【9】Quantum Cutting in Tm31Yb31-Codoped Lanthanum Aluminum

Quantum Cutting in Tm 31/Yb 31-Codoped Lanthanum AluminumGermanate GlassesQiang Zhang,z ,y Bin Zhu,z Yixi Zhuang,z Guorong Chen,J Xiaofeng Liu,z ,y Guang Zhang,z ,y Jianrong Qiu,zand Danping Chen w ,zzKey Laboratory of Material Science and Technology for High Power Lasers,Shanghai Institute of Optics and FineMechanics,Chinese Academy of Sciences,Shanghai 201800,ChinayGraduate School of the Chinese Academy of Sciences,Beijing 100039,ChinazState Key Laboratory of Silicon Materials,Zhejiang University,Hangzhou 310027,ChinaJKey Laboratory for Ultrafine Materials of Ministry of Education,School of Materials Science and Engineering,EastChina University of Science and Technology,Shanghai 200237,ChinaIn the present paper,Tm 31/Yb 31-codoped lanthanum aluminum germanate glasses are prepared by simply melting and quench-ing method.Quantum cutting (QC)between Tm 31and Yb 31ions in these samples is realized;in the QC process,one blue photon at about 467nm is cut into two near-infrared photons at about 1l m.The maximum energy transfer efficiency and the corresponding QC efficiency can reach as high as 59.9%and 159.9%,respectively.I.IntroductionLUMINESCENTmaterials with quantum efficiency (QE)largerthan unity under ultraviolet or visual excitation are of great importance for effective energy transfer.The former has been researched for decades as mercury-free phosphors or fluorescent lamps,1,2while the latter is being investigated for their potential application as a down-conversion layer in silicon solar cells in recent years.3–11As we know,the high cost per kilowatt-hour of photovoltaic cell limits its contribution.Most energy loss is re-lated to the spectral mismatch of incident photon energy to the energy gap of the solar cell.12By cutting one high-energy photon into two low-energy photons with energy slightly larger than the gap of the solar cell,the energy loss related to the thermalization of hot charge carriers can be reduced and the efficiency of solar cells can also be enhanced.12Much work has been carried out to search suitable materials.RE 31–Yb 31(RE 5Tm,Tb,and Pr)couples are the main choices as dopants because Yb 31ions with 1m m emission have simple energy levels and RE ions have a suitable energy level at about two times of the energy gap of the solar cell.13–15From the point of application,glassy materials are more suitable to be used as down-conversion layer in solar cells compared with particulate crystalline materials 3,11,16,17be-cause glassy materials can be shaped as a large plate and have better transparency.Some work has been performed on oxyflu-oride glass ceramic and borogermanate glasses.4,8,14,15However,stability of the glass or glass ceramic is apt to be negatively in-fluenced by the addition of fluoride or borate.In this paper,we focus on the quantum cutting (QC)between Tm 31and Yb 31ions and choose lanthanum aluminum germanate (LAG)glass as a rger solubility of rare earth ions and lower pho-non energy of LAG glass also make it more suitable as host material compared with silicate or borosilicate glasses.II.Experimental ProcedureTm 0.05Yb x La 0.95Àx AlGe 2O 7(x 50,0.1,0.2,0.3,0.4,and 0.5)glasses are prepared by a conventional melting and quenching method using reagent-grade La 2O 3,Al 2O 3,GeO 2,Tm 2O 3,and Yb 2O 3.The glasses obtained are hereafter denoted as LAG5Tm,LAG5Tm10Yb,LAG5Tm20Yb,LAG5Tm30Yb,LAG5-Tm40Yb,and LAG5Tm50Yb,respectively.Approximately 25g of powder was ground well and melted in a corundum crucible at 14001C for 1h for each batch.Then,the melt was quenched on an iron plate.All the samples were cut into 10mm Â10mm Â2.5mm and polished for measurements.The glass transition tem-perature (T g )and onset crystallization temperature (T x )of blank LAG glass were analyzed using differential thermal analysis (DTA)method at a heating rate of 101C/min.The absorption spectra were measured with a JASCO V-570UV/VIS spectro-photometer (JASCO International Co.Ltd.,Tokyo,Japan).The measurements of excitation,emission spectra,and decay curves were carried out using a FLS920fluorescence spectrophotometer (Edinburgh Analytical Instruments,Edinburgh,U.K.).III.Results and DiscussionFigure 1shows the DTA thermogram of blank LAG glass.The glass transition temperature (T g 57521C)and the onset crystal-lization temperature (T x 59701C)are clearly observed.The rel-atively high T g and T x indicate the higher thermal stability of the LAG glass.The absorption spectra of the blank LAG and LAG5Tm20Yb glasses are shown in Fig.2.The absorption spectrum of LAG5Tm20Yb is vertically shifted so that the char-acteristic absorption peaks due to Tm 31and Yb 31can be ob-served more clearly.The strong absorption band assigned to the Yb 31:2F 5/2-2F 7/2transition can be clearly identified in the wavelength range from 950to 1100nm.4Other peaks assigned to 3H 6-3F 4,3F 2,3,1G 4,1D 2transitions of Tm 31ions are also visible at 789,684,467,and 359nm,respectively.6,14The ab-sorption edge fixes at about 300nm for both blank and doped LAG glasses.The inset shows the absorbance curve of LAG5-Tm20Yb with thickness of 2.5mm,and we can see that the 3H 6-1G 4transition at about 467nm is just above twice the energy level of Yb 31ions.H.Ebendorff-Heidepriem—contributing editorThis work was financially supported by the National Natural Science Foundation of China (NNSFC)(grant 60878043and no.6070701).wAuthor to whom correspondence should be addressed.e-mail:d-chen@Manuscript No.26535.Received July 29,2009;approved October 8,2009.J ournalJ.Am.Ceram.Soc.,93[3]654–657(2010)DOI:10.1111/j.1551-2916.2009.03483.xr 2009Shanghai Institute of Optics and Fine Mechanics Journal compilation r 2009The American Ceramic Society654Part of the energy-level diagram of Tm 31and Yb 31ions in LAG glasses has been shown in Fig.3to illustrate the mecha-nism of QC between Tm 31and Yb 31.14In the present system,the proposed NIR QC occurs upon excitation with visible light at the 1G 4level of a Tm 31ion.The excited Tm 31ion transfers its energy to two Yb 31ions through (Tm 31:1G 4,Yb 31:2F 7/2,Yb 31:2F 7/2)-(Tm 31:3H 6,Yb 31:2F 5/2,Yb 31:2F 5/2),and then the two excited Yb 31ions emit two NIR photons by 2F 5/2-2F 7/2transition.14The excitation spectra of Yb 31ions and 1G 4-3F 4transition of Tm 31ions with the corresponding emission spectra have also been measured to provide a convincing evidence for the presence of Tm 31–Yb 31QC.Figure 4presents the excitation spectra of Tm 31:1G 4-3F 4transition (l em 5650nm)(A)and the emission spectra under the excitation of 3H 6-1G 4transition (l ex 5467nm)(B)in the ultraviolet and visible range.There are two obvious excitation peaks in Fig.4(A)at 359and 467nm,which are assigned to the transitions of 3H 6-1D 2and 3H 6-1G 4of Tm 31,respectively.14The intensity of 1G 4-3F 4transition at about 650nm decreases with the addition of Yb 31ions.This indicates that an efficient energy transfer from 1G 4energy level of Tm 31to 2F 5/2energy level of Yb 31ions may occur.6,14To illustrate this energy transfer process,the excitation spectra of Yb 31monitored by 1025nm emission wavelength are depicted in Fig.5(A).We can see that beside a broad chargetransfer band at about 300nm,4there are two excitation peaks at 359and 467nm,which are coincident with that of 1G 4-3F 4transition of Tm 31at 650nm.Thus,Tm 31ion absorbs photons at 467nm and then transfers its energy to Yb 31ions,as shown in Fig.5(B).As the energy gap between 1G 4and 2F 5/2is very large,the phonon-assisted energy transfer process becomes diffi-cult.Hence,efficient energy transfer between Tm 31and Yb 31occurs with the excitation of 467nm monochromatic light and this is a QC process.However,the intensity of Yb 31:2F 5/2-2F 7/2transition becomes weaker as the amount of Yb 31in-creases and the peak wavelength also shows a red shift.We as-cribe this to the concentration quenching and reabsorption of Yb 31ions in the LAG5TmxYb glasses.The energy transfer efficiency Z ET is defined as the ratio of Tm 31ions that are depopulated by ET to Yb 31ions over the total number of Tm 31ions excited.Z ET can be estimated by dividing the integrated intensity of the decay curves of the LAG5TmxYb glasses to that of LAG5Tm 3,10,15,16:Z ET ¼1ÀRI LAG5TmxYb d tRI LAG5Tm d twhere I represents the intensity of 650nm at time t .Figure 6shows the decay curves of LAG5TmxYb 650nm emissionunderFig.2.Absorption spectra of blank lanthanum aluminum germanate (LAG)glass and LAG5Tm20Yb.The inset shows the detailed absorp-tion in the range 300nm B 600nm.The absorption spectrum of LAG5-Tm20Yb is vertically shifted so that the characteristic absorption peaks due to Tm 31and Yb 31can be observed moreclearly.Fig.3.Part of the energy-level diagram of Tm 31and Yb 31ions in lanthanum aluminum germanateglasses.Fig.4.Excitation (l em 5650nm)(A)and emission (l ex 5467nm)(B)spectra of Tm 31inLAG5TmxYb.the excitation of 467nm monochromatic light.The lifetime de-fined as t LAG5TmxYb ¼R 1t 0½I ðt Þ=I 0 d t and the corresponding en-ergy transfer efficiencies are also labeled in the figure.The lifetime of Tm 31emission at 650nm decreases rapidly from 78.8to 31.6m s when the substitution ratio of Yb 31reaches 50%,then we can obtain the biggest energy transfer efficiency 59.9%for LAG5Tm50Yb.The total QE Z QE can be defined as the ratio of photons emitted to the photons that are absorbed.Then,the QE efficiency can be expressed as Z QE 5Z QE0(1ÀZ ET )12Z ET 3,10,14–16in which Z QE0stands for the QE of Tm 31in LAG5Tm glass.The first and second term stand for the numbers of visible and near-infrared photons emitted per high-energy photons absorbed.Assuming that all excited Tm 31and Yb 31ions decay by radiative emission,then we can obtain the upper limit of the QE:Z MQE 511Z ET where Z QE0is set to 1.3,14Hence,the QE reaches as high as 159.9%for LAG5-Tm50Yb and the QE is larger or comparable with that reportedin other glassy materials.This result indicates that LAG is more suitable as a down-conversion layer in solar cells because its stability is better than that of oxyfluoride glass ceramic and borogermanate glasses previously reported.6,14,17IV.ConclusionTm 31–Yb 31-codoped LAG glasses are prepared by a melting and quenching method.The glasses show a capacity of dividing a visible photon into two near-infrared photons with QE up to 159.9%.The glasses show a potential application as a down-conversion layer in silicon solar cells.References1R.T.Wegh,E.V.D.van Loef,and A.Meijerink,‘‘Visible Quantum Cutting Via Downconversion in LiGdF 4:Er 31,Tb 31upon Er 314f(11)—44f(10)5d Exci-tation,’’J.Lumin.,90[3–4]111–22(2000).2R.T.Wegh,H.Donker,K.D.Oskam,and A.Meijerink,‘‘Visible Quantum Cutting in LiGdF 4:Eu 31Through Downconversion,’’Science ,283[5402]663–6(1999).3P.Vergeer,T.J.H.Vlugt,M.H.F.Kox,M.I.den Hertog,J.P.J.M.van der Eerden,and A.Meijerink,‘‘Quantum Cutting by Cooperative Energy Transfer in Yb x Y 1Àx PO 4:Tb 31,’’Phys.Rev.B ,71,014119,11pp (2005).4X.Liu,S.Ye,Y.Qiao,G.Dong,B.Zhu,D.Chen,kshminarayana,and J.Qiu,‘‘Cooperative Downconversion and Near-Infrared Luminescence of Tb 31–Yb 31Codoped Lanthanum Borogermanate Glasses,’’Appl.Phys.B—Lasers Opt.,96[1]51–5(2009).5D.Q.Chen,Y.L.Yu,Y.S.Wang,P.Huang,and F.Y.Weng,‘‘Cooperative Energy Transfer Up-Conversion and Quantum Cutting Down-Conversion in Yb 3:TbF 3Nanocrystals Embedded Glass Ceramics,’’J.Phy.Chem.C ,113[16]6406–10(2009).6kshminarayana,H.C.Yang,S.Ye,Y.Liu,and J.R.Qiu,‘‘Co-Operative Downconversion Luminescence in Tm 31/Yb 31:SiO 2–Al 2O 3–LiF–GdF 3Glasses,’’J.Phys.D:Appl.Phys.,41,175111,6pp (2008).7J.X.Chen,S.Ye,X.Wang,and J.R.Qiu,‘‘Cooperative Quantum Cutting of Nano-Crystalline BaF 2:Tb 31,Yb 31in Oxyfluoride Glass Ceramics,’’Chin.Phys.Lett.,25[6]2078–80(2008).8D.Q.Chen,Y.S.Wang,Y.L.Yu,P.Huang,and F.Y.Weng,‘‘Near-Infrared Quantum Cutting in Transparent Nanostructured Glass Ceramics,’’Opt.Lett.,33[16]1884–6(2008).9D.Q.Chen,Y.S.Wang,Y.L.Yu,P.Huang,and F.Y.Weng,‘‘Quantum Cutting Downconversion by Cooperative Energy Transfer from Ce 31to Yb 31in Borate Glasses,’’J.Appl.Phys.,104,116105,3pp(2008).Fig.5.Excitation (l em 51025nm)(A)and emission (l ex 5467nm)(B)spectra of Yb 31inLAG5TmxYb.Fig.6.Decay curves of Tm 31:650nm emission under the excitation of 467nm light in LAG5TmxYb.656Rapid Communications of the American Ceramic Society Vol.93,No.310Q.Y.Zhang,G.F.Yang,and Z.H.Jiang,‘‘Cooperative Downconversion in GdAl 3(BO 3)(4):RE 31,Yb 31(RE 5Pr,Tb,and Tm),’’Appl.Phys.Lett.,91[5]051903,3pp (2007).11Q.Y.Zhang,C.H.Yang,and Y.X.Pan,‘‘Cooperative Quantum Cutting in One-Dimensional (Yb x Gd 1Àx )Al–3(BO 3)(4):Tb 31Nanorods,’’Appl.Phys.Lett.,90[2]021107,3pp (2007).12B.S.Richards,‘‘Luminescent Layers for Enhanced Silicon Solar Cell Performance:Down-Conversion,’’Sol.Energy Mater.Sol.Cells.,90[9]1189–207(2006).13Q.Y.Zhang and X.F.Liang,‘‘Cooperative Downconversion in Y 3Al 5O 12:RE 31,Yb 31(RE 5Tb,Tm,and Pr)Nanophosphors,’’rm.Dis.,16[7]755–8(2008).14S.Ye,B.Zhu,J.Luo,J.X.Chen,kshminarayana,and J.R.Qiu,‘‘En-hanced Cooperative Quantum Cutting in Tm 31–Yb 31Codoped Glass Ceramics Containing LaF 3Nanocrystals,’’Opt.Express ,16[12]8989–94(2008).15S.Ye,B.Zhu,J.X.Chen,J.Luo,and J.R.Qiu,‘‘Infrared Quantum Cutting in Tb 31,Yb 31Codoped Transparent Glass Ceramics Containing CaF 2Nano-crystals,’’Appl.Phys.Lett.,92,141112,3pp (2008).16Q.Y.Zhang,C.H.Yang,Z.H.Jiang,and X.H.Ji,‘‘Concentration-Depen-dent Near-Infrared Quantum Cutting in GdBO 3:Tb 31,Yb 31Nanophosphors,’’Appl.Phys.Lett.,90[6]061914,3pp (2007).17X.Liu,Y.Qiao,G.Dong,S.Ye,B.Zhu,kshminarayana,D.Chen,and J.Qiu,‘‘Cooperative Downconversion in Yb 31RE 31(RE 5Tm or Pr)Codoped Lanthanum Borogermanate Glasses,’’Opt.Lett.,33[23]2858–60(2008).&March 2010Rapid Communications of the American Ceramic Society657。