, Tb³⁺ → Yb³⁺ and Ce³⁺ → Tb³⁺ → Yb³⁺ possible ET processes in the tri-doped Ce³⁺–Tb³⁺–Yb³⁺ material. Results reveal that the introduction of Ce³⁺ as a sensitizer with a large absorption cross-section could efficiently turn the adverse high energy photons in the near ultraviolet region into favorable low energy NIR photons effectively absorbed by silicon solar cells. The Ce³⁺–Tb³⁺–Yb³⁺ tri-doped sample could be a potential DC solar spectral convertor by reducing the energy loss to improve the photoelectric conversion efficiency of silicon solar cells.

Fig. 7 The NIR emission spectra of (a) Ba₂Y(BO₃)₂Cl:0.03Tb³⁺, 0.20Yb³⁺; (b) Ba₂Y(BO₃)₂Cl:0.03Ce³⁺, 0.20Yb³⁺and (c) Ba₂Y(BO₃)₂Cl:0.03Ce³⁺, 0.03Tb³⁺, 0.20Yb³⁺ phosphors.

4. Conclusions

In summary, steady and dynamic photoluminescent properties and the energy transfer mechanism in the Ce³⁺–Tb³⁺–xYb³⁺ tri-doped Ba₂Y(BO₃)₂Cl phosphor were systematically investigated, and four possible ET processes Ce³⁺ → Tb³⁺, Ce³⁺ → Yb³⁺, Tb³⁺ → Yb³⁺ and Ce³⁺ → Tb³⁺ → Yb³⁺ were proposed. As a comparison, the Ce³⁺/Tb³⁺–xYb³⁺ co-doped Ba₂Y(BO₃)₂Cl were also studied. It is found that the energy transfer efficiency of Ce³⁺ → xYb³⁺ and Tb³⁺ → xYb³⁺ in the tri-doped system is larger than that of the double-doped system. The NIR emission intensity of Yb³⁺ in a tri-doped sample under excitation of 355 nm is 1.4 times of Ce³⁺–Yb³⁺ co-doped sample and is 23 times of Tb³⁺–Yb³⁺ co-doped phosphors with optimal composition. Moreover, Ce³⁺ ion significantly enlarges the absorption cross-section in the near ultraviolet region (320–390 nm) due to the allowed 4f → 5d transition and greatly enhances the NIR emission of Yb³⁺ ion where the c-Si solar cell exhibits the greatest spectral response. Results indicate that Ba₂Y(BO₃)₂Cl:Ce³⁺, Tb³⁺, Yb³⁺ phosphor is a promising spectral convertor for enhancing photoelectric conversion efficiency of silicon solar cells.

Acknowledgements

This work was supported by the high-level talent project of Northwest University, National Natural Science Foundation of China (no. 11274251), Ph.D. Programs Foundation of Ministry of Education of China (20136101110017), Technology Foundation for Selected Overseas Chinese Scholar, Ministry of Personnel of China (excellent), Natural Science Foundation of Shaanxi Province (no. 2014JM1004) and Foundation of Key Laboratory of Photoelectric Technology in Shaanxi Province (12JS094).

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