Near-infrared luminescence and energy transfer processes in LaOF:Nd³⁺, Yb³⁺

Abstract

Nd³⁺ and Yb³⁺ singly and codoped samples of LaOF were prepared by a hydrothermal synthesis method and the phase purity of samples was examined by X-ray powder diffraction. The Yb³⁺ concentration dependence of luminescence and decay times of LaOF:Nd³⁺, Yb³⁺ were investigated. Cross-relaxation energy transfer from Nd³⁺ (⁴D_3/2 → ⁴G_5/2) to Yb³⁺ (²F_7/2 → ²F_5/2) was confirmed not only based on the excitation spectra, but also by the decay properties of the Nd^{3+ 4}D_3/2 state. However, the dominant process was multi-phonon relaxation from Nd³⁺ higher levels to ⁴F_3/2, followed by energy transfer to Yb^{3+ 2}F_5/2. In addition, host energy transfer to Yb³⁺ and back energy transfer from Yb³⁺ (²F_5/2 → ²F_7/2) to Nd³⁺ (⁴I_9/2 → ⁴F_3/2) were also revealed. The present study could be rewarding in the design of efficient spectral converters applied in silicon-based solar cells.

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1. Introduction

At present, the commercial single-junction mono- and poly-crystalline Si solar cells dominate the photovoltaic market. However, crystalline Si wafer cells are characterized by relatively low efficiencies, considerably below the theoretical limits and below results that have been obtained in the laboratory.¹ The energy losses mainly originate from two causes: first, the photons with energy lower than the bandgap of Si are not absorbed by the solar cells; second, a huge part of the energy of photons with energy larger than the band gap is wasted through thermalization of charge carriers. Among the different approaches to improve the conversion efficiency of sunlight to electricity in solar cells, down-conversion (DC) or quantum cutting (QC), which means one high energy photon is converted into two low energy photons, is one of the interesting ways proposed to reduce the energy losses.²

Some of the lanthanide ions are very suitable to be used in DC materials because of their rich energy level structures that allow for efficient spectral conversion.³ Trivalent ytterbium has only two simple energy levels of 4f¹³ configuration with excited state at around 10 000 cm⁻¹ (1.24 eV), matching with the band-gap energy of silicon (1.12 eV). Therefore, Yb³⁺ is a very favourable activator, of which emission can be efficiently absorbed by silicon solar cells with very little thermalization losses. Some lanthanide ions with excited state twice the energy of Yb^{3+ 2}F_5/2 state can be the sensitizers of Yb³⁺. Such as Ce³⁺,^4,5 Pr³⁺,^6–8 Nd³⁺,^9,10 Tb³⁺,¹¹ and so on, codoped with Yb³⁺ in some hosts, have been investigated on the energy transfer processes and luminescence properties.

For the sensitizer of Nd³⁺, the DC from the Nd^{3+ 4}G_9/2 level via the cross-relaxation process from Nd³⁺ (⁴G_9/2 → ⁴F_3/2) to Yb³⁺ (²F_7/2 → ²F_5/2), followed by a second energy transfer step from the ⁴F_3/2 level of Nd³⁺ to Yb³⁺, are preferable processes for quantum cutting materials. It is reported that in YF₃:Nd³⁺, Yb³⁺ the first step energy transfer process is very inefficient due to the fast multi-phonon relaxation from ⁴G_9/2 state to lower states. But efficient cross-relaxation energy transfer from higher level ⁴D_3/2 to Yb³⁺ was observed.⁹ In LaOBr with lower phonon energy, more efficient first step energy transfer was observed.¹² The second step energy transfer process was almost confirmed in all reported materials.^9,12,13 In some hosts, back energy transfer from Yb³⁺ to Nd³⁺ was proposed.^13,14 Because of the low absorption coefficient of parity forbidden 4fⁿ–4fⁿ transitions, some research is focusing on finding sensitizers with large absorption coefficient to enhance the absorptivity in the blue-near UV range, such as by host sensitization.^10,15,16 As discussed in literature,⁹ the cross-relaxation energy transfer from higher level ⁴D_3/2 to Yb³⁺ was verified only by excitation spectra, decay curves are needed to validate this process. In addition, the research on sensitization of Nd³⁺ doped in host lattices with low phonon energy is crucial in the view point of developing efficient spectral converter for practical application in solar cells.

In this paper, we choose lanthanum oxyfluoride (LaOF) as the host, which has high thermal and chemical stability and superior luminescence quantum efficiency associated with low maximum phonon energy of hosts.¹⁷ The research focuses on the study of energy transfer processes between Nd³⁺ and Yb³⁺ and the host sensitization process, not only from the view point of steady state spectra, but also the decay curves of Nd³⁺ and Yb³⁺ emissions. The Yb³⁺ concentration dependence of decay of Nd^{3+ 4}D_3/2 and ⁴F_3/2 and Yb^{3+ 2}F_5/2 states will also be investigated.

2. Experimental

2.1 Sample preparation

Crystalline powder samples of LaOF:x% Nd³⁺ (x = 0.5, 1.0, 3.0, 5.0 and 10.0) and LaOF:1.0% Nd³⁺, x% Yb³⁺ (x = 0, 1.0, 2.0, 3.0 and 5.0) were prepared by hydrothermal method followed a heat-treatment process.¹⁸ The stoichiometric of 4 mmol LnCl₃·6H₂O (Ln = La, Nd, Yb) and 8 mmol trisodium citrate (99.0%) (Cit³⁻) were dissolved in deionized water to form 40 ml of solution, then 4 mmol NaF (98.0%) and 4 mmol NaCO₃ (99.8%) were added into the solution. After vigorously stirring for 1 h, the obtained solution was transferred into a Teflon bottle (50 ml), held in a stainless steel autoclave, sealed, and maintained at 180 °C for 24 h. As the autoclave cooled to room temperature naturally, the products were collected by centrifugal and washed with deionized water and ethanol in sequence. The precipitates LnCO₃F were obtained and then dried in air at 80 °C for 12 h. Finally the precursors were annealed at 600 °C in air for 4 h with a heating rate of 1 °C min⁻¹.

2.2 Characterization

The phase purity of the prepared samples was examined by an X-ray diffractometer (Japan, Shimadzu, XRD-6000) with Cu Kα (λ = 0.15406 nm) over the angular range 20° ≤ 2θ ≤ 80°, operating at 40 kV and 30 mA. The morphology and composition of the samples were examined using a field-emission scanning electron microscopy (FE-SEM, S-4800, Hitachi) equipped with an energy-dispersive X-ray (EDX) spectrometer. Excitation and emission spectra were recorded by a fluorescence spectrophotometer FLS980 (Edinburgh Instruments) equipped with a 450 W xenon arc lampas the excitation source and two detectors of Hamamatsu R928P photomultiplier tube (PMT, 200–870 nm) and InGaAs (900–2050 nm). Lifetime measurements were performed upon the pulsed excitation of 355 nm from a YAG:Nd laser with pulse duration of 10 ns and repetition frequency of 10 Hz. The luminescence was measured by a Zolix Omni-λ3005 monochromator with a grating blazed at 1250 nm and a detector of Hamamatsu PMT R5108 (400–1200 nm). The decay profiles were collected with a digital oscilloscope (RIGOLDS1104Z). All the measurements were carried out at room temperature.

3. Results and discussion

3.1 Powder X-ray diffraction, crystal structure and morphology

XRD patterns of LaOF:10.0% Nd³⁺ and LaOF:1.0% Nd³⁺, 5.0% Yb³⁺ as representatives were shown in Fig. 1. The positions and relative intensity of diffraction peaks agree well with the standard PDF card no. 06-0281 of LaOF, suggesting that the doping of Nd³⁺ and Yb³⁺ ions has no influence on the crystal structure of LaOF and no other phase was induced. The crystal structure of LaOF is rhombohedral, with space group of R3̄m (no. 166) and Z = 6. La³⁺ ions occupy sites with C_3v symmetry.¹⁹ The SEM image of micro particle LaOF:0.5% Nd³⁺, 1.0% Yb³⁺ is shown in Fig. S1,† the average size of the sample is 160 nm. EDS spectrum of it in Fig. S2† confirms the presence of all expected elements in LaOF.

Fig. 1 XRD patterns of prepared samples LaOF:10.0% Nd³⁺, LaOF:1.0% Nd³⁺, 5.0% Yb³⁺ and standard PDF card of LaOF.

3.2 Luminescence properties of LaOF:Nd³⁺

The emission spectrum of LaOF:1.0% Nd³⁺ upon Nd^{3+ 4}I_9/2 → ⁴D_1/2 excitation at 352.5 nm is shown in Fig. 2. The peaks ranging from 370 to 660 nm are attributed to transitions from ⁴D_3/2 and ²P_3/2 to ⁴I_J states. The peaks centered at 887 nm, 1059.5 nm and 1334 nm originate from ⁴F_3/2 to ⁴I_9/2, ⁴I_11/2 and ⁴I_13/2 transitions, respectively.²⁰ The inset in Fig. 2 shows the concentration dependence of Nd³⁺ integrated emission intensity in the range of infrared; all the conditions of measurement were uniform. With the increase of the doping concentration of Nd³⁺, the integrated emission intensity increases and reaches maximum at the concentration of 1.0%, then the intensity begins to decrease which is due to the concentration quenching of luminescence.

Fig. 2 Emission spectra of LaOF:1.0% Nd³⁺ upon ⁴D_1/2 state excitation at 352.5 nm. Inset shows the variation of Nd^{3+ 4}F_3/2 state integrated emission intensity with the concentration of Nd³⁺.

The excitation spectrum of LaOF:1.0% Nd³⁺ monitoring Nd^{3+ 4}F_3/2 → ⁴I_11/2 transition at 1059.5 nm is presented in Fig. 3. All the excitation peaks originating from Nd³⁺ 4f³ to 4f³ transitions were assigned in the figure.²⁰ The presence of excitation peaks from Nd³⁺ higher energy levels monitoring Nd^{3+ 4}F_3/2 state emission reveals that the feeding of ⁴F_3/2 state from these excited states through multi-phonon relaxation or cross-relaxation processes. From the established energy level diagram of Nd³⁺, the energy gaps of the neighboring higher levels are very small, therefore, multi-phonon relaxation to ⁴F_3/2 state should be the dominate process.⁹

Fig. 3 Excitation spectrum of LaOF:1.0% Nd³⁺ monitoring the emission of Nd³⁺ at 1059.5 nm.

3.3 Luminescence properties of LaOF:Nd³⁺, Yb³⁺

The dependence of emission spectra of LaOF:1.0% Nd³⁺, x% Yb³⁺ on the doping concentration of Yb³⁺ (x = 0, 1.0, 2.0 and 5.0) is demonstrated in Fig. 4. Upon Nd³⁺ excitation at 352.5 nm, the emission from Yb^{3+ 2}F_5/2 to ²F_7/2 transition (976 nm) appears in addition to the emissions of Nd^{3+ 4}F_3/2 state (887 nm, 1059.5 nm and 1334 nm).

Fig. 4 Emission spectrum of LaOF:1.0% Nd³⁺, x% Yb³⁺ (x = 0, 1.0, 2.0 and 5.0) upon excitation of Nd^{3+ 4}D_1/2 state at 352.5 nm.

The presence of the Yb³⁺ emission upon Nd³⁺ excitation indicates the occurrence of energy transfer from Nd³⁺ to Yb³⁺. From the comparison of relative emission intensity of Nd³⁺ and Yb³⁺ with the concentration of Yb³⁺ in Fig. 4, the emission intensity from Nd³⁺ decreases all along, whereas the emission intensity of Yb³⁺ increases first and reaches maximum at concentration of 2.0%, and then decreases. The decrease of Nd³⁺ emission intensity and increase of Yb³⁺ emission intensity with Yb³⁺ concentration are due to the increasing energy transfer efficiency from Nd³⁺ to Yb³⁺. However, concentration quenching occurs with further increase the content of Yb³⁺. As discussed in Section 3.2, electron on the Nd³⁺ higher excited state is dominated by non-radiative relaxation to the lower ⁴F_3/2 state. Based on this, the energy transfer route from Nd³⁺ (⁴F_3/2 → ⁴I_9/2) to Yb³⁺ (²F_7/2 → ²F_5/2) is proposed, the occurrence of which will further be confirmed according to the decay properties of related states. From the energy level structures of Nd³⁺ and Yb³⁺, the route of cross-relaxation energy transfer from the higher energy levels of Nd³⁺ to Yb³⁺ is also possible, which will be discussed in the following section.

Fig. 5 shows the excitation spectra of LaOF:1.0% Nd³⁺, x% Yb³⁺ (x = 0, 2.0 and 5.0) monitoring the emission of Nd³⁺ at 1059.5 nm and Yb³⁺ at 976 nm, respectively, and of LaOF:1.0% Yb³⁺ monitoring the emission of Yb³⁺ at 976 nm. The spectra in Fig. 5(b) and (c) were normalized at 748 nm of Nd^{3+ 4}F_7/2 excitation peak. In the excitation spectra of LaOF:1.0% Nd³⁺ monitoring the emission of Nd³⁺ at 1059.5 nm in Fig. 5(a), the characterized excitation peaks of Nd³⁺ are present and the corresponding transitions were indicated in the spectra. For the sample of LaOF singly doped with Yb³⁺, monitoring the emission of Yb³⁺ at 976 nm, a broad excitation band situating at around 256 nm emerges, which is originated from host lattice excitation of LaOF.^18,21 The appearance of host excitation band monitoring the emission of Yb³⁺ reveals the energy transfer from host to Yb³⁺. In the excitation spectra of Nd³⁺ doped LaOF, the host excitation band is not present, showing the absence of energy transfer from host to Nd³⁺.

Fig. 5 Excitation spectra of LaOF: 1.0% Nd³⁺ (a), LaOF: 1.0% Nd³⁺, 2.0% Yb³⁺ (b) and LaOF: 1.0% Nd³⁺, 5.0% Yb³⁺ (c) monitoring the emission of Nd³⁺ at 1059.5 nm and Yb³⁺ at 976 nm, respectively and of LaOF: 1.0% Yb³⁺ (a) monitoring the emission of Yb³⁺ at 976 nm. The spectra in (b) and (c) were normalized at 748 nm.

From the excitation spectra of LaOF:Nd³⁺, Yb³⁺ monitoring the emission of Yb³⁺ in Fig. 5(b) and (c), the characterized excitation peaks of Nd³⁺ and the excitation band related to host lattice are present, demonstrating the energy transfer from Nd³⁺ and host to Yb³⁺. The host lattice excitation band also appears in addition to the characterized excitation peaks of Nd³⁺ in the excitation spectra monitoring the emission of Nd³⁺. As shown in Fig. 5(a), there is no direct energy transfer from host to Nd³⁺, but when Yb³⁺ was codoped, energy transfer from host to Nd³⁺ occurs, indicating that the host excitation energy is firstly transferred to Yb³⁺, and then Yb³⁺ transfers the energy to Nd³⁺. The back energy transfer from Yb³⁺ to Nd³⁺ was also proposed in 0.8CaSiO₃–0.2Ca₃(PO₄)₂ eutectic glass.¹⁴

From the comparison of normalized excitation spectra in Fig. 5(b) and (c), the excitation intensity of energy levels Nd^{3+ 2}G_7/2 and higher ones is enhanced monitoring the emission of Yb³⁺. This illustrates that cross-relaxation energy transfer from higher Nd³⁺ states to Yb³⁺ occurs, otherwise, no enhancement will appear in the excitation spectra with Yb³⁺ emission. Based on the energy level diagrams of Nd³⁺ and Yb³⁺, the potential cross-relaxation energy transfer processes will be proposed.

3.4 Decay properties of LaOF:Nd³⁺, Yb³⁺

In order to further clarify the energy transfer processes between Nd³⁺ and Yb³⁺, luminescence decay curves were obtained for the Nd³⁺ and Yb³⁺ emission upon excitation into Nd^{3+ 4}D_1/2 state at 355 nm. Fig. 6 presents the decay curves of Nd^{3+ 4}D_3/2 state in LaOF:1.0% Nd³⁺, x% Yb³⁺ (x = 2.0, 3.0 and 5.0). The average decay times were provided in the figure. With the increase of Yb³⁺ concentration, the decay time of Nd^{3+ 4}D_3/2 state is decreasing, further revealing the occurrence of cross-relaxation energy transfer from Nd^{3+ 4}D_3/2 state to Yb^{3+ 2}F_5/2 state.

Fig. 6 Decay curves of Nd^{3+ 4}D_3/2 state emission in LaOF:1.0% Nd³⁺, x% Yb³⁺ (x = 2.0, 3.0 and 5.0) upon Nd^{3+ 4}D_1/2 state excitation at 355 nm.

The decay curves of Nd^{3+ 4}F_3/2 state emission (885 nm) in LaOF:1.0% Nd³⁺, x% Yb³⁺ (x = 0, 2.0, 3.0 and 5.0) upon excitation into the ⁴D_1/2 state at 355 nm were shown in Fig. 7. The luminescence decay curves of the Nd^{3+ 4}F_3/2 emission (885 nm) in the sample without doping of Yb³⁺ can be described by a single exponential with a lifetime of about 668 μs. With the doping of Yb³⁺ and concentration increasing, the lifetime of Nd^{3+ 4}F_3/2 state gradually decreases with average time from 522 μs to 168 μs and become non exponential. This demonstrates that energy transfer from ⁴F_3/2 energy level of Nd³⁺ to Yb³⁺ occurs and the transfer efficiency increases with the increase of Yb³⁺ concentration. The non-exponential character of the decay of Nd^{3+ 4}F_3/2 emission is originated from the random distribution of Yb³⁺ around Nd³⁺.

Fig. 7 Decay curves of Nd^{3+ 4}F_3/2 state emission in LaOF:1.0% Nd³⁺, x% Yb³⁺ (x = 0, 2.0, 3.0 and 5.0) upon Nd^{3+ 4}D_1/2 state excitation at 355 nm.

If energy transfer from Nd^{3+ 4}F_3/2 state to Yb^{3+ 2}F_5/2 state occurs, there should be a rising process for the emission of Yb^{3+ 2}F_5/2. Hence, the decay curves of Yb^{3+ 2}F_5/2 state are presented in Fig. 8. The rising process of Yb^{3+ 2}F_5/2 state emission becomes faster with the increase of Yb³⁺ concentration, which is consistent with the decay trends of Nd^{3+ 4}F_3/2 state. In addition, the decay time of Yb^{3+ 2}F_5/2 state decreases with the increase of Yb³⁺ content, indicating the occurrence of concentration quenching.

Fig. 8 Decay curves of Yb^{3+ 2}F_5/2 state emission in LaOF:1.0% Nd³⁺, x% Yb³⁺ (x = 2.0, 3.0 and 5.0) upon Nd^{3+ 4}D_1/2 state excitation at 355 nm.

3.5 Energy transfer diagram

According to the discussions on the luminescence spectra and decay curves of LaOF:Nd³⁺, Yb³⁺, the diagram demonstrating the energy transfer processes among host, Nd³⁺ and Yb³⁺ was established. Upon Nd^{3+ 4}D_1/2 state excitation at 352.5 nm, there are two potential energy transfer paths from Nd³⁺ to Yb³⁺ and the corresponding energy levels of energy transfer were indicated by the arrows in Fig. 9. Based on the matching of energy between levels of Nd³⁺ and Yb³⁺, the first possible energy transfer path is from Nd³⁺ (⁴D_3/2 → ⁴G_5/2) to Yb³⁺ (²F_7/2 → ²F_5/2). This transfer path was supported by the excitation spectra in Fig. 5(b) and (c) and decay curves of Nd^{3+ 4}D_3/2 state in Fig. 6. The small enhancement of Nd^{3+ 4}D_3/2 state excitation peak, which was also observed in YF₃:Nd³⁺, Yb³⁺, indicates that the cross-relaxation energy transfer efficiency is inefficient, originating from the fast multi-phonon relaxation to the lower levels. The energy transfer path 2 from Nd³⁺ (⁴F_3/2 → ⁴I_9/2) to Yb³⁺ (²F_7/2 → ²F_5/2) which is the dominant process has been proposed in many literatures.^9,10,12,16 The abnormal behavior is the enhancement of ⁴G_5/2 excitation peak in Fig. 5(b) and (c), from the level of which quantum cutting process is impossible to happen.

Fig. 9 Energy level diagram of Nd³⁺ and Yb³⁺ ions and the potential energy transfer processes proposed.

From the excitation spectrum in Fig. 5(a), energy transfer from host to Yb³⁺ is present and there is no direct energy transfer from host to Nd³⁺. The mechanism responsible for the energy transfer is not clear. The excitation spectra of Nd³⁺ and Yb³⁺ codoped LaOF indirectly reveal that the transfer of energy from Yb³⁺ to Nd³⁺indicated by path 3. the back energy transfer is unfavorable to the efficiency of spectral converters. The host lattice has strong absorption with efficient energy transfer to Yb³⁺, but absorption lies in the range with weak sun light intensity. Adjusting the host absorption to longer wavelength (300–500 nm) or searching for appropriate hosts for sensitization is necessary in the future research.

4. Conclusions

The energy transfer processes in LaOF:Nd³⁺, Yb³⁺ have been investigated. The emission and excitation spectra of LaOF:Nd³⁺ demonstrate that the multi-phonon relaxation from Nd^{3+ 4}D_3/2 to ⁴F_3/2 state is dominated and with transitions to ⁴I_J states. For Nd³⁺ and Yb³⁺ codoped LaOF, cross-relaxation energy transfer from Nd³⁺ (⁴D_3/2 → ⁴G_5/2) to Yb³⁺ (²F_7/2 → ²F_5/2) and direct energy transfer from Nd³⁺ (⁴F_3/2 → ⁴I_9/2) to Yb³⁺ (²F_7/2 → ²F_5/2) were observed and further confirmed by the Yb³⁺ concentration dependence of decay properties of related states. These energy transfer efficiencies increase with Yb³⁺ concentration. From the excitation spectra of Nd³⁺ or Yb³⁺ singly doped LaOF, energy transfer from host to Yb³⁺ and no energy transfer from host to Nd³⁺ were revealed. The appearance of host excitation band in the excitation spectrum of LaOF:Nd³⁺, Yb³⁺ with Nd^{3+ 4}F_3/2 emission indirectly confirms the back energy transfer from Yb³⁺ (²F_5/2 → ²F_7/2) to Nd³⁺ (⁴I_9/2 → ⁴F_3/2).

Although cross-relaxation energy transfer from Nd³⁺ to Yb³⁺ occurs, the efficiency is very low due to the fast non-radiative relaxation to Nd^{3+ 4}F_3/2 state. The back energy transfer from Yb³⁺ to Nd³⁺ could also decrease the efficiency of infrared emissions. The host sensitization of Yb³⁺ is very interesting and greatly enhances the absorptivity, but the host absorption should be adjusted to blue-near UV range to match well with the solar spectrum. The knowledges on the energy transfer processes (host, Nd³⁺ and Yb³⁺) in LaOF:Nd³⁺, Yb³⁺ would be great beneficial to the development of efficient spectral converters for silicon-based solar cells.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 51302182), The Natural Science Foundation of Shanxi Province (2014011017-3 and 2016011030), Program for the Top Young Academic Leaders of Higher Learning Institutions of Shanxi, Program for the Outstanding Innovative Teams of Higher Learning Institutions of Shanxi, and school fund of Taiyuan University of Technology (tyut-rc201361a, tyut-rc201497a, 2015QN068 and 1205-04020102). The authors are indebted to Prof. Fangtian You of Beijing Jiaotong University for decay curves measurements. One of the authors Qiufeng Shi also thank Ural Federal University for the support of literature resources.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra19953d

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