Efficient near ultraviolet to near infrared downconversion photoluminescence of La2GeO5: Bi3+, Nd3+ phosphor for silicon-based solar cells

Efficient near ultraviolet to near infrared downconversion photoluminescence of La2GeO5: Bi3+, Nd3+ phosphor for silicon-based solar cells

Optical Materials 85 (2018) 523–530 Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat Effi...

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Optical Materials 85 (2018) 523–530

Contents lists available at ScienceDirect

Optical Materials journal homepage: www.elsevier.com/locate/optmat

Efficient near ultraviolet to near infrared downconversion photoluminescence of La2GeO5: Bi3+, Nd3+ phosphor for silicon-based solar cells

T

Jianming Lia,b, Shaoan Zhangc,∗∗, Haoming Luob, Zhongfei Mua,∗, Zhenzhang Lic, Qingping Dud, Junqin Fenga, Fugen Wue a

Experimental Teaching Department, Guangdong University of Technology, Waihuan Xi Road, No.100, Guangzhou, 510006, PR China School of Chemical Engineering and Light Industry, Guangdong University of Technology, Waihuan Xi Road, No.100, Guangzhou, 510006, PR China Basic Teaching Department, Guangzhou Maritime University, Hongshan Three Road, No. 101, Guangzhou, 510725, PR China d School of Environmental Science and Engineering, Guangdong University of Technology, Waihuan Xi Road, No.100, Guangzhou, 510006, PR China e School of Materials and Energy, Guangdong University of Technology, Waihuan Xi Road, No.100, Guangzhou, 510006, PR China b c

A R T I C LE I N FO

A B S T R A C T

Keywords: La2GeO5: Bi3+ Nd3+ Quantum cutting Energy transfer Downconversion materials

Recently, downconversion materials have attracted considerable research due to large promotion of the efficiency of silicon-based solar cells through the absorption of solar radiation via quantum cutting. However, due to lack of ideal downconversion material, it is emergent to seek novel downconversion phosphors with large ultraviolet absorption and high quantum efficiency. Herein, we reported the photoluminescence performance and dynamics of a downconversion phosphor La2GeO5: Bi3+, Nd3+. This phosphor presents a strong ultraviolet absorption and emits a series of intense near infrared emissions by quantum cutting. The emission energy from the Nd3+ matches the energy gap of silicon-based solar cells which can improve the solar cell efficiency. Energy transfer from Bi3+ to Nd3+ in La2GeO5 occurs, proving that energy transfer mechanism of Bi3+→ Nd3+ is an electric dipole-dipole (DMSO‑d6) interaction. The energy transfer efficiency of Bi3+→Nd3+ and theoretical quantum efficiency for La2GeO5: Bi3+, Nd3+ are calculated to be as high as 91% and 191%, respectively. La1.92GeO5:0.03Bi3+, 0.05Nd3+ phosphor presents a promising thermal behavior with an activation energy of 0.152 eV. Thus, the development of near-infrared downconversion Bi3+, Nd3+ co-doped phosphors might open up a new approach to achieve high efficiency silicon solar cells by means of quantum cutting.

1. Introduction At present, consumption of fossil fuels has been dramatically increasing due to improvements in the quality of life, industrialization of developing nations and increase of the world population [1,2]. Renewable solar energy is attracting much interest due to its clean, pollution-free, persistence and inexhaustible, etc. Theoretically, the photon conversion efficiency of solar energy reaches maximally 30% for a crystalline silicon (c-Si) solar cell with a band gap of ∼1.12 eV [3,4]. However, c-Si solar cells mismatch with the low energy photons (λ > 1100 nm) and high energy photons (λ < 400 nm), and these parts of energy are gone by thermalization losses. Fortunately, downconversion (DC) materials can largely enhance the efficiency of silicon solar cells by spectral modifications [5–8]. Especially, rare-earth doped DC materials can absorb a high energy (ultraviolet (UV)) photon and ∗

emit two or more low energy (visible and near-infrared (NIR) photons which just match with solar cell response region ∼1000 nm), which is called as quantum cutting [9,10]. Theoretically, Dexter in 1957 predicted that quantum efficiency (QE) of DC materials can maximally reach 200% with an appreciable efficiency [10]. A large number of Ln3+-Yb3+/Nd3+ (Ln3+ = Tm3+, Pr3+, Tb3+, Ho3+) codoped NIR quantum cutting phosphors are reported [11–17]. However, most Ln3+Yb3+/Nd3+ ions codoped glasses or phosphors show weak NIR emission since the absorption of rare earth in UV region corresponding to 4f4f transition is parity-forbidden. Thus, we should better provide an ideal sensitizer which can enhance the UV absorption of Yb3+/Nd3+ doped phosphors. Interestingly, bismuth, with [Xe]4f145d106s26p3 electronic configuration, is considered as the wonder metal due to its diverse oxidation states and multi-type electronic structure [18]. As a favored sensitizer

Corresponding author. Corresponding author. E-mail addresses: [email protected] (S. Zhang), [email protected] (Z. Mu).

∗∗

https://doi.org/10.1016/j.optmat.2018.09.024 Received 3 July 2018; Received in revised form 13 September 2018; Accepted 16 September 2018 0925-3467/ © 2018 Elsevier B.V. All rights reserved.

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or an activator, the photoluminescence performance of Bi3+ has been extensively investigated. As far as we know, Bi3+ is one of the good sensitizers, presenting a strong UV absorption band of 1S0→3P1 or 1P1 transition and giving a broad emission in the visible region due to the 3 P1→1S0 transition. For promoting the efficiency of silicon solar cells, the development of Bi3+-Yb3+/Nd3+ doped DC phosphors are the topical subject of investigation. Research on Bi3+-Yb3+/Nd3+ codoped DC materials are investigated in different hosts such as GdNdO3 [7], CaTiO3 [19] and YVO4 [20], etc. In the above hosts, a high NIR quantum cutting can be achieved by virtue of energy transfer processes between Bi3+ and Ln3+. In our previous work, it is found that Bi-doped La2GeO5 phosphor exhibits a blue-greenish broad band at 480 nm with a high QE and excellent thermal stability [21]. The broad emission of La2GeO5:Bi3+ overlaps well with several 4f-4f excitation absorption of Nd3+, whose emission energy is more than double of the energy of the 4 F3/2 level of Nd3+ ions. In this paper, we add Nd3+ into La2GeO5:Bi3+ and achieve a series of NIR emissions from La2GeO5: Bi3+, Nd3+. To perform this study, the photoluminescence excitation (PLE) and emission (PL) spectra and lifetime dynamics are investigated as a function of the Nd3+ concentration. Our research results shows that energy transfer efficiency of Bi3+→Nd3+ in La2GeO5 is as high as 90% and the theoretical QE yields for La2GeO5:Bi3+, Nd3+ is calculated to be 191%. Furthermore, La1.92GeO5:0.03Bi3+, 0.05Nd3+ phosphor presents a good thermal stability with an activation energy of 0.152 eV. Thus, a new approach is provided for us to further improve the efficiency of silicon solar cells by means of quantum cutting for Bi and Nd codoped phosphor. 2. Experimental section 2.1. Synthesis La2GeO5:Bi3+, La2GeO5:Nd3+ and La2GeO5: Bi3+, Nd3+ samples were prepared by the traditional solid state method. The raw materials are La2O3 (99.999%), Eu2O3 (99.999%), GeO2 (99.999%), Nd2O3 (99.999%) and Bi2O3 (99.99%). All of the raw materials with a stoichiometric ratio were mixed and put into crucibles. Then, they are subsequently heated at 1573 K for 5 h. Finally, the as-synthesized sample was cooled to room temperature and grinded. All of the calculations in this work are based on the density functional theory. The projector augmented wave (PAW) method is implemented in the Vienna ab-initio simulation package [22]. In this approach, the exchange-correlation energy has been approximated using the generalized gradient approximations of Perdew-Burke-Ernzerhof [23,24]. The energy cutoff is 600 eV for the PAW basis functions.6 First, the validity of the pseudopotentials was tested. The conjugate gradient optimization method was used in the optimization of the electronic structure, and a 5 × 5 × 5 k-point mesh generated by the Monkhors-Pack scheme was employed in the Brillouin zone sampling. All atoms in the supercell were fully relaxed until the forces acting on each atom were less than 0.01 eVÅ−1.

Fig. 1. (a) Observed (black dots) and calculated (red solid line) XRD patterns and difference profile (blue solid line) of Rietveld refinement for La2GeO5 host; (b) crystal structure of La2GeO5. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

were measured from 300 K to 600 K by a Jobin Yvon Triax 320 fluorospectrometer equipped with double excitation monochromators and a homemade high-temperature sample heater.

3. Results and discussion 3.1. Crystal structure and phase characterization To better understand the photoluminescence properties based on the crystal structure, the X-ray crystallographic data of La2GeO5 was refined by the general structure analysis system (GSAS) method [25]. The refinement started with the crystallographic data of La2GeO5 (ICSD No. 55100) as the initial model. The refined results show that the data and the structural model are in good agreement. The results of refinement are shown in Fig. 1a along with the profile comparison of experimental and calculated values. The calculated crystallographic parameters for La2GeO5 sample are shown in Table 1. As shown in Fig. 1b, the crystal structure of La2GeO5 host consists of isolated GeO4 tetrahedra and two La sites (La(1)O9 and La(2)O7) [26,27]. The La(2) position is located within the layers of GeO4 tetrahedra while the La(1) ions are above and below the layers with the O(5) ion bridging the two La ions. If Bi3+ and Nd3+ are doped into La2GeO5

2.2. Characterization The phase purity of the prepared phosphors was measured by an Xray diffractometer (XRD, BRUKER D8 ADVENCE) with Cu Kα1 radiation (wavelength = 0.15406 nm) at 36 kV tube voltage and 20 mA tube current. The photoluminescence excitation (PLE), photoluminescence (PL) emission and decay curves were recorded by an Edinburgh Instruments spectrophotometer FLS 920, equipped with a red-sensitive photomultiplier (Hamamatsu R928P) in Peltier-cooled housing, in the single photon counting mode and with the aid of an integration sphere. A 450 W ozone-free xenon was used as the excitation source for the steady-state measurements. Quantum yields of the phosphors were measured on a quantum yields measurement system (C9920-02, Hamamatsu Photonics K.K., Japan). The high-temperature PL spectra 524

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Table 1 Refined Structural Parameters for La2GeO5 host with P21/c space group. Cell parameters

a = 7.10975(4) Å, b = 7.46333(2) Å, c = 9.60172(9)Å, V = 485.7732(1)Å3 α = 90.00534°, β = 107.5466°, γ = 90.00374°

R factor

Rexp = 2.25, Rwp = 5.96, Rp = 4.43, GOF = 2.65

Atom

site

x

y

z

Occ.

La1 La2 Ge3 O4 O5 O6 O7 O8

4f 6h 6h 2a 2a 6h 6h 12i

0.02136 0.38175 0.29737 0.10646 0.10701 0.2964 0.3743 0.3923

0.62526 0.64389 0.09028 0.13795 0.61386 0.5634 0.5445 0.23268

0.23354 0.08165 0.04119 0.99794 0.94898 0.35987 0.75563 0.53998

1 1 1 1 1 1 1 1

Fig. 2. XRD patterns of La1.97GeO5:0.03Bi3+, La1.95GeO5:0.05Nd3+ and La1.92GeO5:0.03Bi3+, 0.05 Nd3+ samples along with the standard JCPDS card No 40-1183. Fig. 3. (a) Diffuse reflectance spectra of La2GeO5 host and (b) shows the optical band gap energies of La2GeO5 host.

host, they will prefer to occupy La3+ not Ge4+ sites due to their similar ionic radii (La3+, r = 0.117 nm; Bi3+, r = 0.103 nm; Nd3+, r = 0.104 nm; Ge4+, r = 0.039 nm). To study the effect of activator doping on the crystal structure of La2GeO5 host, the XRD patterns of La1.97GeO5: 0.03Bi3+, La1.95GeO5: 0.05Nd3+ and La1.92GeO5: 0.03Bi3+, 0.05 Nd3+ samples were examined in Fig. 2. Comparison among them indicates that all diffraction peaks are well consistent with the standard JCPDS card No.40-1183. Clearly, the XRD patterns suggest that the trace amounts of Bi3+ and Nd3+ codoped ions can still keep the crystal structure of La2GeO5 host intact. A small shift in the diffraction peaks is not observed. This indicates the very high solubility of Bi3+ ions in La2GeO5 which can be understood on the basis of the above analysis of the crystal structure of La2GeO5 host.

(αhν )2 = A (hν − Eg )

(1)

where A is a constant related to the host,α is the absorption coefficient and hν is the photon energy. According to Kubelka-Munk theory [29], the absorption coefficient (α) in equation (1) can be approximately replaced with

F (R∞) =

(1−R∞)2 2R∞

(2)

in which R∞ is the reflectance of an infinitely thick material. It can be substituted with experimentally observed reflectance. Since the transmittance of our samples is effectively zero, the Tauc plot of La2GeO5 host is depicted and fitted in Fig. 3b, giving the bandgap Eg = 5.852 eV. Although an exact discussion of transitions from VB to CB requires calculation of the excited-state electronic structure, the calculated static electronic band, as an approximation, can still provide important information. The calculated band structure in Fig. 4 reveals that monoclinic La2GeO5 host is an insulator with a direct bandgap (Eg) of 3.894 eV (The zero of energy was set at the Fermi level). In Fig. 4, the upper VB is mainly composed of O-2p, La-5p and Ge-4p states, which is situated in the energy range of −4.03eV ∼ −0.06eV. The CB is separated into two parts: the upper energy parts are contributed mostly by

3.2. Optical band gap energy and electronic structure Fig. 3a presents the diffuse reflectance spectra of La2GeO5 host. The spectrum of La2GeO5 host exhibits a platform of high reflection in the wavelength range from 400 to 800 nm and then starts to decrease dramatically from 400 nm to 200 nm. The absorption of La2GeO5 host is related to the excitation of electrons from host lattices’ valence band (VB) to the conduction band (CB). Based on the UV absorption edges of La2GeO5 host, the host bandgap (Eg) could be estimated by the following equation [28]: 525

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Fig. 4. Computed band structure and density of states for La2GeO5 host.

La-5d orbitals, and the lower parts in La2GeO5, which are also the bottom of the CB, are formed mainly by the Ge-4s orbitals. Therefore, the interband transition might be approximately viewed as one from O2p to Ge-4s orbitals. By comparison between optical and calculated values of band gap for La2GeO5, a difference is found. The inconsistency results mainly from the errors introduced from the measurement of the reflectance spectrum and the estimation by using equations (1)–(3).

3.3. Photoluminescence and energy transfer of Bi3+-Nd3+ Fig. 5 presents the PLE and PL spectra of La1.97GeO5: 0.03Bi3+ (λem = 480 nm, a; λex = 316 nm, b) and excitation spectrum of La1.92GeO5: 0.03Bi3+, 0.05Nd3+ (λem = 1058 nm, c) and La1.95GeO5: 0.05Nd3+ (λem = 1058 nm, d). The PLE spectrum of La1.97GeO5: 0.03Bi3+ (λem = 480 nm, black) in Fig. 5a consists of a strong broad absorption band around 316 nm with a hump at 245 nm, which are ascribed to the 1S0 →3P1 and 1S0 →3P0 transitions of Bi3+, respectively. Under excitation at 316 nm, La1.97GeO5:0.03Bi3+ sample exhibits a broad emission (λex = 316 nm, red) ranging from 375 nm to 600 nm, which originates from the 3P1 → 1S0 electronic transitions of Bi3+ ions, as shown in Fig. 5b. From Fig. 5c, one can see that La1.95GeO5: 0.05Nd3+ (λem = 1058 nm, blue) in the UV region show weak absorption and all the excitation peaks in the 200–800 nm region originates from the spin-forbidden 4f→4f transition of Nd3+. From Fig. 5b and c, one can see that a emission broad band from La1.97GeO5: 0.03Bi3+ overlaps with the excitation absorption of

Fig. 6. PL spectra of La1.97-xGeO5: 0.03Bi3+, xNd3+ (x = 0.005, 0.01, 0.03, 0.05, 0.08, 0.10 and 0.15) in the visible (a) and NIR (b) range under 316 nm excitation at room temperature.

La1.95GeO5:0.05Nd3+ ranging from 300 to 600 nm, implying that energy transfer from Bi3+ to Nd3+ might occur in La2GeO5. To provide direct evidence of energy transfer from Bi3+ to Nd3+, the PLE spectra of La1.92GeO5: 0.03Bi3+, 0.05Nd3+ (λem = 1058 nm, green) and La1.95GeO5: 0.05Nd3+ (λem = 1058 nm, blue) were measured. In Fig. 5d, the strong broad absorption around 316 nm in excitation spectrum of La1.92GeO5:0.03Bi3+, 0.05Nd3+ sample was attributed to the spin-allowed 1S0→3P1 transition from of Bi3+ and other weak excitation peaks are associated with 4f-4f transitions of Nd3+. The presence of excitation bands from Bi3+ ions is found in the excitation spectrum of Nd3+ ions, indicating that energy transfer from Bi3+ to Nd3+ in La1.92GeO5: 0.03Bi3+, 0.05Nd3+ takes place, indeed. In Fig. 6a, La1.97-xGeO5: 0.03Bi3+, xNd3+ presents a strong bluishgreen emission ranging from 370 nm to 600 nm, which originates from the from the 3P1 → 1S0 electronic transitions of Bi3+ ions. Obviously, its emission intensity suffers a decrease with the Nd3+ doping concentration increasing. The reason for this is that activated energy of Bi3+ is transferred to Nd3+ ions. Fig. 6b shows the NIR emission spectrum of La1.97-xGeO5: 0.03Bi3+, xNd3+ under UV light excitation. It consists of a series of line peaks at 815 nm, 876 nm, 895 nm, 925 nm, 1058 nm and 1328 nm, which are ascribed to the Nd3+ characteristic intra-4f transitions of 2D5/2 → 4F3/2, 2P1/2 → 4F3/2, 4F3/2 → 4I9/2, 2P1/2 → 4F5/2, 4F3/ 4 4 4 2 → I11/2 and F3/2 → I13/2, respectively [7]. As shown in Fig. 6b, the emission intensity of Nd3+ in the near infrared range are greatly enhanced, the optimum concentration of Nd3+ in La1.97-xGeO5: 0.03Bi3+, xNd3+ is approximately 0.05. A further increase of Nd3+ concentration

Fig. 5. PLE and PL spectra of La1.97GeO5: 0.03Bi3+ (λem = 480 nm, a; λex = 316 nm, b); Excitation spectrum of La1.92GeO5:0.03Bi3+, 0.05Nd3+ (λem = 1058 nm, c) and La1.95GeO5: 0.05Nd3+ (λem = 1058 nm, d). 526

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Fig. 8. Decay lifetimes of Bi3+ dependent on the Nd3+ doping concentration in La1.97-xGeO5: 0.03Bi3+, xNd3+ and dependence of the energy transfer efficiency (η) on Nd3+ (x) content (x = 0, 0.005, 0.01, 0.03, 0.05, 0.08, 0.10 and 0.15).

Fig. 7. Decay curves of La2GeO5: 0.03Bi3+, xNd3+ (x = 0, 0.005, 0.01, 0.03, 0.05, 0.08, 0.10 and 0.15) monitoring at 480 nm under 316 nm excitation.

results in the decrease of its emission intensity due to the concentration quenching effect. Actually, at higher concentration, the distance among the Nd3+ ions becomes smaller than the critical distance and the excitation energy easily migrates to the quenching centers. Meanwhile, the emission intensity of Bi3+ in the visible region continuously decreases. All the above prove that energy transfer from Bi3+ to Nd3+ in La1.97-xGeO5: 0.03Bi3+, xNd3+ occurs. For better understanding the energy transfer from Bi3+ to Nd3+ in La2GeO5, the decay curves of La1.97-xGeO5: 0.03Bi3+, xNd3+ (x = 0, 0.005, 0.01, 0.03, 0.05, 0.08, 0.10 and 0.15) samples were measured as a function of Nd3+ concentration in Fig. 7. All decay curves were found following the double exponential decay equation as follows [30,31]: t

t

It = A1 e− τ1 + A2 e− τ2

increased gradually; meanwhile, energy transfer efficiency ηT increases with increasing Nd3+ content, both of which further confirms energy transfer from Bi3+ to Nd3+ ions. By aids of equation (6), energy transfer efficiency (ηT) from Bi3+ to Nd3+ is calculated to be 14%, 20%, 40%, 50%, 71%, 82% and 91% for the Nd3+ concentration x = 0.005, 0.01, 0.03, 0.05, 0.08, 0.10 and 0.15, respectively. Without the regard of the nonradiative losses, the maximum value QE of La2GeO5: Bi3+, Nd3+ is calculated to be approximately 191%. All the calculated data of decay lifetimes (τave), energy transfer efficiency (ηET) and the theoretical QE (ηQE) were listed in Table 2. Meanwhile, A comparison of Bi3+ and Nd3+ codoped quantum cutting phosphors in different host is made, as shown in Table 3. Based on Dexter's energy transfer theory about multipolar interactions, the possible mechanism on the energy transfer from Bi3+ to Nd3+ is discussed in the following. The multipolar-multipolar interaction process can be illuminated as follows [32,33]:

(3)

Where τ1 and τ2 are exponential component of the decay time and A1 and A2 are fitting constants, respectively. It is the luminescence intensity at times t. The average decay lifetimes (τave) can be evaluated by the equation [30,31]:

τave

A1 τ12 + A2 τ22 = A1 τ1 + A2 τ2

η0 ∝ c n /3 ηs

(4)

Where η0 and ηs are the luminescence quantum efficiency of Bi3+ in the absence and presence of Nd3+, respectively; C is the total content of Bi3+ and Nd3+, n = 6, 8 and 10, which corresponds to dipole-dipole (DMSO‑d6), dipole-quadrupole (d-q), and quadrupole-quadrupole (q-q) interactions, respectively. Generally, the ratio of η0 / ηs can be estimated by the corresponding emission integrated intensity (I0/ Is ). In this case, I0/ Is versus Cn/3 plots are depicted and the optimal linear relation (R2 = 0.99684) was obtained when n = 6, as shown in Fig. 9.

As Nd3+ content increases from 0 to 0.005, 0.01, 0.03, 0.05, 0.08, 0.10 to 0.15, the τave value of Bi3+ emission at 480 nm decreases from 0.520, 0.447, 0.414, 0.311, 0.257, 0.148, 0.094 to 0.054 us, as shown in Fig. 8. The decay lifetime reduction of Bi3+ is an evidence for energy transfer from Bi3+ to Nd3+. This strongly proves that an energy transfer from Bi3+ to Nd3+ ions occurs. With the above decay lifetimes, the energy transfer efficiency (ηET) from Bi3+ to Eu3+ can be calculated by the following equation [32,33]:

ηET = 1 −

τs τ0

Table 2 Decay lifetimes, energy transfer efficiency ηΕΤ (%), and theoretical QE (ηQE) of La1.97-xGeO5: 0.03Bi3+, xNd3+ (x = 0.005, 0.01, 0.03, 0.05, 0.08, 0.10 and 0.15).

(5)

where τs0 and τs are the lifetime of Bi3+ in the absence and presence of Nd3+, respectively. Furthermore, the theoretical QE yields of all phosphor materials were calculated by using the following relation between ηQE and ηET [ 12, 14, 15]:

ηQE = ηH (1 − ηET ) + 2ηET

Nd3+ concentration (x value)

(6)

0 0.005 0.01 0.03 0.05 0.08 0.10 0.15

where ηH stands for luminescent QE of La1.97GeO5: 0.03Bi , xNd lattice without. Nd3+ ion. Assuming that the non-radiative transitions from the sensitizer Bi3+ in La2GeO5 host is almost negligible, ηET is set to be 1. As was clearly shown in Fig. 8, the decay lifetimes of the sensitizer Bi3+ in La2GeO5 reduced when the activator Nd3+ doping concentration was 3+

(7)

3+

527

La1.97-xGeO5: 0.03Bi3+, xNd3+ τ (us)

ηΕΤ (%)

ηQE (%)

0.520 0.447 0.414 0.311 0.257 0.148 0.094 0.054

0 14 20 40 50 71 82 91

100 114 120 140 150 171 182 191

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Table 3 Energy transfer efficiency ηΕΤ (%) and theoretical QE (ηQE) of Bi3+ and Nd3+ codoped phosphors in different hosts. Phosphors

ηET

Theoretical QE

Reference

GdNdO4: Bi3+, Nd3+ YVO4: Bi3+, Nd3+ Gd2O3: Bi3+, Nd3+ La2GeO5: Bi3+, Nd3+ Lu2GeO5: Bi3+, Nd3+

72% 90% 64.1% 91% 67.8%

172% 190% 164.1% 191% 167.8%

[7] [34] [35] This work In press

Therefore, the dipole-dipole interaction is responsible for energy transfer from Bi3+ to Nd3+ in La2GeO5. 3.4. Thermal quenching Fig. 10a illustrates the temperature-dependent PL spectra of Nd3+ in NIR region of La1.92GeO5: 0.03Bi3+, 0.05Nd3+ under 316 nm excitation. Obviously, the relative PL intensity of Nd3+gradually decreases as the temperature increases from 293 K to 573 K. The phenomenon is called as thermal quenching effect, which is originated from the temperature-dependent electron-phonon interaction, in which the excited luminescent center is thermally activated through phonon interaction, and then thermally released through the crossing point between the excited state and the ground state. This nonradiative transition probability by thermal activation is strongly dependent on temperature, resulting in the decrease of emission intensity. To evaluate the resistance ability against thermal quenching, the activation energy (ΔEa), which is key indicator for the thermal quenching, can be calculated by the Arrhenian equation [32,33]:

It =

I0 1 + Ae(−ΔE/κ B T)

(8)

where It is the intensity at a given temperature, I0 is the initial intensity, A is a constant, ΔE is the activation energy for thermal quenching, kB is the Boltzmann constant (8.629 × 10−5 eV/K). Plotting ln(I0/IT-1) vs l/ kBT gives a straight line plot up to 623 K with an activation energy of 0.152 eV, as shown in Fig. 10b. These results indicate that La1.92GeO5: 0.03Bi3+, 0.05Nd3+ phosphor shows a good resistance to thermal quenching and degradation.

Fig. 10. (a) Temperature-dependent PL spectra of La1.92GeO5: 0.03Bi3+, 0.05Nd3+ and the inset gives the temperature dependence of the normalized integrated intensity of Nd3+ for La1.92GeO5: 0.03Bi3+, 0.05Nd3+; (b) Relationship of ln[(I0/I(T))-1] versus 1/κT for La1.97(GeO4)O: 0.03Bi3+.

absorption band of La2GeO5: Bi3+, Nd3+ phosphor in the UV region is largely enhanced. Under excitation at UV light, When excited at UV region, the energy could be efficiently absorbed by Bi3+ ions via Bi3+: 1 S0→3P1 transition. The Bi3+: 3P1 excited levels can either relax radiatively to the 1S0 ground state, producing broad band luminescence in the green range (Bi(1)) or in the blue range (Bi(2)), or transfer energy to

3.5. The DC mechanism of La2GeO5: Bi3+, Nd3+ The energy transfer process from Bi3+ to Nd3+ is summarized in Fig. 11 which depicts the energy level diagrams of the Bi3+ and Nd3+ emission centers in La2GeO5 host. With the introduction of Bi3+, the

Fig. 9. Dependence of Ιs0/Ιs on Cn/3 ((a) n = 6, (b) n = 8, (c) c = 10) in La1.97GeO5: 0.03Bi3+, xNd3+ (x = 0.005, 0.01, 0.03, 0.05, 0.08, 0.10 and 0.15), respectively. 528

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Fig. 11. Scheme of Nd3+ luminescence by energy transfer from Bi3+ to Nd3+ in La1.92GeO5: 0.03Bi3+, 0.05Nd3+.

the Nd3+. Meanwhile, the 3P1 and 3P0 states of the Bi3+ are resonant with 2D5/2 and 2P1/2 levels of Nd3+ ion, excitation energy from 3P1 and 3 P0 states is transferred to 2D5/2 and 2P1/2 levels of Nd3+ through the energy transfer process. The excited 2D5/2 and 2P1/2 states of Nd3+ ions jumps to 4F3/2 and 4F5/2 states accompanying by the emission of 812 nm (2D5/2 → 4F3/2), 888 nm (2D5/2 → 4F5/2), 875 nm (2P1/2 → 4F3/2) and 925 nm (2P1/2 → 4F5/2) NIR photons. Subsequently, the population in the 4F5/2 state relaxes to the 4F3/2 state nonradiatively. Finally, Nd3+ ions at the 4F3/2 level relax to the ground state through 4F3/2 → 4I9/2 (888 nm) and 4F3/2 → 4I11/2 (1064 nm) transitions. In this way, we get two 888 nm photons (2D5/2 → 4F5/2 and 4F3/2 → 4I9/2) emission by cascade and thus a two-photon quantum cutting process, while we obtain 812 nm (2D5/2 → 4F3/2), 875 nm (2P1/2 → 4F3/2), 925 nm (2P1/ 4 4 4 4 4 2 → F5/2) and 1064 nm ( F3/2 → I11/2) and 1328 nm ( F3/2 → I13/2) emissions through the QC process.

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