1. Introduction

Nowadays, solar energy has become most promising energy resource for its advantages of being inexhaustible and pollution-free [1]. However, the theoretical maximum conversion efficiency of silicon solar cells is only 31% based on the Shockley-Queisser analysis [2]. It is well-known that a large part of the energy losses that limit the efficiency is related to the spectral mismatch between the incident solar spectrum and the spectral response of solar cells: photons with energy smaller than the band-gap of silicon solar cells (E_g = 1.12 eV) will not be absorbed and photons with energy larger than the band gap is lost as heat [3–5]. The necessary improvement of silicon solar cell performance has led to extensive research activity over the past years [3–6]. In order to enhance the efficiency of solar cells, modification of the solar spectrum through the use of lanthanide phosphors is a promising way, which can be divided into three ways including downconversion (DC), photoluminescence (PL) and up-conversion (UC) [4–7]. Among them, DC is most beneficial for solar cells with a smaller band-gap where thermal losses are predominantly. This process is also known as quantum cutting (QC) where one higher energy ultraviolet (UV) or visible (VIS) photon is “cut” into two lower energy near-infrared (NIR) photons [8,9]. Recently, several groups reported DC from UV or VIS light to NIR light using rare-earth ion pairs [3–10]. Lanthanide ions are very well suited to be used in DC process because they have the rich energy level structure that allows for efficient spectral conversion. Among all lanthanides, Yb³⁺ ion has a relatively simple electronic structure of two energy-level manifolds: ²F_7/2 ground state and ²F_5/2 excited state around 10,000 cm⁻¹ in NIR region, which located just above the band gap of Si (9090 cm⁻¹). Accordingly, cooperative DC has also been reported for Tb³⁺/Yb³⁺, Pr³⁺/Yb³⁺ and Tm³⁺/Yb³⁺ couples in many host materials [8,10,11]. More recently, Meijerink et al. found that there is efficient energy transfer (ET) from Nd³⁺ to Yb³⁺ couples in YF₃ host, and DC is observed from the ⁴D_3/2 level of Nd³⁺ with estimated quantum efficiency up to 140% [12].

Fluorides host materials are generally interesting acting as UC and DC luminescence because of their low maximum phonon energy, which reduces multiphonon relaxations [12,13]. However, the chemical, physical and optical stability are very important for the practical application of designed materials, especially as the spectral conversion layer for the outdoor solar cell. In order to keep the balance of low phonon energy and good stability, rare earth oxyhalides, representing by LaOBr, will be a good choice acting as the host materials. In this paper, our objective is to make an attempt in utilizing the UV-VIS part of the solar spectrum to enhance the NIR response of silicon solar cells, so that the UV or VIS to NIR QC luminescence, energy transfer and QC mechanism in Nd³⁺-Yb³⁺ doped LaOBr have been studied. The dependence of the Yb³⁺ doping concentration on the NIR emissions, lifetimes, and QE from the LaOBr:Nd³⁺/Yb³⁺ phosphors has been investigated in detail.

2. Experiments

Crystalline powder samples of LaOBr:Nd³⁺ and LaOBr:Nd³⁺/Yb³⁺ were prepared by the traditional high temperature solid-state method. The Nd³⁺ concentration was kept constant at 5% while the Yb³⁺ concentration was varied between 0 and 10% (0, 1, 3, 5, or 10%). Stoichiometric amounts of starting materials La₂O₃, Yb₂O₃ and Er₂O₃ (99.995%, Shanghai Yuelong Non-Ferrous Metals Limited, China) and NH₄Br (analytical reagent, A. R., 99.5%, Beijing Fine Chemical Company, China), were thoroughly mixed in an agate mortar. Some excessive NH₄Br (20%) is necessary for loss of Br source at high temperature. Then the mixtures were fired at 600 °C for 2 h in air to produce the final samples. The X-ray diffraction (XRD) measurements were carried out on a SHIMADZU XRD-6000 diffractometer using Cu Kα radiation (λ = 0.15405 nm), operating at 40 kV, 30 mA. The morphologies of the samples were observed using a scanning electron microscope (Tescan Vega, XM 5136). The NIR emission and excitation spectra were recorded on a JOBIN YVON FL3-21 spectro-fluorometer with a 150 W Xe lamp as the excitation lamp and the R1033P photomultiplier for signal detection. Luminescent decay curves were measured by using the same spectrofluorometer, and the 363 nm pulsed laser radiation was used as the excitation source. All the measurements were performed at room temperature.

3. Results and Discussions

LaOBr has been reported to crystallize in the form of tetragonal phase with space group P4/nmm and a = 4.1479 (0) Å, c = 7.3928 (0) Å, V = 127.19 (0) ų, Z = 2. In our experiment, the LaOBr product was obtained by the simple solid-state method. As shown in Fig. 1 , it gives the XRD pattern of as-prepared LaOBr:5%Nd³⁺,5%Yb³⁺ sample and the JCPDS Card file of No. 88-0070 of LaOBr. By careful comparison between the given XRD patterns, their positions and intensities of the main peaks are nearly the same. It indicates that the crystal structure of the obtained LaOBr is in good agreement with the standard value from the JCPDS file. Furthermore, the inset of Fig. 1 shows the SEM image of LaOBr:5%Nd³⁺,5%Yb³⁺. SEM result shows that rectangular-like phosphor grains with an average diameter of about 1~2 μm are uniformly distributed. The observed micro-sheets particle morphology of LaOBr is related with the layer structure of (LaO)_nⁿ⁺ cation and Br^- anion layers along the crystallographic c direction of LaOBr crystal [14].

 

Fig. 1 XRD pattern of as-prepared LaOBr:5%Nd³⁺,5%Yb³⁺ and JCPDS Card (No. 88-0070) of LaOBr, and the inset shows the SEM image of LaOBr:5%Nd³⁺,5%Yb³⁺.

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Figure 2 (right) shows the room temperature NIR PL spectrum under excitation of 363 nm (Nd³⁺:⁴I_9/2-⁴D_1/2) in LaOBr:5%Nd³⁺ sample. The result shows that upon excitation at 363 nm, NIR emission is observed around 1082 nm and 1370 nm, which originate from the ⁴F_3/2-⁴I_11/2 and ⁴F_3/2-⁴I_13/2 transitions of Nd³⁺, respectively. As pointed out by Meijerink, there is a high possibility for the nonradiative relaxation of high energy emitting levels, such as ⁴D_1/2, ⁴D_3/2 and ²P_3/2, to the ⁴F_3/2 level, so that one can observe the ⁴F_3/2-⁴I_J transitions emission peaks [12]. Therefore, DC process of single Nd³⁺ in the LaOBr host is probably not possible. However, as shown in Fig. 2 (left), it gives the photoluminescence excitation (PLE) spectrum of the Nd³⁺:⁴F_2/3-⁴I_11/2 emission (1082 nm). The result shows that NIR emission of Nd³⁺ contains many absorption lines in the UV and VIS region, which benefit to the solar light absorption for the enhancement of the NIR response of silicon solar cells. Several excitation lines are identified from this spectrum, which are assigned to the electronic transitions of ⁴I_9/2→⁴D_1/2, ⁴I_9/2→⁴D_3/2, ⁴I_9/2→²P_3/2, ⁴I_9/2→⁴G_9/2, ⁴I_9/2→²G_9/2, ⁴I_9/2→⁴G_11/2, ⁴I_9/2→⁴G_5/2, ⁴I_9/2→²G_7/2, ⁴I_9/2→²H_11/2, ⁴I_9/2→⁴F_9/2, ⁴I_9/2→⁴F_7/2, ⁴I_9/2→⁴S_3/2, ⁴I_9/2→⁴F_5/2, and ⁴I_9/2→²H_3/2, respectively. The inset of Fig. 2 shows the variation of the Nd³⁺ emission intensities (1082 nm) as a function of the Nd³⁺ doping concentrations in LaOBr:xNd³⁺. It is found that the optimum Nd³⁺ doping concentration is 5 mol % in the LaOBr host, which has the highest NIR emission intensity in this series of samples.

 

Fig. 2 PLE spectrum of the Nd³⁺:⁴F_2/3-⁴I_11/2 emission (1082 nm) and NIR PL spectrum under excitation of 363 nm (Nd³⁺:⁴I_9/2-⁴D_1/2) in LaOBr:5%Nd³⁺, and the inset shows the tendency of the Nd³⁺ emission intensities (1082 nm) as a function of te Nd³⁺ doping concentrations in LaOBr:xNd³⁺.

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Figure 3 (left) shows the PLE spectra of the Nd³⁺:⁴F_2/3-⁴I_11/2 emission (1082 nm) and the Yb³⁺:²F_2/5-²F_2/7 emission (1020 nm) in LaOBr:5%Nd³⁺,5%Yb³⁺. It is found that the profiles and the relative intensities of the excitation spectra obtained by two different monitoring wavelengths of 1082 and 1020 nm appear similarly. This is a direct proof on the energy transfer between Nd³⁺ and Yb³⁺ because the Yb³⁺ ions only have two simple energy-level manifolds: ²F_7/2 ground state and ²F_5/2 excited state. So that the NIR emission at 1020 nm for Yb³⁺ should come from the absorption of Nd³⁺ under excitation of 363 nm (Nd³⁺:⁴I_9/2-⁴D_1/2) and the energy transfer between Nd³⁺ and Yb³⁺.

 

Fig. 3 Left side: PLE spectra of Nd³⁺:⁴F_2/3-⁴I_11/2 emission (1082 nm) and the Yb³⁺:²F_2/5-²F_2/7 emission (1020 nm) in LaOBr:5%Nd³⁺,5%Yb³⁺. Right side: The dependence of the NIR PL spectra upon excitation of 363 nm (Nd³⁺:⁴I_9/2-⁴D_1/2) on Yb³⁺ doping concentrations in LaOBr:5%Nd³⁺,xYb³⁺.

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To investigate the possible QC mechanism and conversion efficiency in Nd³⁺–Yb³⁺ couple, a series of samples have been prepared with a fixed Nd³⁺ concentration at 5 mol % and various Yb³⁺ concentration (x = 0, 1, 3, 5, 10 mol %). The NIR PL spectra upon excitation of 363 nm (Nd³⁺:⁴I_9/2-⁴D_1/2) in LaOBr:5%Nd³⁺,xYb³⁺ samples are given in Fig. 3 (right side). When Yb³⁺ is absent, only emissions from Nd³⁺ were observed around 1082 nm and 1370 nm, as discussed in Fig. 2. In the Nd³⁺/Yb³⁺ codoped samples, typical Yb³⁺ emission near 1020 nm (ascribed to Yb³⁺: ²F_2/5-²F_2/7 transition) appears and NIR emissions of Nd³⁺ are observed around 987 nm, 1082 nm and 1370 nm, which originate from the ⁴F_3/2-⁴I_9/2, ⁴F_3/2-⁴I_11/2 and ⁴F_3/2-⁴I_13/2 transitions of Nd³⁺, respectively. As is also shown in Fig. 3, the NIR emission intensity of Yb³⁺ increases with increasing Yb³⁺ content, along with the decrease of Nd³⁺ emissions, which strongly suggests the existence of ET from Nd³⁺ to Yb³⁺. Furthermore, when the doping concentration of Yb³⁺ reaches 10 mol %, the Yb³⁺ emission decreases, which should be attributed to the concentration quenching (CQ) of Yb³⁺ at high Yb³⁺ concentration. In order to further prove the variation of the NIR emissions of Nd³⁺ and Yb³⁺, Fig. 4(a) shows the dependence of the Yb³⁺ emission intensities (at 1020 nm) and Nd³⁺ emission intensity (at 1082 nm) on the Yb³⁺ doping concentration under excitation at 363 nm. It is noticed that with increasing Yb³⁺ concentrations, the NIR PL intensity of the Yb³⁺ increases rapidly, while the Nd³⁺ NIR emission decreases monotonically. However, a decrease has been clearly observed when Yb³⁺ concentration is set to more than 5 mol % due to the CQ effect.

 

Fig. 4 (a) The dependence of the NIR emission intensities at 1020 nm (Yb³⁺:²F_2/5-²F_2/7) and 1082 nm (Nd³⁺:⁴F_2/3-⁴I_11/2) on the doping concentrations. (b) Lifetime and ETE as a function of the Yb³⁺ doping concentrations in LaOBr:5%Nd³⁺,xYb³⁺.

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To gain further insight in the energy transfer processes between Nd³⁺ and Yb³⁺, luminescence decay curves were recorded. Luminescence decay curves of Nd³⁺ emission (⁴F_2/3 level) upon excitation in the ⁴D_3/2 level of Nd³⁺ in LaOBr:5%Nd³⁺,xYb³⁺ samples are shown in Fig. 5 . For the sample without any Yb³⁺, nearly single exponential luminescence decay is observed and fitted with a lifetime of 160 μs by using the equation of I(t) = I₀ + A exp(-t/τ) [15]. Addition of Yb³⁺ results in faster and nonexponential decay, which is attributed to ET from Nd³⁺ ions to neighboring Yb³⁺ ions [16]. It is found from Fig. 5 that there is a rapid decrease of the decay time upon raising the Yb³⁺ concentration from 1 to 10 mol %, which shows that energy transfer from Nd³⁺ to Yb³⁺ is efficient, consistent with the rapid increase of the Yb³⁺-emission intensity (at 1020) upon adding Yb³⁺ observed in Fig. 3. Since the other curves are nonexponential, the decay process of theses samples are characterized by average lifetime (τ), which can be calculated by using the following equation [15,16],

 

Fig. 5 Decay curves of the Nd³⁺:⁴F_2/3-⁴I_11/2 (1082 nm) under excitation of 363 nm with different Yb³⁺ concentration (x) in LaOBr:5%Nd³⁺,xYb³.^.

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In order to obtain a comprehensive understanding on the NIR luminescence and ET process in LaOBr:Nd³⁺,Yb³⁺ under excitation of UV light, part of the energy-level diagram of Nd³⁺ and Yb³⁺ in LaOBr host has been shown in Fig. 6 . Yb³⁺ has no other levels in the UV and VIS regions, so that the cross relaxation (CR) and energy transfer from Nd³⁺ and Yb³⁺ should be dominant for the NIR emissions of Yb³⁺. As shown in Fig. 6, Nd³⁺ ions can absorb the UV photon (363 nm) and jump from the ⁴I_9/5 ground state to the ⁴D_3/2 excited state, and reach the low energy state of ⁴G_9/2 after the relaxation process [12]. Accordingly, the DC process for the Nd³⁺/Yb³⁺ couple happens via sequential two-step energy transfer. Part of the energy at ⁴G_9/2 level is transferred to Yb³⁺ via CR process: Nd³⁺ (⁴G_9/2→⁴F_3/2), Yb³⁺ (²F_7/2→²F_5/2), populating the ²F_5/2 level of Yb³⁺ [12, 19]. In a second step the remaining energy can be transferred to a second Yb³⁺ ion from the ⁴F_3/2 level of Nd³⁺ to the ²F_5/2 level of Yb³⁺ [19], which can then emit another NIR photon, or otherwise emission can occur from the ⁴F_3/2 level of Nd³⁺. As we have observed in Fig. 3, the NIR emissions of Nd³⁺ are also observed around 987 nm, 1082 nm and 1370 nm, which originate from the ⁴F_3/2-⁴I_9/2, ⁴F_3/2-⁴I_11/2 and ⁴F_3/2-⁴I_13/2 transitions of Nd³⁺, respectively.

 

Fig. 6 Schematic energy-level diagram of the CR (1) and ET (2) processes for near-infrared emission of the Nd³⁺, Yb³⁺ couple in LaOBr:Nd³⁺,Yb³⁺ under excitation of UV light (λ_ex = 363 nm).

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4. Conclusions

In conclusion, LaOBr:Nd³⁺/Yb³⁺ has been prepared via a high temperature solid-state method, and downconversion (DC) for the Nd³⁺/Yb³⁺ couple has been investigated in this system. It is observed from the ⁴D_3/2 level of Nd³⁺ with estimated quantum efficiency up to 128.8% when the concentration quenching effect of Yb³⁺ is considered. After the relaxation process from the Nd^{3+ 4}D_3/2 excited state to the low energy state of ⁴G_9/2, DC from the ⁴G_9/2 level via the cross-relaxation process Nd³⁺ (⁴G_9/2→⁴F_3/2), Yb³⁺ (²F_7/2→²F_5/2), followed by a second energy transfer step from the ⁴F_3/2 level of Nd³⁺ to Yb³⁺, could possibly lead to the emission of two IR photons from Yb³⁺. The development of near-infrared (NIR) quantum cutting (QC) phosphors could open up a new approach in achieving high efficiency silicon-based solar cells.