1. Introduction

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% [1]. 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 [4]. 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⁺²F_5/2→²F_7/2 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.

2. Experimental

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].

 

Fig. 1. XRD patterns of the 0.5Tm³⁺-8Tb³⁺ doped as-made glass and glass ceramics obtained by 4, 8, 24 and 40 h thermal treatment at 720°C, respectively. The left hand inset describes the main diffraction peak position of LaF₃ nanocrystal, and the right hand inset describes the thermal treatment duration dependent lattice parameters.

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Fig. 2 (a). optical images of the as-made glass with 0.5Tm³⁺-8Yb³⁺ doping and the corresponding glass ceramics obtained by 4, 8, 24 and 40h thermal treatment, (b) and (c): HRTEM images of the 24h thermal treated glass ceramic.

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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 Tm^{3+ 3}H₆→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 ²F_7/2 multiplet to two different Stark levels of the ²F_5/2 multiplet [13].

 

Fig. 3. Left side: Excitation spectra of Tm³⁺ 651 nm emission monitored in 0.5Tm³⁺ single doped glass ceramic (blue line) and of Yb³⁺ 1016 nm emission monitored in 0.5Tm³⁺-8Yb³⁺ codoped glass ceramic (green line). Right side: Emission spectra of 0.5Tm³⁺ single doped (black line) and 0.5Tm³⁺-8Yb³⁺ codoped (red line) glass ceramics under the excitation of 468 nm.

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Fig. 4 Schematic energy level diagram of Tm³⁺ and Yb³⁺ with transitions that may be involved in the cooperative energy transfer.

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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 Yb^{3+ 2}F_5/2→²F_7/2 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, η_QE, 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 ²F_7/2 ground state and the ²F_5/2 excited state) and the energy migration among Yb³⁺ ions in the nanocrystals is not as efficient as in the bulk [14], 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, ${\eta }_{\mathrm{ET}}=1-\frac{\int I_{x-\mathrm{Yb}}\mathrm{dt}}{\int I_{0-\mathrm{Yb}}\mathrm{dt}},$ 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 η_QE=η_Tm-r(1-η_ET)+2η_ET, where the quantum efficiency for Tm³⁺ ions, η_Tm-r, is set to 1 [8].

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.

Table 1. The integrated intensity of Tm³⁺ decay curves

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Fig. 5. Thermal treatment duration dependent quantum efficiency.

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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.

4. Conclusion

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.