Effects of Heat Treatment and Yb 3+ Concentration on the Downconversion Emission of Er 3+ /Yb 3+ Co-Doped Transparent Silicate Glass-Ceramics

The SiO 2 -Al 2 O 3 -BaF 2 -TiO 2 -CaF 2 transparent silicate glass-ceramics containing BaF 2 nanocrystals were successfully prepared by heat treatment process through conventional melting method. Effects of heat treatment processes and Yb 3+ concentration on the downconversion (DC) emission of the co-doped Er 3+ /Yb 3+ transparent silicate glass-ceramics were investigated. With the increase of temperatures and times of heat treatment process, the DC emission intensity of the co-doped Er 3+ /Yb 3+ glass-ceramics was significantly enhanced. At the same time, with the increase of Yb 3+ concentration, the value of DC intensity of Er 3+ /Yb 3+ co-doped bands centered at 849, 883 and 1533 nm is maximized when the concentration of Yb 3+ reaches 2.5 mol.%. When the concentration exceed 2.5 mol. %, the DC emission intensity of Er 3+ /Yb 3+ co-doped bands centered at 849, 883 and 1533 nm was decreased, owing to the self-quenching effect. It’s interesting that the DC emission intensity of Er 3+ /Yb 3+ co-doped band centered at 978 nm didn’t quench when the Yb 3+ concentration exceed 2.5 mol. %. At the same time, the DC mechanism and ET processes between Yb 3+ and Er 3+ ions were discussed.


Introduction
In recent years, the silicon solar cells (Si-SC) are widely used to produce electric energy, it is considered a green and inexhaustible source of energy. Therefore, many studies have developed to enhance the emission spectrum of Si-SC energy [1][2][3][4] . Usually, there are two processes that contribute to the increase in emission solar cells (SC) spectrum, which is the downconversion (DC) and the upconversion (UC) of rare earth (RE 3+ ) ions. Among them, the DC emission of the single doped Er 3+ and the co-doped Er 3+ with others RE 3+ ions is a promising way to increase the efficiency spectrum of SC [5][6][7] .
In reality, the solar spectrum is within the wavelength range of 300-2500 nm 8 , whereas the band-gap of the Si-SC converts only a small band around (1.000 nm at full efficiency into the electricity. The spectrum below the band-gap is not absorbed at all, and the spectrum above the band-gap is fully absorbed but is converted into electricity with high thermal losses. This spectral mismatch are caused a major loss of energy. Therefore, the researchers have interested in improve DC luminescence intensity of the co-doped RE 3+ ions to deliver the highest spectrum efficiency for the SC energy 2, [9][10][11] . Among the existing trivalent RE 3+ ions, the Yb 3+ has a relatively simple electronic structure of two energy-level manifolds: the 2 F 7/2 ground state and 2 F 5/2 excited state around (1000 nm in the near-infrared (NIR) region, which located just above the band-gap of Si-SC 1,12 . Similar to Yb 3+ , the Er 3+ also is one of most efficient ions combining to enhance SC spectrum because it has a favorable energy level structure with 4 I 15/2 → 4 I 11/2 transition corresponding to NIR emission of about 980 nm. Therefore, enhancement on the DC emission can be achieved by combine of the co-doped Er 3+ /Yb 3+ , through energy transfer (ET) process between between Er 3+ and Yb 3+ ions. There upon the energy is transferred to two Yb 3+ ions via a resonant ET process. Finally, the Yb 3+ ions will emit the two required photons with the band-gap energy of Si-SC 13 .
In 2009, L. Aarts et al., 14 have investigated the DC emission for SC in NaYF 4 : Er 3+ , Yb 3+ . This result indicated that the desired DC process from the 4 F 7/2 level has very low efficiency due to fast multi-phonon relaxation from the 4 F 7/2 to 4 S 3/2 level via the intermediate 2 H 11/2 level. Recently, In the paper of M.B. de la Mora et al., 15 mentioned the materials for DC in SC: Perspectives and challenges.
Results of this paper affirmed among different options, downconversion is an appealing way to harvest the efficiency in solar cells because it permits to optimize the solar spectrum usage 15 . With the purposed to improved efficiency photoluminescence for the solar cells application. In previous studies, we have investigated enhancement of upconversion emission of Er 3+ /Yb 3+ co-doped transparent silicate glass-ceramics containing BaF 2 nanocrystals by effects of Mn 2+ concentrations 16 and heat treatment processes 17 . In this work, we continues to investigation the effects of the heat treatment processes and Yb 3+ concentration on the DC emission intensity of the co-doped Er 3+ /Yb 3+ transparent silicate glass-ceramics containing BaF 2 nanocrystals. At the same time, the mechanism of DC and ET processes between Yb 3+ and Er 3+ ions are also proposed and discussed.

Experimental Details
The glasses were prepared according to a conventional melt-quenching method.  Table 1. Mixtures with a sufficient weight of approximately 10 g, compacted into a platinum crucible, were set in an electric furnace. The electric furnace in this study manufactured by Nabertherm, Germany. After holding at 1500 ºC for 45 min under air atmosphere in an electric furnace, the melts were quenched by putting them onto a polished plate of stainless steel. According to the glass transition temperature (T g ) of differential thermal analysis which was determined by differential scanning calorimeter (DTA-60AH SHIMADZU) with a heating rate of 10 ºC/min under a nitrogen atmosphere.
The samples were cut into the size of 10×10×2 mm 3 and polished for optical measurements. To identify the crystallization phase, XRD (X-ray diffraction) analysis was carried out with a powder diffractometer (BRUKER AXS GMBH) using CuKα radiation. The sizes, shape, structure and component compositions of the asprepared nanocrystals were characterized by transmission electron microscopy (TEM, JEM-2100) at 200 kV. The reflectance spectra in the wavelength range of 350-1800 nm were measured on a Hitachi U-4100 spectrophotometer. The DC spectra in the wavelength range of 800-1650 nm and lifetime curves were measured on an Edinburgh Instruments FLS980 fluorescence spectrometer using a µF920 microsecond flash lamp as the excitation source and detected using a liquid-nitrogen-cooled PbS detector upon excitation at 410 nm. All spectral, DTA, XRD, TEM measurements were conducted at ambient temperatures.

Results and Discussion
To characterize the thermal stability of the prepared SiO 2 -Al 2 O 3 -BaF 2 -TiO 2 -CaF 2 glass system, a DTA curve of SEY-1 glass sample was measured and showed in Fig.1.  As can be seen in this figure, three temperature parameters: the glass transition temperature (T g ) was located around 554 ºC, the crystallization onset temperature (T x1 = 675 ºC), two crystallization peaks temperatures (T p1, T p2 ) are located around 685 o C and 773 o C, respectively. Therefore, the transparent silicate glass-ceramics can be prepared by heat-treat in the first crystallization peak near 665 ºC, by controlling the appropriate crystallization temperature and process. Besides, between ~710ºC and 773 ºC, an endothermic reaction occurs. It's also the crystallization onset temperature (T x2 ) and the T x2 is determined value around 753ºC. The difference ΔT between the crystallization onset temperature T x1 and the glass transition temperature T g (ΔT = T x1 -T g ) is used as a rough indicator of glass thermal stability, and the ΔT = 675 ºC -554 ºC = 121ºC > 100 ºC indicating the prepared glass is stable and suitable for applications such as fiber amplifiers and solar cells, etc. Based on the analysis results of the DTA curve, all the prepared glasses were heattreated within the range of 665 o C to 773 o C. However, when glass-ceramics samples heat-treated up to 695 o C, the glassceramics sample is no longer transparent glass-ceramics. The optical images of glass-ceramics samples heat treatment at ~600, 685, 695 and 773 o C as shown in inset of Fig. 1.
The transparent silicate glass-ceramics was prepared and the nanocrystals structures in the glass-ceramics were monitored by XRD. The XRD patterns of glass-ceramics after heat treatment at different temperatures are shown in Fig. 2 (a). From the results of Fig. 2(a) shows when the increase of processing temperature from 600 up to 685 ºC, crystal size of BaF 2 nanocrystals was increased from 10.7 up to 17.9 nm. Relationship between crystal size with the heat treatment temperatures are shown in the Fig. 2  Also from the result of the Fig. 2(a), the precursor glass sample presents a broad diffraction curve characteristic of the amorphous state, while in the patterns of transparent silicate glass-ceramics, the intense diffraction peaks are clearly observed, indicating that microcrystallites are successfully precipitated during thermal treatment. The diffraction pattern of the crystalline element is typical of a face-centered-cubic and these diffraction peaks around 2θ(degree) = 26º, 30º, 43º, 50º and 53º can be assigned respectively to the (111), (200), (220), (311) and (222) planes of the BaF 2 cubic phase.
The XRD patterns of glass-ceramics after heat treatment at different times are shown in Fig. 2 (c). From the results of Fig. 2(c) shows when the increase of processing times from 10 up to 30h, crystal size of BaF 2 nanocrystals was increased from 17.6 up to 19.9 nm. Relationship between crystal size with the heat treatment times are shown in the Fig. 2(d).
The crystallites size D for a given (hkl) plane was estimated from the XRD patterns following the Scherrer equation:  Figs. 2 (c & d). Clearly, in this figure, the increase of the heat treatment temperatures and times were led to the crystal size increased, similar to the result of our previous works [17][18][19] .
The TEM image of SEY-0.2E2.5Y-685 transparent silicate glass-ceramics sample is shown in Fig. 3. From result of Fig. 3, it demonstrates that the BaF 2 nanocrystals were distributed homogeneously among the glass matrix and the mean sizes of nanocrystals were about 18-19 nm, which was similar to those calculated by Debye-Scherrer equation. The HRTEM image of the SEY-0.2E2.5Y-685 transparent silicate glass-ceramics sample is shown in inset of Fig. 3. As from this figure, the lattice spacing of (111) was estimated about 0.334 nm.
The DC emission spectra of the SEY-0.2E2.5Y-10h, SEY-0.2E2.5Y-15h, SEY-0.2E2.5Y-20h, SEY-0.2E2.5Y-25h, and SEY-0.2E2.5Y-30h transparent glass-ceramics samples, under excitation 410 nm are shown in Fig. 6. Similar in the case of changing heat treatment temperatures, the DC emission intensity of the Er 3+ /Yb 3+ co-doped bands centered at 824, 849, 883, 918, 978, 1265 and 1533 nm were strongly increased with the increase of heat treatment times from 10 to 30 h. These results confirms that the heat treatment processes greatly affects the DC emission intensity of Er 3+ /Yb 3+ co-doped transparent silicate glass-ceramics.   Furthermore, the effect of Yb 3+ concentration on the DC emission intensity of Er 3+ /Yb 3+ co-doped transparent silicate glass-ceramics were also presented follows. The DC emission spectra of SEY-0.2E0Y, SEY-0.2E1.0Y, SEY-0.2E1.5Y, SEY-0.2E2.0Y, SEY-0.2E2.5Y and SEY-0.2E3.0Y transparent glass-ceramics samples, under 410 nm excitation are shown in Fig. 7. As shown in the Fig. 7, in the DC process, the Yb 3+ ions act as an efficient sensitizer. While Er 3+ fixed concentration, with the increase of Yb 3+ concentration, the DC emission intensity of Er 3+ /Yb 3+ co-doped bands centered at 849, 883 and 1533 nm were strongly increased and reaches its maximum value when the content of Yb 2 O 3 is 2.5 mol. %. When the concentration exceed 2.5 mol. %, the DC emission intensity of Er 3+ /Yb 3+ co-doped bands centered at 849, 883 and 1533 nm was decreased. This result may be owing to the reasons mainly of the self-quenching effect can be attributed to the cluster or the ions pair between the Yb 3+ ions is possibly formed in high the Yb 3+ concentration 21 . Further, the increase of Yb 3+ concentration has enhanced the probability of interaction between the Yb 3+ ions and some impurity, such as OH − impurities was born from atmospheric moisture during melting 22 . Therefore, the Yb 3+ could not effectively absorb the pumping energy leading to the quenching of the DC emission intensities.
This phenomenon can be explained by these reasons: Firstly, as the molarity of Er 3+ ions increased, the increased luminescent centers lead the emission intensity bands centered at 824, 849, 883, 918, 1265 and 1533 nm significantly increased. Secondly, the possible ET from Yb 3+ to Er 3+ ions, contribute to the emission intensity bands centered at 824, 849, 883, 918, 1265 and 1533 nm improved while emission intensity bands centered at 978 nm decreased. The mechanism of the ET from Yb 3+ to Er 3+ ions was proposed as above section.

Conclusions
In study of this article, the effects of heat treatment and Yb 3+ concentration on the DC emission of Er 3+ /Yb 3+ co-doped in transparent silicate glass-ceramics containing BaF 2 nanocrystals were successfully investigated. Comparison with the precursor glass, the DC luminescence of Er 3+ /Yb 3+ co-doped transparent glass-ceramics has significantly enhanced after heat treatment process changing temperatures and times. With the increase of Yb 3+ concentration, the DC emission intensity of Er 3+ / Yb 3+ co-doped bands centered at 849, 883 and 1533 nm were strongly increased and reaches its maximum at 2.5 mol. % Yb 3+ concentration.
When the concentration exceed 2.5 mol. %, the DC emission intensity of Er 3+ /Yb 3+ co-doped bands centered at 849, 883 and 1533 nm was decreased, owing to the selfquenching effect. Whereas the DC emission intensity band centered 978 nm, corresponding to the transitions: 4 I 11/2 → 4 I 15/2 of Er 3+ and 2 F 5/2 → 2 F 7/2 of Yb 3+ didn't quench when the Yb 3+ concentration exceed 2.5 mol. %. At the same time, we deem that there was possibly an energy transition process from the 2 F 5/2 → 2 F 7/2 transition of Yb 3+ to the 4 I 11/2 → 4 I 15/2 and 4 F 9/2 → 4 I 13/2 transitions of Er 3+ ions. In addition, the data presented for this study might provide useful information for further development of the DC in transparent silicate glass-ceramics associated with the ET between Yb 3+ and Er 3+ ions. These materials are promising for applications in enhancing conversion efficiency of SC.