Tm3+-Yb3+ codoped transparent oxyfluoride glass ceramics containing LaF3 nanocrystals were obtained by thermal treatment on the asmade glasses. The formation of LaF3 nanocrystals and the incorporation of Tm3+ and Yb3+ into LaF3 nanocrystal lattice were confirmed by X-ray diffraction and high resolution transmission electron microscopy. Infrared quantum cutting involving Yb3+ 950–1100 nm (2F5/2→2F7/2) emission was achieved upon the excitation of the 1G4 energy level of Tm3+ at 468 nm. We measured the photoluminescence properties of these glass ceramics. We also investigated the thermal treatment duration dependent quantum efficiency, and found that the quantum efficiency is 13% increased for the 0.5Tm3+-4Yb3+ doped glass ceramic with a maximum value of 144%, and 16% increased for the 0.5Tm3+-8Yb3+ doped glass ceramic with a maximum value of 162%, respectively.
©2008 Optical Society of America
The high energy of the vacuum ultraviolet (VUV) photons makes it theoretically possible to achieve two visible photon emissions for every incident VUV photon with quantum efficiency close to 200% . This visible quantum cutting has been widely investigated due to its application on mercury-free fluorescent lamps and plasma display panels [2, 3]. Recently, quantum cutting has also been applied for photovoltaic applications . In silicon solar cells, a single electron-hole pair is generated when the incoming photon energy is above 1.1 eV, with the excess energy being lost to heat. The thermalization of charge carriers generated by the absorption of high-energy photons is one of the major loss mechanisms leading to low energy conversion efficiencies of solar cells. The quantum cutting process can divide a high-energy photon into two or more photons with lower energy, which could largely reduce the thermalization loss [5–7]. Rare earth ions with abundant energy levels are good candidates for quantum cutting, and the infrared quantum cutting has been reported in Tb3+-Yb3+ co-doped systems [8–10].
In this paper, infrared quantum cutting is achieved in Tm3+-Yb3+ codoped transparent glass ceramics containing LaF3 nanocrystals. The infrared emission around 1000 nm originated from Yb3+2F5/2→2F7/2 transition is observed under the excitation of Tm3+3H6→1G4 absorption due to the cooperative energy transfer from one Yb3+ ion to two nearest neighboring Tm3+ ions simultaneously. In addition, we found that the quantum efficiency in the glass ceramics increases with increasing thermal treatment duration. This oxyfluoride glass ceramics are promising for applications because they not only have the advantages of high chemical and mechanical stabilities, but also contain nanosize crystallite phases that can improve the optical properties without loss of transparency.
Oxyfluoride glasses with compositions of 45SiO2-12Na2O-23Al2O3-20LaF3-0.5Tm3+-xYb3+(x=0, 4, and 8) were prepared by using high purity (99.99%) SiO2, Na2O, Al2O3, LaF3, TmF3 and YbF3 as raw materials, which were mixed homogeneously and melted at 1450°C for 40 min in a covered corundum crucible in air. The melts were poured onto a cold brass plate and then pressed by another plate. Differential thermal analysis (DTA) measurements were carried out in the SDT Q600 differential thermal analyzer at a heating rate of 10°C /min, from which the crystallization temperature of the glass was determined (750°C). The thermal treatment temperature was selected at 720 °C for obtaining transparent glass-ceramics. Transparent glass ceramics were obtained by thermal treatment at 720 °C for 4, 8, 12, 16, 24, 32, and 40h. X-ray diffraction measurements were carried out using a Rigaku D/MAX-RA diffractometer with Cu Kα as the incident radiation source. The microstructures of the samples were observed by JEM-2010 high resolution transmission electron microscopy (HRTEM). The excitation and emission spectra and the fluorescence decay curves in both the visible and infrared regions were recorded with a FLS920 fluorescence spectrophotometer.
3. Results and discussion
Figure 1 shows the XRD patterns of the 0.5Tm3+-8Tb3+ doped as-made glass and glass ceramics. It is observed that the as-made glass is completely amorphous with no diffraction peaks. After crystallization by thermal treatment at 720°C for 4h, 8h, 24h and 40h, the XRD patterns show intense diffraction peaks, which can all be assigned to the hexagonal LaF3 phase. The longer the thermal treatment duration, the more dominate the XRD peaks, which indicates more LaF3 nanocrystals are formed. Additionally, the position of the main diffraction peak moves to the larger angle side with increasing thermal treatment duration (as shown in the left hand inset), which indicates the contraction of the lattice parameters. The thermal treatment duration dependent lattice parameters of the hexagonal LaF3 phase obtained from the XRD peak positions are described in the right hand inset. This contraction is due to the substitution of Tm3+ and Yb3+ for La3+ in LaF3 lattice. Since the radius of Tm3+ and Yb3+ are smaller than La3+, the influence of the size mismatch on the lattice parameter become dominant when the LaF3 crystals grow better during the crystallization process, which leads to the contraction of the lattice parameters with increasing thermal treatment duration. Such incorporation of rare earth ions into fluoride nanocrystals during crystallization have been reported in other oxyfluoride glass ceramics [10–12].
Figure 2 (a) shows the optical images of the as-made glass with 0.5Tm3+-8Yb3+ doping and the corresponding glass ceramics obtained by 4h, 8h, 24h, and 40h thermal treatment at 720°C, respectively. All the samples keep transparency even after 40h thermal treatment, which is important for a downconversion layer placed in front of a solar cell. Figs. 2(b) and 2(c) present the HRTEM images of the 24h thermal treated glass ceramic. The HRTEM observations reveal that the hexagonal LaF3 nanocrystals are formed and the size distribution is 30–60 nm.
Figure 3 depicts the excitation and emission spectra of 0.5Tm3+ single-doped and 0.5Tm3+-8Yb3+ codoped glass ceramics. The excitation band corresponding to Tm3+ 651 nm emission is situated at 468 nm (blue line), which can be assigned to 3H6→1G4 transition. In the 0.5Tm3+-8Yb3+ codoped glass ceramic, the excitation spectra of Yb3+ infrared emission at 1016 nm (green line) is in good agreement with Tm3+ 3H6→1G4 absorption, which indicates the performance of energy transfer. The emission spectra of 0.5Tm3+ single doped and 0.5Tm3+-8Yb3+ codoped glass ceramics were recorded under 468 nm excitation. The emission intensities of Tm3+ at 651 nm and 786 nm decrease with the introduction of Yb3+. For the codoped sample, Yb3+ emission in the infrared region is observed under the excitation of Tm3+3H6→1G4 absorption. This is another evidence of the energy transfer from Tm3+ to Yb3+. In addition, Yb3+ emission in the infrared region consists of two peaks, a sharper peak located at 980 nm and a broader peak centered at 1016 nm, which are originated from the transitions of the lowest Stark level of the 2F7/2 multiplet to two different Stark levels of the 2F5/2 multiplet .
Figure 4 draws the schematic energy levels with transitions which may be involved in the cooperative energy transfer process from one Tm3+ ion to two Yb3+ ions. With excitation at 468 nm, Tm3+ emissions occur at 651 and 786 nm (shown in Fig. 3), which can be assigned to the transitions from 1G4 to 3F4 and 3H5 levels, respectively. Meanwhile, two near-infrared photons due to the Yb3+ 2F5/2→2F7/2 transitions are obtained from one absorbed visible photon.
The energy transfer efficiency ηET is defined as the ratio of donors that are depopulated by energy transfer to the acceptors over the total number of donors being excited. In the present system, Tm3+ acts as the donor and Yb3+ as the acceptor. The total quantum efficiency, ηQE, can be defined as the ratio of the photons emitted to the photons absorbed, assuming that all excited Yb3+ ions decay radiatively. This assumption leads to an upper limit of quantum efficiency, and the actual quantum efficiency maybe lower due to concentration quenching. Since Yb3+ ions have only two manifolds (the 2F7/2 ground state and the 2F5/2 excited state) and the energy migration among Yb3+ ions in the nanocrystals is not as efficient as in the bulk , the concentration quenching effect is neglectable in the glass ceramics investigated with the highest Yb3+ doping of 8%.
The energy transfer efficiency can be obtained experimentally by dividing the integrated intensity of the decay curves of the Tm3+-Yb3+ codoped glass ceramics to the integrated intensity of the Tm3+ single doped curve, where I represents for the intensity and x (x=4, 8) the Yb3+ concentration, respectively. The relation between the transfer efficiency and the quantum efficiency is linear and is defined as ηQE=ηTm-r(1-ηET)+2ηET, where the quantum efficiency for Tm3+ ions, ηTm-r, is set to 1 .
The luminescence decays of Tm3+ were monitored in the Yb3+ free and the Yb3+ doped glass ceramics obtained by thermal treatment on the as-made glasses for various duration. The integrated intensity of these decay curves are given in Table 1, according to which, the thermal treatment duration dependent quantum efficiency is calculated and depicted in Fig. 5. From these two efficiency curves, we observe that with increasing thermal treatment duration, the quantum efficiency is 13% increased for the 0.5Tm3+-4Yb3+ doped glass ceramic with a maximum value of 144%, and 16% increased for the 0.5Tm3+–8Yb3+ doped glass ceramic with a maximum value of 162%, respectively. This enhancement can be explained as follow: during the crystallization process of thermal treatment, the rare earth ions of Tm3+ and Yb3+ could be incorporated into the LaF3 nanocrystals. The fluoride crystal provides the distinct advantage of low phonon frequencies compared to oxide materials, which leads to more efficient energy transfer from Tm3+ to Yb3+ as well as a higher quantum efficiency. This result indicates that the crystallization process is necessary for getting excellent quantum efficiency.
The glass ceramics begin to lose their transparency after 40 h thermal treatment, which can be ascribed to the formation of larger LaF3 crystals. Since the transparency is an essential property for the downconversion layers placed in front of the solar cells, the optimum thermal treatment duration should be less than 40 h.
In summary, Tm3+-Yb3+ codoped transparent oxyfluoride glass ceramics containing LaF3 nanocrystals were obtained by thermal treatment on the as-made glasses for various hours. The formation of LaF3 nanocrystals and the incorporation of Tm3+ and Yb3+ into LaF3 crystals were confirmed by XRD and HRTEM. Excitation, emission and decay measurements were performed to prove the occurrence of cooperative energy transfer from one Tm3+ ion to two Yb3+ ions, which leads to 950–1100 nm infrared emission subsequently. The thermal treatment duration dependent quantum efficiency is calculated, which is 13% increased for the 0.5Tm3+-4Yb3+ doped glass ceramic with a maximum value of 144%, and 16% increased for the 0.5Tm3+-8Yb3+ doped glass ceramic with a maximum value of 162%, respectively, after thermal treatment at 720°C for 40 hours. These oxyfluoride glass ceramics are promising for applications on the solar cells due to the combination advantages of high mechanical stabilities and good optical properties.
This work was financially supported by National Nature Science Foundation of China (Grant No. 50672087 and No. 60778039), National Basic Research Program of China (2006CB806007) and National High Technology Program of China (2006AA03Z304). This work was also supported by the program for Changjiang Scholars and Innovative Research Team in University (IRT0651).
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