LaOBr:Nd3+/Yb3+ has been prepared via a high temperature solid-state method, and near-infrared (NIR) quantum cutting (QC) luminescence in this system has been demonstrated. NIR luminescence of LaOBr:Nd3+/Yb3+ has been investigated by excitation, emission spectra and lifetime measurements, respectively. After absorption of a single 363 nm photon, downconversion (DC) occurs from the Nd3+ 4G9/2 level via the cross-relaxation process Nd3+ (4G9/2→4F3/2), Yb3+ (2F7/2→2F5/2), followed by a second energy transfer step from Nd3+ (4F3/2 level) to Yb3+ (2F5/2 level), leading to the emission of two IR photons from Yb3+, which is a promising avenue to boost the efficiency of solar cells with a twofold increase in the photon number.
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Nowadays, solar energy has become most promising energy resource for its advantages of being inexhaustible and pollution-free . However, the theoretical maximum conversion efficiency of silicon solar cells is only 31% based on the Shockley-Queisser analysis . 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 (Eg = 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: 2F7/2 ground state and 2F5/2 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 4D3/2 level of Nd3+ with estimated quantum efficiency up to 140% .
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.
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) Å3, 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 .
Figure 2 (right) shows the room temperature NIR PL spectrum under excitation of 363 nm (Nd3+:4I9/2-4D1/2) 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 4F3/2-4I11/2 and 4F3/2-4I13/2 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 4D1/2, 4D3/2 and 2P3/2, to the 4F3/2 level, so that one can observe the 4F3/2-4IJ transitions emission peaks . 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+:4F2/3-4I11/2 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 4I9/2→4D1/2, 4I9/2→4D3/2, 4I9/2→2P3/2, 4I9/2→4G9/2, 4I9/2→2G9/2, 4I9/2→4G11/2, 4I9/2→4G5/2, 4I9/2→2G7/2, 4I9/2→2H11/2, 4I9/2→4F9/2, 4I9/2→4F7/2, 4I9/2→4S3/2, 4I9/2→4F5/2, and 4I9/2→2H3/2, 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.
Figure 3 (left) shows the PLE spectra of the Nd3+:4F2/3-4I11/2 emission (1082 nm) and the Yb3+:2F2/5-2F2/7 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: 2F7/2 ground state and 2F5/2 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+:4I9/2-4D1/2) and the energy transfer between Nd3+ and Yb3+.
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+:4I9/2-4D1/2) 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+: 2F2/5-2F2/7 transition) appears and NIR emissions of Nd3+ are observed around 987 nm, 1082 nm and 1370 nm, which originate from the 4F3/2-4I9/2, 4F3/2-4I11/2 and 4F3/2-4I13/2 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.
To gain further insight in the energy transfer processes between Nd3+ and Yb3+, luminescence decay curves were recorded. Luminescence decay curves of Nd3+ emission (4F2/3 level) upon excitation in the 4D3/2 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) . Addition of Yb3+ results in faster and nonexponential decay, which is attributed to ET from Nd3+ ions to neighboring Yb3+ ions . 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],Eq. (1), the lifetime of Nd3+ 4F2/3 (1082 nm) in LaOBr:5%Nd3+,xYb3+ can be determined as 160, 133, 124, 114 and 94 μs for x = 0, 1, 3, 5, and 10 mol %, respectively. The dependence of lifetime as a function of the Yb3+ doping concentrations is also given in Fig. 4(b. From these luminescence decay curves, the ET efficiency _(ETE, ηETE) and the total theoretical quantum efficiency (QE, ηQE) can be estimated by the following equations [17,18]:11,13]. Accordingly, it has been determined that the ETE from the 4F2/3 level are 16.9%, 22.5%, 28.8% and 41.3% for samples with 1, 3, 5 and 10 mol % Yb3+, respectively. In Fig. 4(b), ETE is plotted as a function of Yb3+ doping concentration. It is presented that the ETE increases monotonously and achieves a maximum value of 41.3%. Supposing there is no loss of nonradiation by defects and impurities, a maximum QE of 141.3% is obtained. However, by taking into account CQ at Yb3+ doping beyond 10 mol%, the maximum QE should be 128.8% at the Yb3+ doping concentration of 5 mol %.
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 4I9/5 ground state to the 4D3/2 excited state, and reach the low energy state of 4G9/2 after the relaxation process . Accordingly, the DC process for the Nd3+/Yb3+ couple happens via sequential two-step energy transfer. Part of the energy at 4G9/2 level is transferred to Yb3+ via CR process: Nd3+ (4G9/2→4F3/2), Yb3+ (2F7/2→2F5/2), populating the 2F5/2 level of Yb3+ [12, 19]. In a second step the remaining energy can be transferred to a second Yb3+ ion from the 4F3/2 level of Nd3+ to the 2F5/2 level of Yb3+ , which can then emit another NIR photon, or otherwise emission can occur from the 4F3/2 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 4F3/2-4I9/2, 4F3/2-4I11/2 and 4F3/2-4I13/2 transitions of Nd3+, respectively.
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 4D3/2 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 Nd3+ 4D3/2 excited state to the low energy state of 4G9/2, DC from the 4G9/2 level via the cross-relaxation process Nd3+ (4G9/2→4F3/2), Yb3+ (2F7/2→2F5/2), followed by a second energy transfer step from the 4F3/2 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.
This present work was supported by the National Natural Science Foundations of China (Grant No.51002146, 51272242), the Ph.D. Programs Foundation of Ministry of Education of China (Grant No. 20090022120002) and the Fundamental Research Funds for the Central Universities (2010ZY35, 2011YYL131).
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