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Host sensitized near-infrared emission in Nd3+-Yb3+ Co-doped Na2GdMg2V3O12 phosphor
Author’s Accepted Manuscript Host Sensitized Near-infrared Emission in Nd3+ Yb3+ Co-doped Na2GdMg2V3O12 Phosphor Dan Song, Chongfeng Guo, Jin Zhao, Hao Suo, Xiaoqi Zhao, Xianju Zhou, Guozhen Liu www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(16)30677-0 http://dx.doi.org/10.1016/j.ceramint.2016.05.072 CERI12877
To appear in: Ceramics International Received date: 2 May 2016 Revised date: 11 May 2016 Accepted date: 11 May 2016 Cite this article as: Dan Song, Chongfeng Guo, Jin Zhao, Hao Suo, Xiaoqi Zhao, Xianju Zhou and Guozhen Liu, Host Sensitized Near-infrared Emission in Nd3+ Yb3+ Co-doped Na2GdMg2V3O12 Phosphor, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.05.072 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Host Sensitized Near-infrared Emission in Nd3+ -Yb3+ Co-doped Na2GdMg2V3O12 Phosphor Dan Songa, Chongfeng Guoa,b*, Jin Zhaoa, Hao Suoa, Xiaoqi Zhaoa, Xianju Zhouc, Guozhen Liub a
National Key Laboratory of Photoelectric Technology and Functional Materials (Culture Base) in Shaanxi
Province, National Photoelectric Technology and Functional Materials & Application of Science and Technology International Cooperation Base, Institute of Photonics & Photon-Technology, Northwest University, Xi’an 710069, China; b
ARC Centre of Excellence for Nanoscale Biophotonics (CNBP), Macquarie University, North Ryde 2109,
Australia c
School of Sciences, Chongqing University of Posts and Telecommunications, Chongqing 400065, China
---------------------------------------------------------------*Author to whom correspondence should be addressed: Institute of Photonics & Photon-Technology；Northwest University, Xi’an 710069, China; Tel. & Fax.: 86-29-88302661 E-mail: [email protected] (Prof. Guo)
Abstract Yb3+/Nd3+ singly and co-doped Na2GdMg2V3O12 phosphors with near-infrared (NIR) emission were synthesized via sol-gel method. The phase purity and structure of samples were characterized by X-ray diffraction (XRD), the photoluminescence emission (PL) and excitation (PLE) spectra along with decay curves were also measured. Near infrared (NIR) emissions (850-1150 nm) from acceptors Yb3+ or Nd3+ matching well with the response curve of the silicon solar cell were obtained, in which VO43- groups acted as sensitizers by capturing near ultraviolet photons which are not absorbed efficiently by silicon solar cell and transferred them to Yb3+/Nd3+ by energy transfer processes. The NIR emission intensities of the Nd3+-Yb3+co-doped samples Na2GdMg2V3O12 were enhanced greatly in comparison with that of Nd3+/Yb3+ singly doped samples, and the possible energy transfer processes were also discussed in detail. Results indicate that the obtained samples are potential solar spectral down-conversion (DC) convertors to enhance the conversion efficiency of the silicon solar cells. 1
Keywords: Phosphor; Near-infrared; Energy transfer; Vanadate garnet.
1. Introduction As the environmental degradation and energy crisis are becoming hot issues, more attentions and efforts are focused on seeking green and renewable energy source to replace traditional fossil fuel. The sunlight is an inexhaustible renewable pollution-free clean energy, which could be efficiently converted into electrical energy using photovoltaic solar cells [1, 2]. It is estimated that solar cell will provide about 16% global electricity by 2050, which stimulates more interests from governments, companies and experts [3, 4]. As we all know, there are many types of solar cells, such as high efficient semiconductor, low-cost dye-sensitized, soft organic, perovskite solar cells and so on [5, 6]; but crystalline silicon (c-Si) solar cells are still dominating the present commercial market due to their mature technology. However, the conversion efficiency of silicon solar cells is only about 18%, which is far below the theoretical maximum value of 30% (the Shockley-Queisser limit) because of the spectral mismatch between solar spectrum (300-2500 nm, AM 1.5G) and the response spectrum of c-Si solar cells (Eg ≈ 1.12 eV, λ ≈ 1100 nm) [7, 8]. Silicon solar cell absorbs photons with higher energy than its bandgap Eg (E﹥Eg) and converts each absorbed photon to a pair of carriers; whereas photons with energy less than Eg (E﹤Eg) tend to be transmitted rather than absorbed. Simultaneously, the photons with energy higher than Eg loss their excess energy through thermalization of “hot” electron-hole pairs [9]. In order to reduce the spectral mismatch losses and enhance the photovoltaic conversion efficiency, up-conversion (UC) and down-conversion (DC) phosphors have been proposed to tune the solar spectrum to match the spectral response of c-Si solar cells. The former absorbs two or more lower-energy photons to induce one higher-energy photon; the latter could convert the high-energy photons into near-infrared (NIR) photons by energy transfer [10]. For NIR DC materials used for c-Si solar cells, the Yb3+ ion is known as the most suitable activator due to its simple 4f13 configuration and only one excited state 2F7/2 (～10000 cm-1) just above the band edge of crystalline silicon [11]. RE3+-Yb3+ (RE3+ = Tb3+, Pr3+, Tm3+, Ho3+, Er3+ or Nd3+) and Ce3+/Eu2+-Yb3+ co-doped DC materials have been extensively investigated, in which RE3+ and Ce3+/Eu2+ ions play the role of sensitizer to absorb ultraviolet-visible light and then transfer the absorbed energy to nearby Yb3+ to emit NIR light [12-17]. However, the absorption 2
peaks of Tb3+, Pr3+, Tm3+, Ho3+, Er3+ or Nd3+ originate from the forbidden 4f → 4f transitions, which naturally have weak intensity and narrow bandwidth. The broadband absorptions of sensitizers Ce3+/Eu2+ from the allowed 4f-5d transitions have to be prepared at reduced atmosphere, leading to the reduction of partial Yb3+ to Yb2+ accompanied with the decrease of NIR emission from Yb3+ [18]. In addition, some self-activated compounds, such as vanadate, niobate, tungstate and molybdate，have strong absorption in near ultraviolet (NUV) range and intense emission from blue to yellow region without any dopants, which could efficiently transfer their absorbed energy to the activator ions and the host-sensitized NIR DC phosphor could be realized through doping Yb3+ [19-22]. Besides Yb3+, rare earth ion Nd3+ can also emit NIR light from 800 to 1150 nm from the transitions of 4F3/2 to 4I9/2,
11/2,
which makes Nd3+ as the optional
NIR luminescent center ion for application in Si solar cell similar to Yb3+ [23]. Here, we investigated the NIR emission properties of Yb3+/Nd3+ singly and co-doped Na2GdMg2V3O12 samples, in which self-activated vanadate garnet VO43- group acts as the sensitizer to harvest the incident sunlight and transfers the absorbed energy to activator Yb3+/Nd3+ leading to NIR emission. The detailed energy transfer (ET) mechanisms are systematically discussed through the measurement of vis-NIR emission spectra and decay curves. Results indicate that the NIR emission intensity could be greatly enhanced in Nd3+-Yb3+ co-doped Na2GdMg2V3O12 phosphors in comparison with that of Yb3+ or Nd3+ solely doped phosphors. The co-doped samples could efficiently convert NUV and visible light to NIR emission, which allows it to be applied in enhancing the conventional efficiency of c-Si solar cells by modifying solar spectrum.
2.Experimental All Yb3+/Nd3+ singly and co-doped Na2GdMg2V3O12 samples were synthesized by a sol-gel method. The stoichiometric amounts of raw materials, including high purity Gd2O3, Yb2O3 and Nd2O3 (99.99%) and analytical purity (A.R.) Na2CO3, Mg(NO3)2·6H2O and NH4VO3, were weighed. In a typical process, lanthanide oxides Gd2O3, Yb2O3 and Nd2O3 were firstly dissolved in dilute HNO3 (A.R.) solution at 120 ℃ under constant stirring and the excessive HNO3 was evaporated to remove. Then appropriate volume of distilled water and Na2CO3, Mg(NO3)2·6H2O were added and stirred to form a transparent solution. Subsequently, NH4VO3 and the chelating 3
agent citric acid C6H8O7·H2O (A.R.) were added into above solution, and the molar ratio of total chelate metal cations to citric acid was fixed at 1: 2. The obtained highly transparent orange solution was placed in 70 ℃ oven for 24 h and then increase to 120 ℃ for 24 h to get deep-brown dried foamy precursors. The dried precursors were pre-heated at 500 ℃ for 5 h, and further calcined at 850 ℃ for 5 h in the air. Finally, the as-prepared samples were naturally cooled to room temperature and grounded to get sample powder. X-ray diffraction (XRD) patterns of the Na2GdMg2V3O12: Nd3+/Yb3+ phosphors were used to characterize the structure through a Rigaku-Dmax 3C powder diffractometer (Rigaku Corp., Tokyo, Japan) with Cu-Kα radiation (λ = 1.5406 Å). The photoluminescence (PL), photoluminescence excitation (PLE) spectra and decay curves were measured with an FLS920 spectrofluorometer (Edinburgh Corp., UK) equipped with 450 W Xenon lamp. The temperature dependent spectra were measured by an Oxford OptistatDN2 nitrogen cryogenics temperature controlling system (duration of stay at the measured temperature is 0.5 h). All the measurements of photoluminescent spectra were repeated three times to reduce error.
3.Results and discussion 3.1 Phase and structure The property of sample is mainly determined by its structure, thus the phase purities and structures of Yb3+/Nd3+ solely and Nd3+-Yb3+ co-doped Na2GdMg2V3O12 samples as well as the blank sample were characterized by XRD patterns, as shown representatively in Fig. 1. All the diffraction peaks of samples agree well with those of the standard Na2YMg2V3O12 (JCPDS card No. 49-0412), and no any impurity peaks appear [24]. Above results indicate that compounds Na2GdMg2V3O12 and Na2YMg2V3O12 adopt the same cubic garnet structure, belonging to the space group of Ia-3d (230), but the calculated lattice constants of Na2GdMg2V3O12 (a = b = c = 12.414 Å) are slightly bigger than those of Na2YMg2V3O12 (a = b = c = 12.397 Å) due to the larger ionic radius of Gd3+ (RGd = 1.05 Å) than that of Y3+ (RY = 1.02 Å) [25]. In addition, it also implies that the Yb3+/Nd3+ ions successfully incorporate into the lattice and occupy the sites of Gd3+ due to their similar radius and the same valence. It is noted that all samples in our experiments show similar XRD patterns. 3.2 Luminescent properties of Yb3+ singly doped Na2GdMg2V3O12 4
In order to investigate the luminescent properties of Yb3+ doped Na2GdMg2V3O12, the PLE and PL spectra of samples are presented in Fig. 2a and b as a function of Yb3+ concentration, respectively. The intense excitation band ranging from 250 to 400 nm centered at 325 nm is found by monitoring at around 520 nm visible light assigned to the broad-band absorption of the charge transfer (CT) band of O2- - V5+ in the host rather than that of O2--Yb3+, and the latter usually locates at a shorter wavelength below 250 nm in inorganic compounds [22]. The broad intense absorption from host is also observed by monitoring at 972 nm near infrared Yb3+ emission, which indicates the presence of energy transfer from host to Yb3+. The emission spectra cover the visible and NIR region with various Yb3+ concentrations under 325 nm excitation in Fig. 2b. In the visible range (400-700nm), the broad unsymmetrical emission band centered at 520 nm is from the charge-transfer (CT) of an electron from the 2p orbital of O2- to the vacant 3d orbital of V5+ in tetrahedral [VO4]3- group with Td symmetry [26]. The unsymmetrical emission band is attributed to the partially allowed transitions from 3T2→1A1 and 3T1→1A1. In the NIR region (900-1150 nm), an intense broad emission peaked at 972 nm is observed from 2F5/2 to 2F7/2 states of Yb3+, in which a series of sharp line emissions are attributed to the transitions among the different stark levels of 2
F5/2 and 2F7/2 [27]. This is another proof for the occurrence of energy transfer from host to Yb3+.
In addition, it is observed that the PL intensity of [VO4]3- intrinsic host emission reduces monotonously with the increasing of Yb3+ doping concentration from 0 to 0.2, whereas the NIR emission intensity first increases and then decreases after reaching the maximum at about x = 0.12 due to the concentration quenching effect. The decreased emission of host companied with the enhanced Yb3+ emission in NIR region with the increase of Yb3+ is also powerful evidence for the appearance of ET from VO43- to Yb3+. To further investigate the process of ET, the decay curves of VO43- emission at 520 nm in Na2GdMg2V3O12: xYb3+ (x = 0-0.20) with the 325 nm excitation were displayed in Fig. 2c. With the increase of Yb3+ concentration from 0 to 0.2, the decay curves deviate from mono-exponential and can be fitted to be a bi-exponential modal described as [28]:
I ( t ) =0I +1A e x pt( 1- / 2A) + e xt 2p ( - /
) (1)
where I(t) and I0 are the luminescence intensities at time t and 0, A1 and A2 are pre-exponentials factors related with lifetime components τ1 and τ2, respectively. The average lifetimes can be calculated by the following equation [29]:
5
taverage =(A112 +A2 22 )/(A11 +A2 2 )
(2)
And the energy transfer efficiency (ETE) ηET can be estimated based on the equation:
ET =1- / 0
(3)
where τ0 and τ stand for the lifetime values of VO43- emissions (520 nm) without and with Yb3+-doped samples. As the Yb3+ concentration increase, the lifetimes keep decreasing from 6.24 to 3.8 μs; simultaneously, the energy transfer efficiency (ETE) exhibits a monotonous increase from 6.1% to 41.5% with the content of Yb3+ increasing to 0.2, as shown in Fig. 2d. Above results indicate that more energy absorbed by host is transferred to Yb3+ with increasing the Yb3+ concentration, which leads to the declining emission intensity and lifetime of 520 nm as well as the enhancing of NIR emission at 972 nm. 3.3 Luminescent properties of Nd3+-doped Na2GdMg2V3O12 As a typical emitting center in NIR region, Nd3+ ions emission ranges from 850 to 1150 nm and matches well with the response curve of Si-based solar cells. Photo-luminescent properties of Nd3+ doped Na2GdMg2V3O12 were investigated in order to show its potential possibility in this field. Fig. 3a shows the PLE and PL spectra of the Na2GdMg2V3O12: Nd3+ phosphors. The PLE spectrum of sample monitoring at 1060 nm includes a broad band and a series of sharp line absorptions；the former band centered at 325 nm is assigned to the absorption from the CT band of VO43-, the latter sharp lines peaked at 482, 532, 592, 684, 738 and 823 nm are ascribed to the f-f transitions of Nd3+: 4I9/2 → 2K15/2 + 2G9/2, 4I9/2 → 2K13/2 + 4G9/2 + 4G7/2, 4I9/2 → 2G5/2 + 2G7/2, 4I9/2 → 4
F9/2, 4I9/2 → 4F7/2 + 4S3/2 and 4I9/2 → 4F5/2, respectively [30, 31]. Whereas monitoring at 520 nm,
only one broad absorption band from host was observed. This host absorption band could be observed as monitoring at host emission or NIR emission from Nd3+, which implies that the ET occurrence from host to Nd3+. For the PL spectrum (dot line) with excitation at 325 nm, it covers visible and NIR region from 400-1150 nm, consisting of a broad band emission in visible region from host centered at 520 nm and a series of sharp line emissions ranging from 800 to 1150 nm in NIR region due to the electronic f-f transitions from 4F3/2 to 4IJ (J = 9/2, 11/2) of Nd3+. Comparing the PLE and PL spectrum, a significant overlap was found between the host emission and the Nd3+ absorption lines, which is expected for efficient energy transfer from host to Nd3+ and the concaves in the visible luminescence at 592 nm ascribed to the re-absorption of Nd3+ : 4I9/2 → 6
2
G5/2 + 2G7/2 [32] . Inset of Fig. 3a shows the integrated PL intensity in visible and NIR region
varying with dopant Nd3+ content, and the similar trend is also found in Na2GdMg2V3O12: Yb3+ phosphors. It is clearly observed that the intensity of host emission mono-directionally drop, whereas the NIR intensity first increase and then decline after reaching the maximum at y=0.05 due to the concentration quenching effect. Above luminescent results not only prove the appearance of energy transfer but also present broader absorption range, which is more appropriate for the solar cell. In addition, the energy transfer efficiency is also estimated according to the decrease of host emission lifetime. The decay curves of host emission are shown in Fig. 3b with various Nd3+ concentration, in which it is found that the decay rate of host emission become faster with increasing Nd3+ content due to the absorption by the Nd3+ ion and ET from VO43- to Nd3+. And the lifetimes are calculated to decrease from 6.24 to 2.1 μs according to Eq (1) and (2) with the concentration of Nd3+ reaching 0.10; the ETE from VO43- to Nd3+ was also estimated based on the various values of host emission lifetime and shown in Fig. 3c as function of Nd3+ content. The decreasing lifetime of sensitizer and increasing of ETE further give strong evidence to the ET process from VO43- to Nd3+. 3.4 Luminescence in Nd3+-Yb3+ co-doped Na2GdMg2V3O12 As discussed above, Yb3+ or Nd3+ ions solely doped Na2GdMg2V3O12 both emit strong NIR emission ranging from 800 to 1150 nm, which can be efficiently absorbed by Si-based solar cell. Previous publications have also proved the occurrence of efficient energy transfer from Nd3+ to Yb3+ [15, 33, 34], therefore it is expected that the introduction of Nd3+ could enhance the NIR emission intensity of Yb3+. Fig. 4a displays the PLE and PL spectra of Na2GdMg2V3O12: 0.05Nd3+, 0.04Yb3+ as representative of Nd3+-Yb3+ co-doped samples. For the PLE spectra monitoring at 520 nm from host VO43-: 3T1, 3T2 → 1A1, 972 nm from Yb3+: 2F5/2 → 2F7/2 and 1060 nm from Nd3+: 4
F3/2 → 4I11/2, they all include a broad absorption band in 200-400 nm region with the maximum at
325 nm, which implies the occurrence of energy transfer from host to Yb3+ or Nd3+. Besides the host broad absorption, a series of characteristic peaks from the f-f transitions of Nd3+ are also observed as the monitoring wavelength is 972 nm of Yb3+ and 1060 nm of Nd3+, which illuminates that Nd3+ can directly absorb visible light and transfer partial absorbed energy to Yb3+ in Nd3+-Yb3+ co-doped Na2GdMg2V3O12. In order to prove our deduction, the PL spectra excited at 592 nm (Nd3+: 4I9/2 → 2G5/2 + 2G7/2; dot line) and 325 nm (host absorption; dash line) are shown in 7
Fig. 4a. The profiles of PL spectra are similar in 850 to 1150 nm NIR region and dominated by 972 (Yb3+) and 1060 nm (Nd3+) except the unsymmetrical band emission from host under excitation of 325 nm light, including a series of sharp lines emission peaked at 950, 972, 1016 and 1034nm (diamond indexed) from the transitions of Yb3+ and 885, 935, 1060 and 1097 nm (asterisk indexed) from transitions of Nd3+. In order to find optimal composition of sample, Nd3+-Yb3+ co-doped Na2GdMg2V3O12 were investigated with fixed Nd3+ at 0.05 and various Yb3+ concentration. Insets of Fig. 4a present the PL intensity changes with Yb3+ concentrations when excited at 325 and 592 nm, respectively. It is clearly observed that the emission intensity of acceptors Yb3+ (972 nm) first increase and then decrease after reaching the maximum at about x = 0.04 whether excitation at 325 nm from host or 592 nm from Nd3+. However, the emission intensity of host emission centered at 520 nm and Nd3+ emission peaked at 1060 nm monotonously decrease, which indicates the appearance of energy transfer from VO43-/Nd3+ to Yb3+. In addition, the decay curves of host emission and 1060 nm emission from Nd3+ were also measured as function of Yb3+ contents by excited at 325 and 592 nm, respectively. The corresponding lifetimes were calculated based on Eq (1) and (2), and it is found that their lifetimes decline with the growth of Yb3+, as shown in Fig. 4a and 4b; the ETE was also estimated according to Eq (3), it is clearly found that the ETE is higher with 325 nm excitation than that of excitation at 592 nm, because the former is a sum of ETE including VO43→ Nd3+, VO43- →Yb3+ and VO43- → Nd3+ → Yb3+ whereas the latter is ETE of Nd3+ → Yb3+. Generally, the working temperature of solar cells in outdoor is usually changed with the environment, such as the season, region and the intensity of sun radiation. Thus, the temperature-dependent NIR emission spectra of Na2GdMg2V3O12: 0.05Nd3+, 0.04Yb3+ were measured and presented in Fig.5. As the temperature increasing from 280 to 420 K, the PL intensities monotonously decrease but the peak positions and shapes have no significant changes, as illuminated in Fig. 5a. The NIR emission intensity at 380 K decreases to 63.02% of initial intensity (as displayed at Fig. 5b), which indicate that Na2GdMg2V3O12: 0.05Nd3+, 0.04Yb3+ phosphor has better thermal stability than that of other Nd3+-Yb3+ doped materials [35]. Therefore, the present phosphor is an ideal luminescent converter candidate used for silicon-based solar cells. 3.5 Energy Transfer processes in Nd3+-Yb3+ co-doped Na2GdMg2V3O12 According to the analysis of above results, four energy transfer processes may be existed in 8
the Nd3+-Yb3+ co-doped Na2GdMg2V3O12 phosphors, including (a) VO43- → Yb3+, (b) VO43- → Nd3+, (c) Nd3+ → Yb3+ and (d) VO43- → Nd3+ → Yb3+. In order to clearly illustrate the possible complicated energy transfer processes, the schematic energy level diagram with feasible transitions are shown in Fig. 6. In the present system, under the excitation of near ultraviolet (n-UV) light, VO43- groups are excited by absorbing the n-UV photons as a result of electron transfer from O-2p valence band to V-3d conduction band, and then relax non-radiatively to the lower 3d level; finally, the excited VO43- relax to ground state with a yellow-greenish emission or transfer partial energy to the nearby Yb3+/Nd3+ ions through ET process. For the energy transfer VO43- → Yb3+, there are two possible routes claimed by previous publications: (a1) Cooperative energy transfer (CET) process [36]: one excited [VO4]3- ion transfer its energy to two nearby Yb3+ and emit two NIR photons, in which the quantum efficiency is beyond 100%, but no convincing experimental evidence or data could support CET process; (a2) Single- photon ET [37]: the energy is transferred from the higher excited [VO4]3- to close O2--Yb3+ charge transfer state (CTS) by the help of multi-phonons and then Yb3+ undergoes fast intra-ion thermal relaxation to the 2F5/2 state, inducing a single NIR photon emission for each UV photon excitation. For the energy transfer VO43- → Nd3+ (b)，part of absorbed energy by VO43- group is transferred to Nd3+ and promote the electrons of Nd3+ from 4I9/2 ground state to 4G9/2 or 7G9/2, 4G5/2 level accompanied by thermal energy losses, and further relax to the 4F3/2 level through non-radiative transitions resulting a series of sharp line emissions from 4F3/2 to 4IJ (J = 9/2, 11/2) of Nd3+ dominated by 1060 nm. When excited by the wavelength of 592 nm (Nd3+: 4I9/2 → 2G5/2 + 2G7/2), the energy transfer (c) Nd3+ → Yb3+ takes place. The electrons of Nd3+ are pumped to 2G5/2 level and then relax to 4F3/2 by non-radiative relaxation, followed by a 1060 nm NIR emission from 4F3/2 to 4I11/2 transition of Nd3+ or transfer part of energy to one nearby Yb3+ ion leading to 972 nm NIR emission from 2F5/2 to 2F7/2 transition of Yb3+. Given that above-mentioned three ET processes, the energy transfer (d) VO43- → Nd3+ → Yb3+ is easy to understand. Upon excitation by 325 nm n-UV light, part of excited state electrons relax to the ground state by emitting a broad band yellow-greenish emission; part of excited state electrons transfer energy to Nd3+, then the excited Nd3+ ions act as a bridge to transfer part of accepted energy to Yb3+ inducing the 972 nm NIR emission of 2F5/2 → 2F7/2. 3.6 Comparison of NIR emission in Nd3+/Yb3+ single or co-doped Na2GdMg2V3O12 Above results illuminate that NIR emission ranging from 850 to 1150 nm could be achieved 9
in Nd3+/Yb3+ single or co-doped Na2GdMg2V3O12 phosphors. In order to compare the NIR emission intensity, the NIR spectra of Na2GdMg2V3O12:0.04Yb3+, Na2GdMg2V3O12:0.05Nd3+ and Na2GdMg2V3O12: 0.05Nd3+, 0.04Yb3+ samples are shown in a comparable method along with the solar spectrum and the response curve of c-Si solar cell, as displayed in Fig. 7. It is observed that these NIR emission spectra excited at 325 nm match well with the spectral response of silicon solar cells. The NIR emission intensity increases with the order of IYb＜INd＜INd-Yb, which is consistent with the results of energy transfer efficiency. The integrated intensity of Na2GdMg2V3O12: 0.05Nd3+, 0.04Yb3+ or Na2GdMg2V3O12: 0.05Nd3+ is 5.9 or 2.1 times of that of Na2GdMg2V3O12: 0.04Yb3+ in the region from 850 to 1150 nm, respectively. This is due to the more efficient energy transfer in the co-doped Nd3+-Yb3+ sample than that of Nd3+/Yb3+ solely doped Na2GdMg2V3O12. Moreover, the PLE spectrum indicates that Nd3+-Yb3+ co-doped system could absorb the UV-vis light below 600nm (red dash line) and give intense NIR emission in 850-1150 nm and broad-band visible light in 400-700nm (red solid line). Results reveal that the introduction of Nd3+ could broaden the absorption band and enhance the NIR emission of Na2GdMg2V3O12: Yb3+. The Nd3+-Yb3+ co-doped sample could be a potential DC solar spectral convertor by improve the photoelectric conversion efficiency of silicon solar cells.
4.Conclusion Broadband excitation and strong NIR emission spectra converters were obtained by Nd3+/Yb3+ singly or co-doped Na2GdMg2V3O12. In present systems, VO43- groups serve as sensitizers by capturing 300-400 nm near ultraviolet photons those are not absorbed efficiently by silicon solar cell, and then transfer part of the absorbed energy to Yb3+/Nd3+ accepters inducing strong NIR emission (850-1150 nm) matching well with the response curve of the silicon solar cells. The possible energy processes VO43- → Nd3+, VO43 -→ Yb3+, Nd3+ → Yb3+ have been confirmed and discussed through their spectra and decay times. In the co-doped system, four possible ET processes, including VO43- → Nd3+, VO43 -→ Yb3+, Nd3+ → Yb3+ and VO43- → Nd3+ → Yb3+ were proposed. By comparison, it is found that the PL intensity of Na2GdMg2V3O12: 0.05Nd3+, 0.04Yb3+ or Na2GdMg2V3O12: 0.05Nd3+ is 5.9 or 2.1 times of that of Na2GdMg2V3O12: 0.04Yb3+ in NIR region from 850 to 1150 nm, respectively. This indicates that the ET process VO43- → Nd3+ is more efficient than VO43- → Yb3+; and the introduction of Nd3+ could broaden the absorption 10
band from NUV to visible range and enhance the NIR emission, making Na2GdMg2V3O12: Nd3+, Yb3+ phosphor become an excellent spectral convertor candidate for enhancing conversion efficiency of silicon solar cells.
Acknowledgments This work was supported by National Natural Science Foundation of China (No. 11274251), Ph.D. Programs Foundation of Ministry of Education of China (20136101110017), Technology Foundation for Selected Overseas Chinese Scholar, Ministry of Personnel of China (excellent), Natural Science Foundation of Shaanxi Province (No.2014JM1004) and Foundation of Key Laboratory of Photoelectric Technology in Shaanxi Province (12JS094).
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of Nd3+-doped La2BaZnO5 phosphor, J. Solid State Chem. 215 (2014) 22-25. [24] D. Song, C. F Guo, T. Li, Luminescence of the self-activated vanadate phosphors Na2LnMg2V3O12 (Ln=Y, Gd), Ceram Int. 41 (2015) 6518-6524. [25] G. Gundiah, Y. Shimomura, N. Kijima, A.K. Cheetham, Novel red phosphors based on vanadate garnets for solid state lighting applications, Chem. Phys. Lett. 455 (2008) 279-283. [26] J. C. Zhou, F. Huang, J. Xu, H. Chen, Y. S Wang, Luminescence study of a self-activated and rare earth activated Sr3La(VO4)3 phosphor potentially applicable in W-LEDs, J. Mater. Chem. C 3 (2015) 3023-3028. [27] G. J. Gao, M. Y. Peng, L. Wondraczek, Spectral shifting and NIR down-conversion in Bi3+-Yb3+ co-doped Zn2GeO4, J. Mater. Chem. C 2 (2014) 8083-8088. [28] Y. Luo, Z. G. Xia, B. F. Lei, Y. L. Liu, Structural and luminescence properties of Sr2VO4Cl and Sr5(VO4)3Cl: self-activated luminescence and unusual Eu3+ emission, RSC Adv. 3 (2013) 22206-22212. [29] H. Suo, C. F Guo, Z. Yang, S. S. Zhou, C. K. Duan, M. Yin, Thermometric and optical heating bi-functional properties of upconversion phosphor Ba5Gd8Zn4O21:Yb3+/Tm3+, J. Mater. Chem. C 3 (2015) 7379-7385. [30] Q. L. Xiao and W. Y. Chen, Ultraviolet to near-infrared conversion in Nd3+ doped strontium cerate Nanophosphors, J. Alloys Compd. 631 (2015) 272-275. [31] J. Y Sun, Y. N. Sun, C. Cao, Z. G Xia and H. Y. Du, Near-infrared luminescence and quantum cutting mechanism in CaWO4:Nd3+, Yb3+, Appl. Phys. B 111 (2013) 367-371. [32] J. J. Zhou and Y. Teng, Broadband spectral conversion of visible light to near-infrared emission via energy transfer from Ce3+ to Nd3+/Yb3+ in YAG, J. Mater. Res. 26 (2011) 689-692. [33] L. L. Liu, M. M. Li, Sh. Z. Cai, Y. M. Yang, Y. H. Mai, Near-infrared quantum cutting in Nd3+ and Yb3+ doped BaGd2ZnO5 phosphors, Opt. Mater. Express 5 (2015) 756-763. [34] J. M. Meijer, L. Aarts, B. M. van der Ende, T. J. H. Vlugt, A. Meijerink, Downconversion for solar cells in YF3: Nd3+, Yb3+, Phys. Rev. B 81 (2010) 035107. [35] M Piasecki, E Mandowska, A Herrmann, D. Ehrt, A. Majchrowski, L. R. Jaroszewicz, M. G. Brik, I. V. Kityk, Tailoring Nd3+ luminescence characteristics by Yb3+ doping in K5Nd(MoO4)4 , RbNd(WO4)2 and NdAl3(BO3)4 crystal matrices, J. Alloy. Compd.639 (2015) 577-582. [36] X. R. Cheng, L. Su, Y. Q. Wang, X. Zhu, X. T. Wei, Y. Y. Wang, Near-infrared quantum cutting in YVO4: Yb3+ thin-films via down-conversion, Opt. Mater. 34 (2012) 1102-1106. [37] X.T. Wei, S. Huang, Y. H. Chen, C. X. Guo, M. Yin, W. Xu, Energy transfer mechanisms in Yb3+ doped YVO4 near-infrared down-conversion phosphor, J. Appl. Phys. 107 (2010) 103107. 13
Figure captions
Fig. 1 XRD patterns of Na2GdMg2V3O12, Na2GdMg2V3O12: Yb3+, Na2GdMg2V3O12: Nd3+ and Na2GdMg2V3O12: Nd3+, Yb3+ as well as the standard XRD data of Na2YMg2V3O12 (JCPDS No. 49-0412) as a reference.
Fig. 2 PLE (a) and PL (b) spectra of samples Na2GdMg2V3O12: xYb3+ (x=0, 0.04, 0.08, 0.10, 0.12, 0.15 and 0.20) as well as decay lifetime of host emission (c); calculated lifetime and energy transfer efficiency (d) as function of the concentration of Yb3+.
Fig. 3 PLE (black and blue solid line) and PL (green and red dash line) spectra of the sample doped with Nd3+ (a) together with decay lifetime of host emission (b), and calculated lifetime and ET efficiency (c) as function of Nd3+ in samples Na2GdMg2V3O12: yNd3+ (y = 0, 0.01, 0.03, 0.05, 0.07, and 0.10). Inset is the PL intensity of Nd3+ and host emission as function of Nd3+ concentration.
Fig. 4 PLE and PL spectra of 0.05Nd3+-0.04Yb3+ doped sample (a) and the lifetime along with ET efficiency as function of the Yb3+ concentration with excitation wavelength at 325 (b) and 592 nm (c) respectively. Insets are the PL intensity of host emission (520nm), Yb3+ (972nm) and Nd3+ (1060nm) varing with Yb3+ concentration under 325 and 592 nm excitation.
Fig. 5 Temperature-dependent NIR emission spectra (a) and the integrated emission intensity (b) of sample Na2GdMg2V3O12: 0.05Nd3+, 0.04Yb3+.
Fig. 6 Schematic energy level diagram and possible ET processes in Nd3+-Yb3+ co-doped Na2GdMg2V3O12 phosphors.
Fig. 7 PLE (dash line, λem=972nm) in Nd3+-Yb3+ co-doped Na2GdMg2V3O12 sample and PL (solid line, λex=325nm) spectra in Nd3+/Yb3+ single or co-doped Na2GdMg2V3O12 phosphors in comparison with the Solar spectrum and spectral response curve of c-Si solar cell.
14
Figures
Fig. 1 XRD patterns of Na2GdMg2V3O12, Na2GdMg2V3O12: Yb3+, Na2GdMg2V3O12: Nd3+ and Na2GdMg2V3O12: Nd3+, Yb3+ as well as the standard XRD data of Na2YMg2V3O12 (JCPDS No. 49-0412) as a reference.
Fig. 2 PLE (a) and PL (b) spectra of samples Na2GdMg2V3O12: xYb3+ (x=0, 0.04, 0.08, 0.10, 0.12, 0.15 and 0.20) as well as decay lifetime of host emission (c); calculated lifetime and energy transfer efficiency (d) as function of the concentration of Yb3+.
15
Fig. 3 PLE (black and blue solid line) and PL (green and red dash line) spectra of the sample doped with Nd3+ (a) together with decay lifetime of host emission (b), and calculated lifetime and ET efficiency (c) as function of Nd3+ in samples Na2GdMg2V3O12: yNd3+ (y = 0, 0.01, 0.03, 0.05, 0.07, and 0.10). Inset is the PL intensity of Nd3+ and host emission as function of Nd3+ concentration.
Fig. 4 PLE and PL spectra of 0.05Nd3+-0.04Yb3+ doped sample (a) and the lifetime along with ET efficiency as function of the Yb3+ concentration with excitation wavelength at 325 (b) and 592 nm (c) respectively. Insets are the PL intensity of host emission (520nm), Yb3+ (972nm) and Nd3+ (1060nm) varing with Yb3+ concentration under 325 and 592 nm excitation.
16
Fig. 5 Temperature-dependent NIR emission spectra (a) and the integrated emission intensity (b) of sample Na2GdMg2V3O12: 0.05Nd3+, 0.04Yb3+.
Fig. 6 Schematic energy level diagram and possible ET processes in Nd3+-Yb3+ co-doped Na2GdMg2V3O12 phosphors.
17
Fig. 7 PLE (dash line, λem=972nm) in Nd3+-Yb3+ co-doped Na2GdMg2V3O12 sample and PL (solid line, λex=325nm) spectra in Nd3+/Yb3+ single or co-doped Na2GdMg2V3O12 phosphors in comparison with the Solar spectrum and spectral response curve of c-Si solar cell.
18
PII: DOI: Reference:
S0272-8842(16)30677-0 http://dx.doi.org/10.1016/j.ceramint.2016.05.072 CERI12877
To appear in: Ceramics International Received date: 2 May 2016 Revised date: 11 May 2016 Accepted date: 11 May 2016 Cite this article as: Dan Song, Chongfeng Guo, Jin Zhao, Hao Suo, Xiaoqi Zhao, Xianju Zhou and Guozhen Liu, Host Sensitized Near-infrared Emission in Nd3+ Yb3+ Co-doped Na2GdMg2V3O12 Phosphor, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.05.072 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Host Sensitized Near-infrared Emission in Nd3+ -Yb3+ Co-doped Na2GdMg2V3O12 Phosphor Dan Songa, Chongfeng Guoa,b*, Jin Zhaoa, Hao Suoa, Xiaoqi Zhaoa, Xianju Zhouc, Guozhen Liub a
National Key Laboratory of Photoelectric Technology and Functional Materials (Culture Base) in Shaanxi
Province, National Photoelectric Technology and Functional Materials & Application of Science and Technology International Cooperation Base, Institute of Photonics & Photon-Technology, Northwest University, Xi’an 710069, China; b
ARC Centre of Excellence for Nanoscale Biophotonics (CNBP), Macquarie University, North Ryde 2109,
Australia c
School of Sciences, Chongqing University of Posts and Telecommunications, Chongqing 400065, China
---------------------------------------------------------------*Author to whom correspondence should be addressed: Institute of Photonics & Photon-Technology；Northwest University, Xi’an 710069, China; Tel. & Fax.: 86-29-88302661 E-mail: [email protected] (Prof. Guo)
Abstract Yb3+/Nd3+ singly and co-doped Na2GdMg2V3O12 phosphors with near-infrared (NIR) emission were synthesized via sol-gel method. The phase purity and structure of samples were characterized by X-ray diffraction (XRD), the photoluminescence emission (PL) and excitation (PLE) spectra along with decay curves were also measured. Near infrared (NIR) emissions (850-1150 nm) from acceptors Yb3+ or Nd3+ matching well with the response curve of the silicon solar cell were obtained, in which VO43- groups acted as sensitizers by capturing near ultraviolet photons which are not absorbed efficiently by silicon solar cell and transferred them to Yb3+/Nd3+ by energy transfer processes. The NIR emission intensities of the Nd3+-Yb3+co-doped samples Na2GdMg2V3O12 were enhanced greatly in comparison with that of Nd3+/Yb3+ singly doped samples, and the possible energy transfer processes were also discussed in detail. Results indicate that the obtained samples are potential solar spectral down-conversion (DC) convertors to enhance the conversion efficiency of the silicon solar cells. 1
Keywords: Phosphor; Near-infrared; Energy transfer; Vanadate garnet.
1. Introduction As the environmental degradation and energy crisis are becoming hot issues, more attentions and efforts are focused on seeking green and renewable energy source to replace traditional fossil fuel. The sunlight is an inexhaustible renewable pollution-free clean energy, which could be efficiently converted into electrical energy using photovoltaic solar cells [1, 2]. It is estimated that solar cell will provide about 16% global electricity by 2050, which stimulates more interests from governments, companies and experts [3, 4]. As we all know, there are many types of solar cells, such as high efficient semiconductor, low-cost dye-sensitized, soft organic, perovskite solar cells and so on [5, 6]; but crystalline silicon (c-Si) solar cells are still dominating the present commercial market due to their mature technology. However, the conversion efficiency of silicon solar cells is only about 18%, which is far below the theoretical maximum value of 30% (the Shockley-Queisser limit) because of the spectral mismatch between solar spectrum (300-2500 nm, AM 1.5G) and the response spectrum of c-Si solar cells (Eg ≈ 1.12 eV, λ ≈ 1100 nm) [7, 8]. Silicon solar cell absorbs photons with higher energy than its bandgap Eg (E﹥Eg) and converts each absorbed photon to a pair of carriers; whereas photons with energy less than Eg (E﹤Eg) tend to be transmitted rather than absorbed. Simultaneously, the photons with energy higher than Eg loss their excess energy through thermalization of “hot” electron-hole pairs [9]. In order to reduce the spectral mismatch losses and enhance the photovoltaic conversion efficiency, up-conversion (UC) and down-conversion (DC) phosphors have been proposed to tune the solar spectrum to match the spectral response of c-Si solar cells. The former absorbs two or more lower-energy photons to induce one higher-energy photon; the latter could convert the high-energy photons into near-infrared (NIR) photons by energy transfer [10]. For NIR DC materials used for c-Si solar cells, the Yb3+ ion is known as the most suitable activator due to its simple 4f13 configuration and only one excited state 2F7/2 (～10000 cm-1) just above the band edge of crystalline silicon [11]. RE3+-Yb3+ (RE3+ = Tb3+, Pr3+, Tm3+, Ho3+, Er3+ or Nd3+) and Ce3+/Eu2+-Yb3+ co-doped DC materials have been extensively investigated, in which RE3+ and Ce3+/Eu2+ ions play the role of sensitizer to absorb ultraviolet-visible light and then transfer the absorbed energy to nearby Yb3+ to emit NIR light [12-17]. However, the absorption 2
peaks of Tb3+, Pr3+, Tm3+, Ho3+, Er3+ or Nd3+ originate from the forbidden 4f → 4f transitions, which naturally have weak intensity and narrow bandwidth. The broadband absorptions of sensitizers Ce3+/Eu2+ from the allowed 4f-5d transitions have to be prepared at reduced atmosphere, leading to the reduction of partial Yb3+ to Yb2+ accompanied with the decrease of NIR emission from Yb3+ [18]. In addition, some self-activated compounds, such as vanadate, niobate, tungstate and molybdate，have strong absorption in near ultraviolet (NUV) range and intense emission from blue to yellow region without any dopants, which could efficiently transfer their absorbed energy to the activator ions and the host-sensitized NIR DC phosphor could be realized through doping Yb3+ [19-22]. Besides Yb3+, rare earth ion Nd3+ can also emit NIR light from 800 to 1150 nm from the transitions of 4F3/2 to 4I9/2,
11/2,
which makes Nd3+ as the optional
NIR luminescent center ion for application in Si solar cell similar to Yb3+ [23]. Here, we investigated the NIR emission properties of Yb3+/Nd3+ singly and co-doped Na2GdMg2V3O12 samples, in which self-activated vanadate garnet VO43- group acts as the sensitizer to harvest the incident sunlight and transfers the absorbed energy to activator Yb3+/Nd3+ leading to NIR emission. The detailed energy transfer (ET) mechanisms are systematically discussed through the measurement of vis-NIR emission spectra and decay curves. Results indicate that the NIR emission intensity could be greatly enhanced in Nd3+-Yb3+ co-doped Na2GdMg2V3O12 phosphors in comparison with that of Yb3+ or Nd3+ solely doped phosphors. The co-doped samples could efficiently convert NUV and visible light to NIR emission, which allows it to be applied in enhancing the conventional efficiency of c-Si solar cells by modifying solar spectrum.
2.Experimental All Yb3+/Nd3+ singly and co-doped Na2GdMg2V3O12 samples were synthesized by a sol-gel method. The stoichiometric amounts of raw materials, including high purity Gd2O3, Yb2O3 and Nd2O3 (99.99%) and analytical purity (A.R.) Na2CO3, Mg(NO3)2·6H2O and NH4VO3, were weighed. In a typical process, lanthanide oxides Gd2O3, Yb2O3 and Nd2O3 were firstly dissolved in dilute HNO3 (A.R.) solution at 120 ℃ under constant stirring and the excessive HNO3 was evaporated to remove. Then appropriate volume of distilled water and Na2CO3, Mg(NO3)2·6H2O were added and stirred to form a transparent solution. Subsequently, NH4VO3 and the chelating 3
agent citric acid C6H8O7·H2O (A.R.) were added into above solution, and the molar ratio of total chelate metal cations to citric acid was fixed at 1: 2. The obtained highly transparent orange solution was placed in 70 ℃ oven for 24 h and then increase to 120 ℃ for 24 h to get deep-brown dried foamy precursors. The dried precursors were pre-heated at 500 ℃ for 5 h, and further calcined at 850 ℃ for 5 h in the air. Finally, the as-prepared samples were naturally cooled to room temperature and grounded to get sample powder. X-ray diffraction (XRD) patterns of the Na2GdMg2V3O12: Nd3+/Yb3+ phosphors were used to characterize the structure through a Rigaku-Dmax 3C powder diffractometer (Rigaku Corp., Tokyo, Japan) with Cu-Kα radiation (λ = 1.5406 Å). The photoluminescence (PL), photoluminescence excitation (PLE) spectra and decay curves were measured with an FLS920 spectrofluorometer (Edinburgh Corp., UK) equipped with 450 W Xenon lamp. The temperature dependent spectra were measured by an Oxford OptistatDN2 nitrogen cryogenics temperature controlling system (duration of stay at the measured temperature is 0.5 h). All the measurements of photoluminescent spectra were repeated three times to reduce error.
3.Results and discussion 3.1 Phase and structure The property of sample is mainly determined by its structure, thus the phase purities and structures of Yb3+/Nd3+ solely and Nd3+-Yb3+ co-doped Na2GdMg2V3O12 samples as well as the blank sample were characterized by XRD patterns, as shown representatively in Fig. 1. All the diffraction peaks of samples agree well with those of the standard Na2YMg2V3O12 (JCPDS card No. 49-0412), and no any impurity peaks appear [24]. Above results indicate that compounds Na2GdMg2V3O12 and Na2YMg2V3O12 adopt the same cubic garnet structure, belonging to the space group of Ia-3d (230), but the calculated lattice constants of Na2GdMg2V3O12 (a = b = c = 12.414 Å) are slightly bigger than those of Na2YMg2V3O12 (a = b = c = 12.397 Å) due to the larger ionic radius of Gd3+ (RGd = 1.05 Å) than that of Y3+ (RY = 1.02 Å) [25]. In addition, it also implies that the Yb3+/Nd3+ ions successfully incorporate into the lattice and occupy the sites of Gd3+ due to their similar radius and the same valence. It is noted that all samples in our experiments show similar XRD patterns. 3.2 Luminescent properties of Yb3+ singly doped Na2GdMg2V3O12 4
In order to investigate the luminescent properties of Yb3+ doped Na2GdMg2V3O12, the PLE and PL spectra of samples are presented in Fig. 2a and b as a function of Yb3+ concentration, respectively. The intense excitation band ranging from 250 to 400 nm centered at 325 nm is found by monitoring at around 520 nm visible light assigned to the broad-band absorption of the charge transfer (CT) band of O2- - V5+ in the host rather than that of O2--Yb3+, and the latter usually locates at a shorter wavelength below 250 nm in inorganic compounds [22]. The broad intense absorption from host is also observed by monitoring at 972 nm near infrared Yb3+ emission, which indicates the presence of energy transfer from host to Yb3+. The emission spectra cover the visible and NIR region with various Yb3+ concentrations under 325 nm excitation in Fig. 2b. In the visible range (400-700nm), the broad unsymmetrical emission band centered at 520 nm is from the charge-transfer (CT) of an electron from the 2p orbital of O2- to the vacant 3d orbital of V5+ in tetrahedral [VO4]3- group with Td symmetry [26]. The unsymmetrical emission band is attributed to the partially allowed transitions from 3T2→1A1 and 3T1→1A1. In the NIR region (900-1150 nm), an intense broad emission peaked at 972 nm is observed from 2F5/2 to 2F7/2 states of Yb3+, in which a series of sharp line emissions are attributed to the transitions among the different stark levels of 2
F5/2 and 2F7/2 [27]. This is another proof for the occurrence of energy transfer from host to Yb3+.
In addition, it is observed that the PL intensity of [VO4]3- intrinsic host emission reduces monotonously with the increasing of Yb3+ doping concentration from 0 to 0.2, whereas the NIR emission intensity first increases and then decreases after reaching the maximum at about x = 0.12 due to the concentration quenching effect. The decreased emission of host companied with the enhanced Yb3+ emission in NIR region with the increase of Yb3+ is also powerful evidence for the appearance of ET from VO43- to Yb3+. To further investigate the process of ET, the decay curves of VO43- emission at 520 nm in Na2GdMg2V3O12: xYb3+ (x = 0-0.20) with the 325 nm excitation were displayed in Fig. 2c. With the increase of Yb3+ concentration from 0 to 0.2, the decay curves deviate from mono-exponential and can be fitted to be a bi-exponential modal described as [28]:
I ( t ) =0I +1A e x pt( 1- / 2A) + e xt 2p ( - /
) (1)
where I(t) and I0 are the luminescence intensities at time t and 0, A1 and A2 are pre-exponentials factors related with lifetime components τ1 and τ2, respectively. The average lifetimes can be calculated by the following equation [29]:
5
taverage =(A112 +A2 22 )/(A11 +A2 2 )
(2)
And the energy transfer efficiency (ETE) ηET can be estimated based on the equation:
ET =1- / 0
(3)
where τ0 and τ stand for the lifetime values of VO43- emissions (520 nm) without and with Yb3+-doped samples. As the Yb3+ concentration increase, the lifetimes keep decreasing from 6.24 to 3.8 μs; simultaneously, the energy transfer efficiency (ETE) exhibits a monotonous increase from 6.1% to 41.5% with the content of Yb3+ increasing to 0.2, as shown in Fig. 2d. Above results indicate that more energy absorbed by host is transferred to Yb3+ with increasing the Yb3+ concentration, which leads to the declining emission intensity and lifetime of 520 nm as well as the enhancing of NIR emission at 972 nm. 3.3 Luminescent properties of Nd3+-doped Na2GdMg2V3O12 As a typical emitting center in NIR region, Nd3+ ions emission ranges from 850 to 1150 nm and matches well with the response curve of Si-based solar cells. Photo-luminescent properties of Nd3+ doped Na2GdMg2V3O12 were investigated in order to show its potential possibility in this field. Fig. 3a shows the PLE and PL spectra of the Na2GdMg2V3O12: Nd3+ phosphors. The PLE spectrum of sample monitoring at 1060 nm includes a broad band and a series of sharp line absorptions；the former band centered at 325 nm is assigned to the absorption from the CT band of VO43-, the latter sharp lines peaked at 482, 532, 592, 684, 738 and 823 nm are ascribed to the f-f transitions of Nd3+: 4I9/2 → 2K15/2 + 2G9/2, 4I9/2 → 2K13/2 + 4G9/2 + 4G7/2, 4I9/2 → 2G5/2 + 2G7/2, 4I9/2 → 4
F9/2, 4I9/2 → 4F7/2 + 4S3/2 and 4I9/2 → 4F5/2, respectively [30, 31]. Whereas monitoring at 520 nm,
only one broad absorption band from host was observed. This host absorption band could be observed as monitoring at host emission or NIR emission from Nd3+, which implies that the ET occurrence from host to Nd3+. For the PL spectrum (dot line) with excitation at 325 nm, it covers visible and NIR region from 400-1150 nm, consisting of a broad band emission in visible region from host centered at 520 nm and a series of sharp line emissions ranging from 800 to 1150 nm in NIR region due to the electronic f-f transitions from 4F3/2 to 4IJ (J = 9/2, 11/2) of Nd3+. Comparing the PLE and PL spectrum, a significant overlap was found between the host emission and the Nd3+ absorption lines, which is expected for efficient energy transfer from host to Nd3+ and the concaves in the visible luminescence at 592 nm ascribed to the re-absorption of Nd3+ : 4I9/2 → 6
2
G5/2 + 2G7/2 [32] . Inset of Fig. 3a shows the integrated PL intensity in visible and NIR region
varying with dopant Nd3+ content, and the similar trend is also found in Na2GdMg2V3O12: Yb3+ phosphors. It is clearly observed that the intensity of host emission mono-directionally drop, whereas the NIR intensity first increase and then decline after reaching the maximum at y=0.05 due to the concentration quenching effect. Above luminescent results not only prove the appearance of energy transfer but also present broader absorption range, which is more appropriate for the solar cell. In addition, the energy transfer efficiency is also estimated according to the decrease of host emission lifetime. The decay curves of host emission are shown in Fig. 3b with various Nd3+ concentration, in which it is found that the decay rate of host emission become faster with increasing Nd3+ content due to the absorption by the Nd3+ ion and ET from VO43- to Nd3+. And the lifetimes are calculated to decrease from 6.24 to 2.1 μs according to Eq (1) and (2) with the concentration of Nd3+ reaching 0.10; the ETE from VO43- to Nd3+ was also estimated based on the various values of host emission lifetime and shown in Fig. 3c as function of Nd3+ content. The decreasing lifetime of sensitizer and increasing of ETE further give strong evidence to the ET process from VO43- to Nd3+. 3.4 Luminescence in Nd3+-Yb3+ co-doped Na2GdMg2V3O12 As discussed above, Yb3+ or Nd3+ ions solely doped Na2GdMg2V3O12 both emit strong NIR emission ranging from 800 to 1150 nm, which can be efficiently absorbed by Si-based solar cell. Previous publications have also proved the occurrence of efficient energy transfer from Nd3+ to Yb3+ [15, 33, 34], therefore it is expected that the introduction of Nd3+ could enhance the NIR emission intensity of Yb3+. Fig. 4a displays the PLE and PL spectra of Na2GdMg2V3O12: 0.05Nd3+, 0.04Yb3+ as representative of Nd3+-Yb3+ co-doped samples. For the PLE spectra monitoring at 520 nm from host VO43-: 3T1, 3T2 → 1A1, 972 nm from Yb3+: 2F5/2 → 2F7/2 and 1060 nm from Nd3+: 4
F3/2 → 4I11/2, they all include a broad absorption band in 200-400 nm region with the maximum at
325 nm, which implies the occurrence of energy transfer from host to Yb3+ or Nd3+. Besides the host broad absorption, a series of characteristic peaks from the f-f transitions of Nd3+ are also observed as the monitoring wavelength is 972 nm of Yb3+ and 1060 nm of Nd3+, which illuminates that Nd3+ can directly absorb visible light and transfer partial absorbed energy to Yb3+ in Nd3+-Yb3+ co-doped Na2GdMg2V3O12. In order to prove our deduction, the PL spectra excited at 592 nm (Nd3+: 4I9/2 → 2G5/2 + 2G7/2; dot line) and 325 nm (host absorption; dash line) are shown in 7
Fig. 4a. The profiles of PL spectra are similar in 850 to 1150 nm NIR region and dominated by 972 (Yb3+) and 1060 nm (Nd3+) except the unsymmetrical band emission from host under excitation of 325 nm light, including a series of sharp lines emission peaked at 950, 972, 1016 and 1034nm (diamond indexed) from the transitions of Yb3+ and 885, 935, 1060 and 1097 nm (asterisk indexed) from transitions of Nd3+. In order to find optimal composition of sample, Nd3+-Yb3+ co-doped Na2GdMg2V3O12 were investigated with fixed Nd3+ at 0.05 and various Yb3+ concentration. Insets of Fig. 4a present the PL intensity changes with Yb3+ concentrations when excited at 325 and 592 nm, respectively. It is clearly observed that the emission intensity of acceptors Yb3+ (972 nm) first increase and then decrease after reaching the maximum at about x = 0.04 whether excitation at 325 nm from host or 592 nm from Nd3+. However, the emission intensity of host emission centered at 520 nm and Nd3+ emission peaked at 1060 nm monotonously decrease, which indicates the appearance of energy transfer from VO43-/Nd3+ to Yb3+. In addition, the decay curves of host emission and 1060 nm emission from Nd3+ were also measured as function of Yb3+ contents by excited at 325 and 592 nm, respectively. The corresponding lifetimes were calculated based on Eq (1) and (2), and it is found that their lifetimes decline with the growth of Yb3+, as shown in Fig. 4a and 4b; the ETE was also estimated according to Eq (3), it is clearly found that the ETE is higher with 325 nm excitation than that of excitation at 592 nm, because the former is a sum of ETE including VO43→ Nd3+, VO43- →Yb3+ and VO43- → Nd3+ → Yb3+ whereas the latter is ETE of Nd3+ → Yb3+. Generally, the working temperature of solar cells in outdoor is usually changed with the environment, such as the season, region and the intensity of sun radiation. Thus, the temperature-dependent NIR emission spectra of Na2GdMg2V3O12: 0.05Nd3+, 0.04Yb3+ were measured and presented in Fig.5. As the temperature increasing from 280 to 420 K, the PL intensities monotonously decrease but the peak positions and shapes have no significant changes, as illuminated in Fig. 5a. The NIR emission intensity at 380 K decreases to 63.02% of initial intensity (as displayed at Fig. 5b), which indicate that Na2GdMg2V3O12: 0.05Nd3+, 0.04Yb3+ phosphor has better thermal stability than that of other Nd3+-Yb3+ doped materials [35]. Therefore, the present phosphor is an ideal luminescent converter candidate used for silicon-based solar cells. 3.5 Energy Transfer processes in Nd3+-Yb3+ co-doped Na2GdMg2V3O12 According to the analysis of above results, four energy transfer processes may be existed in 8
the Nd3+-Yb3+ co-doped Na2GdMg2V3O12 phosphors, including (a) VO43- → Yb3+, (b) VO43- → Nd3+, (c) Nd3+ → Yb3+ and (d) VO43- → Nd3+ → Yb3+. In order to clearly illustrate the possible complicated energy transfer processes, the schematic energy level diagram with feasible transitions are shown in Fig. 6. In the present system, under the excitation of near ultraviolet (n-UV) light, VO43- groups are excited by absorbing the n-UV photons as a result of electron transfer from O-2p valence band to V-3d conduction band, and then relax non-radiatively to the lower 3d level; finally, the excited VO43- relax to ground state with a yellow-greenish emission or transfer partial energy to the nearby Yb3+/Nd3+ ions through ET process. For the energy transfer VO43- → Yb3+, there are two possible routes claimed by previous publications: (a1) Cooperative energy transfer (CET) process [36]: one excited [VO4]3- ion transfer its energy to two nearby Yb3+ and emit two NIR photons, in which the quantum efficiency is beyond 100%, but no convincing experimental evidence or data could support CET process; (a2) Single- photon ET [37]: the energy is transferred from the higher excited [VO4]3- to close O2--Yb3+ charge transfer state (CTS) by the help of multi-phonons and then Yb3+ undergoes fast intra-ion thermal relaxation to the 2F5/2 state, inducing a single NIR photon emission for each UV photon excitation. For the energy transfer VO43- → Nd3+ (b)，part of absorbed energy by VO43- group is transferred to Nd3+ and promote the electrons of Nd3+ from 4I9/2 ground state to 4G9/2 or 7G9/2, 4G5/2 level accompanied by thermal energy losses, and further relax to the 4F3/2 level through non-radiative transitions resulting a series of sharp line emissions from 4F3/2 to 4IJ (J = 9/2, 11/2) of Nd3+ dominated by 1060 nm. When excited by the wavelength of 592 nm (Nd3+: 4I9/2 → 2G5/2 + 2G7/2), the energy transfer (c) Nd3+ → Yb3+ takes place. The electrons of Nd3+ are pumped to 2G5/2 level and then relax to 4F3/2 by non-radiative relaxation, followed by a 1060 nm NIR emission from 4F3/2 to 4I11/2 transition of Nd3+ or transfer part of energy to one nearby Yb3+ ion leading to 972 nm NIR emission from 2F5/2 to 2F7/2 transition of Yb3+. Given that above-mentioned three ET processes, the energy transfer (d) VO43- → Nd3+ → Yb3+ is easy to understand. Upon excitation by 325 nm n-UV light, part of excited state electrons relax to the ground state by emitting a broad band yellow-greenish emission; part of excited state electrons transfer energy to Nd3+, then the excited Nd3+ ions act as a bridge to transfer part of accepted energy to Yb3+ inducing the 972 nm NIR emission of 2F5/2 → 2F7/2. 3.6 Comparison of NIR emission in Nd3+/Yb3+ single or co-doped Na2GdMg2V3O12 Above results illuminate that NIR emission ranging from 850 to 1150 nm could be achieved 9
in Nd3+/Yb3+ single or co-doped Na2GdMg2V3O12 phosphors. In order to compare the NIR emission intensity, the NIR spectra of Na2GdMg2V3O12:0.04Yb3+, Na2GdMg2V3O12:0.05Nd3+ and Na2GdMg2V3O12: 0.05Nd3+, 0.04Yb3+ samples are shown in a comparable method along with the solar spectrum and the response curve of c-Si solar cell, as displayed in Fig. 7. It is observed that these NIR emission spectra excited at 325 nm match well with the spectral response of silicon solar cells. The NIR emission intensity increases with the order of IYb＜INd＜INd-Yb, which is consistent with the results of energy transfer efficiency. The integrated intensity of Na2GdMg2V3O12: 0.05Nd3+, 0.04Yb3+ or Na2GdMg2V3O12: 0.05Nd3+ is 5.9 or 2.1 times of that of Na2GdMg2V3O12: 0.04Yb3+ in the region from 850 to 1150 nm, respectively. This is due to the more efficient energy transfer in the co-doped Nd3+-Yb3+ sample than that of Nd3+/Yb3+ solely doped Na2GdMg2V3O12. Moreover, the PLE spectrum indicates that Nd3+-Yb3+ co-doped system could absorb the UV-vis light below 600nm (red dash line) and give intense NIR emission in 850-1150 nm and broad-band visible light in 400-700nm (red solid line). Results reveal that the introduction of Nd3+ could broaden the absorption band and enhance the NIR emission of Na2GdMg2V3O12: Yb3+. The Nd3+-Yb3+ co-doped sample could be a potential DC solar spectral convertor by improve the photoelectric conversion efficiency of silicon solar cells.
4.Conclusion Broadband excitation and strong NIR emission spectra converters were obtained by Nd3+/Yb3+ singly or co-doped Na2GdMg2V3O12. In present systems, VO43- groups serve as sensitizers by capturing 300-400 nm near ultraviolet photons those are not absorbed efficiently by silicon solar cell, and then transfer part of the absorbed energy to Yb3+/Nd3+ accepters inducing strong NIR emission (850-1150 nm) matching well with the response curve of the silicon solar cells. The possible energy processes VO43- → Nd3+, VO43 -→ Yb3+, Nd3+ → Yb3+ have been confirmed and discussed through their spectra and decay times. In the co-doped system, four possible ET processes, including VO43- → Nd3+, VO43 -→ Yb3+, Nd3+ → Yb3+ and VO43- → Nd3+ → Yb3+ were proposed. By comparison, it is found that the PL intensity of Na2GdMg2V3O12: 0.05Nd3+, 0.04Yb3+ or Na2GdMg2V3O12: 0.05Nd3+ is 5.9 or 2.1 times of that of Na2GdMg2V3O12: 0.04Yb3+ in NIR region from 850 to 1150 nm, respectively. This indicates that the ET process VO43- → Nd3+ is more efficient than VO43- → Yb3+; and the introduction of Nd3+ could broaden the absorption 10
band from NUV to visible range and enhance the NIR emission, making Na2GdMg2V3O12: Nd3+, Yb3+ phosphor become an excellent spectral convertor candidate for enhancing conversion efficiency of silicon solar cells.
Acknowledgments This work was supported by National Natural Science Foundation of China (No. 11274251), Ph.D. Programs Foundation of Ministry of Education of China (20136101110017), Technology Foundation for Selected Overseas Chinese Scholar, Ministry of Personnel of China (excellent), Natural Science Foundation of Shaanxi Province (No.2014JM1004) and Foundation of Key Laboratory of Photoelectric Technology in Shaanxi Province (12JS094).
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Figure captions
Fig. 1 XRD patterns of Na2GdMg2V3O12, Na2GdMg2V3O12: Yb3+, Na2GdMg2V3O12: Nd3+ and Na2GdMg2V3O12: Nd3+, Yb3+ as well as the standard XRD data of Na2YMg2V3O12 (JCPDS No. 49-0412) as a reference.
Fig. 2 PLE (a) and PL (b) spectra of samples Na2GdMg2V3O12: xYb3+ (x=0, 0.04, 0.08, 0.10, 0.12, 0.15 and 0.20) as well as decay lifetime of host emission (c); calculated lifetime and energy transfer efficiency (d) as function of the concentration of Yb3+.
Fig. 3 PLE (black and blue solid line) and PL (green and red dash line) spectra of the sample doped with Nd3+ (a) together with decay lifetime of host emission (b), and calculated lifetime and ET efficiency (c) as function of Nd3+ in samples Na2GdMg2V3O12: yNd3+ (y = 0, 0.01, 0.03, 0.05, 0.07, and 0.10). Inset is the PL intensity of Nd3+ and host emission as function of Nd3+ concentration.
Fig. 4 PLE and PL spectra of 0.05Nd3+-0.04Yb3+ doped sample (a) and the lifetime along with ET efficiency as function of the Yb3+ concentration with excitation wavelength at 325 (b) and 592 nm (c) respectively. Insets are the PL intensity of host emission (520nm), Yb3+ (972nm) and Nd3+ (1060nm) varing with Yb3+ concentration under 325 and 592 nm excitation.
Fig. 5 Temperature-dependent NIR emission spectra (a) and the integrated emission intensity (b) of sample Na2GdMg2V3O12: 0.05Nd3+, 0.04Yb3+.
Fig. 6 Schematic energy level diagram and possible ET processes in Nd3+-Yb3+ co-doped Na2GdMg2V3O12 phosphors.
Fig. 7 PLE (dash line, λem=972nm) in Nd3+-Yb3+ co-doped Na2GdMg2V3O12 sample and PL (solid line, λex=325nm) spectra in Nd3+/Yb3+ single or co-doped Na2GdMg2V3O12 phosphors in comparison with the Solar spectrum and spectral response curve of c-Si solar cell.
14
Figures
Fig. 1 XRD patterns of Na2GdMg2V3O12, Na2GdMg2V3O12: Yb3+, Na2GdMg2V3O12: Nd3+ and Na2GdMg2V3O12: Nd3+, Yb3+ as well as the standard XRD data of Na2YMg2V3O12 (JCPDS No. 49-0412) as a reference.
Fig. 2 PLE (a) and PL (b) spectra of samples Na2GdMg2V3O12: xYb3+ (x=0, 0.04, 0.08, 0.10, 0.12, 0.15 and 0.20) as well as decay lifetime of host emission (c); calculated lifetime and energy transfer efficiency (d) as function of the concentration of Yb3+.
15
Fig. 3 PLE (black and blue solid line) and PL (green and red dash line) spectra of the sample doped with Nd3+ (a) together with decay lifetime of host emission (b), and calculated lifetime and ET efficiency (c) as function of Nd3+ in samples Na2GdMg2V3O12: yNd3+ (y = 0, 0.01, 0.03, 0.05, 0.07, and 0.10). Inset is the PL intensity of Nd3+ and host emission as function of Nd3+ concentration.
Fig. 4 PLE and PL spectra of 0.05Nd3+-0.04Yb3+ doped sample (a) and the lifetime along with ET efficiency as function of the Yb3+ concentration with excitation wavelength at 325 (b) and 592 nm (c) respectively. Insets are the PL intensity of host emission (520nm), Yb3+ (972nm) and Nd3+ (1060nm) varing with Yb3+ concentration under 325 and 592 nm excitation.
16
Fig. 5 Temperature-dependent NIR emission spectra (a) and the integrated emission intensity (b) of sample Na2GdMg2V3O12: 0.05Nd3+, 0.04Yb3+.
Fig. 6 Schematic energy level diagram and possible ET processes in Nd3+-Yb3+ co-doped Na2GdMg2V3O12 phosphors.
17
Fig. 7 PLE (dash line, λem=972nm) in Nd3+-Yb3+ co-doped Na2GdMg2V3O12 sample and PL (solid line, λex=325nm) spectra in Nd3+/Yb3+ single or co-doped Na2GdMg2V3O12 phosphors in comparison with the Solar spectrum and spectral response curve of c-Si solar cell.
18