Efficient Near-Infrared Quantum Cutting in Tm3＋/yb3＋ Codoped LiYF4 Single Crystals for Solar Photovoltaic

Downconversion （DC） with emission of two near-infrared photons about 1000 nm for each blue photon absorbed was obtained in thulium （Tm3＋） and ytterbium （Yb3＋） codoped yt- trium lithium fluoride （LiYF4） single crystals grown by an improved Bridgman method. The luminescent properties of the crystals were measured through photoluminescence excitation, emission spectra and decay curves. Luminescence between 960 and 1050 nm from yb3＋： 2Fs/2--＋2FT/2 transition, which was originated from the DC from Tm3＋ ions to Yb3＋ ions, was observed under the excitation of blue photon at 465 nm. Moreover, the energy transfer processes were studied based on the Inokuti-Hirayama model, and the results indicated that the energy transfer from Tm3＋ to Yb3＋ was an electric dipole-dipole interaction. The max- imum quantum cutting efficiency approached with 0.49mo1% Tm3＋ and 5.99mo1% Yb3＋. increasing the energy efficiency of crystalline energy part of the solar spectrum. up to 167.5% in LiYF4 single crystal codoped Application of this crystal has prospects for Si solar cells by photon doubling of the high

Near-infrared Quantum Cutting in Pr^(3+)-Yb^(3+) Co-doped Oxyfluoride Glass Ceramics Containing CaF_2 Nanocrystals

The Pr3＋-Yb3＋ co-doped oxyfluoride glass-ceramics containing CaF2 nanocrystals were obtained by thermal treatment on the as-made glasses. The structure of fluoride nanocrystals was investigated. The light-emitting mechanism of pr3＋-yb3＋ in the near infrared region was proposed and the fluorescence lifetime and quantum efficiency was calculated. The results indicate that the main phase in the oxyfluoride glass- ceramics is CaF2 nanocrystal sized at 30 nm. Energy dispersive spectroscopy （EDS） and X-ray diffraction （XRD） have proved the incorporation of Pr3＋ and Yb3＋ into CaF2 nanocrystal lattice, Near-infrared quantum cutting involving Yb3＋ 980 nm and 1 015 tun （2F5/2→2F7/2） emission has been achieved upon the excitation of the 3P0 energy level of Pr3＋ at 482 nm. The fluorescence lifetime decreases sharply and quantum efficiency increases with increasing Yb3＋ concentration, and the optimal quantum efficiency reaches 191%.

Concentration effect of the near-infrared quantum cutting of 1788-nm luminescence of Tm^(3+) ion in(Y_(1-x)Tm_x)_3Al_5O_(12) powder phosphor

In the present paper, the concentration effect of near-infrared quantum cutting of Tm3＋ ion in （Y1-xTmx）3Al5Ol2 powder phosphor is studied by means of experiments and calculations. In addition, the absorption spectra, visible-to-near- infrared excitation and emission spectra, and fluorescence lifetimes are measured. It is found that （Y1-xTmx）3Al5O12 powder phosphor has a strong four-photon near-infrared quantum cutting luminescence of 1788.0-nm 3F4 →3H6 fluores- cence of Tm3＋ ion, when excited by 357.0-nm light. It is also found that the up-limit of the four-photon near-infrared quantum cutting luminescence efficiency of （Yo.700Tmo.300）3Al5 O12 powder phosphor is approximately 302.19%. To the knowledge of the authors, this is the first time that a near-infrared quantum cutting efficiency up-limit exceeding 300% has been reported. The results of this manuscript are valuable in aiding the probing of the new generation Ge solar cell.

Infrared quantum cutting conversion luminescence of Tb(0.7)Yb(3):FOV

The infrared quantum cutting of oxyfluoride nanophase vitroceramics Tb（0.7）Yb（3）：FOV has been studied in the present paper. The actual quantum cutting efficiency formula calculated from integral fluorescence intensity is extended to the case of Tb（0.7）Yb（3）：FOV. The visible and the infrared fluorescence spectra and their integral fluorescence intensities are measured from static fluorescence experiment. Lifetime curve is measured from dynamic fluorescence experiment. It is found that the total actual quantum cutting efficiency n of the excited 5D4 level is about 93.7%, and that of excited （5D3, 5G6） levels is 93.5%. It is also found that the total theoretical quantum cutting efficiency upper limit ？~x^Yb of the 485.5 nm excited 5D4 level is about 121.7%, and that of 378.5 nm excited （5D3, 5G6） levels is 137.2%.

Efficient near-infrared quantum cutting by Ce^(3+)-Yb^(3+) couple in GdBO_3 phosphors

Ce3＋ and Yb3＋ co-activated GdBO3 phosphors were prepared by a conventional solid-state reaction method. X-ray powder diffraction, photoluminescent spectra and decay curves were used to characterize their structural and luminescent properties. An efficient near-infrared （NIR） quantum cutting （QC） from the phosphors was observed, which involved the emission of two low-energy NIR photons （around 971 nm） from an absorbed ultra-violet （UV） photon at 358 nm via a cooperative energy transfer （CET） from Ce3＋ to Yb3＋ ions. The theoretical quantum efficiency was calculated and the maximum efficiency approached up to 164% before reaching the critical concentration quenching threshold. Our results demonstrated that these phosphors might find potential application in improving the efficiency of silicon based solar cells.

Efficient near-infrared quantum cutting in Tb3＋,Yb3＋ codoped LuPO_4 phosphors

Tb^（3＋） and Yb^（3＋） codoped LuPO_4 phosphors were prepared by the reverse-strike co-precipitation method.The obtained LuPO_4：Tb^（3＋）,Yb^（3＋） phosphors were characterized by X-ray diffraction（XRD）,photoluminescence（PL） spectra and decay kinetics to understand the near-infrared quantum cutting（QC） phenomena.The XRD results showed that all the phosphors exhibited good crystallinity and had a pure tetragonal phase of LuPO_4.The experimental results showed that the strong green emission around 545 nm from Tb^（3＋）（~5D_4→~7F_5） and near-infrared（NIR） emission at 1003 nm from Yb^（3＋）（~2F_（5/2）→~2F_（7/2）） of LuPO_4：Tb^（3＋）,Yb^（3＋）phosphors were observed under 489 nm excitation,respectively.The Yb^（3＋） concentration dependence on luminescent properties and lifetimes of both the visible and NIR emissions were also investigated.The quenching concentration of Yb^（3＋） ions approached as high as 10 mol.%.The excellent luminescence properties of the LuPO_4：Tb^（3＋）,Yb^（3＋） indicated its potential application in improving the energy conversion efficiency of the silicon based solar cells by converting one blue photon to two NIR ones.

Multi-photon quantum cutting in Gd_(2)O_(2)S:Tm^(3+) to enhance the photo-response of solar cells

Conventional photoluminescence(PL)yields at most one emitted photon for each absorption event.Downconversion(or quantum cutting)materials can yield more than one photon by virtue of energy transfer processes between luminescent centers.In this work,we introduce Gd2O2S:Tm^(3+) as a multi-photon quantum cutter.It can convert near-infrared,visible,or ultraviolet photons into two,three,or four infrared photons of,1800 nm,respectively.The cross-relaxation steps between Tm^(3+) ions that lead to quantum cutting are identified from(time-resolved)PL as a function of the Tm^(3+) concentration in the crystal.A model is presented that reproduces the way in which the Tm^(3+) concentration affects both the relative intensities of the various emission lines and the excited state dynamics and providing insight in the quantum cutting efficiency.Finally,we discuss the potential application of Gd2O2S:Tm^(3+) for spectral conversion to improve the efficiency of next-generation photovoltaics.

VUV Spectroscopy of GdPO_4∶Eu^(3+) and GdBO_3∶Eu^(3+)

The emission and the excitation spectra of GdPO4 ： Eu^3＋ and GdBO3： Eu^3 ＋ prepared by solid state reaction method were investigated using the synchrotron radiation source of SUPERLUMI station of HASYLAB. The energy transfer between Gd^3＋ and Eu^3＋ was discussed with the probability of quantum cutting process. In the excitation spectra monitoring the red emission from Eu^3＋ , the distinct lines corresponding to the intraconfigurational 4f-4f transitions from Gd^3＋ were observed for both samples, indicating an efficient energy transfer from host Gd^3＋ ions to the doped Eu^3＋ ions. The efficient energy transfer is necessary for the quantum cutting process based on the two-step energy transfer from Gd^3＋ to Eu^3＋ . However, the overlapping of the lines corresponding to Gd^3＋ ：^8S7/2→^6GJ and the broad excitation band （180 - 270 nm） due to Eu^3＋- O^2- charge transfer states （CTS） around 200 nm cause excitation energy on ^6GJ levels to dissipate into CTS by direct energy transfer, unfavorable to the cross relaxation energy transfer between Gd^3＋ and Eu^3＋, thus unfavorable to the quantum cutting process. With the help of the general rules governing the energy positions of Eu^3＋-O^2- ：CTS, the suggestions concerning searching suitable oxide hosts for Gd^3＋-Eu^3＋ quantum cutting were made.

Cooperative energy acceptor of three Yb^(3+) ions

Our previous work first reported the cooperative sensitized luminescence from Cu2＋ or Pb2＋ by three clustered Yb^3＋ ions, in which three NIR photons can be converted into a high energy photon. Could a reverse process happen that a high energy photon is cut into three NIR photons？ This work demonstrated an example of three-photon quantum cutting （QC） phosphor, CaF2：Ce^3＋,Yb^3＋, in which three clustered Yb^3＋ ions （Yb^3＋-trimer） cooperatively and indirectly received a 306 nm ultraviolet （UV） photon energy transferred from a Ce^3＋ ion in 5d excited state and emitted three 975 nm near-infrared （NIR） photons. The cluster destruction experiments were designed to verify the necessity of the presence of Yb^3＋-trimers for QC. The dynamical analysis on luminescence of Ce^3＋ ions confirmed the energy transfer from Ce^3＋ ions to Yb^3＋-trimers. The doping concentration effect on luminescence was investigated. Furthermore, the maximum energy transfer （ET） efficiency and the corresponding QC efficiency were estimated to be 61% and 222%, respectively. Therefore, the reported three-photon QC phosphor has an attractive prospect in efficiently harvesting solar energy for silicon solar cells.

Energy transfer and luminescent properties of Tb^(3+)and Tb^(3+),Yb^(3+)doped CNGG phosphors

In this work,calcium niobium gallium garnet(Ca_(3)Nb_(1.6875)Ga_(3.1875)O_(12)-CNGG)ceramic samples singledoped with Tb^(3+)and co-doped with Tb^(3+)and Yb^(3+)ions were sintered by the solid-state reaction method.The structural characterization of the samples was carried out by X-ray diffraction measurements.The optimal concentration of Tb^(3+)ions corresponding to the maximum luminescence in the green spectral range in CNGG:x at%Tb(x=0.1,0.5,1,2,3,4,and 5)was determined to be 4 at%.The timeresolved luminescence of the^(5)D_(4)level(Tb^(3+))in the CNGG:x at%Tb samples was analysed to explore the quenching mechanisms involved in the Tb^(3+)green emission.Co-doped CNGG:4 at%Tb,y at%Yb(y=0.5,2,4,6,8,and 10)ceramics were prepared and investigated.It is shown that CNGG:4 at%Tb,y at%Yb phosphors exhibit intense green luminescence under ultra-violet(UV),visible(VIS),and near-infrared(NIR)excitation,thus demonstrating the presence of simultaneous down-conversion(DC)and upconversion(UC)processes.The dependence of the UC luminescence intensity on the diode laser pumping power was measured and the results indicate a two-photon process based on cooperative energy transfer(CET).Under UV excitation,the lifetime of the^(5)D_(4)(Tb^(3+))level slowly increases with increase of Yb^(3+)concentration,suggesting the energy transfer from Yb^(3+)to Tb^(3+)ions,while under NIR excitation,the lifetime of the^(5)D_(4)(Tb^(3+))level decreases with increase of Yb^(3+)ions concentration,indicating the presence of a strong energy transfer from Tb^(3+)to Yb^(3+)ions.The highest energy transfer efficiency ofη_(ET)≈42%was determined for the CNGG:4 at%Tb,10 at%Yb sample.The obtained results indicate that CNGG:(Tb^(3+),Yb^(3+))could be efficient new yellowish-green-emitting phosphors.

Cooperative energy transfer in Tm^(3+)and Yb^(3+) co-doped phosphate glasses

An efficient near-infrared （NIR） quantum cutting （QC） in Tm3＋ and Yb3＋ co-doped phosphate glasses was demonstrated, which involved the emission of two NIR photons from an absorbed visible photon via a cooperative energy transfer （CET） from Tm3＋ to Yb3＋ ions. Judd-Ofelt （J-O） theory was used to calculate the intensity parameters （ 2 , 4 , 6 ）, the radiative transition rates （Ar ）, and radiative transition lifetime （τ rad ） of Tm3＋ . Based on Inokuti-Hirayama＇s model, the energy transfer processes were studied and results indicated that the energy transfer of the electric dipole-dipole （Edd） was dominant in this system. Quantum efficiency related to Yb 3＋ concentration was calculated, and the maximum QE efficiency reached 169.8%.