Tb4+-Yb3+ co-doped NaYF4 nanoparticles (NPs) are prepared by sintering the as-synthesized NaYF4:Tb3+, Yb3+ NPs at 380°C under air atmosphere. The oxidization of Tb3+ ions to Tb4+ ions in NaYF4 NPs after sintering is demonstrated through X-ray photoelectron spectroscopy (XPS). The near-infrared (NIR) downconversion (DC) luminescence of Tb4+-Yb3+ couple is measured and investigated for the first time. The results show that DC luminescence of Tb4+-Yb3+ couple enhance obviously compared with Tb3+-Yb3+ couple in as-synthesized sample. The enhancement factor is about 14 and 19 excited at 379nm and 487nm, respectively. On analyzing the exponential dependence of NIR fluorescence intensity on the pumping power, we reveal that the energy transfer (ET) mechanism from Tb4+ to Yb3+ in NaYF4 NPs occurs by the single-step ET process. Our study may provide a promising DC layer on the top of silicon-based solar cells to improve the photovoltaic conversion efficiency.
© 2016 Optical Society of America
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 . 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 .
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 . 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 . 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.
We chose the inorganic fluoride hexagonal NaYF4 nanoparticles (NPs) as DC host due to its low phonon frequencies and high chemical stability . Hexagonal NaYF4:15%Tb3+, 10%Yb3+ NPs were synthesized through coprecipitation method as follows : 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 . 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.
Figure 2 shows the visible emission spectra (λex = 379nm) and corresponding excitation spectra (λem = 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→7Fj (j = 6, 5, 4, 3)) in NaYF4:Tb4+,Yb3+ NPs decrease dramatically , 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 .
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 2F5/2→2F7/2 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.
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.
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 .
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 2F5/2→2F7/2 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.
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.
National Natural Science Foundation of China (NFSC) (11204039, 51202033); Natural Science Foundation of Fujian Province of China (2015J01243); Science Foundation of the Educational Department of Fujian Province of China (JA13084).
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