We report on conversion of near-ultraviolet and visible radiation ranging from 250 to 500nm into near-infrared emission by a Ca8Mg(SiO4)4Cl2: Eu2+, Er3+ phosphor. Efficient 1530-1560nm Er3+ emission (4I13/2→4I15/2) was detected under the excitation of Eu2+ (4f→5d) absorption band as a result of energy transfer from Eu2+ to Er3+, which is confirmed by both steady state and time-resolved emission spectra. The laser power dependent emission intensity changes were investigated to analysis the energy transfer mechanism. Energy transfer from Eu2+ to Er3+ followed by a multi-photon quantum cutting of Er3+ is proposed. The result indicates that the phosphor has potential application in enhancement of conversion efficient of germanium solar cells because the energy difference of Er3+: 4I13/2→4I15/2 transition matches well with the bandgap of Ge (Eg~0.785 eV).
©2010 Optical Society of America
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 2F5/2 and 2F7/2 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+: 4I13/2→4I15/2 transition. Recently, Chen et al. proposed efficient near infrared three-photon quantum cutting excited by visible light in Er3+ ions. 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.
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.
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. 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 O2- 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 (8S7/2)→4f65d1 transitions: the sub band at about 280nm is due to the transition from the ground state 8S7/2 to the crystal field split eg (4f65d1) state, and the peaks at 335, 375 and 430nm vicinities correspond to the t2g (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 7FJ levels. 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+.  This process may promote the reduction of Eu3+, resulting in the increase of luminescence intensity due to Eu2+.
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 Er3+ 4I13/2 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 Er3+ 4I13/2 energy level caused by the energy feeding of Eu2+ and the energy level transition process in Er3+ ions from high energy level to 4I13/2 level.
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.
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 4G11/2. 4G11/2 level of one Er3+ ion is excited, by absorbing one photon, decays to 4F7/2 and to 4S3/2, respectively; at the same time 4I13/2 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) (4S3/2→4I9/2, 4I15/2→4I13/2 and 4I9/2→4I13/2, 4I15/2→4I13/2). Therefore, four Er3+ ions may be excited at the 4I13/2 level for each absorbed photon. Moreover, process (5) brings two Er3+ ions in the 4I9/2 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.
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 4I13/2 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.
This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 50872123, and 50802083), National Basic Research Program of China (Grant No. 2006CB8060007), and the Program for Changjiang Scholars and Innovative Research Team in University (Grant No. IRT0651).
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