Near-infrared quantum cutting in Nd3+ and Yb3+ Doped BaGd2ZnO5 phosphors

Linlin Liu; Mingming Li; Shuzhen Cai; Yanmin Yang; Yaohua Mai

doi:10.1364/OME.5.000756

1. Introduction

The application of solar cells is of great significance. The conversion from sunlight to electricity using solar cell devices represents a promising approach to green and renewable energy. However, the efficiency limit of the conventional crystalline silicon (c-Si) solar cells was estimated to be 29% in the early 1960s (Shockley Queisser limit) [1]. The main difficulty in improving the conversion efficiency of solar energy is the spectral mismatch between the energy distribution of photons in the incident solar spectrum and the band gap of a semiconductor material [2–4]. Among the different approaches to improve the solar cells efficiency, the quantum cutting (QC) is one of the interesting ways proposed since the thermalization effect of the electrons is a main limitation to the conversion efficiency of solar cells [3,5,6]. Owing to the abundant energy levels and narrow emission spectra lines, rare earth (RE) ions play a great role in the QC down-conversion (DC) process. Yb³⁺ ion has been extensively applied for its near-infrared (NIR) emission around 980nm (²F_5/2→²F_7/2), which is just above the band-gap of the Si-based solar cells. Recently, rare earth (RE) doped DC luminescent materials, such as Er³⁺-Yb³⁺ [7–10], Ho³⁺-Yb³⁺ [11,12], Tm³⁺-Yb³⁺ [13,14], Ce³⁺-Yb³⁺ [15], Dy³⁺-Yb³⁺ [16–18], Tb³⁺-Yb³⁺ [19–23], have been used in solar cells to improve solar energy conversion efficiencies.

In quantum cutting process, the low phonon energy and high chemical stability are the critical factors. Low phonon energy can prevent the non-radiative energy loss of multi-phonon relaxation of doped rare earth ions and enhance the energy transfer efficiency [24,25]. Besides, the chemical stability is an essential condition to the solar cells [25]. The halide and oxide have been proposed in other studies. The halide material has the low phonon energy but chemical stability is poorer. The oxide possesses good chemical stability but their phonon energy is higher. As a new host material, BaGd₂ZnO₅ has the low phonon energy and high chemical stability. It has been reported in previous studies [26]. The phonon energy of this host material is only 360 cm⁻¹ [27,28].

In this paper, the NIR quantum cutting of Nd³⁺, Yb³⁺ doped BaGd₂ZnO₅ phosphors was studied through the visible and NIR light emission and excitation spectra and energy level diagram. With the measurement of lifetime decay curves, the quantum efficiency and energy transfer efficiency were also calculated.

2. Experimental

2.1Sample preparation

BaGd₂ZnO₅: Nd³⁺, Yb³⁺ were synthesized via high temperature solid-state method. BaCO₃,ZnO,Gd₂O₃,Nd₂O₃ and Yb₂O₃ (A.R.) were used as the starting materials. Chemicals were weighed in the stoichiometric ratio and then grinded adequately in the agate mortars. The homogeneous material was calcined at 1300 °C for 3h in the muffle furnace and then cooled down to the room temperature.

2.2 Characteristic

X-ray powder diffraction (XRD) is carried out in the 2θ range 10-80° using a Bruker D8 advance X-ray diffracmeter (Bruker Optics, Ettlingen, Germany) with Cu Kα radiation. The emission spectra and excitation spectra were measured by the combined time resolved and steady state fluorescence spectrometers (FLS980, Edinburgh Instruments) equipped with two photomultiplier tubes (PMT) (Hamamatsu R928, U = 900V and a nitrogen cooled (−85°C) R5509-73, U = 1500V), as well as continuous wavelength 450W ozone free xenon (Xe) lamp as excitation sources. Decay curves were measured using FLS920 (Edinburgh Instruments) with a microsecond Xe lamp as the excitation source.

3. Results and discussion

3.1 Crystal structure

Figure 1(a) shows the XRD patterns of Nd³⁺, Yb³⁺ doped BaGd₂ZnO₅ phosphors and the standard card data (JCPDS No.49-0518). The positions and relative intensities of the diffraction peaks accord well with the data reported in the JCPDS standard card (No.49-0518). The radii of Yb³⁺ and Nd³⁺ are similar to Gd³⁺, so the XRD patterns of the samples with different doping concentrations are similar. It indicates that the pure-phase BaGd₂ZnO₅ phosphors are obtained. Figure 1(b) shows the schematic crystal structure of BaGd₂ZnO₅ seen from the c axis, which belongs to the orthorhombic. Furthermore, the lattice constants of BaGd₂ZnO₅ are: a = 7.15731 Å, b = 12.49399 Å, c = 5.77427 Å and V = 516.35 Å³.

Fig. 1 (A) XRD patterns of BaGd₂ZnO₅: 1%Nd³⁺, x%Yb³⁺ (x = 0, 1, 3, 7, 10) and standard card data (JCPDS No.49-0518); (B) the crystal structure of BaGd₂ZnO₅ seen from the c axis.

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3.2 Fluorescence spectra

In order to investigate the luminescence properties, the visible light photoluminescence of BaGd₂ZnO₅: 1%Nd³⁺, x%Yb³⁺ (0, 1, 3, 5, 7, 10) were measured. Figures 2(a) and 2(b) show the excitation spectra monitored at 423 nm emission light derived from ²P_1/2→⁴I_9/2 transition of Nd³⁺ and emission spectra excited by 359 nm ultra violet (UV) light derived from ⁴D_3/2→⁴I_9/2 transition, respectively. The excitation peak is shown in 359 nm, and emission peaks at 423 nm and 457 nm are obtained. The intensity decreases gradually with the concentration of Yb³⁺ increasing from 0 to 10%, which is shown in the inset. It indicates that the energy transfer (ET) from Nd³⁺ to Yb³⁺ occurs, which decreases the population of Nd³⁺ excited state level.

Fig. 2 Visible light photoluminescence of BaGd₂ZnO₅: 1%Nd³⁺, x%Yb³⁺ (x = 0, 1, 3, 5, 7, 10): (A) excitation spectra (λ_em = 423 nm); (B) emission spectra (λ_ex = 359 nm); inset is the varying of emission peak intensity of 423 nm and 457 nm with Yb³⁺ concentration.

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For further studying the properties of ET from Nd³⁺ to Yb³⁺, the excitation and emission spectra of x%Yb³⁺ doped BaGd₂ZnO₅: 1%Nd³⁺ (x = 0, 1, 3, 5, 7, 10) were measured and analyzed. Figure 3(a) shows the excitation spectra monitored at 978 nm derived from the ²F_5/2→²F_7/2 transition of Yb³⁺. The excitation peaks at 336 nm, 359 nm, 437 nm and 480 nm are obtained, and the intensities of 978 nm emission increase firstly and then decrease. The difference is that the maximums of 336 nm, 359 nm, 437 nm and 480 nm peaks correspond to 5%Yb³⁺, 7%Yb³⁺, 1%Yb³⁺ and 1%Yb³⁺, respectively, which are shown in the inset of Fig. 3(a). It indicates that the quenching concentrations for these four peaks are different. Furthermore, the shape of 359 nm excitation peak monitored at 978 nm is similar to that monitored at 423 nm in Fig. 2(a). The quenching concentration for 359 nm excitation peak is up to7%, so it is chosen as the excitation light source to measure the emission spectra. Figure 3(b) shows the emission spectra of BaGd₂ZnO₅: 1%Nd³⁺, x%Yb³⁺ (λ_ex = 359 nm), and the NIR emission peaks are located at 978 nm and 1071 nm, which are derived from ²F_5/2→²F_7/2 transition of Yb³⁺ and ⁴F_3/2→⁴I_11/2 transition of Nd³⁺, respectively. In Nd³⁺ singly doped BaGd₂ZnO₅ phosphors, the 978 nm (Yb^{3+ 2}F_5/2→²F_7/2) emission light is not observed. However, intensities of these peaks increase when adding Yb³⁺ into BaGd₂ZnO₅: 1%Nd³⁺ phosphors. The intensity of 978 nm emission increases firstly and then decreases because of concentration quenching, but that of 1071 nm emission decreases gradually with the Yb³⁺ concentration increasing. It further indicates that the ET from Nd³⁺ to Yb³⁺ occurs from all above. Furthermore, the quenching concentration of Yb³⁺ under the excitation of 359 nm is 7%, so it is a useful highly doped material for silicon-based solar cells. Interestingly, the intensities variety of 336 nm, 437 nm and 480 nm excitation peaks proves that the ET also occurs under the excitation of these lights. However, the 978 nm emission intensities excited by these peaks are weak compared with 359 nm. So, we mainly studied the 359 nm excitation peak in this paper.

Fig. 3 Near-infrared photoluminescence of BaGd₂ZnO₅: 1%Nd³⁺, x%Yb³⁺ (x = 0, 1, 3, 5, 7, 10): (A) excitation spectra (λ_em = 978 nm), inset shows the excitation peaks intensities varying with Yb³⁺ concentrations; (B) emission spectra (λ_ex = 359 nm); inset is the relation between emission peak intensity of 423 nm and 457 nm and Yb³⁺ concentration.

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For studying the ET mechanism, the energy level diagram of Nd³⁺ and Yb³⁺ is shown in Fig. 4.Upon 359 nm excitation, the ⁴D_3/2→⁴I_9/2 transition occurs, and then the electrons jump to the ²P_1/2 by multi-phonon relaxation (MPR). After that, the ²P_1/2→⁴I_9/2 transition occurs, and 423 nm blue light is obtained. Part of electrons in ²P_1/2 jump to ⁴G_11/2 by MPR, and the 457 nm blue light is observed. From Fig. 2 and Fig. 3, the ET from Nd³⁺ to Yb³⁺ has been proved. The energies of 423 nm and 457 nm are approximately twice the energy of the Yb^3+
2F_5/2→²F_7/2 transition, so the 978 nm NIR emission comes from 423 nm and 457 nm photons of Nd³⁺ as the way of quantum cutting, which can be described as follows:

{}^{2}P_{1 / 2} (N d^{3 +}) + 2 {}^{2}F_{7 / 2} (Y b^{3 +}) \to {}^{4}I_{9 / 2} (N d^{3 +}) + 2 {}^{2}F_{5 / 2} (Y b^{3 +})

2 {}^{2}F_{5 / 2} (Y b^{3 +}) \to 2 {}^{2}F_{7 / 2} (Y b^{3 +})

Figure 5 shows the luminescence decay curves of BaGd₂ZnO₅: 1%Nd³⁺, x%Yb³⁺ (x = 0, 1, 3, 5, 7, 10), which corresponds to the 423 nm emission of ²P_1/2→⁴I_9/2 transition, and the excitation wavelength is 359 nm. With Yb³⁺ concentration increasing, the lifetime gradually decreases. The inset clearly shows the lifetime varying process upon Yb³⁺ contents. Combining the photoluminescence properties proposed above, the decrease of lifetime of 423 nm emission comes from the other path that ET from Nd³⁺ to Yb³⁺. With the Yb³⁺ concentration increasing, more energy is transferred to Yb³⁺, so the lifetime of ²P_1/2 energy level decreases. So, the ET from Nd³⁺ to Yb³⁺ is proved again. Furthermore, the lifetime decay curves of 457 nm derived from ⁴G_11/2→⁴I_9/2 transition are given in Fig. 6, and the similar lifetime varying tendency is observed. The reason is that the electrons of ⁴G_11/2 energy level come from the MPR of ²P_1/2 level, and the energy of this level is also transferred to Yb³⁺.

Fig. 4 The energy level diagram of Nd³⁺ and Yb³⁺ and the ET mechanism between them.

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Fig. 5 Luminescence decay curves of BaGd₂ZnO₅: 1%Nd³⁺, x%Yb³⁺ (x = 0, 1, 3, 5, 7, 10) (λ_ex = 359 nm, λ_em = 423 nm), the inset is the lifetime varying process upon Yb³⁺ concentration.

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Fig. 6 Luminescence decay curves of BaGd₂ZnO₅: 1%Nd³⁺, x%Yb³⁺ (x = 0, 1, 3, 5, 7, 10) (λ_ex = 359 nm, λ_em = 457nm).

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Table 1 shows the lifetime, energy transfer efficiency (ETE) and quantum efficiency (QE) of BaGd₂ZnO₅: 1%Nd³⁺, x%Yb³⁺ (x = 0, 1, 3, 5, 7, 10) (λ_ex = 359 nm, λ_em = 423 nm). The ETE and QE are calculated by lifetime as follows [29–31]:

τ = \int_{- \infty}^{+ \infty} \frac{I (t)}{I_{0}} d t

η_{E T} = 1 - \frac{τ_{x % Y b^{3 +}}}{τ_{0 Y b^{3 +}}}

η_{Q E} = η_{N d} β (1 - η_{E T}) + 2 η_{Y b} η_{E T}

where x % represents the Yb³⁺ concentration and I denotes the decay intensity. τ is the lifetime and η_ET is the energy transfer efficiency. η_Nd and η_Yb stand for the luminescent quantum efficiency of the ⁴F_3/2 level of Nd³⁺ and the ²F_5/2 level of Yb³⁺, respectively.

β

for the population ratio of the energy levels ⁴F_3/2 to ²P_1/2. Assuming that all the excited Yb³⁺ ions and the residual excited Nd³⁺ ions decay radiatively, i.e., η_Nd = η_Yb =

β

= 1 [15, 30, 32], which is an ideal scenario, the upper limited values of the total down-conversion QE are calculated to be 100% to 131.9% for samples with Yb³⁺ contents from 0% to 10%.

Table 1. Lifetime (τ), energy transfer efficiency (η_ET) and quantum efficiency (η_QE) of BaGd₂ZnO₅: 1%Nd³⁺, x%Yb³⁺ (x = 0, 1, 3, 5, 7, 10) (λ_ex = 359 nm, λ_em = 423 nm).

View Table

4. Conclusion

In this paper, Nd³⁺ and Yb³⁺ doped BaGd₂ZnO₅ phosphors were synthesized and investigated as NIR quantum cutting luminescence materials. The composition and structure have been studied by the X-ray powder diffraction. Upon the excitation of 359 nm light, the 423 nm, 457 nm and 1071 nm light of Nd³⁺ are obtained, and the intensities of these emission light all decrease with Yb³⁺ concentration increasing. With Yb³⁺ adding into the BaGd₂ZnO₅: Nd³⁺, the 978 nm NIR light of Yb³⁺ is also obtained under the 359 nm excitation. The intensities of 978 nm light increase firstly and then decrease because of concentration quenching, and the high quenching concentration is 7%. It indicates that the ET from Nd³⁺ to Yb³⁺ occurs. The maximum of ET is 31.9%. This material has a potential application in solar cells to progress the conversation efficiency.

Acknowledgments

This work was supported by National Science Foundation of China (No. 11474083), Hebei Province Department of Education Fund (ZD2014069).

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x/%(Yb³⁺ concentration)	Lifetime(τ)/ μs	η_ET/%	η_QE/%
0	7.685( $\pm$ 0.005)	0	100
1	7.679( $\pm$ 0.005)	0.08	100.08
3	7.034( $\pm$ 0.005)	8.47	108.47
5	6.340( $\pm$ 0.005)	17.50	117.50
7	6.162( $\pm$ 0.005)	19.82	119.82
10	5.231( $\pm$ 0.005)	31.93	131.93

Near-infrared quantum cutting in Nd³⁺ and Yb³⁺ Doped BaGd₂ZnO₅ phosphors

Abstract

1. Introduction

2. Experimental

2.1Sample preparation

2.2 Characteristic

3. Results and discussion

3.1 Crystal structure

3.2 Fluorescence spectra

4. Conclusion

Acknowledgments

References and links

Cited By

Figures (6)

Tables (1)

Equations (5)

Optical Materials Express