1. Introduction

The increasing demand for solar energy, due to its green and inexhaustible advantage, has put how to improve the photovoltaic conversion efficiency of solar cells at the forefront of research [1]. The mismatch between the solar spectrum and the band gap energy of silicon semiconductor limits the photovoltaic conversion efficiency of silicon-based solar cells, because photons with energy lower than the band gap cannot be absorbed, while for photons with energy larger than the band gap, the excess energy is lost by thermalization of hot charge carriers [2–4]. Herein, there are many routes to improve the conversion efficiency, and one of them is the downconversion (DC) [5–10]. The DC process can convert ultraviolet-visible (UV-Vis) photon (300-600nm) into near-infrared (NIR) photon (~1000nm), which can be efficiently absorbed by silicon-based solar cells [1].

RE³⁺-Yb³⁺ (RE = Tb, Ho, and Pr) couple have been demonstrated with optical spectroscopy for NIR DC in various hosts [2, 6, 9]. However, these DC materials are still far from practical application, because the absorption of the sensitizer RE³⁺ ion arisen from the parity-forbidden 4ƒ-4ƒ transitions are naturally weak in intensity, narrow in bandwidth, and usually give emission in UV-Vis region [11]. In this article, we report an efficient NIR DC luminescence between Tb⁴⁺-Yb³⁺ couple, which is observed for the first time to our knowledge. Tb⁴⁺ ion might be an ideal broadband sensitizer for Yb³⁺ ion due to its charge transfer (CT) state located at 300nm-600nm [12, 13]. This broad CT state covers the high-energy part of the solar spectrum and matches twice the energy of Yb³⁺ ion. Moreover, the Tb⁴⁺ ion has the same electron configuration as Gd³⁺ ion (4ƒ⁷). Thus, its excited 4ƒ levels lie above the CT state, which result in that it absorbs high-energy photon but doesn’t give any emission in UV-Vis region in any host materials [14]. Therefore, the sensitizer Tb⁴⁺ ion could efficiently transfer the absorbed energy to activator Yb³⁺ ion without any emission in UV-Vis region, which will provide a better NIR DC system for silicon-based solar cells to improve the photovoltaic conversion efficiency.

2. Experimental

We chose the inorganic fluoride hexagonal NaYF₄ nanoparticles (NPs) as DC host due to its low phonon frequencies and high chemical stability [15]. Hexagonal NaYF₄:15%Tb³⁺, 10%Yb³⁺ NPs were synthesized through coprecipitation method as follows [16]: 0.4550g YCl₃·6H₂O (99.99%), 0.0776g YbCl₃·6H₂O (99.99%) and 0.1120g TbCl₃·6H₂O (99.99%) were mixed with 12ml oleic acid (OA, 90%) and 30ml 1-octadecene (ODE, 90%) in a 100ml flask and heated to 130°C to form a homogeneous solution, and then cooled down to room temperature. 20ml methanol (A.R.) solution containing 0.2g NaOH (A.R.) and 0.2963g NH₄F (A.R.) was slowly added into the flask and the mixture were stirred for 30min to ensure that all fluoride has consumed completely. Subsequently, the mixture was slowly heated to 130°C to evaporate methanol, then heated up to 300°C rapidly and maintained for 1h under argon atmosphere. After the solution was cooled down naturally, NaYF₄ NPs were precipitated from the solution with ethanol, washed with ethanol for three times, collected by centrifugation and baked in 60°C. Finally, the as-synthesized NaYF₄ NPs were sintered at 380°C under air atmosphere, yielding the final Tb⁴⁺-Yb³⁺ co-doped NaYF₄ NPs. Sintering at 380°C could oxidize the Tb³⁺ ion to Tb⁴⁺ ion, and avoid the NaYF₄ lattice structure be destroyed [17]. The chemical reagents (YCl₃·6H₂O, YbCl₃·6H₂O, TbCl₃·6H₂O, OA, and ODE) were purchased from Sigma-Adrich. Methanol, NaOH and NH₄F were supplied by Sinopharm Chemical Reagent Company (Shanghai). All chemicals were used directly without further purification.

As-prepared samples were characterized by X-ray diffraction (XRD, MiniFlexII, Rigaku), X-ray photoelectron spectroscopy (XPS, ESCALAB 250, Thermo Scientific), fluorescence spectra (Fluorolog 3-22 spectrofluorometer, Horiba Jobin Yvon), scanning electron microscope (SEM, SU8010, Hitachi). Specifically, the fluorescence spectra in the same figure were measured by the same spectrofluorometer in one experiment, with the same measuring conditions (temperature, slit width, placement of samples, optical path, etc). Thus, the intensity of these spectra in one figure is comparable.

3. Results and discussion

Figure 1(a) illustrates the XRD patterns of the NaYF₄: x%Tb,10%Yb (x = 5,10,15,20,30) NPs after sintering at 380°C for 2h. All of the experimental diffraction peaks match well with those of hexagonal NaYF₄ phase (JCPDS 16-0334), which indicates that the NaYF₄ lattice structure are not destroyed after 380°C sintering. In order to confirm that Tb³⁺ ions are oxidized to Tb⁴⁺ ions after sintering, the Tb 4d XPS spectrum of NaYF₄:15%Tb,10%Yb NPs after sintering is measured. As shown in Fig. 1(b), the photoelectron line at ~147 eV belongs to Tb³⁺ ions while the photoelectron line at ~150 eV belongs to Tb⁴⁺ ions [14, 18], which indicates part of Tb³⁺ ions are oxidized to Tb⁴⁺ ions during the sintering process. In addition, according to the inset of Fig. 1(b), the NaYF₄ NPs have an average diameter of ~20nm and a large specific surface, which will promote the oxidation of Tb³⁺ ions to Tb⁴⁺ ions. To distinguish from as-synthesized NaYF₄:Tb³⁺,Yb³⁺ NPs, we use NaYF₄:Tb⁴⁺,Yb³⁺ NPs to represent the NaYF₄ NPs after 380°C sintering.

 

Fig. 1 (a) XRD patterns of the NaYF₄: x%Tb,10%Yb (x = 5,10,15,20,30) NPs after sintering. (b) Tb 4d photoelectron spectrum of NaYF₄:15%Tb,10%Yb NPs after sintering. Inset: SEM image of NaYF₄:15%Tb,10%Yb NPs before sintering.

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Figure 2 shows the visible emission spectra (λ_ex = 379nm) and corresponding excitation spectra (λ_em = 544nm) of NaYF₄:Tb³⁺,Yb³⁺ NPs and NaYF₄:Tb⁴⁺,Yb³⁺ NPs. We can observe the intensity of major excitation peaks at 350, 368, 379, and 487nm (Tb³⁺:⁷F₆→⁵D₂, ⁵L₁₀, ⁵D₃, ⁵D₄) and the major emission peaks at 490, 544, 585 and 620nm (Tb³⁺:⁵D₄→⁷F_j (j = 6, 5, 4, 3)) in NaYF₄:Tb⁴⁺,Yb³⁺ NPs decrease dramatically [7], which further indicates part of Tb³⁺ ions are oxidized to Tb⁴⁺ ions after sintering at 380°C. In addition, the excitation/emission peaks of Tb⁴⁺ ions aren’t observed in NaYF₄:Tb⁴⁺,Yb³⁺ NPs, which is in agreement with that Tb⁴⁺ ion absorbs high-energy photon but doesn’t give any emission in UV-Vis region [14].

 

Fig. 2 (a) UV-Vis excitation spectra (λ_em = 544nm) and (b) emission spectra (λ_ex = 379nm) of NaYF₄: Tb³⁺, Yb³⁺ NPs (dash lines) and NaYF₄: Tb⁴⁺, Yb³⁺ NPs (solid lines).

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In order to research the influence of Tb⁴⁺ ion on the NIR DC luminescence in NaYF₄ NPs, the NIR emission spectra of NaYF₄:Tb³⁺,Yb³⁺ NPs and NaYF₄:Tb⁴⁺,Yb³⁺ NPs under 379nm and 487nm excitation are measured. As shown in Fig. 3(a) and Fig. 3(b), it can be clearly observed that the major emission peak is located at 977nm, attributed to the ²F_5/2→²F_7/2 transitions of Yb³⁺ ion. Contrast with NaYF₄:Tb³⁺,Yb³⁺ NPs, the NIR emission intensity of Yb³⁺ ion in NaYF₄:Tb⁴⁺,Yb³⁺ NPs enhances obviously. The enhancement factor is about 14 and 19 excited at 379nm and 487nm, respectively.

 

Fig. 3 (a),(b) NIR emission spectra under 379nm and 487nm excitation; (c) UV-Vis excitation spectra monitoring at 977nm of NaYF₄:Tb³⁺,Yb³⁺ NPs (dash lines) and NaYF₄:Tb⁴⁺, Yb³⁺ NPs (solid lines), respectively.

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Figure 3(c) illustrates the excitation spectra of NaYF₄:Tb³⁺,Yb³⁺ NPs and NaYF₄:Tb⁴⁺,Yb³⁺ NPs monitored at 977nm. Compared with the NaYF₄:Tb³⁺,Yb³⁺ NPs, the weak excitation peaks of Tb³⁺ ion still can be observed in the excitation spectrum of NaYF₄:Tb⁴⁺,Yb³⁺ NPs, indicating part of Tb³⁺ ions remains after sintering at 380°C. Moreover, a strong broad excitation band from 300nm to 600nm can be observed in the excitation spectrum of NaYF₄:Tb⁴⁺,Yb³⁺ NPs, which may arise from (i) Tb⁴⁺ ions; (ii) Tb³⁺ ions; (iii) oxygen defects; (iv) Yb²⁺ ions; (v) Yb³⁺ ions. Firstly, the possibility arising from Tb³⁺ ions can be easily excluded because Tb³⁺ ions doesn’t have characteristic excitation band located at 300nm to 600nm. Secondly, if this broad excitation band is originated from oxygen defects, the oxygen defects will transfer absorbed energy to Tb³⁺ ions and we should observe an excitation band from 300nm to 600nm in the excitation spectrum of NaYF₄:Tb⁴⁺,Yb³⁺ NPs monitored the emission of Tb³⁺ ions at 544nm, which is opposite to the experimental result (See Fig. 2). Thus, the possibility arising from oxygen defects also can be excluded. To further confirm this broad excitation band is originated from Tb⁴⁺ ions, the excitation spectra monitored at 977nm of NaYF₄:Tb⁴⁺,Yb³⁺ NPs doped different Tb concentration are measured. As shown in Fig. 4(a), the excitation band isn’t observed when only doped Yb³⁺ ions, which can exclude the possibility arising from Yb²⁺ ions or Yb³⁺ ions. Moreover, the excitation band intensity of NaYF₄:Tb⁴⁺,Yb³⁺ NPs increases with increasing doped Tb concentration from 0% to 15%, and then decreases with the further increase of doped Tb concentration mainly ascribed to the concentration quenching effect that Tb⁴⁺ ions migrate the absorbed energy to defects, which convincingly demonstrates that this broad excitation band stems from the CT transition of Tb⁴⁺ ions. In addition, from the inset of Fig. 4(a), the relative rate of Tb⁴⁺ and Tb³⁺ doesn’t change with Tb concentration, because the relative rate of excitation peak area of the Tb⁴⁺ and Tb³⁺ is nearly unchanged doped with different Tb concentration. Therefore, we can conclude that an energy transfer (ET) from Tb⁴⁺ ion to Yb³⁺ ion occurs in NaYF₄ NPs after sintering.

 

Fig. 4 (a) UV-Vis excitation spectra (λ_em = 977nm) of the NaYF₄:x%Tb⁴⁺,10%Yb³⁺ NPs. Inset: the excitation peak area of Tb³⁺ and Tb⁴⁺ in the NaYF₄: x%Tb⁴⁺, 10%Yb³⁺ NPs (x = 0,5,10,15,20,30). (b) NIR emission spectra (λ_ex = 379nm) of NaYF₄:Tb⁴⁺,Yb³⁺ NPs under different sintering time.

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In addition, the influence of sintering time on NIR DC luminescence in NaYF₄ NPs is investigated. As shown in Fig. 4(b), the NIR emission intensity of NaYF₄:Tb⁴⁺,Yb³⁺ NPs increases with increasing sintering time from 0h to 2h and then decreases with the further increase of sintering time. The increase NIR emission intensity is ascribed to that Tb³⁺ ions are oxidized to Tb⁴⁺ ions under sintering at air atmosphere, while the decrease intensity after further sintering is due to that oxygen defects will occur and absorb the energy [19].

Tb⁴⁺-Yb³⁺ couple have a broad and strong excitation band in UV-Vis region, which may have two different ET mechanisms in the DC process, similar to Ce³⁺-Yb³⁺ couple [20, 21]. One mechanism involves DC by cooperative ET, which would yield two NIR photons for each UV-Vis photon excitation. The other mechanism of single-step ET yields only a single NIR photon for each UV-Vis photon excitation. To judge the ET mechanism from Tb⁴⁺ ions to Yb³⁺ ions in NaYF₄ NPs, the pumping power dependence curves for the luminescence of Yb³⁺ ions at 977nm are measured and plotted on a double logarithmic scale. We know that the relationship between the NIR emission intensity (I) and pumping power (P) is I∝Pⁿ, where n is the corresponding photon number involved in the DC process [22, 23]. As shown in Fig. 5(a), the intensities of NIR emission exhibited linear dependence on the pumping power. The number of photon n determined from the slope coefficient of the linear-fitting line is 1.025 and 1.026 excited at 379nm and 487nm, respectively, which demonstrates the single-step ET mechanism in NaYF₄:Tb⁴⁺,Yb³⁺ NPs. In order to illustrate the NIR DC luminescence process of Tb⁴⁺-Yb³⁺ couple in NaYF₄ NPs, the energy levels diagram of Tb⁴⁺ ion and Yb³⁺ ion are shown in Fig. 5(b). In this system, Tb⁴⁺ ion is doped as a sensitizer and Yb³⁺ ion is doped as an activator. Initially, the doped Tb⁴⁺ ions are excited at 379nm or 487nm from the ground level ⁷F₆ to the CT state. Then, the single-step ET process occurs from an excited Tb⁴⁺ ion to neighbor Yb³⁺ ion in the ground level. Finally, the NIR DC luminescence at 977nm is emitted from the transition ²F_5/2→²F_7/2 of the excited Yb³⁺ ion. It should be stressed that the CT transition of Tb⁴⁺ ion has a very broad and strong absorption but does not give any emission in UV-Vis region, which ensures that Tb⁴⁺ ion can efficiently transfer the absorbed high-energy to Yb³⁺ ion for NIR DC luminescence. Therefore, an efficient NIR DC luminescence is achieved through single-step ET process from Tb⁴⁺ ion to Yb³⁺ ion in NaYF₄ NPs.

 

Fig. 5 (a) Log-log plot for dependency of 977nm NIR emission intensity on pumping power in NaYF₄:Tb⁴⁺,Yb³⁺ NPs excited at 379nm and 487nm, respectively. (b) Energy levels diagrams of Tb⁴⁺-Yb³⁺ couple in the NIR DC energy transfer.

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4. Conclusions

In summary, we use a facile strategy to prepare Tb⁴⁺-Yb³⁺ co-doped NaYF₄ NPs by sintering the as-synthesized NaYF₄:Tb³⁺,Yb³⁺ NPs at 380°C under air atmosphere. Tb⁴⁺ ion appears attributed to the oxidation of Tb³⁺ ion during sintering process, which is demonstrated by XPS spectrum. The NIR DC luminescence of Tb⁴⁺-Yb³⁺ couple is measured and investigated. The results show that the NIR DC luminescence of Tb⁴⁺-Yb³⁺ couple has an efficient enhancement compared with Tb³⁺-Yb³⁺ couple in as-synthesized sample. The enhancement factor is about 14 and 19 excited at 379nm and 487nm, respectively. This is due to that the broad and strong CT state of Tb⁴⁺ ion located at UV-Vis region absorbs high-energy photon but doesn’t give any emission. We reveal that the ET mechanism from Tb⁴⁺ ions to Yb³⁺ ions in NaYF₄ NPs occurs by the single-step ET process through the exponential dependence curves of NIR fluorescence intensity on the pumping power. We also research the influence of sintering time on NIR DC luminescence and find the optimal sintering time is 2h. Our study may provide a promising DC layer for silicon-based solar cells to improve the photovoltaic conversion efficiency.