Conversion of broadband UV-visible light to near infrared emission by Ca₁₄Zn₆Al₁₀O₃₅: Mn⁴⁺, Nd³⁺/Yb³⁺

Abstract

Efficient Ca₁₄Zn₆Al₁₀O₃₅: Mn⁴⁺, Nd³⁺/Yb³⁺ spectral conversion materials have been prepared by a sol–gel method. The Ca₁₄Zn₆Al₁₀O₃₅: Mn⁴⁺, Nd³⁺/Yb³⁺ materials can efficiently shift the short-wavelength sunlight in 250–550 nm spectral regions into near infrared emission which matches the higher sensitivity region of Si-based solar cells. The maximal energy transfer efficiency is 76.0% and 80.4% in Mn⁴⁺, Nd³⁺ and Mn⁴⁺, Yb³⁺ co-doped samples when excited at 460 nm, respectively. A dipole–dipole interaction is responsible for the energy transfer sensitization processes from Mn⁴⁺ to Nd³⁺/Yb³⁺ ions, which has been confirmed by Dexter's theory and the Yokota–Tanimoto model.

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1. Introduction

Solar energy is free, clean and abundant in the world and thus is regarded as an ideal replaceable energy for mankind in the future. The capacity of photovoltaic devices to transform sunlight directly into electricity promises a prime technology for solar energy utilization. Currently, the photovoltaic solar cell market is predominated by single-junction crystalline and polycrystalline Si solar cells with a conversion efficiency of about 15%.^1,2 Nevertheless, the theoretical maximum conversion efficiency of Si-based solar cells is higher than 30%.^3,4 In principle, photons with energy higher than the bandgap of photovoltaic devices are absorbed, but the Si-based solar cell works most efficiently under irradiation of the light in the vicinity of 1000 nm.

To enhance the efficiency of the Si-based solar cell, down-shifting is a promising technique by which short-wavelength sunlight can be converted to light in near-infrared (NIR) region where the photovoltaic solar cell is more sensitive. Lanthanide ions were usually used to obtain NIR light by down shifting because of their rich energy-level structure permitting abundant NIR emission. Among the lanthanide ions, Nd³⁺ and Yb³⁺ are particularly noteworthy because their NIR emissions take place at around 900(1060) nm and 980 nm, respectively, which are just above the band edge of Si semiconductor where the solar cell exhibits excellent spectral response. However, the Nd³⁺ and Yb³⁺ ions exhibit weak and narrow absorption due to their parity-forbidden 4f–4f transitions, and as a result, only a small part of ultraviolet and visible sunlight can be converted into NIR emission. In order to realize broadband spectral conversion, the f–d transition lanthanide ions (e.g., Ce³⁺, Eu²⁺, Yb²⁺),^5,6 transition metal ions (e.g., Cr³⁺, Mn²⁺)^7–9 and semiconductor quantum dots (e.g., CdSe)^10–12 have been widely attempted as sensitizers in Nd³⁺/Yb³⁺ doped materials. Nevertheless, enhancing the sensitized Nd³⁺/Yb³⁺ NIR emission with excitation at UV-visible light is still a difficult problem to be solved.

In recent years, tetravalence manganese ion (Mn⁴⁺) doped luminescent materials have been investigated and attracted much attention.^13–22 Mn⁴⁺ ions enter into the octahedral sites as substitutes and exhibit deep red luminescence.^18,19,23 More importantly, the Mn⁴⁺–O²⁻ charge transfer transition and ⁴A₂ → ⁴T₁, ⁴A₂ → ⁴T₂ spin-allowed d–d transitions of Mn⁴⁺ exhibit a strong and continuous UV and visible absorption, which indicates the possible application for broadband light conversion. Among these phosphors, the Mn⁴⁺ doped Ca₁₄Zn₆Al₁₀O₃₅ material is especially attractive because of its high luminescence quantum efficiency, good thermal and chemical stability, and ease of preparation.^13,22 Furthermore, the Ca²⁺ sites can be replaced by Nd³⁺/Yb³⁺ ions because of their similar ion radius. This implies that efficient energy transfer from Mn⁴⁺ to Nd³⁺/Yb³⁺ can possibly take place in the Ca₁₄Zn₆Al₁₀O₃₅ matrix. Hence the Mn⁴⁺, Nd³⁺/Yb³⁺ co-doped Ca₁₄Zn₆Al₁₀O₃₅ materials promise to be broadband spectral converters to obtain efficient NIR emission (around 1000 nm).

In this paper, two spectral conversion materials, i.e., Ca₁₄Zn₆Al₁₀O₃₅: Mn⁴⁺, Nd³⁺ and Ca₁₄Zn₆Al₁₀O₃₅: Mn⁴⁺, Yb³⁺ have been prepared. The luminescence properties and energy transfer mechanism between Mn⁴⁺ and Nd³⁺/Yb³⁺ ions were investigated in detail.

2. Experimental

2.1. Materials synthesis

The spectral conversion materials were prepared through a sol–gel procedure. Al(NO₃)₃(H₂O)₉ (99.9%), Nd(NO₃)₃(H₂O)₆ (99.9%), Yb(NO₃)₃(H₂O)₆ (99.9%), Mn(NO₃)₂(H₂O)₄ (99.9%), Ca(NO₃)₂(H₂O)₄ (99.9%), Zn(NO₃)₂(H₂O)₆ (99.9%), C₆H₈O₇ (99.9%) were used as raw materials. Firstly, the reagents were dissolved in distilled water with the stoichiometric ratio of Ca_14−x/yZn₆Al_10−zO₃₅ : Mn_z, Nd_x/Yb_y (x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.225, 0.25, 0.275, 0.30, 0.40; y = 0.0, 0.2, 0.4, 0.6, 0.7, 0.8, 0.9, 1.0, 1.2; z = 0.0, 0.2, 0.4, 0.5, 0.55, 0.6, 0.65, 0.7, 0.8, 1.0) and subsequently mixed by stirring. Then the C₆H₈O₇ aqueous solutions were dripped into the nitrate solutions with the cation–ligand ratio cation : C₆H₈O₇ = 1 : 1.5 accompanied by constant stirring. After standing for 12 h, the solutions were dried at 100 °C for 24 hours to obtain precursors. Then the precursors were carried out to remove the organic matter by heating the temperature to 600 °C for 2 h and then cooled to room temperature. After grinding, the powders were heated to 1000 °C with a ramp rate of 5 °C min⁻¹ under air condition and calcined at that temperature for 2 h to obtain the final products.

2.2. Characterization

The crystalline phases of the samples were analyzed by X-ray diffraction (XRD) on a Bruker D8 advanced equipment, using Cu tube with Cu/K (k = 0.1541 nm) radiation. X-ray photoelectron spectroscopy (XPS) was measured on a K-Alpha 1063 (Thermo Fisher Scientific) with a focused monochromatic Al Ka X-ray beam (12 kV, 6 mA, 5 × 10⁻⁹ torr). The photoexcitation spectra (PLE) and visible-near-infrared photoluminescence (PL) spectra were measured by a monochromator (Zolix Instrument, Omni-λ320i) coupled with photomultiplier (PMTH-S1-CR131) and near infrared sensitive detector (DInGaAs 2600-TE), in which a monochromator (Zolix Instrument Omni-λ320) coupled with a 150 W xenon lamp was used to provide the monochromatic exciting light. The luminescence decay curves were analyzed by a PTI QM 40 spectrofluorometer, using a pulse xenon lamp as the excitation source. The diffuse refection spectra of the samples were measured by a Varian Cary 100 UV-Vis Spectrofluorometer with a DRA-CA-30I Diffuse Reflectance Accessory. The morphology of the prepared samples was characterized by a JSM-6610 scanning electron microscope (SEM).

3. Results and discussion

Fig. 1 shows the X-ray diffraction (XRD) patterns of Ca_14−x/yZn₆Al_10−zO₃₅: Mn_z, Nd_x/Yb_y (x = 0.25, y = 0.4, 0.6, 0.8, z = 0.2, 0.6). The diffraction peaks of the samples can be identified by comparison with the standard XRD data of Ca₁₄Zn₆Al₁₀O₃₅ (JCPDS 50-0426). Ca₁₄Zn₆Al₁₀O₃₅ has a cubic structure with space group F23. In Ca₁₄Zn₆Al₁₀O₃₅ crystal structure, four of the five independent positions occupied by Zn and Al are in the tetrahedral coordination, with the average Zn–O distances of 1.951 Å and average Al–O distances of 1.719, 1.794 and 1.891 Å, respectively. The fifth independent position is in the octahedral coordination with the Al–O distance of 1.936 Å. In addition, Ca²⁺ has three different coordination environments. Two of them are in octahedral with the average Ca–O distances of 2.336 and 2.346 Å, and the third independent Ca²⁺ is in a seven-coordinated polyhedron with an average Ca–O distance being equal to 2.498 Å.²⁴ The schematic of Ca₁₄Zn₆Al₁₀O₃₅ crystal structure is shown in Fig. 2. Fig. 3 shows the Mn XPS core level spectrum for Ca₁₄Zn₆Al_9.4O₃₅: Mn_0.6. The smoothed XPS spectrum shows the peak of Mn 2p_3/2 with a binding energy at 642.3 eV (see Fig. 3). It is known that the peaks of Mn²⁺ 2p_3/2 (MnO), Mn³⁺ 2p_3/2 (Mn₂O₃) and Mn⁴⁺ 2p_3/2 (MnO₂) locate at 641.7, 641.8 and 642.4 eV, respectively.²⁵ Therefore, the Mn element predominantly behaves as the state of Mn⁴⁺ in the Ca₁₄Zn₆Al₁₀O₃₅ host. It is commonly accepted that Mn⁴⁺ ions are preferentially accommodated at the Al³⁺ sites in the lattice with an octahedral coordination.^13,22 It can be seen that Ca²⁺ site is likely to be replaced by Nd³⁺/Yb³⁺ ion without significant structural changes when a small amount of Nd³⁺/Yb³⁺ are introduced, due to the similar ion radius between Ca²⁺ and Nd³⁺/Yb³⁺ (Ca²⁺: radius = 0.100 nm; Nd³⁺: radius = 0.098 nm; Yb³⁺: radius = 0.086 nm). Considering the different valences of Ca²⁺ and Nd³⁺/Yb³⁺, charge compensation is required. The commonly accepted view is that the formative Ca vacancies can compensate charge imbalance.^26–28 When Nd³⁺/Yb³⁺ ions are co-doped into Ca₁₄Zn₆Al₁₀O₃₅ matrix, the Ca vacancies might form and they could keep the electroneutrality of the compound.

Fig. 1 Power X-ray diffraction patterns of Ca_14−x/yZn₆Al_10−zO₃₅: Mn_z, Nd_x/Yb_y (x = 0.25, y = 0.4, 0.6, 0.8, z = 0.2, 0.6).

Fig. 2 Schematic of the Ca₁₄Zn₆Al₁₀O₃₅ crystal structure.

Fig. 3 Mn 2p XPS spectrum of Ca₁₄Zn₆Al_9.4O₃₅: Mn_0.6 sample.

The diffuse reflection spectra of Ca₁₄Zn₆Al₁₀O₃₅ doped with Mn⁴⁺, Nd³⁺ and Yb³⁺ ions are shown in Fig. 4. In the Mn⁴⁺ single-doped and Mn⁴⁺, Nd³⁺/Yb³⁺ co-doped samples, it can be seen two strong absorption peaks at 370 nm and 465 nm originating from the spin-allowed Mn⁴⁺: ⁴A₂ → ⁴T₁ and Mn⁴⁺: ⁴A₂ → ⁴T₂ transitions, which are similar to the experimental results reported previously.²⁹ The absorption band between 300 and 350 nm is due to the charge transfer transition of Mn⁴⁺–O²⁻. In addition, the weak and narrow absorption peaks of Nd³⁺ are also observed in Ca₁₄Zn₆Al₁₀O₃₅: Mn⁴⁺, Nd³⁺. The high absorption efficiency of the above mentioned samples in the wide UV-visible light region are beneficial to the realization of the short-wavelength light down-shifting.

Fig. 4 Normalized diffuse reflection spectra of Ca₁₄Zn₆Al_9.8O₃₅: Mn_0.2, Ca_13.75Zn₆Al_9.8O₃₅: Mn_0.2, Nd_0.25 and Ca_13.2Zn₆Al_9.8O₃₅: Mn_0.2, Yb_0.8 samples.

Fig. 5 presents the normalized PLE and/or PL spectra of Mn⁴⁺, Nd³⁺, Yb³⁺ single-doped, Mn⁴⁺, Nd³⁺/Yb³⁺ co-doped samples. Excitation into the absorption band at 460 nm gives an intense broad red emission around 710 nm originating from the ²E → ⁴A₂ spin-forbidden transition of Mn⁴⁺ ions in Mn⁴⁺ single-doped sample. The excitation also gives luminescence bands which are located at 900, 1060 nm in Mn⁴⁺, Nd³⁺ co-doped and 980 nm in Mn⁴⁺, Yb³⁺ co-doped samples, originating from Nd³⁺: ⁴F_3/2 → ⁴I_9/2, Nd³⁺: ⁴F_3/2 → ⁴I_11/2 and Yb³⁺: ²F_5/2 → ²F_7/2 transitions, respectively. The PLE spectra monitored at 710 nm of Mn⁴⁺ single-doped sample shows three broad peaks around 320, 380 and 470 nm, which can be attributed to the charge transfer transition of Mn⁴⁺–O²⁻ and spin-allowed transitions of Mn⁴⁺: ⁴A₂ → ⁴T₁, Mn⁴⁺: ⁴A₂ → ⁴T₂ corresponding well with the absorption spectrum. It is noteworthy that the PLE spectra shapes of the Mn⁴⁺, Nd³⁺ co-doped sample monitored at 1060 nm and the Mn⁴⁺, Yb³⁺ co-doped sample monitored at 980 nm are quite similar to the Mn⁴⁺ single-doped sample (see Fig. 5(a)–(c)). Only weak and discrete PLE peaks in visible region caused by the f–f transitions of Nd³⁺ appear in the Nd³⁺ single-doped sample and no PLE peak in visible region is observed in the Yb³⁺ single-doped sample (see Fig. 5(a) and (b)). Obviously, the typical broad and intense excitation bands in the Mn⁴⁺, Nd³⁺/Yb³⁺ co-doped samples monitored at the NIR region also originate from the charge transfer transition of Mn⁴⁺–O²⁻ and spin-allowed transitions of Mn⁴⁺: ⁴A₂ → ⁴T₁, Mn⁴⁺: ⁴A₂ → ⁴T₂. The characteristics of the above PLE spectra demonstrate that the NIR luminescence of Nd³⁺/Yb³⁺ in Mn⁴⁺, Nd³/Yb³⁺ co-doped samples is generated by the energy transfer sensitization from Mn⁴⁺ to Nd³⁺/Yb³⁺ ions.

Fig. 5 PLE spectra (left) and/or PL spectra (right) for the Ca_13.75Zn₆Al_9.8O₃₅: Mn_0.2, Nd_0.25 and Ca_13.75Zn₆Al₁₀O₃₅: Nd_0.25 samples (a), the Ca_13.2Zn₆Al_9.8O₃₅: Mn_0.2, Yb_0.8 and Ca_13.2Zn₆Al₁₀O₃₅: Yb_0.8 samples (b), and the Ca₁₄Zn₆Al_9.8O₃₅: Mn_0.2 samples (c).

In order to optimize the NIR emission performance, the PL for the Ca_14−x/yZn₆Al_10−zO₃₅: Mn_z, Nd_x/Yb_y materials with different Mn⁴⁺ and Nd³⁺/Yb³⁺ doping concentrations were systemically investigated under excitation at 460 nm. When the Mn⁴⁺ content is fixed (z = 0.2), it is found that the NIR PL intensity in both the Mn⁴⁺, Nd³⁺ co-doped and Mn⁴⁺, Yb³⁺ co-doped samples increases at first with the increase of the content of rare earth ions, reaching the maximum at x = 0.25 and y = 0.8, respectively, and then decreases gradually as the result of the concentration quenching, just as shown in Fig. 6(a) and (b). When the Nd³⁺ and Yb³⁺ doping contents are fixed at x = 0.25 and y = 0.8, the NIR PL intensity of Nd³⁺ and Yb³⁺ can be further enhanced significantly by increasing the Mn⁴⁺ doping content (see Fig. 6(c) and (d)), respectively. The NIR emission intensities of the Nd³⁺ and Yb³⁺ ions both reach their maximum at z = 0.6. Compared with the Ca_13.75Zn₆Al₁₀O₃₅: Nd_0.25 and Ca_13.2Zn₆Al₁₀O₃₅: Yb_0.8 samples, the NIR luminescence intensity is enhanced by 338 times at 1060 nm for Ca_13.75Zn₆Al_9.4O₃₅:Mn_0.6, Nd_0.25 and 306 times at 980 nm for Ca_13.2Zn₆Al_9.4O₃₅:Mn_0.6, Yb_0.8, respectively. The enhancement of the NIR emission induced by co-doped Mn⁴⁺ indicates the efficient energy transfer from Mn⁴⁺ to Nd³⁺/Yb³⁺ ions.

Fig. 6 PL spectra of Ca_14−xZn₆Al_9.8O₃₅: Mn_0.2, Nd_x (a), Ca_14−yZn₆Al_9.8O₃₅: Mn_0.2, Yb_y (b), Ca_13.75Zn₆Al_10−zO₃₅: Mn_z, Nd_0.25 (c) and Ca_13.2Zn₆Al_10−zO₃₅ : Mn_z, Yb_0.8 (d) excited at 460 nm.

The energy transfer efficiency depends on how well the acceptor energy levels match the frequencies of the donor emission. As we compare the emission spectra of Mn⁴⁺ single-doped sample with the excitation spectra of Nd³⁺ single-doped sample, we can find a good spectral overlap between the Mn⁴⁺: ²E emission and the Nd³⁺: ⁴F_9/2, ⁴F_7/2, ⁴S_3/2 excitation, as shown in Fig. 7(a). Therefore, it enables the efficient sensitization NIR emission by nonradiative resonant energy transfer from Mn⁴⁺ to Nd³⁺ ions via the process Mn⁴⁺: ²E + Nd³⁺: ⁴I_9/2 → Mn⁴⁺: ⁴A₂ + Nd³⁺: ⁴F_9/2, ⁴F_7/2, ⁴S_3/2. From Fig. 7(b), and we can see that there is relatively large energy gap between the Mn⁴⁺: ²E level and Yb³⁺: ²F_5/2 level. It seems that efficient sensitization NIR emission by resonant energy transfer from Mn⁴⁺ to Yb³⁺ is unlikely. However, efficient energy transfer from Mn⁴⁺ to Yb³⁺ can still take place in the Mn⁴⁺, Yb³⁺ co-doped samples. From the emission spectrum of Mn⁴⁺: ²E → ⁴A₂ shown in Fig. 7(b), we can see that the emission sideband almost extends from the peak at 715 nm to 850 nm, which indicates the strong electron–phonon coupling in Ca₁₄Zn₆Al₁₀O₃₅. The strong electron–phonon coupling is beneficial to the phonon-assisted energy transfer.³⁰ Therefore the NIR luminescence of Yb³⁺ might be mainly generated by phonon-assisted energy transfer from Mn⁴⁺ to Yb³⁺.

Fig. 7 The overlapping between the emission spectra of Mn⁴⁺ and the excitation spectra of Nd³⁺ monitored at 1060 nm (a)/Yb³⁺ monitored at 980 nm (b).

The excitation/emission and energy transfer pathways for the Mn⁴⁺, Nd³⁺/Yb³⁺ ion couples in Ca₁₄Zn₆Al₁₀O₃₅ are illustrated in Fig. 8. Firstly, the Mn⁴⁺ ions are excited into their charge-transfer or ⁴T₁, ⁴T₂ excited states by the irradiation. Then the excited Mn⁴⁺ ions rapidly relax to their metastable ²E state, and the energy transfer takes place via Mn⁴⁺: ²E + Nd³⁺: ⁴I_9/2 → Mn⁴⁺: ⁴A₂ + Nd³⁺: ⁴F_9/2, ⁴F_7/2, ⁴S_3/2 or Mn⁴⁺: ²E + Yb³⁺: ²F_7/2 → Mn⁴⁺: ⁴A₂ +Yb³⁺: ²F_5/2. The following nonradiative relaxation among the intra-4f shell energy levels performs the population of Nd³⁺: ⁴F_3/2 and Yb³⁺: ²F_5/2 levels, resulting in the 900, 1060 and 980 nm emissions, respectively.

Fig. 8 Energy-transfer and electron-transition scheme of Mn⁴⁺, Nd³⁺/Yb³⁺ in Ca₁₄Zn₆Al₁₀O₃₅.

In order to further understand the energy transfer process and estimate the energy transfer sensitization efficiency, the luminescence decay curves of the Mn⁴⁺: ²E → ⁴A₂ emission were measured in the Ca_14−x/yZn₆Al_9.8O₃₅: Mn_0.2, Nd_x/Yb_y (from x = 0.00 to 0.40; from y = 0.0 to 1.2) samples under excitation at 460 nm, as shown in Fig. 9. It can be seen that the decay rate increases with the increase of Nd³⁺/Yb³⁺ contents. The effective lifetime of the Mn⁴⁺ luminescence is expressed as

where I(t) is the time dependent luminescence intensity of Mn⁴⁺ and τ is the decay lifetime of Mn⁴⁺ luminescence. The calculated decay lifetimes are all listed in Table 1. It can be seen that the lifetime of Mn⁴⁺ luminescence decreases monotonously from 1.123 to 0.413 ms with the Nd³⁺ content increasing from 0.00 to 0.40 and from 1.123 to 0.652 ms with the Yb³⁺ content increasing from 0.0 to 1.2. The Nd³⁺/Yb³⁺ concentration dependence of the decay lifetime proves the nonradiative energy transfer process from Mn⁴⁺ to Nd³⁺/Yb³⁺. The energy transfer efficiency (η_ET) can be calculated by³¹Where τ and τ₀ are the lifetimes of the Mn⁴⁺ luminescence in the Mn⁴⁺, Nd³⁺/Yb³⁺ co-doped and Mn⁴⁺ single-doped cases, respectively. The estimated energy transfer efficiencies are listed in Table 1. As expected, with the increase of Nd³⁺/Yb³⁺ content, the energy transfer efficiency gradually increases due to the decrease of distance between donor (Mn⁴⁺) and acceptor (Nd³⁺/Yb³⁺). It should be pointed out that the near infrared emission intensity will decrease when the Nd³⁺/Yb³⁺ doping concentration exceeds a certain value, although the Mn⁴⁺ → Nd³⁺/Yb³⁺ energy transfer probability always increases with the enhancement of Nd³⁺/Yb³⁺ doping concentration in our experiment. This means that fluorescence quenching is inevitable because of the intensified Nd³⁺–Nd³⁺ or Yb³⁺–Yb³⁺ interaction at high Nd³⁺/Yb³⁺ concentration.

Fig. 9 Decay curves of Mn⁴⁺ luminescence in Ca_14−xZn₆Al_9.8O₃₅: Mn_0.2, Nd_x (a)/Ca_14−yZn₆Al_9.8O₃₅: Mn_0.2, Yb_y (b) monitoring at 710 nm excited at 460 nm light.

Table 1 The effective decay lifetime (τ) of Mn⁴⁺ luminescence and the energy transfer efficiency (η_ET) of Mn⁴⁺ → Nd³⁺/Yb³⁺

Ca14−xZn6Al9.8O35: Mn0.2, Ndx			Ca14−yZn6Al9.8O35: Mn0.2, Yby
Content (x)	τ (ms)		Content (y)	τ (ms)
0.00	1.123	0.000	0.0	1.123	0.000
0.05	0.901	0.198	0.2	0.924	0.177
0.10	0.783	0.303	0.4	0.899	0.199
0.15	0.666	0.407	0.6	0.806	0.282
0.20	0.575	0.488	0.8	0.749	0.333
0.25	0.525	0.533	1.0	0.716	0.363
0.30	0.486	0.567	1.2	0.652	0.419
0.40	0.413	0.632

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It is known that the energy transfer types include radiation reabsorption, exchange interaction, and multipolar interaction. For the Ca₁₄Zn₆Al₁₀O₃₅: Mn⁴⁺, Nd³⁺/Yb³⁺, the energy transfer based on radiation reabsorption can be neglected because the structure of the emission spectra of Mn⁴⁺ is hardly changed in Ca₁₄Zn₆Al₁₀O₃₅: Mn⁴⁺, Nd³⁺/Yb³⁺ phosphors with Nd³⁺/Yb³⁺ content increasing (see Fig. 10). If the energy transfer takes place by exchange interaction, the critical distance between the donor and acceptor should be short enough (<5 Å).³² The distance (R_C) between the Mn⁴⁺ and Nd³⁺/Yb³⁺ ions were estimated with the following equation³³

Here x_C is the critical concentration of the doped ions, V is the volume of the unit cell of Ca₁₄Zn₆Al₁₀O₃₅ (3286.7 ų). N is the number of Ca²⁺ ions in the unit cell (N = 86). The critical concentration are estimated to be about 0.032 and 0.107 for the Ca₁₄Zn₆Al₁₀O₃₅: Mn⁴⁺, Nd³⁺ and Ca₁₄Zn₆Al₁₀O₃₅: Mn⁴⁺, Yb³⁺ from the total concentration of the Mn⁴⁺ and Nd³⁺/Yb³⁺ ions at which the energy transfer efficiency is 50%. Thus by using eqn (3), the critical distances are estimated to be 13.1 Å in Ca₁₄Zn₆Al₁₀O₃₅: Mn⁴⁺, Nd³⁺ and 8.8 Å in Ca₁₄Zn₆Al₁₀O₃₅: Mn⁴⁺, Yb³⁺. These values are all larger than the typical critical distance for exchange interaction (<5 Å), indicating that the exchange interaction plays an unimportant role in the Mn⁴⁺ → Nd³⁺/Yb³⁺ energy transfer process. Thus the energy transfer should be performed via electric multipolar interaction.

Fig. 10 Emission spectra of Mn⁴⁺ in Ca_14−xZn₆Al_9.8O₃₅: Mn_0.2, Nd_x (a)/Ca_14−yZn₆Al_9.8O₃₅: Mn_0.2, Yb_y (b) excited at 460 nm light.

Base on Dexter's theory of multipolar interaction and Reisfeld's approximation, the type of multipolar interaction between donor and acceptor ions can be expressed in the following equation:^34–36

τ₀ and τ are the luminescence decay lifetimes of the donor (Mn⁴⁺) in the absence and presence of the acceptor (Nd³⁺/Yb³⁺), respectively. C is the acceptor (Nd³⁺/Yb³⁺) concentration, and n = 6, 8 and 10 represent the dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively. With the experimental data, the τ₀/τ–C^n/3 plots are illustrated in Fig. 11. The best linear relationship is obtained when n = 6 for both Mn⁴⁺, Nd³⁺ co-doped and Mn⁴⁺, Yb³⁺ co-doped samples, indicating that a dipole–dipole interaction is predominantly responsible for the energy transfer of Mn⁴⁺ → Nd³⁺/Yb³⁺.

Fig. 11 Dependence of τ₀/τ of Mn⁴⁺ on Ca₁₄Zn₆Al₁₀O₃₅: Mn⁴⁺, Nd³⁺ (a, b, c)/Ca₁₄Zn₆Al₁₀O₃₅: Mn⁴⁺, Yb³⁺ (d, e, f) at C^6/3, C^8/3, and C^10/3, respectively.

The luminescence decay curves of Mn⁴⁺ can also be further analyzed by the generalized Yokota–Tanimoto (Y–T) model:^37,38

with , where τ₀ is the decay constant of the donor (Mn⁴⁺) luminescence in the single-doped sample, S is the parameter of multipolar interaction that represents the dipole–dipole (S = 6), dipole–quadrupole (S = 8), and quadrupole–quadrupole (S = 10) interactions, respectively. C is the acceptor (Nd³⁺/Yb³⁺) concentration. Γ(x) is the gamma function, and C^(S)_DA is the donor–acceptor energy transfer parameter. a_i and b_i are the approximant coefficients involved in the electric multipolar interaction. D is the diffusion parameter that characterizes the excitation diffusion among donor ions. In principle, the excited donor ions can transfer their energy to the acceptor ions directly, or after the energy transfer between the nearest donor ions via excitation diffusion until an acceptor ion is reached. It is found in our experiment that the luminescence decay curves of Mn⁴⁺ in Mn⁴⁺ single-doped samples almost have no change as Mn⁴⁺ doping content increases from z = 0.2 to 0.6, as shown in Fig. 12(a). The result indicates that the Mn⁴⁺ radiation decay curve is mainly determined by its spontaneous emission of the isolated Mn⁴⁺ ions and the diffusion among the Mn⁴⁺ ions is of no importance. Thus the diffusion parameter of D can be set to 0. In this generalized Y–T model, the obtained best fitting is at S = 6 for Mn⁴⁺, Nd³⁺ co-doped and Mn⁴⁺, Yb³⁺ co-doped samples, as shown in Fig. 12(b) and (c). This further indicates the energy transfer between the Mn⁴⁺ and Nd³⁺/Yb³⁺ ions is performed via dipole–dipole interaction.

Fig. 12 Decay curves of Mn⁴⁺ luminescence for Ca₁₄Zn₆Al_10−zO₃₅: Mn_z (z = 0.2, 0.6) (a), decay curves of Mn⁴⁺ luminescence and its fitted curves based on Y–T model for Ca_13.75Zn₆Al_9.8O₃₅: Mn_0.2, Nd_0.25 (b) and for Ca_13.2Zn₆Al_9.8O₃₅: Mn_0.2, Nd_0.8 (c). The decay curves are recorded at 710 nm emission under excitation at 460 nm.

The SEM images of the Ca_13.75Zn₆Al_9.4O₃₅: Mn_0.6, Nd_0.25 and Ca_13.2Zn₆Al_9.4O₃₅: Mn_0.6, Yb_0.8 phosphors are shown in Fig. 13(a) and (b). The phosphors present coherent flake structure. Thus they are likely to be fabricated onto the front surface of solar cells as planar down-shifting layer (shown in Fig. 13(c)). Based on eqn (1) and (2), it is found that the calculated energy transfer efficiency for the Ca_13.75Zn₆Al_9.4O₃₅: Mn_0.6, Nd_0.25 and Ca_13.2Zn₆Al_9.4O₃₅: Mn_0.6, Yb_0.8 samples are as high as 76.0% and 80.4%, respectively. Moreover, the integrated NIR luminescent intensity of Yb³⁺ is 1.12 orders higher than that of Nd³⁺ in Ca_13.75Zn₆Al_9.4O₃₅: Mn_0.6, Nd_0.25 and Ca_13.2Zn₆Al_9.4O₃₅: Mn_0.6, Yb_0.8 under same excitation conditions (excited by 460 nm light) and the Si-based solar cell is more sensitive to the 980 nm light emitted by Yb³⁺ than the 900, 1060 nm light emitted by Nd³⁺. Thus the Ca_13.2Zn₆Al_9.4O₃₅: Mn_0.6, Yb_0.8 phosphor might be more suitable for spectral down-shifting application.

Fig. 13 SEM images of Ca_13.75Zn₆Al_9.4O₃₅: Mn_0.6, Nd_0.25 (a) and Ca_13.2Zn₆Al_9.4O₃₅: Mn_0.6, Yb_0.8 (b) and schematic diagram of down-shifting layer (c).

4. Conclusions

In summary, novel NIR phosphors of Ca₁₄Zn₆Al₁₀O₃₅: Mn⁴⁺, Nd³⁺/Yb³⁺ with high-efficiency are prepared. The Ca₁₄Zn₆Al₁₀O₃₅: Mn⁴⁺, Nd³⁺/Yb³⁺ phosphors exhibit strong absorption due to the Mn⁴⁺–O²⁻ charge transfer transition and Mn⁴⁺ (3d)-electronic spin-allowed transitions and give intense near infrared emission caused by the energy transfer from Mn⁴⁺ to Nd³⁺/Yb³⁺. It means that the phosphors can possibly be used as spectral conversion materials to enhance the efficiency of Si-based solar cells. The spectral conversion mechanism is investigated in detail according to their excitation–emission spectra and luminescence decay performance. The dipole–dipole interaction induced energy transfers are responsible for the strong NIR emission of Nd³⁺/Yb³⁺ under the excitation of the light ranging from 250 to 550 nm.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the National Science Foundation of China (No. 51372214 & 61233010), Project of Department of Science and Technology of Hunan Province of China (No. 2014FJ3124), and the Open Project of State Key Laboratory of Rare Earth Resources Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Science (RERU2013017).

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