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Zhuang et al. Vol. 26, No. 11 /November 2009 /J. Opt. Soc. Am. B 2185

Broadband downconversion from oxygen-deficientcenters to Yb3+ in germanate glasses

Yixi Zhuang,1 Yu Teng,1 Jiajia Zhou,1 Song Ye,1 Xiaofeng Liu,2,3 Geng Lin,2,3 Jian Ruan,2,3 and Jianrong Qiu1,*1State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, China

2State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics,Chinese Academy of Sciences, Shanghai 201800, China

3Graduate School of the Chinese Academy of Sciences, Beijing 100039, China*Corresponding author: qjr@zju.edu.cn

Received June 18, 2009; revised September 25, 2009; accepted October 1, 2009;posted October 2, 2009 (Doc. ID 112871); published October 28, 2009

We report broadband downconversion from ultraviolet to near-infrared in Yb3+-doped oxygen-deficientSrO–Al2O3–GeO2 glasses. Oxygen-deficient centers are introduced in germanate glasses by adding metal Alinstead of corresponding oxide �Al2O3�. Tunable luminescence related to these defects is observed. Conversionof broadband ultraviolet light to blue-green tunable luminescence and to near-infrared emission is detected inglasses. The energy transfer efficiency increases with increasing Yb2O3 concentration. The dependence of en-ergy transfer efficiency on wavelength is also discussed. © 2009 Optical Society of America

OCIS codes: 160.2750, 160.2220, 300.6340.

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. INTRODUCTIONn upsurge in the research of efficient utilization of solarnergy has been brought from the viewpoint of the energyrisis [1,2]. One of the attempts is to improve the solar cellfficiency by spectral modulation. In silicon solar cells, aingle electron-hole pair is generated when the incominghoton energy is above 1.1 eV, with the excess energy be-ng lost to heat. The thermalization of charge carriersenerated by the absorption of high-energy photons is onef the major loss mechanisms leading to low energy con-ersion efficiencies of solar cells. The quantum cuttingrocess, which divides a high-energy photon into two orore photons with lower energy, could significantly re-

uce the thermalization loss [3–5].Recently, near-infrared (NIR) quantum cutting via co-

perative downconversion has been widely witnessed inany rare-earth-ion-doped phosphors, glasses, and glass

eramics [6–10]. Tb3+–Yb3+, Tm3+–Yb3+, and Pr3+–Yb3+

ere codoped in those materials where Tb3+, Tm3+, andr3+ act as the absorption centers, and Yb3+ act as accep-ors. However, the absorption bandwidth of rare-earthons is rather narrow due to the nature of f–f transitions,hich may limit the practical applications in solar cell.roadband downconversion shows obvious advantages in

ransferring high-energy photons in a wide wavelengthegion. Some research groups have reported broadbandownconversion based on energy transfer between defi-ient centers or Ce3+ and Yb3+ [11–13]. They focused onaterials whose absorption band is located in the region

f 400 to 500 nm. It should be noted that the band from00 to 400 nm shows low intensity in the AM 1.5 solarpectrum but relatively strong in the AM 0 solar spec-rum. Broad downconversion from UV �300 to 400 nm� toIR shows potential application of solar energy utiliza-

ion especially in an extraterrestrial situation.

0740-3224/09/112185-4/$15.00 © 2

Defects in materials can be controlled, leading to a dra-atic performance improvement over the “ideal” material

14]. We have demonstrated that tunable luminescencean be realized through controlling the formation of oxy-en defects in glasses. Efficient blue, white, and red lumi-escence can be tuned by adjusting the excitation sourcerom 300 nm to 370 nm [15]. Meanwhile, conversion ofear-ultraviolet radiation into visible and NIR emissionshrough energy transfer between silicon–oxygen relatedefects and Yb3+ has also been observed [12,13].In this research, benefits of the tunability of lumines-

ence and application in spectral modulation from defectsere combined, and tunable downconversion from UV toIR was demonstrated.

. EXPERIMENTlasses with compositions of 40SrO–28Al2O3–4Al–0GeO2−xYb2O3 (x=0, 0.5, 1.0, 2.0, 3.0, 4.0, 5.0, and.0 mol. %) and 40SrO–30Al2O3–30GeO2 were preparedy the conventional melt-quenching method. Analyticaleagents of SrCO3, GeO2, Al2O3, Al, and Yb2O3 were useds raw materials. Each batch of about 20 g was mixed ho-ogeneously and filled in a corundum crucible with a lid.he batches were melted at 1600 °C for 1 h in air andast onto a stainless-steel slab. The as-prepared glassesere cut into 10 mm�10 mm�1.0 mm and polished forptical measurements. For the sake of convenience,amples A, B, C, D, E, F, G, and H were used to refer tohose Al-contained glasses doped with 0, 0.5, 1.0, 2.0, 3.0,.0, 5.0, and 6.0 mol. % Yb2O3, respectively (listed inable 1). The one sample free of Al and Yb2O3 wasarked as A0 for comparison with sample A. The excita-

ion, emission spectra, and the fluorescence decay curvesn both the visible and NIR regions were recorded at room

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2186 J. Opt. Soc. Am. B/Vol. 26, No. 11 /November 2009 Zhuang et al.

emperature with a FLS920 fluorescence spectrophotom-ter (Edinburgh Instrument Ltd., UK).

. RESULTS AND DISCUSSIONo obvious visible luminescence was observed in sample0, while intense visible emission was detected in sample(substituting 4 mol Al for 2 mol Al2O3). The deficiency

f oxygen in the glass matrix may be a key point of theppearance of luminescence. It is generally consideredhat Ge-related oxygen defects are the origins of the vis-ble luminescence [15,16].

Figure 1 shows a two-dimensional contour line plot ofuorescence intensity for sample A. Similar plots were ob-ained from samples B–H and are not shown here. Therere two peaks of intensity at coordinates [310 (excitationavelength, nm) 365 (emission wavelength, nm)] and

310, 470]. These peaks correspond to several primaryransitions between energy bands of oxygen defects. Fur-hermore, the emission central wavelength shifts toonger wavelength under longer wavelength excitation,enoted as two dotted lines in the plot.Figure 2 shows the excitation and emission spectra of

amples A, D, and H. Under excitation at 310 nm, two vis-ble emission bands at 380 and 470 nm were observed inll samples. From sample A to D and to H, with more Yb3+

oped in the matrices, these emission bands, especiallyhe 470 nm one, turn weak. It’s believed that the lost en-rgy is transferred to neighboring Yb3+ through downcon-ersion processes. As a result, NIR emission from Yb3+

an be observed in all Yb3+-doped samples. The Yb3+

mission in the NIR region consists of two peaks: a sharpeak at 980 nm and a broader peak at 1030 nm. Theseeaks are attributed to the transitions from the lowesttark level of the 2F7/2 multiplet to two different Stark

evels of the 2F5/2 multiplet, respectively [17]. It should be

Table 1. Al and Yb2O3 Concentrationsin SrO-Al2O3-Al-GeO2-Yb2O3 Glasses

Samples A0 A B C D E F G H

Al/mol% 0 4 4 4 4 4 4 4 4b2O3/mol% 0 0 0.5 1.0 2.0 3.0 4.0 5.0 6.0

ig. 1. (Color online) Contour plot of fluorescence intensity forample A. The dotted lines mark the excitation dependent emis-ion peaks.

oted that the intensity of NIR emission depends on Yb3+

oncentration. The emission intensity rises up as the dop-ng concentration increases and quenches down with too

uch Yb3+. In our case, the most intense NIR emissionas observed in sample D with 2.0 mol. % Yb2O3.The excitation spectra of sample H is depicted in the

eft part of Fig. 2. The spectra monitored at 470 show aarrow band centered at 310 nm. However, a broad bandith FWHM of 80 nm was observed under 980 nm moni-

oring. The result shows that broader absorption band-idth is available in this case, compared with RE3+–Yb3+

RE=Tb, Pr, or Tm) codoped materials.When Ge-related oxygen-deficient centers were intro-

uced in the samples doped with Al, a defect energy-levelystem was established. Several energy bands in the re-ion of 2.5 to 5 eV are attributed to Ge2+ [15,16]. A Ge2+

oordinated with two oxygen atoms can be generatedhen deficiency of oxygen is surrounded or when a nor-al structure is exposed to high energy radiation. Thee2+ deficient center is related to three typical energyands, as shown in the left part of Fig. 3. With an efficientxcitation at 310 nm, intense broad emission centered at65 and 470 nm was generated. When Yb3+ were dopednd sited around oxygen deficient centers energy down-onversion occurred. In the process of second-order energy

ig. 2. (Color online) Left: Excitation spectra of sample H under80 and 470 monitoring. Middle: Visible emission spectra ofample A, D, and H at 310 nm excitation. Right: NIR emissionpectra of sample A, D, and H at 310 nm excitation.

ig. 3. Schematic energy level diagram of defect centers andb3+. Solid, dotted, and short-dotted arrows represent optical

ransitions, nonradiative energy transfer processes, and nonradi-tive relaxation, respectively.

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Zhuang et al. Vol. 26, No. 11 /November 2009 /J. Opt. Soc. Am. B 2187

ransfer, one high-energy photon from an oxygen-deficiententer excites two Yb3+ ions simultaneously. NIR lumi-escence of 980 nm attributed to Yb3+: 2F5/2→ 2F7/2 can bebserved. A schematic energy-level diagram of defect cen-ers and Yb3+ and energy transfer is illuminated in Fig. 3.t should be noted that there are several mechanisms ofecond-order energy transfer; however, cooperative en-rgy transfer is dominant between high-energy and low-nergy photons in a Tb3+–Yb3+ ion couple [18]. It is mostossible that the same mechanism is responsible for thenergy transfer in our case.

Figure 4 depicts the luminescence decay curves of70 nm emission with 310 nm excitation. With the intro-uction of Yb3+, the luminescence decay curves in Fig. 4how nonexponential characteristics, and a mean decayifetime �m is given by Eq. (1):

�m =�t0

�

�I�t�/I0�dt, �1�

here I�t� is the luminescence intensity as a function ofime t and I0 is the maximum of I�t�, which occurs at thenitial time t0. The lifetime of 470 nm luminescence inb3+ free sample is 230 �s, and it decreases down to26 �s when 12 mol. % of Yb3+ is introduced. This can bettributed to a depopulation of the defects excited-statenergy level due to the presence of Yb3+ acceptors as anxtra decay pathway.

The energy-transfer efficiency �ET is defined as the ra-io of donors that are depopulated by energy transfer tohe acceptors over the total number of donors being ex-ited [18]. In our case, oxygen defects act as the donors,nd Yb3+ ions act as the acceptors.

�ET = 1 −�m−xYb

�m−0Yb, �2�

here x represents the Yb3+ concentration. The inset ofig. 4 shows the energy transfer efficiency from defectenters to Yb3+ as a function of doping concentration.rom the chart, energy-transfer efficiency for downcon-ersion rises monotonically with more doped Yb3+.

Samples A and H were chosen to investigate the effectf wavelength of both the ultraviolet excitation and blue-reen emission on the energy-transfer efficiency. Read

ig. 4. (Color online) Luminescence decay curves of 470 nmmission at 310 nm excitation. The inset shows calculated energyransfer efficiency as a function of Yb3+ concentration.

rom Fig. 1, pairs of excitation and emission wavelength,uch as [300, 456], were listed at the bottom of Fig. 5. Lu-inescence decay curves of samples A and H were re-

orded at each pair of excitation and emission. Corre-ponding energy-transfer efficiencies of sample H atifferent test conditions were calculated using Eq. (2) andre charted in Fig. 5. From the chart, the energy-transferfficiency culminates at [310, 470] and descends at longerxcitation and emission wavelength.

From Fig. 2, a broad band centered at 325 nm was ob-erved in the excitation spectra monitored at 980 nm;owever, the most efficient energy transfer occurs at310,470]. Some points can be made from the distinction:i) greater intensity may bring larger energy-transfer ef-ciency. The emission intensity excited by 310 nm isreater than any other excitation, although the emissionenters at 470 nm excited by 310 nm. (ii) Concentrationuenching is neglected in Eq. (2) for calculating thenergy-transfer efficiency. That approximation may bringeviation between energy-transfer efficiency and NIRmission intensity.

. CONCLUSIONSn summary, broadband downconversion from ultravioleto near-infrared in Yb3+-doped oxygen-deficientrO–Al2O3–GeO2 glasses is reported. The oxygen defectsre introduced by adding metal Al instead of correspond-ng oxide �Al2O3�. Broad ultraviolet absorption leads to in-ense blue-green luminescence and NIR emission due toooperative downconversion from oxygen-deficient centerso Yb3+. The energy-transfer efficiency increases with in-reasing Yb2O3 concentration. The energy-transfer effi-iency depends on wavelength and shows a maximum at10 nm excitation. These results show the Yb3+-dopedxygen-deficient glasses may have some potential applica-ion values for the enhancement of photovoltaic conver-ion efficiency of silicon solar cells.

CKNOWLEDGMENTShis work was financially supported by the Nationalatural Science Foundation of China (NSFC) (grants

ig. 5. (Color online) Wavelength-dependent energy transfer ef-ciency of cooperation downconversion in sample H. The x axis isouples of excitation wavelength and corresponding emissionentral wavelength determined from Fig. 1.

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2188 J. Opt. Soc. Am. B/Vol. 26, No. 11 /November 2009 Zhuang et al.

0672087, 50872123, and 50802083), the National Basicesearch Program of China (2006CB806000b), and therogram for Changjiang Scholars and Innovative Re-earch Team in University (IRT0651).

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