Near-infrared quantum cutting in Ho3+, Yb3+-codoped BaGdF5 nanoparticles via first-and second-order energy transfers
© Guo et al.; licensee Springer. 2012
Received: 5 October 2012
Accepted: 9 November 2012
Published: 22 November 2012
Infrared quantum cutting involving Yb3+ 950–1,000 nm (2 F5/2 → 2 F7/2) and Ho3+ 1,007 nm (5S2,5F4 → 5I6) as well as 1,180 nm (5I6 → 5I8) emissions is achieved in BaGdF5: Ho3+, Yb3+ nanoparticles which are synthesized by a facile hydrothermal route. The mechanisms through first- and second-order energy transfers were analyzed by the dependence of Yb3+ doping concentration on the visible and infrared emissions, decay lifetime curves of the 5 F5 → 5I8, 5S2/5F4 → 5I8, and 5 F3 → 5I8 of Ho3+, in which a back energy transfer from Yb3+ to Ho3+ is first proposed to interpret the spectral characteristics. A modified calculation equation for quantum efficiency of Yb3+-Ho3+ couple by exciting at 450 nm was presented according to the quantum cutting mechanism. Overall, the excellent luminescence properties of BaGdF5: Ho3+, Yb3+ near-infrared quantum cutting nanoparticles could explore an interesting approach to maximize the performance of solar cells.
KeywordsNear infrared quantum cutting First- and second-order energy transfers Back energy transfer BaGdF5: Ho3+ Yb3+
Lanthanide (Ln) ions could exhibit both efficient upconversion (UC) and downconversion (DC) emission properties , where the UC process converts low-energy light, usually near infrared (NIR) or infrared, to higher energies, ultraviolet or visible, via multiple absorptions or energy transfers (ETs). In contrast, DC process is the conversion of higher-energy photons into lower-energy photons . For the time being, DC of NIR luminescence (i.e., NIR quantum cutting (QC)), which down-converts one incident UV-blue photon into two NIR photons (approximately 1,000 nm), has attracted more attention for their application in silicon solar cells by modifying the incident light wavelength .
As it is well known to us that the solar spectrum and the bandgap energy of silicon semiconductor do not match each other, thus photons with energy lower than the bandgap could not be absorbed, while for photons with energy larger than the bandgap, the excess energy is lost by thermalization of hot charge carriers . Take these sources of energy loss into account for the solar spectrum; the maximum energy efficiency is 30% only for a crystalline Si solar cell with a bandgap of 1.12 eV . Considering this, if the conversion of one UV or visible photon into two NIR photons with energies about 1.12 eV is realized through QC in a silicon solar cell, the energy loss related to thermalization of hot charge carriers can be reduced, and the solar cell efficiency will be enhanced greatly to satisfy the application requirement .
To obtain high NIR QC efficiency, other Ln3+ ions are generally codoped with Yb3+ in the hosts, and it is commonly demonstrated in Ln3+-Yb3+ (Ln = Tb, Tm, Pr, Er, Nd, and Ho) couple-codoped materials . Generally speaking, there are two mechanisms involved in the NIR QC in Ln3+-Yb3+ couple : one is second-order cooperation of energy transfer (CET) based on one donor and two acceptor ions, and the other is first-order resonance energy transfer (ET). Nevertheless, CET process is not as efficient as resonance ET [7–9], but high CET efficiency would be realized at high Yb3+ concentration. Thus, a NIR QC via resonant first-order ET process seems more favorable for luminescent materials, which requires an intermediate energy level of donor ion to resonantly excite acceptor ions by a two-step ET process. There are many reports about second-order CET [2, 10–14]; however, there are few reports about first- and second-order ETs occurring simultaneously in one research system.
On the other hand, to fulfill the requirements of NIR QC, host materials should have the energy of phonons as low as possible in order to reduce probabilities of multiphonon relaxations between spaced energy levels of Ln3+ ions. Cubic BaGdF5, a tri-fluoride compound, has a wide bandgap and low phonon energy which is a suitable NIR QC matrix . Meanwhile, Ho3+ ion has favorable metastable energy levels and considerable energy match between Yb3+ and Ho3+, so Ho3+/Yb3+ couple could be a good choice in NIR QC investigation. However, NIR QC reports in Ho3+/Yb3+-codoped materials are limited in NaYF4 and glass ceramic [7, 9]. In addition, considering NIR nanomaterials is convenient in the practical application of the coating for the solar cells. Herein, we prepared BaGdF5: Ho3+, Yb3+ nanoparticles with different Yb3+ concentrations by a trisodium citrate (Cit3−)-assisted hydrothermal method, which is less finicky, low-cost, and effective for large-scale production. Furthermore, NIR QC via first- and second-order resonant ET processes and a back ET from Yb3+ to Ho3+ in BaGdF5: Ho3+, Yb3+ nanoparticles is firstly investigated, and the corresponding quantum efficiencies (QE) are also calculated.
BaGd1 − 0.01 − x%Ybx%Ho0.01 F5 (0 ≤ x ≤ 20) samples were prepared by a hydrothermal process. Firstly, 1 mmol rare earth oxides Gd2O3, Yb2O3, and Ho2O3 were dissolved in dilute HNO3 solution, and the residual HNO3 was removed by heating and evaporation, resulting in the formation of clear solution of RE(NO3)3 (RE = Gd, Yb, Ho). Cit3− aqueous solution was added into the Ba(NO3)2·2H2O and RE(NO3)3 solution to form metal-Cit complex. After vigorous stirring for 30 min, aqueous solution containing 4 mmol NaBF4 was poured into the above solution, and pH value of the mixture was adjusted to about 4.5 by adding diluted HCl or NH3·H2O. After additional agitation for 15 min, the as-obtained mixed solution was transferred into a 50-ml teflon autoclave, which was tightly sealed and maintained at 180°C for 24 h. As the autoclave was cooled to room temperature naturally, the resulting precipitates were separated via centrifugation, dried in oven at 80°C for 12 h.
The XRD patterns were obtained on Rigaku D/max-2400 powder diffractometer (Rigaku Corporation, Tokyo, Japan) using Cu Kα radiation (1.5405 Å) at 40 kV and 60 mA. The size, shape, and structure of the as-prepared samples were characterized by SEM (S-4800). Emission and excitation measurements were performed using an Edinburgh Instruments' FLS920 fluorescence spectrometer (Livingston, UK), and a 0.3-m double excitation monochromator and two emission monochromators to record the emission spectra in the wavelength range of 200–850 nm (with a Hamamatsu R928 photomultiplier tube, Bridgewater, NJ, USA) or in the wavelength range of 850–1650 nm (with a liquid nitrogen-cooled Hamamatsu R5509-72 PMT). All spectra were measured at room temperature.
Results and discussion
The fitting results of parameters of the double-exponential decays
Yb3+ concentration (mol%)
486 nm (5 F3 → 5I8)
545 nm (5S2 → 5I8)
651 nm (5 F5 → 5I8)
Also, the corresponding lifetime values as well as ηCET are calculated, which are summarized in the inset of Figure 5a. Unexpectedly, ηCET is calculated to be as high as 92% in the BaGdF5: 1% Ho3+, 15% Yb3+. In this case, the number of the remaining photons relaxed to the lower energy levels than 5 F3 to give other emission is very small. Besides, double-exponential decay curves of Ho3+: 5 F4, 5S2 → 5I8 emission at 545 nm as well as 5 F5 → 5I8 emission at 651 nm with logarithmic coordinates are plotted with various Yb3+ concentrations in Figure 5b, c, and the highest resonant ET efficiencies for CR1(ηCR1) and CR2(ηCR2) are calculated to be 71% and 69%, respectively, which are also efficient. All the above results just indicate that the Yb3+: 2 F5/2 → 2 F7/2 emission should demonstrate stronger intensity and higher quenching concentration than those of Ho3+: 5I6 → 5I8 emission.
However, the NIR emission spectra in Figure 3 show distinct results. Based on these experimental results and combining the energy levels of Yb3+ and Ho3+, we brought up a novelty back ET process from Yb3+ to Ho3+ (Yb3+(2 F5/2) + Ho3+(5I8) → Yb3+(2 F7/2) + Ho3+(5I6) + hν) which may be occurring in the NIR QC system, as shown in the Figure 5d, since the back ET phenomenon widely exists in UC for Yb3+, Ho3+-doped materials [20, 21]. This ET not only increases the Ho3+: 5 F5 → 5I8 emission intensity but also reduces the Yb3+: 2 F5/2 → 2 F7/2 emission intensity, resulting in the spectral features in Figure 3.
where the last two terms stand for the extra QEs for CTE and CR1 processes, respectively. The total ηQE for BaGdF5: 15% Yb3+, 1% Ho3+ is calculated to 192% by this formulation. This so high QE partly results from the Ho3+5I65I8 emission (1,180 nm), which is useless to enhance the efficiency of Si solar cells, since this emission does not match the spectral response of Si solar cells.
Unlike the common situation that two emitting photons are from the acceptor Yb3+ ions, both the donor (Ho3+) and the acceptor (Yb3+) could emit NIR photons under blue light excitation. The visible emissions decrease with the introduction of the Yb3+ ions, while the NIR emissions at 980 nm and 1,180 nm are greatly enhanced. The quenching concentration of Ho3+ is higher than that of Yb3+. The fluorescence decay lifetimes of 5 F3 → 5I8, 5 F4, 5S2 → 5I8, and 5 F5 → 5I8 emissions of Ho3+ donors were recorded and calculated as a function of Yb3+ concentration. It could be concluded that NIR emissions are mainly through second- and first-order ET processes: Ho3+(5 F3) → 2Yb3+(2 F5/2) + hν, Ho3+(5S2) + Yb3+(2 F7/2) → Ho3+(5I6) + Yb3+(2 F5/2) + hν by spectra and decay curve analysis. The corresponding QE are calculated to be 192% in BaGdF5: 1% Ho3+, 15% Yb3+, so BaGdF5: Ho3+, Yb3+ nanoparticles could open up an approach in designing ultra-efficient photonic devices, for the application in low bandgap solar cells and thermo-photovoltaic energy conversion, etc.
LG, JZ, YzW, and PD are all Ph.D. candidates, and YhW is a Distinguished Young Scholar at the Department of Materials Science, School of Physical Science and Technology, Lanzhou University.
This work is supported by the National Science Foundation for Distinguished Young Scholars (no. 50925206). We express thanks to professor Xueyuan Chen and doctor Haomiao Zhu of the Fujian Institute of Research on the Structure of Matter for their assistance in measuring the luminescence spectra.
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