1. Introduction

Utilizing UC materials to convert the sub-bandgap photons to higher-energy photons, which could be absorbed by the single-junction solar cells, is an alternative to improve the conversion efficiency of solar cells [1–5]. Efficient photon upconversion phenomena were reported with singlet emission of organic molecules sensitized by organic chromophores with triplet-triplet annihilation in solutions, but suffered low efficiency in solid state for device application [6]. Lanthanide ions in solids are most common for photon UC management because of their long-lived intermediate energy level. However, the most significant problem of this kind of upconversion materials lies in the high pumping energy density and narrow excitation line of lanthanide ions [7–14]. Sensitizers, such as noble metals with plasmon coupling effects [15–17] are used to enhance upconversion efficiency of lanthanide ions. Nevertheless, sensitizers with broadband absorption feature would benefit upconversion performance [6,10]. Furthermore, it is more practical for UC materials to be excitable by noncoherent sunlight. Most recently, It is reported [18] that dye molecules with broadband absorption characteristic could efficiently sensitize the UC luminescence of β-NaYF₄:Er³⁺,Yb³⁺, which provides the upconverted emission from lanthanide ions under a broadband low power excitation [19]. Herein, to get high energy transfer efficiency, short distance and the matching of energy levels between sensitizer(dye molecules) and acceptor(Yb³⁺-Er³⁺) are required for resonant energy transfer pathway [19,20]. It is known that transition metal(TM) ions have the broadband absorption feature, which is commonly utilized to sensitize Stokes luminescence of lanthanide ions [21,22]. Analogously, they could also be used as a broadband sensitizer for UC luminescence [7,10], for instance, the Cr⁵⁺-Er³⁺ [23], V³⁺-Er³⁺ [24,25] and V³⁺-Pr³⁺ [26] systems. However, these systems show UC luminescence only at cryogenic temperature. It is scarce for TM-lanthanide ions codoped systems to show room-temperature UC luminescence since the nonradiative relaxation rate of TM ions is generally large [7,10,23,24,27–29], resulting in depopulation of the intermediate energy level of TM ions involved in the UC process. Then it is preferable to design TM ion as sensitizer for the long-lived intermediate excited energy level of lanthanide ions in which the UC process occurs. In this way, TM ion broadband-sensitized UC luminescence of Yb³⁺-Er³⁺, which is considered as a high efficient UC luminescence system [30], may probably be obtained at room-temperature. Experience reveals that the material host with desired room-temperature UC luminescence should have proper sites for accommodating the TM ions and lanthanide ions, and the transition metal ions should not absorb the desired emission of lanthanide ions [10,28,29].

Cr³⁺-sensitized Stokes luminescence of lanthanide ions is extensively studied in d-f bimetallic complexes [21,31]. There is also several work about Cr³⁺-sensitized UC of lanthanide ions in d-f organometallic complexes, all of which were studied at cryogenic temperature [31]. It is reported that La₃Ga₅GeO₁₄:Cr³⁺ exhibited excellent near infrared emission at around 980 nm with broadband absorption at 370~520 nm and 560~800 nm [32,33]. It can be deduced that Cr³⁺ could efficiently transfer its absorbed energy to Yb³⁺ in La₃Ga₅GeO₁₄ through resonant energy transfer based on the matching of emission energy level of Cr³⁺ and absorption energy level of Yb³⁺ at around 980 nm, which are proved by many previous work [10,21,22,24]. The absence of absorption at 520~560 nm makes Cr³⁺ probable to sensitize Yb³⁺-Er³⁺ with room-temperature UC luminescence of Er³⁺ in this wavelength range, which is further convinced by the fact that La³⁺ site could accommodate Yb³⁺/Er³⁺ and Ga³⁺ site could accommodate Cr³⁺ with less distortion defects as emission killer. Fortunately, we observed the room-temperature broadband-noncoherent-light excitable UC luminescence in this system. This contribution will give an insight on the TM-sensitized room-temperature UC luminescence phenomenon and energy transfer mechanism involved.

2. Experimental

All samples were synthesized by high-temperature solid state reaction. Stoichiometric amounts of La₂O₃ (99.99%), Ga₂O₃ (analytical reagent, A.R.), GeO₂ (99.99%), Cr₂O₃ (A.R.), Yb₂O₃ (99.99%), Er₂O₃ (99.99%) and NH₄Cl (A.R., 10 wt%) were weighted and mixed thoroughly. NH₄Cl was added as flux. The mixture was calcined at 1000 °C for 4 h and 1300 °C for another 4 h sequentially, with an intermediate grinding. Phase purity was checked by PANalytical X’ pert Pro X-ray diffractometer with Cu anode target (Kα₁ = 1.54059 Å). XRD data for refinement was collected on a Bruker D8 ADVANCE X-ray diffractometer with Cu target and Ni filter. The step-scan mode was used with setup of 40 KV × 40 mA, step 0.01°, 0.2 s/step. Rietveld refinement was conducted by TOPAS-Academic program. Diffuse reflection spectra were carried out on a LAMBDA 950 UV–vis–NIR Spectrophotometer (PE) with BaSO₄ powder as a reference. Photoluminescence excitation and near infrared emission spectra were measured on a FLS920 spectrometer (Edinburgh Instruments Ltd.) equipped with a continuous 450 W xenon lamp as an excitation source(detector Hamamatsu R5509-72). UC emission spectra were recorded on the Jobin-Yvon TRIAX320 spectrofluorimeter equipped with a R928 photomultiplier tube as the detector and a 976 nm laser diode (LD, Coherent Corp.) as monochromatic light source. Additionally, a solar-500 Xe lamp (500 W, Beijing NBeT Group Co. Ltd., China) was used as the broadband noncoherent light source for UC luminescence. Lens concentrator combined with a 590 nm Cut-off filter and a 380~800 nm bandpass filter were utilized in order to get sufficient focus power and the desired wavelengths of 590~800 nm. Broadband UC excitation spectra, UC emission spectra, UC decay curves and near infrared emission decay curves were measured with a customized UV to mid-infrared steady-state and phosphorescence lifetime spectrometer (FSP920-C, Edinburgh) equipped with a digital oscilloscope (TDS3052B, Tektronix) and a tunable mid-band OPO pulse laser as excitation source (410-2400 nm, 10Hz, pulse width≤5 ns, Vibrant 355II, OPOTEK). Excitation power was detected by a Coherent FieldMate laser power meter. All the measurements were performed at room temperature.

3. Results and discussion

3.1 Diffuse reflection spectra and Stokes luminescence

For ease of reference and clarity, the abbreviation of LGG:CYE is used for La_1-y-zGa_5-xGeO₁₄:xCr³⁺,yYb³⁺,zEr³⁺. To study the energy transfer mechanism among Cr³⁺, Yb³⁺ and Er³⁺, a series of samples with different contents of these ions were synthesized. XRD patterns reveal that all the samples are analogous with ICSD #20783 except some traced β-Ga₂O₃. It should be noted that the distance between La-Ga is much shorter than that of La-La according to the refinement results. Figure 1 shows the diffuse reflection spectra of LGG samples with different dopant ions. LGG:C has broad absorption bands in the range of 370~520 nm (⁴A₂→⁴T₁) and 560~800 nm(⁴A₂→⁴T₂). The LGG:YE sample shows some sharp absorption peaks, which are depicted as the intra-energy-level transitions of Yb³⁺/Er³⁺ in Fig. 1. It is worth noting that LGG:C has little absorption in the range of 520~560 nm while LGG:YE has some absorption peaks, implying that the ²H_11/2→⁴I_15/2 UC emission of Er³⁺ in LGG:CYE may be observed when excited by longer-wavelength light. Furthermore, the broadband absorption at 560~800 nm of LGG:C suggests the possibility of broadband excitation for the UC emission.

 

Fig. 1 Diffuse reflection spectra of the samples.

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To investigate Cr³⁺-sensitized UC luminescence of Yb³⁺-Er³⁺, it is necessary to study the energy transfer mechanism among Cr³⁺, Yb³⁺ and Er³⁺. Figure 2 depicts the excitation and Stokes luminescence spectra of LGG:C, LGG:CE and LGG:CYE. The emission spectrum of LGG: C gives a broadband peak, which is ascribed to ⁴T₂→⁴A₂ transition of Cr³⁺ [32]. While that of LGG:CYE exhibits the characteristic ²F_5/2→²F_7/2 transition of Yb³⁺ at ~976 nm (even at low Yb³⁺ contents) and ⁴I_13/2→⁴I_15/2 transition of Er³⁺ at ~1540 nm when photostimulating Cr³⁺, which implies that energy transfer from Cr³⁺ to Yb³⁺/ Er³⁺ takes place. Similar Er³⁺ emission spectrum of LGG:CE is observed at ~1540 nm, while the emission at ~985 nm is mostly the characteristic of ⁴I_11/2→⁴I_15/2 of Er³⁺. The excitation spectrum of LGG:CYE are composed of three bands at 550~770 nm and 375~520 nm, which are assigned to ⁴A₂→⁴T₂ and ⁴A₂→⁴T₁ transition of Cr³⁺, respectively. The analogous excitation spectra for LGG:C and LGG:CYE (only one is shown in Fig. 2) also convinces the occurrence of the energy transfer from Cr³⁺ to Yb³⁺/Er³⁺. The variations of Stokes luminescence intensities and decay curves of LGG:CE and LGG:CYE samples on different Cr³⁺ contents and Yb³⁺ contents are illustrated in Fig. 3 and Fig. 4, respectively. For LGG: xCr³⁺, 0.12Yb³⁺, 0.06Er³⁺, both Yb³⁺ and Er³⁺ emission intensities go up firstly with x increases and then decline. Another point should be noted that Yb³⁺ emission intensity is much stronger than that of Er³⁺ emission. And for LGG: xCr³⁺, 0.06Er³⁺, Er³⁺ emission intensities change little with x. Yb³⁺emission intensity continuously increases while that of Er³⁺ emission goes in the contrary way for LGG: 0.10Cr³⁺, yYb³⁺, 0.06Er³⁺. These phenomena evidently suggest that energy transfer from Cr³⁺ to Yb³⁺ is efficient and Cr³⁺ to Er³⁺ is inefficient. The decay curves of Yb³⁺ emission in Fig. 4(a) and Fig. 4(c) also prove that efficient energy transfer from Cr³⁺ to Yb³⁺ exists in LGG:CYE. Figure 4(b) and Fig. 4(d) with decay curves of Er³⁺ emission suggests inefficient energy transfer from Cr³⁺ to Er³⁺ since they change little with Cr³⁺ contents and Yb³⁺ contents, which is consistent with Fig. 3. Another important conclusion can be made according to Fig. 3(a) and Fig. 4(a) that there is energy quench of Yb³⁺ emission with the increase of Cr³⁺ contents, which suggests less Cr³⁺ contents is required in the design of Cr³⁺-sensitized UC emission of Yb³⁺-Er³⁺. Furthermore, Yb³⁺ is essential for the Cr³⁺-sensitized UC emission of Er³⁺ since energy transfer from Cr³⁺ to Er³⁺ is inefficient.

 

Fig. 2 Stokes luminescence and excitation spectra of LGG:C, LGG:CE and LGG:CYE(all the spectra are normalized).

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Fig. 3 Stokes luminescence intensity variations of LGG: xCr³⁺, 0.12Yb³⁺, 0.06Er³⁺, LGG: xCr³⁺, 0.06Er³⁺(a) and LGG: 0.10Cr³⁺, yYb³⁺, 0.06Er³⁺(b) on Cr contents x and Yb contents y.

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Fig. 4 Stokes luminescence decay curves of LGG: xCr³⁺, 0.12Yb³⁺, 0.06Er³⁺ (a, b) and LGG: 0.10Cr³⁺, yYb³⁺, 0.06Er³⁺ (c, d)

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3.2 UC luminescence pumped by a 976 nm laser diode

To better understand the process of Cr³⁺-sensitized UC luminescence of Yb³⁺-Er³⁺, UC process of Yb³⁺-Er³⁺ and Yb³⁺-Er³⁺-Cr³⁺ should be figured out since Yb³⁺ could be directly pumped by a 976 nm laser diode, which is demonstrated in Fig. 5.Intense UC emission ²H_11/2→⁴I_15/2 and ⁴S_3/2→⁴I_15/2 of Er³⁺ are observed for LGG: 0.12Yb³⁺, 0.06Er³⁺, and two-photon UC process is proposed based on the slopes of Er³⁺ emission intensity on pump power are about 2 in Fig. 5(lower, inset). UC emission ⁴T₂→⁴A₂ of Cr³⁺ can be obtained for LGG: 0.10Cr³⁺, 0.12Yb³⁺, 0.06Er³⁺ except UC emission of Er³⁺, whereas, the slopes of the different emission peak intensities on pump power are obviously deviated from 2 (larger than 1), which may probably be ascribed to large depopulation rate of intermediate energy level in the UC process caused by forward and backward energy transfer among Yb³⁺, Er³⁺ and Cr³⁺ [29,34]. Figure 6(a) is the UC luminescence spectra of LGG: xCr³⁺, 0.12Yb³⁺, 0.06Er³⁺. It can be seen that the UC emission intensity of Er³⁺ declines dramatically and monotonically with Cr³⁺ content x increases, while that of Cr³⁺ varies little. For x = 0.30 in the inset of Fig. 6(a), UC emission of Er³⁺ is totally quenched and a single peak of Cr³⁺ is observed. Thus, it can be concluded that Cr³⁺ would quench the UC luminescence of Yb³⁺-Er³⁺, probably owing to the complex forward and backward energy transfer between Yb³⁺/Er³⁺ and nearest surrounded Cr³⁺. Since there is an absorption peak at around 976 nm for Er³⁺(see Fig. 1), UC luminescence spectra of LGG: xCr³⁺, 0.06Er³⁺ are recorded in Fig. 6(b). It can be seen that UC emission intensity of Er³⁺ is much weaker without Yb³⁺, and UC process form Er³⁺ to Cr³⁺ is also inefficient. The same conclusion that Cr³⁺ would quench the UC luminescence of Er³⁺ can be made. Thus, the content of Cr³⁺ should be less in Cr³⁺-sensitized UC luminescence of Yb³⁺-Er³⁺.

 

Fig. 5 UC luminescence spectra of LGG: 0.10Cr³⁺, 0.12Yb³⁺, 0.06Er³⁺ (upper) and LGG: 0.12Yb³⁺, 0.06Er³⁺ (lower) pumped by a 976 nm laser diode(~60 mW mm⁻²). Inset of each graph is the intensity dependence of different UC emission peak(integrated intensity) on pump power (in Log-Log plot).

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Fig. 6 UC emission of LGG: xCr³⁺, 0.12Yb³⁺, 0.06Er³⁺ (a) and LGG: xCr³⁺, 0.06Er³⁺ (b) pumped by a 976 nm laser diode(~60 mW mm⁻²). Inset of graph(a) is the enlarge spectrum of sample with x = 0.30.

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3.2 Broadband sensitized UC luminescence and the related model

When pumped by 620 nm OPO tunable pulse laser, the emission spectrum of LGG: 0.05Cr³⁺, 0.12Yb³⁺, 0.06Er³⁺ sample evidently demonstrates the ²H_11/2→⁴I_15/2 and ⁴S_3/2→⁴I_15/2 emission of Er³⁺ while it almost shows little emission intensity at the same pump power for LGG: 0.12Yb³⁺, 0.06Er³⁺ sample in Fig. 7(a).The former is about 30 times stronger than the latter (integrated intensity) upon excitation of monochromic light of 620 nm. This phenomenon indicates the sensitization effect of Cr³⁺ for the UC luminescence of Yb³⁺-Er³⁺ in LGG:CYE, based on the fact that there is almost little absorption for Yb³⁺ and Er³⁺ at ~620 nm(see Fig. 1). The striking broad band (full width at half maximum is 33 nm) in Fig. 7(b) again convinces the ⁴A₂→⁴T₂ sensitization of Cr³⁺ for the 548 nm emission of Er³⁺, because it is evidentlydifferent from the absorption(excitation) peak of Er³⁺ at this wavelength range(the magenta line in Fig. 7(b)). It should be pointed out that it is scarce for UC luminescence of TM-lanthanide ions codoped systems at room temperature [7,10,28,29]. The UC luminescence decay curves of ⁴S_3/2→⁴I_15/2 emissions of Er³⁺ excited by 620 nm pulsed laser are shown in Fig. 8.As seen in Fig. 8(a), UC luminescence of Er³⁺ sensitized by Cr³⁺ directly is inefficient because almost no decay signal of Er³⁺ is observed for y = 0, which is in accord with the conclusion of Fig. 3. With Yb³⁺ contents y increase, the decay curves apparently prolong, which can be interpreted by the energy transfer from Cr³⁺ to Er³⁺ through Yb³⁺ as a “bridge” in the UC process. Another point should be noted that an obvious rise-up in the early stage of decay curves with the increase of y is observed, indicating that the UC mechanism is energy transfer upconversion(ETU) [10,27]. The decay curves decline with the raise of Cr³⁺ contents x in Fig. 8(b), suggesting large contents of Cr³⁺ hamper the energy transfer from Cr³⁺ to Yb³⁺- Er³⁺, which also agree with the conclusion of Fig. 3, Fig. 4 and Fig. 6. The UC luminescence decay time of Er³⁺ in LGG: 0.10Cr³⁺, 0.12Yb³⁺, 0.06Er³⁺ is around 170 µs, which is larger than that of Stokes luminescence of Er³⁺ (75 µs). This may be caused by the much more complicated energy transfer process involved in the UC luminescence [35].

 

Fig. 7 (a)UC luminescence spectra of LGG: 0.05Cr³⁺, 0.12Yb³⁺, 0.06Er³⁺ and LGG: 0.12Yb³⁺, 0.06Er³⁺ pumped by an OPO pulsed laser(620 nm) with the same power density (about 50 mW mm-²); (b) the monitored 548 nm UC emission intensity pumped by different wavelength(blue filled circle); for comparison, excitation peak at around 650 nm of LGG: 0.06Er³⁺ is also illustrated(magenta line).

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Fig. 8 UC luminescence decay curves of LGG: 0.10Cr³⁺, yYb³⁺, 0.06Er³⁺ (a) and LGG: xCr³⁺, 0.12Yb³⁺, 0.06Er³⁺ (b) pumped by an OPO pulsed laser (λ_ex = 620 nm, λ_em = 548 nm).

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Strikingly, upon excitation of the concentrated noncoherent broadband 590~800 nm light of the solar simulator, the sample LGG: 0.05Cr³⁺, 0.12Yb³⁺, 0.06Er³⁺ exhibits apparent UC luminescence of Er³⁺ while LGG: 0.12Yb³⁺, 0.06Er³⁺ does not, as shown in Fig. 9(a). This phenomenon also convinces the broadband sensitization effect of Cr³⁺. It is essential to utilize broadband noncoherent solar light in the UC materials for solar cells application. This phosphor puts up the feasibility of broadband noncoherent photons UC management in TM-lanthanide ions codoped inorganic materials system.

 

Fig. 9 (a)UC luminescence spectra of LGG: 0.05Cr³⁺, 0.12Yb³⁺, 0.06Er³⁺ and LGG: 0.12Yb³⁺, 0.06Er³⁺ upon excitation of concentrated broadband noncoherent 590 nm~800 nm light of solar simulator(~18 mW mm⁻²); (b)Sketch of UC model for LGG:CYE.

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The shortest distance of La-La is 4.259 Å, much longer than that of La-Ga(3.438 Å). Crystal structure shows that La³⁺(Yb³⁺) nearest surrounded cations are Ga³⁺(Cr³⁺) ions. According to the conclusions above that energy transfer from Cr³⁺ to Yb³⁺ is efficient, Cr³⁺-Yb³⁺ dimer is proposed to play a role in the UC luminescence process based on that Yb³⁺(La³⁺)-Cr³⁺(Ga³⁺) is close enough to have superexchange interaction in accordance with the previous work [10,36–39]. This could explain why the Stokes luminescence of Yb³⁺ and UC luminescence of Er³⁺ are easily quenched at high Cr³⁺ contents, at which there is more possibility for Cr³⁺ to show up around Yb³⁺ ion. An evident rise-up process in the early stage of decay curves in Fig. 8 suggests that the UC mechanism is ETU. Therefore, a simplified UC model is proposed in Fig. 9(b).︱²F_7/2, ⁴A₂>→︱²F_7/2, ⁴T₂> transition of Cr³⁺-Yb³⁺ dimer could absorb broadband light with some certain energy, then it relaxes to︱²F_5/2, ⁴A₂>, based on the fact that the relaxation rate is much larger than upconversion rate for︱²F_7/2, ⁴T₂> level [9,26]. Two︱²F_5/2, ⁴A₂>→︱²F_7/2, ⁴A₂> transition photons’ energy sequentially transfer to ⁴I_11/2 and ⁴F_7/2 levels of Er³⁺ through resonant energy transfer [10,20,27]. After relaxation from ⁴F_7/2 level, green UC emission of ²H_11/2→⁴I_15/2 and ⁴S_3/2→⁴I_15/2 are obtained.

4. Conclusions

In conclusion, Cr³⁺-sensitized room-temperature UC luminescence of Er³⁺ excited by concentrated broadband noncoherent light is demonstrated in LGG:CYE. Efficient energy transfer from Cr³⁺ to Yb³⁺ and inefficient energy transfer from Cr³⁺ to Er³⁺ make it requisite to utilize Yb³⁺ as “bridge” in the UC process. Low contents of Cr³⁺ would be preferred since large amounts of Cr³⁺ would hamper the UC luminescence of Er³⁺. UC luminescence corresponding to the transitions ²H_11/2→⁴I_15/2 and ⁴S_3/2→⁴I_15/2 of Er³⁺ at around 510 ~560 nm are through ETU mechanism based on Cr³⁺-Yb³⁺ dimer model with superexchange interaction. It may be helpful in designing new photonic materials for applications such as solar cells.