Efficient near-infrared downconversion and energy transfer mechanism in Tb 4 +-Yb 3 + co-doped NaYF 4 nanoparticles

Tb-Yb co-doped NaYF4 nanoparticles (NPs) are prepared by sintering the assynthesized NaYF4:Tb, Yb NPs at 380°C under air atmosphere. The oxidization of Tb ions to Tb ions in NaYF4 NPs after sintering is demonstrated through X-ray photoelectron spectroscopy (XPS). The near-infrared (NIR) downconversion (DC) luminescence of TbYb couple is measured and investigated for the first time. The results show that DC luminescence of Tb-Yb couple enhance obviously compared with Tb-Yb couple in assynthesized 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 Tb to Yb 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 OCIS codes: (160.5690) Rare-earth-doped materials; (250.5230) Photoluminescence; (260.2160) Energy transfer; (160.4236) Nanomaterials. References and links 1. L. Aarts, B. Van der Ende, and A. Meijerink, “Downconversion for solar cells in NaYF4: Er, Yb,” J. Appl. Phys. 106(2), 023522 (2009). 2. D. Chen, Y. Yu, Y. Wang, P. Huang, and F. Weng, “Cooperative energy transfer up-conversion and quantum cutting down-conversion in Yb: TbF3 nanocrystals embedded glass ceramics,” J. Phys. Chem. C 113(16), 6406–6410 (2009). 3. L. Lin, J. Chen, C. Deng, L. Tang, D. Chen, and L. Cao, “Broadband near-infrared quantum-cutting by cooperative energy transfer in Yb-Bi co-doped CaTiO3 for solar cells,” J. Alloys Compd. 640, 280–284 (2015). 4. X. Chen, S. Li, G. J. Salamo, Y. Li, L. He, G. Yang, Y. Gao, and Q. Liu, “Sensitized intense near-infrared downconversion quantum cutting three-photon luminescence phenomena of the Tm:ion activator in Tm:Bi:YNbO4 powder phosphor,” Opt. Express 23(3), A51–A61 (2015). 5. L. Lin, H. Lin, Z. Wang, J. Chen, R. Huang, X. Rao, Z. Feng, and Z. Zheng, “Quantum-cutting of KYF4:Tb,Yb under multiple excitations with high Tb concentration,” Opt. Mater. 36(6), 1065–1069 (2014). 6. K. Deng, T. Gong, L. Hu, X. Wei, Y. Chen, and M. Yin, “Efficient near-infrared quantum cutting in NaYF4: Ho, Yb for solar photovoltaics,” Opt. Express 19(3), 1749–1754 (2011). 7. B. Zheng, S. Xu, L. Lin, Z. Wang, Z. Feng, and Z. Zheng, “Plasmon enhanced near-infrared quantum cutting of KYF4: Tb, Yb doped with Ag nanoparticles,” Opt. Lett. 40(11), 2630–2633 (2015). 8. Y. S. Xu, F. Huang, B. Fan, C. G. Lin, S. X. Dai, L. Y. Chen, Q. H. Nie, H. L. Ma, and X. H. Zhang, “Quantum cutting in Pr-Yb codoped chalcohalide glasses for high-efficiency c-Si solar cells,” Opt. Lett. 39(8), 2225– 2228 (2014). 9. A. Guille, A. Pereira, C. Martinet, and B. Moine, “Quantum cutting in CaYAlO4: Pr, Yb,” Opt. Lett. 37(12), 2280–2282 (2012). 10. I. Terra, L. Borrero-González, J. Carvalho, M. Terrile, M. Felinto, H. Brito, and L. Nunes, “Spectroscopic properties and quantum cutting in Tb–Yb co-doped ZrO2 nanocrystals,” J. Appl. Phys. 113(7), 073105 (2013). #269540 http://dx.doi.org/10.1364/OME.6.002769 Journal © 2016 Received 29 Jun 2016; revised 1 Aug 2016; accepted 1 Aug 2016; published 8 Aug 2016 Vol. 6, No. 9 | 1 Sep 2016 | OPTICAL MATERIALS EXPRESS 2769


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][3][4].Herein, there are many routes to improve the conversion efficiency, and one of them is the downconversion (DC) [5][6][7][8][9][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].RE 3+ -Yb 3+ (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 RE 3+ 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 Tb 4+ -Yb 3+ couple, which is observed for the first time to our knowledge.Tb 4+ ion might be an ideal broadband sensitizer for Yb 3+ ion due to its charge transfer (CT) state located at 300nm-600nm [12,13].This broad CT state covers the highenergy part of the solar spectrum and matches twice the energy of Yb 3+ ion.Moreover, the Tb 4+ ion has the same electron configuration as Gd 3+ ion (4ƒ 7 ).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 Tb 4+ ion could efficiently transfer the absorbed energy to activator Yb 3+ 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.

Experimental
We chose the inorganic fluoride hexagonal NaYF 4 nanoparticles (NPs) as DC host due to its low phonon frequencies and high chemical stability [15].Hexagonal NaYF 4 :15%Tb 3+ , 10%Yb 3+ NPs were synthesized through coprecipitation method as follows [16]: 0.4550g YCl 3 •6H 2 O (99.99%), 0.0776g YbCl 3 •6H 2 O (99.99%) and 0.1120g TbCl 3 •6H 2 O (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 NH 4 F (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, NaYF 4 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 NaYF 4 NPs were sintered at 380°C under air atmosphere, yielding the final Tb 4+ -Yb 3+ co-doped NaYF 4 NPs.Sintering at 380°C could oxidize the Tb 3+ ion to Tb 4+ ion, and avoid the NaYF 4 lattice structure be destroyed [17] 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.
Figure 3(c) illustrates the excitation spectra of NaYF 4 :Tb 3+ ,Yb 3+ NPs and NaYF 4 :Tb 4+ ,Yb 3+ NPs monitored at 977nm.Compared with the NaYF 4 :Tb 3+ ,Yb 3+ NPs, the weak excitation peaks of Tb 3+ ion still can be observed in the excitation spectrum of NaYF 4 :Tb 4+ ,Yb 3+ NPs, indicating part of Tb 3+ 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 NaYF 4 :Tb 4+ ,Yb 3+ NPs, which may arise from (i) Tb 4+ ions; (ii) Tb 3+ ions; (iii) oxygen defects; (iv) Yb 2+ ions; (v) Yb 3+ ions.Firstly, the possibility arising from Tb 3+ ions can be easily excluded because Tb 3+ 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 Tb 3+ ions and we should observe an excitation band from 300nm to 600nm in the excitation spectrum of NaYF 4 :Tb 4+ ,Yb 3+ NPs monitored the emission of Tb 3+ 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 Tb 4+ ions, the excitation spectra monitored at 977nm of NaYF 4 :Tb 4+ ,Yb 3+ NPs doped different Tb concentration are measured.As shown in Fig. 4(a), the excitation band isn't observed when only doped Yb 3+ ions, which can exclude the possibility arising from Yb 2+ ions or Yb 3+ ions.Moreover, the excitation band intensity of NaYF 4 :Tb 4+ ,Yb 3+ 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 Tb 4+ ions migrate the absorbed energy to defects, which convincingly demonstrates that this broad excitation band stems from the CT transition of Tb 4+ ions.In addition, from the inset of Fig. 4(a), the relative rate of Tb 4+ and Tb 3+ doesn't change with Tb concentration, because the relative rate of excitation peak area of the Tb 4+ and Tb 3+ is nearly unchanged doped with different Tb concentration.Therefore, we can conclude that an energy transfer (ET) from Tb 4+ ion to Yb 3+ ion occurs in NaYF 4 NPs after sintering.
In addition, the influence of sintering time on NIR DC luminescence in NaYF 4 NPs is investigated.As shown in Fig. 4(b), the NIR emission intensity of NaYF 4 :Tb 4+ ,Yb 3+ 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 Tb 3+ ions are oxidized to Tb 4+ 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].Tb 4+ -Yb 3+ 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 Ce 3+ -Yb 3+ 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 Tb 4+ ions to Yb 3+ ions in NaYF 4 NPs, the pumping power dependence curves for the luminescence of Yb 3+ 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∝P n , 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 NaYF 4 :Tb 4+ ,Yb 3+ NPs.In order to illustrate the NIR DC luminescence process of Tb 4+ -Yb 3+ couple in NaYF 4 NPs, the energy levels diagram of Tb 4+ ion and Yb 3+ ion are shown in Fig. 5(b).In this system, Tb 4+ ion is doped as a sensitizer and Yb 3+ ion is doped as an activator.Initially, the doped Tb 4+ ions are excited at 379nm or 487nm from the ground level 7 F 6 to the CT state.Then, the single-step ET process occurs from an excited Tb 4+ ion to neighbor Yb 3+ ion in the ground level.Finally, the NIR DC luminescence at 977nm is emitted from the transition 2 F 5/2 → 2 F 7/2 of the excited Yb 3+ ion.It should be stressed that the CT transition of Tb 4+ ion has a very broad and strong absorption but does not give any emission in UV-Vis region, which ensures that Tb 4+ ion can efficiently transfer the absorbed highenergy to Yb 3+ ion for NIR DC luminescence.Therefore, an efficient NIR DC luminescence is achieved through single-step ET process from Tb 4+ ion to Yb 3+ ion in NaYF 4 NPs.

Conclusions
In summary, we use a facile strategy to prepare Tb 4+ -Yb 3+ co-doped NaYF 4 NPs by sintering the as-synthesized NaYF 4 :Tb 3+ ,Yb 3+ NPs at 380°C under air atmosphere.Tb 4+ ion appears attributed to the oxidation of Tb 3+ ion during sintering process, which is demonstrated by XPS spectrum.The NIR DC luminescence of Tb 4+ -Yb 3+ couple is measured and investigated.The results show that the NIR DC luminescence of Tb 4+ -Yb 3+ couple has an efficient enhancement compared with Tb 3+ -Yb 3+ 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 Tb 4+ ion located at UV-Vis region absorbs high-energy photon but doesn't give any emission.We reveal that the ET mechanism from Tb 4+ ions to Yb 3+ ions in NaYF 4 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.