1. Introduction

In recent years, the scope of investigation on quantum cutting (QC) phosphors has been extended to the near infrared (NIR) region because of its potential application in solar cells.[1, 2] There have been extensive investigations on phosphors co-doped with rare earth ions, e.g. Tb³⁺-Yb³⁺, Tm³⁺-Yb³⁺, and Pr³⁺-Yb³⁺, where Tb³⁺, Tm³⁺, and Pr³⁺ act as sensitizers.[3-6] To improve the solar cell efficiency, it is necessary to develop a phosphor which shows an optimized broad and efficient absorption band in near-ultraviolet and visible region, where the solar cells show poor response [7,8], and an emission in the NIR region slightly shorter than the bandgap wavelengths of Si or Ge, where the solar cells exhibit their greatest spectral response. Potential broad and efficient absorption sensitizers are the rare earth ions, e.g. Eu²⁺ and Ce³⁺, which have allowed f-d transition. [8-11] On the other hand, Yb³⁺ ion as a proper luminescent center has been extensively studied because of its energy level difference between ²F_5/2 and ²F_7/2 level matching well with the bandgap of Si (Eg~1.12eV). Similarly, we try to find an appropriate luminescent center for Ge based solar cells, which have been developed as a new type of solar cells with a narrow bandgap. The bandgap of Ge is about 0.785eV, which is a little smaller than the energy level difference of Er³⁺: ⁴I_13/2→⁴I_15/2 transition. Recently, Chen et al. proposed efficient near infrared three-photon quantum cutting excited by visible light in Er³⁺ ions.[12] However, the absorption of Er³⁺ in near-ultraviolet and visible region is very weak and narrow due to the forbidden f-f transition. In this paper, Eu²⁺ is chosen as sensitizer for Er³⁺ in Ca₈Mg(SiO₄)₄Cl₂ phosphor and the multi-photon quantum cutting involved energy transfer process is discussed.

2. Experimental

The preparatory materials of CaCO₃, MgO, SiO₂, and CaC1₂ were weighted with an appropriate stoichiometric ratio. A small amount of high purity Eu₂O₃ (99.99%) and Er₂O₃ (99.99%) were added, and CaC1₂, excessive by about 100%, was used as a flux. After mixing and grinding thoroughly in an agate mortar, the homogeneous mixture was sintered at 1100°C for 2h in a reducing atmosphere. The sintered cake was milled and washed with deionized water to remove the excessive CaCl₂, and the target phosphor sample was obtained with the help of the drying procedure. The three samples were labeled as CMSC: Eu²⁺, CMSC: Er³⁺ and CMSC: Eu²⁺-Er³⁺ corresponding to Eu²⁺ (2 mol. %) singlely doped, Er³⁺ (2 mol. %) singlely doped and Eu²⁺ (2 mol. %)-Er³⁺ (2 mol. %) codoped, respectively. The crystal structure of the samples was checked using an X-ray diffractometer. Excitation, emission spectra and time-resolved emission spectra of the samples were detected using a FLS920 fluorescence spectrophotometer. Samples were excited by monochromatic light from a standard Xe lamp (450 W) passed through a monochromator. The visible and NIR luminescence were detected by photomultiplier PMT R928 and PMT 5509, respectively. The time-resolved emission spectra of Er³⁺ were obtained by μs flash lamp pumping. All the measurements were carried out at room temperature.

3. Results and discussion

Figure 1 shows the XRD patterns of CMSC: Eu²⁺, CMSC: Er³⁺ and CMSC: Eu²⁺-Er³⁺. Compared to the standard data of PDF Card No.80-0449, the three strong diffraction peaks can be indexed to Ca₈Mg(SiO₄)₄Cl₂. And there exists weaker diffraction peaks assigned to Ca₂SiO₄, which are marked with five-pointed star. According to the large difference of the intensity of the strongest diffraction peaks of two phases, the main crystal phase of our three samples is Ca₈Mg(SiO₄)₄Cl₂. The relative intensity ratio between the strongest diffraction peaks of two phases is less than 10%. The existence of a small quantity of second phase has tiny effect on the energy transfer from Eu²⁺ to Er³⁺ in Ca₈Mg(SiO₄)₄Cl₂.

 

Fig. 1 XRD patterns of CMSC: Eu²⁺, CMSC: Er³⁺ and CMSC: Eu²⁺-Er³⁺.

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The host of calcium magnesium chlorosilicate, Ca₈Mg(SiO₄)₄Cl₂, has a cubic crystal structure with the space group of Fd3m. In the crystal lattices, there exist three kinds of cation sites: two Ca sites and one Mg site. Ca atoms occupy both an octahedral site with six oxygen nearest neighbors and a polyhedral site surrounded by six oxygen and two chlorine atoms, which are named as Ca(I) and Ca(II) sites, respectively.[13] Taking the crystal structure into account, the lattice would be distorted and unstable when the Mg²⁺ ion (0.057nm) is replaced with bigger ions (Eu²⁺ (0.117nm) or Er³⁺ (0.089nm)). As the ionic radius of Eu²⁺ or Er³⁺ is closer to that of Ca²⁺, the Ca²⁺ site will preferentially be occupied by Eu²⁺ or Er³⁺. By calculating the value of r⁺/r, which means the ratio of Ca²⁺ and O^2- here, it can be inferred that Eu²⁺ mainly occupies the 8-coordinated Ca(II) site (0.112nm), while Er³⁺ occupies the 6-coordinated Ca(I) site.

Figure 2 shows the excitation and emission spectra of CMSC: Eu²⁺, CMSC: Er³⁺ and CMSC: Eu²⁺-Er³⁺. The broad excitation spectrum of Eu²⁺ extending from 250 to 500nm mainly has two characters. The broad bands are due to the parity allowed 4f⁷ (⁸S_7/2)→4f⁶5d¹ transitions: the sub band at about 280nm is due to the transition from the ground state ⁸S_7/2 to the crystal field split e_g (4f⁶5d¹) state, and the peaks at 335, 375 and 430nm vicinities correspond to the t_2g (4f⁶5d¹) state. In addition, fine structure is observed on the range of 380-480nm, which can be resolved at room temperature. This fine structure is assigned to the splitting of the 4f⁶ configuration in the 4f⁶5d¹ excited state into seven 7F_J levels.[13] The excellent consistency of the shape and location between the excitation spectrum of Er³⁺ monitored at 1544 nm and Eu²⁺ monitored at 505 nm proves the energy transfer from Eu²⁺ to Er³⁺ in Ca₈Mg(SiO₄)₄Cl₂. Besides, the emission of Er³⁺ of CMSC: Eu²⁺-Er³⁺ in near-infrared region presents strong luminescence, which is about 7 times stronger than that of CMSC: Er³⁺ when measured under the same condition. This can be assigned to the strong absorption of Eu²⁺ and the efficient energy transfer from Eu²⁺ to Er³⁺. We noticed that Er³⁺ shows a strong transition very close to the excitation wavelength of 378 nm. Compared with the f-f transition of Er³⁺, the f-d transition of Eu²⁺ at the same region shows efficient absorption due to the large absorption cross section. It is also noticed that, different from the normal energy transfer process, the luminescence intensity of sensitizer i.e. Eu²⁺ in the visible region shows little decrease when Eu²⁺ is codoped with Er³⁺. We suppose that it may be due to the charge balance effect when Er³⁺ ions doped to replace Ca²⁺ ions. The electrons were released by electronegative Ca vacancy and combined with Eu³⁺. [14] This process may promote the reduction of Eu³⁺, resulting in the increase of luminescence intensity due to Eu²⁺.

 

Fig. 2 Excitation and emission spectra of CMSC: Eu²⁺, CMSC: Er³⁺ and CMSC: Eu²⁺-Er³⁺.

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To further prove the energy transfer process from Eu²⁺ to Er³⁺, we measured the time-resolved emission spectra of Er³⁺ infrared emission under 378nm excitation, as shown in Fig. 3 . With increasing the delay time, the intensity of Er³⁺ emission increases within 200 μs and then decreases gradually, which is in accordance of the decay curve of Er³⁺ in CMSC: Eu²⁺-Er³⁺. This corresponds to the population and depopulation processes of Er^{3+ 4}I_13/2 energy level. However, the rise time is about 200μs and longer than lifetime of Eu²⁺ 5d level (~380ns). This corresponds to the population process of Er^{3+ 4}I_13/2 energy level caused by the energy feeding of Eu²⁺ and the energy level transition process in Er³⁺ ions from high energy level to ⁴I_13/2 level.

 

Fig. 3 Time-resolved emission spectra of Er³⁺ near-infrared emission in CMSC: Eu²⁺-Er³⁺ under 378nm excitation.

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Figure 4 shows the laser power dependent emission intensity changes in CMSC: Eu²⁺-Er³⁺ pumped by 375 nm laser. We know that the relationship between the emission intensity and excitation power is I∝p ⁿ. It means that log(I) and log(p) will show liner relationship, as shown in Fig. 4. The slope of Eu²⁺ emission in visible region is equal to 1(n = 1) for one photon energy transfer process, while the slope of Er³⁺ emission in NIR region is smaller than 1 for more than one photon cutting process involved.

 

Fig. 4 Laser power dependent emission intensity changes in CMSC: Eu²⁺-Er³⁺ pumped by 375 nm laser.

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The energy level diagram of Eu²⁺ and Er³⁺ in Ca₈Mg(SiO₄)₄Cl₂ phosphor is shown in Fig. 5 to illustrate a possible energy transfer process between Eu²⁺ and Er³⁺. We give the most likely energy transfer path based on our experiments. The excited energy was efficiently absorbed by the allowed 4f-5d transition of Eu²⁺ ions and then transfer to Er³⁺ ions via a well matched energy level ⁴G_11/2. ⁴G_11/2 level of one Er³⁺ ion is excited, by absorbing one photon, decays to ⁴F_7/2 and to ⁴S_3/2, respectively; at the same time ⁴I_13/2 level is excited (process (1) and (2)). In both cases the highly excited Er³⁺ ion may then follow the de-excitation path described in Fig. 5. (3) and (4) (⁴S_3/2→⁴I_9/2, ⁴I_15/2→⁴I_13/2 and ⁴I_9/2→⁴I_13/2, ⁴I_15/2→⁴I_13/2). Therefore, four Er³⁺ ions may be excited at the ⁴I_13/2 level for each absorbed photon. Moreover, process (5) brings two Er³⁺ ions in the ⁴I_9/2 level. This may determine four, three, or two excitations, depending on whether it is followed or not by process (4). And in process (6) followed by process (2), three excitations may be obtained. Further investigations should be carried out including the direct measurement of quantum efficiency.

 

Fig. 5 Schematic energy level diagram of Eu²⁺ and Er³⁺ in Ca₈Mg(SiO₄)₄Cl₂ phosphor, showing the concept of infrared quantum cutting with visible excitation at 378nm.

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4. Conclusion

Calcium magnesium chlorosilicate phosphor codoped with Eu²⁺ and Er³⁺ was obtained by solid-phase method. The crystal structure before and after rare earth ions doping were identified by XRD patterns. Energy transfer from Eu²⁺ to Er³⁺ is confirmed by the fluorescence spectra and time-resolved emission spectra. The laser power dependent intensity change relationships proved the existence of more than one photon cutting process. A possible mechanism including energy transfer from Eu²⁺ to Er³⁺, and multi-photon emission from ⁴I_13/2 level via cross relaxation has been proposed. The effective NIR quantum cutting process and broad absorption in ultraviolet-visible region is of great significance for improving the efficiency of germanium solar cells.