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Energy transfer mechanisms in Tb3+, Yb3+ codoped Y2O3 downconversion phosphor

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2008 J. Phys. D: Appl. Phys. 41 105406

(http://iopscience.iop.org/0022-3727/41/10/105406)

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IOP PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 41 (2008) 105406 (6pp) doi:10.1088/0022-3727/41/10/105406

Energy transfer mechanisms in Tb3+, Yb3+

codoped Y2O3 downconversion phosphorJun-Lin Yuan1,3, Xiao-Yan Zeng2,3, Jing-Tai Zhao1,4 Zhi-Jun Zhang1,3,Hao-Hong Chen1 and Xin-Xin Yang1

1 Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050,People’s Republic of China2 Institute of Solid State Physics, Chinese Academy of Sciences, PO Box 1129, Hefei 230031,People’s Republic of China3 Graduate School of Chinese Academy of Sciences, Beijing, People’s Republic of China

E-mail: jtzhao@mail.sic.ac.cn (J-T Zhao)

Received 4 December 2007, in final form 28 February 2008Published 25 April 2008Online at stacks.iop.org/JPhysD/41/105406

AbstractTb3+ and Yb3+ co-activated luminescent material that can cut one photon of around 483 nminto two NIR photons of around 1000 nm could be used as a downconversion luminescentconvertor in front of crystalline silicon solar cell panels to reduce thermalization loss of thesolar cell. The Tb3+ → Yb3+ energy transfer mechanisms in the UV–blue region in Y2O3

phosphor were studied by PL excitation spectra and time-resolved luminescence, from whichthe charge transfer mechanism and the cooperative transfer mechanism were identified. Tb3+

ions in the 4f75d1 state relax down to the 5D4 level and cooperatively transfer energy to twoYb3+ ions, which is followed by the emission of two photons (λ ∼ 1000 nm). It was found inthe (Y0.79Tb0.01Yb0.20)2O3 sample that 37% of the Tb3+ ions at the 5D4 level transfer energy totwo neighbouring Yb3+ ions by the cooperative energy transfer mechanism Tb3+

(5D4) → 2Yb3+ (2F5/2). Unfortunately, the high Yb3+ concentration leads to severeconcentration quenching that significantly reduces the external quantum efficiency. Moreover,the energy of the Tb3+ 4f75d1 state can also be lost non-radiatively or transferred to the Yb3+

2F5/2 state via the charge transfer state Tb4+–Yb2+. In conclusion, RE3+ (RE = Ce, Pr, Tb)with strong absorption in the UV region is not an appropriate sensitizer of Tb3+ in Tb3+–Yb3+

codoped downconversion phosphor.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

One major energy loss in Si solar cells is thermalization,which is expected to be considerably reduced if the absorbedUV/blue photon (λ < 500 nm) is cut into two near-infrared (NIR) photons that can be absorbed by Si (λabs <

1100 nm) [1] This scheme could be realized by using a rareearth doped quantum cutting phosphor as a downconversionconvertor in front of solar cell panels [2]. However, mostof the works on the quantum cutting phosphor are focusedon Pr3+ or Gd3+ containing phosphors that can only beefficiently excited by VUV photons (λ = 125–215 nm)[3,4]. Recently, a new quantum cutting phenomenon involving

4 Author to whom any correspondence should be addressed.

efficient cooperative energy transfer (η ∼ 190%) of Tb3+

(5D4) → 2Yb3+ (2F5/2) has been recognized, in which theabsorption of one blue photon of around 483 nm by Tb3+

results in the emission of two Yb3+ atoms (2F5/2–2F7/2)

of around 1000 nm, which is above the absorption edge ofcrystalline Si [2]. Hence, the Tb3+–Yb3+ activator couplewould be ideal for the downconversion luminescent convertorof crystalline Si solar cells. Cooperative energy transfer Tb3+

(5D4) → 2Yb3+ (2F5/2) was observed in various systems,such as KYb(WO4)2 : Tb3+ [5], Tb3+–Yb3+ codoped silicaglass [6], (Yb,Y)PO4 : Tb3+ [7], (Yb,Gd)BO3 : Tb3+ [8] and(Yb,Gd)Al3(BO3)4 : Tb3+ [9].

However, these previous reports only discussed the co-operative transfer behaviour under fixed excitation

0022-3727/08/105406+06$30.00 1 © 2008 IOP Publishing Ltd Printed in the UK

J. Phys. D: Appl. Phys. 41 (2008) 105406 J-L Yuan et al

wavelengths, such as 308 nm [5], 360 nm [6] and 489 nm[7], which were insufficient for revealing the full picture ofTb3+ → Yb3+ energy transfer mechanisms. Moreover, Tb3+

features weak absorption in the UV/blue region due to the for-bidden nature of 4f–4f transitions, and the absorption of Tb3+

could be enhanced by 102 times by its parity-allowed 4f–5dtransitions. Motivated by the deeper understanding of theTb3+ → Yb3+ energy transfer and the designing of Tb3+−Yb3+

codoped phosphors with high quantum efficiency and strongabsorption in the UV/blue region, both static PL excitationspectra and time-resolved luminescence techniques were ap-plied in the Tb3+, Yb3+ codoped Y2O3 phosphor, from whicha competing energy transfer path, the charge transfer mech-anism, was identified and systematically compared with thecooperative energy transfer.

2. Experimental

Powder samples of (Y0.99−xYbxTb0.01)2O3 (x = 0, 0.02,0.05, 0.10, 0.15, 0.20) were prepared by co-precipitationand subsequent calcinations. Rare earth nitrate solutions(0.2 mol L−1) that were obtained by dissolving rare earthoxides (Y2O3, Yb2O3 and Tb4O7, 99.99%) in a hot HNO3

solution were mixed and added into excess NH4HCO3 solution(1 mol L−1) slowly under vigorous stirring for 1 h. Theprecipitates were filtrated, dried at 120 ◦C and heated at1000 ◦C for 8 h in the CO atmosphere. The sampleswere characterized by XRD (Huber Imaging Plate GuinierCamera G670, Cu Kα1, Ge monochromator) and FE-SEM(Sirion 200). Luminescent spectra were obtained from theJobin–Yvon Fluorolog-3-tau system that was equipped witha 450 W Xe lamp and a 50 W flash lamp for steady-stateand time-resolved measurements, respectively. Visual andNIR photons were recorded by a Hamamatsu R928 andan R5509 PMT, respectively. The excitation spectra werecorrected according to the system spectra response. All themeasurements were carried out at room temperature.

3. Results and discussions

3.1. Experimental results

The single phase of cubic Y2O3 was checked by powder XRD(figure 1(a)), whereas a slight lattice shrinkage was observedwith increasing Yb content due to the smaller size of Yb3+

than Y3+. But such a minute structural difference could notbring about a significant change to the optical properties. The(Y0.99−xYbxTb0.01)2O3 phosphor particles are irregular with adiameter of 5–10 µm (figure 1(b)).

Under the excitation of UV photons (λ = 304 nm),both visual emissions of Tb3+ 5D4–7FJ (J = 6, 5, 4, 3, 2)transitions and NIR emissions within 950–1100 nm, whichcorrespond to transitions between Stark levels of 2F5/2 and2F7/2 of Yb3+, were observed (figure 2(a)). As the Yb3+ contentincreases, Tb3+ emissions decrease steadily, whereas Yb3+

emission first reaches a maximum at a concentration of 5% andthen gradually decreases due to the concentration quenchingeffect (figure 2(b)). The excitation spectra of Tb3+ consistof strong spin-allowed (SA) 4f–5d bands within 250–335 nm,

Figure 1. (a) Typical powder XRD pattern of(Y0.99−xYbxTb0.01)2O3 phosphors. (b) The FE-SEM image ofpowder samples.

weak spin-forbidden (SF) 4f–5d bands within 335–368 nm and7F6–5DJ (J = 3, 4) transition peaks at around 376 nm and483 nm, of which the intensities reduced monotonically withincreasing Yb3+content. The intensities of 7F6–5DJ (J = 3, 4)peaks are about 140 times weaker than that of the 4f–5d (SA)bands, due to the fact that the 4f–4f transitions are both parityand SF. It is expected that the stronger absorption of Tb3+ in theUV region due to 4f–5d (SA) transition would lead to the higherpumping rate of the 5D4 level of Tb3+ ions and enhancementof the Tb–Yb quantum cutting emission.

According to [10], the charge transfer band O2−–Yb3+ inY2O3 locates at a relatively high energy region (λ < 250 nm);thus, the observed intense UV-excited NIR emissions canonly originate from the energy transfer from Tb3+ to Yb3+,which is justified by the excitation spectra of Yb3+ 2F5/2–2F7/2

emission (figure 2(d)) that shows principally the same featuresas that of Tb3+: broad 4f–5d bands within 250–368 nm andweak 4f–4f peaks at 376 nm and 483 nm. Supposing that theTb → Yb energy transfer only occurs at the 5D4 level andthere is no alternative Tb → Yb energy transfer path, theexcitation spectra of Yb3+ NIR emission should be of the sameshape as that of Tb3+ emission, and the excitation efficiencyat 483 nm and 376 nm should be 102 times weaker than that at304 nm. Apparently, the postulation is not true according tothe observed excitation spectra of Yb3+ NIR emission. TheTb3+4f–4f peak at 483 nm due to 7F6–5D4 absorption is afactor of 103–104 times weaker than that of the SA band ataround 304 nm. The weak absorption at 483 nm corresponds

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J. Phys. D: Appl. Phys. 41 (2008) 105406 J-L Yuan et al

Figure 2. (a) Typical UV-excited emission spectrum of Y2O3 : Yb3+, Tb3+ phosphors. (b) Visual and NIR emission intensities of Tb3+ andYb3+ as a function of the Yb3+ concentration. (c) Excitation spectra monitoring the Tb3+ 5D4–7F5 emission at 542 nm. (d) Excitation spectraof Yb3+ in Y2O3 : Yb3+, Tb3+ phosphors monitoring the Yb3+ NIR emission at 980 nm.

to Tb3+ (5D4) → 2Yb3+ (2F5/2) cooperative energy transfer,and the strong absorption band in the UV region correspondsto not only the desired Tb3+ (4f75d1) → Tb3+(5D4) → 2Yb3+

(2F5/2) cooperative energy transfer path but also the moreefficient Tb3+(4f75d1) → Yb3+ (2F5/2) competing energytransfer path. The rate of the latter path is 10–102 timesfaster than the rate of intra-ion Tb3+(4f75d1) → Tb3+(5D4)

relaxation. As we see later, the Tb3+(4f75d1) → Yb3+ (2F5/2)

competing transfer path originates from the charge transferinteraction between Tb3+ and Yb3+; hence the mechanism isnamed as the charge transfer mechanism, which was illustratedtogether with the cooperative energy transfer mechanism infigure 3. Possible energy transfer paths in Tb and Yb codopedY2O3 phosphors are shown in figure 4.

3.2. Discussions

Let us first consider the cooperative energy transfer mechanism(figures 3(a) and 4). After the absorption of UV photons,Tb3+ ions are excited up to its 4f75d1 state, from which Tb3+

ions undergo fast intra-ion thermal relaxation down to the

emitting 5D4 level, which is followed by spontaneous 5D4–7FJ

(J = 0−6) emission, cooperative Tb3+(5D4) → 2Yb3+ (2F5/2)

transfer and deactivation by quench centres. Clearly, the Tb3+

4f–5d absorption significantly accelerates the pumping rateof the Tb3+ 5D4 level (rex) and subsequently enhances thecooperative energy transfer to Yb3+ ions. Due to the second-order nature the cooperative energy transfer rates (rCoopT) area factor of 104–108 times smaller than the typical valuesreported for first-order energy transfer rates. Nonetheless,it is still comparable to the spontaneous emission rates (rR)

of Tb3+ and Yb3+(rCoopT ∼ 3500 s−1 in YbPO4 [7]). In therandom uncorrelated process of substituting Yb3+ ions forY3+ in the Y2O3 lattice, a single cooperative acceptor of theYb–Yb type is formed when two Yb3+ ions simultaneously fallwithin the nearest neighbouring coordination sphere of Tb3+.Hence, the start point of observing notable cooperative Tb3+

(5D4) → 2Yb3+ (2F5/2) energy transfer corresponds to theYb3+ content of 16.7%, which is verified in the luminescencedecay curves of Tb3+ emission shown in figure 5.

As shown in figure 5, the decay profile of Tb3+

5D4–7F5 emission in singly doped (Y0.99Tb0.01)2O3 sample

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J. Phys. D: Appl. Phys. 41 (2008) 105406 J-L Yuan et al

Figure 3. Tb3+ → Yb3+ energy transfer mechanisms.(a) Cooperative transfer mechanism Tb3+(5D4) → 2Yb3+ (2F5/2).(b) Configurational coordinate model of luminescence quenchingand Tb3+(4f75d1) → Yb3+ (2F5/2) energy transfer. For displayclarity, the configurational diagrams of Tb and Yb are illustratedseparately. Black solid parabolas are ground states; red dottedparabolas are excited states. The charge transfer state Tb4+–Yb2+ isrepresented by a blue dashed parabola. Vertical solid arrows areoptical transitions, and blue dotted arrows are thermal crossingbetween different states. (Colour online.)

Figure 4. Possible energy transfer paths and different rates inTb–Yb codoped Y2O3 system, in which rex, rCoopT, rCharT rCharT, rR,rD and rNR are the pump rate of Tb3+ 5D4 level, the cooperativetransfer rate, the charge transfer rate, the spontaneous emission rate,the energy diffusion rate between identical ions and thenon-radiative rate, respectively.

is non-exponential, which originates from fast Tb–Tb energydiffusion in the Y2O3 matrix (rD1 � rR1) and energy loss toquench centres, such as Tb4+ centres or defects at the gainsurface. As the content of Yb3+, x, increases from 0 to0.10, there is no apparent change in the decay profile, which

Figure 5. Normalized time-resolved luminescence of Tb3+ 5D4–7F5

emission at 542 nm for various concentrations of Yb3+ under theexcitation of 304 nm. The inset shows the upper limited efficiency(ηM) of cooperative transfer as a function of Yb3+ contents.

suggests that the Tb3+(5D4) → 2Yb3+ (2F5/2) cooperativeenergy transfer is not notable in weakly doped samples witha small possibility of finding a Yb–Yb pair in the nearestneighbouring sphere of the Tb3+ dopant. As x reaches 0.15and 0.20, faster decay curves were observed, which is anindication of fast Tb3+(5D4) → 2Yb3+(2F5/2) cooperativetransfer. The cooperative transfer efficiency (ηCoopT) andtheoretical quantum efficiency (ηth) could be estimated fromequations (1) and (2) [7],

ηCoopT = rCoopT

rR1 + rNR1 + rCoopT<

rCoopT

rR1 + rCoopT

= 1 −∫ ∞

0 Ix(t) dt∫ ∞

0 I0(t) dt= ηM, (1)

ηth = ηTb(1 − ηCoopT) + 2ηYbηCoopT, (2)

where rCoopT, rR1 and rNR1 are the cooperative transfer rate, thespontaneous emission rate and the non-radiative deactivate rateof Tb3+ at the 5D4 level, respectively. By ignoring the quenchprocess, taking the upper limited value of the cooperativeenergy transfer efficiency (ηM) as the approximation ofTb3+(5D4) → 2Yb3+(2F5/2) cooperative transfer ηCoopT) andsetting the quantum efficiencies of Tb3+ and Yb3+ (ηTb and ηYb)as 100%, the upper limited value of the theoretical quantumefficiency (ηth) is calculated to be 103%, 104%, 109%, 130%and 137% for samples with the x value equal to 0.02, 0.05,0.10, 0.15 and 0.20, respectively.

Next let us consider the charge transfer mechanism.Blasse and Grabmaier concluded that the luminescencequenching of a combination of a centre that has a tendencyto become oxidized (Ce3+, Pr3+, Tb3+) with a centre that hasa tendency to become reduced (Eu3+, Yb3+) originates fromthe charge transfer state RE4+ − RE2+ [11–14]. The formationof the charge transfer state involves both photoionization ofTb3+ (Tb3+ → O2−) and electron trapping by Yb3+ (O2− →Yb3+). Besides, the change in the cation valences leads toa significant change in the cation radii (92.3 pm/76 pm forTb3+/Tb4+; 86.8 pm/102 pm for Yb3+/Yb2+) [15] which leadsto a large relaxation space. Consequently, the charge transfer

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J. Phys. D: Appl. Phys. 41 (2008) 105406 J-L Yuan et al

state Tb4+–Yb2+ lies at an energy higher than the 4f75d1 stateof Tb3+ and the O2−–Yb3+ charge transfer state with a largeoffset in the configurational coordinate diagram (figure 3(b)),which leads to another energy loss path of the Tb3+ 4f75d1

state: the charge transfer mechanism.In the configurational coordinate diagram, the crossing

between the Tb4+–Yb2+ parabola and the parabolas of the Tb3+

4f75d1, Tb3+ 7FJ and Yb3+ 2FJ (J = 5/2, 7/2) states indicatesthe pumping of the Tb4+–Yb2+ state by the Tb3+ 4f75d1 state,the pumping of the Yb3+ 2F5/2 state by the Tb4+–Yb2+ state andthe quenching of the Tb4+–Yb2+ state by both the Tb3+ 7FJ andthe Yb3+ 2F7/2 states. The pumping of the Yb3+ 2F5/2 state isfollowed by 2F5/2–2F7/2 emission (λ ∼ 1000 nm). However,no quantum cutting occurs in the Tb3+ (4f75d1) → Yb3+

(2F5/2) charge transfer process, during which luminescencequenching in the Tb–Yb combination is remarkable and only afractional energy from the Tb3+ 4f75d1 state is finally releasedas NIR photons (4f–4f emission in Yb3+).

As for the luminescence efficiency in the Tb3+ (4f75d1) →Yb3+ (2F5/2) process, we are unable to provide a furtherquantitative analysis because the spectra in the visual part andthe NIR part were measured with different photon detectorsand it is difficult to quantitatively compare the efficienciesof Tb3+ emission and Yb3+ emission under UV excitation.Being a first-order interaction, the Tb3+ (4f75d1) → Yb3+

(2F5/2) charge transfer rate (rCharT) is comparable to thatof intra-ion relaxation Tb3+ (4f75d1) → Tb3+ (5D3, 5D4)

(>108 s−1). According to Dexter’s theory of energy transfervia exchange or multi-polar interactions [16], the transfer fromdonor to acceptors in the nearest neighbouring sphere alwaysdominates. In (Y0.99−xYbxTb0.01)2O3 each doped Tb3+ has12 rare earth sites in the nearest neighbouring sphere withthe inter-cation distance within 3.51–4.00 Å. The probabilityof finding one acceptor (Yb3+) in the nearest neighbouringsphere of Tb3+ corresponds to the Yb3+ content of 8.33%.However, concentration quenching takes effect as the Yb3+

content exceeds 5% (figure 2(b)) [17].It should be noted that in real applications of using Tb3+

and Yb3+ codoped luminescent materials as downconversionconvertors in front of the silicon solar cell panel, absorption,the non-radiative process and the competing energy transferpath should be taken into consideration, and a high externalquantum efficiency ηext is required. According to figure 4,ηext could be expressed as

ηext = αrex

rex + rNR3 + rCharT[ηTb(1 − ηCoopT) + 2ηYbηCoopT]

+ αrCharT

rex + rNR3 + rCharTηYb (3)

in which α is the absorption coefficient of the sensitizer (inthis work, Tb3+ itself), rex is the pumping rate of the Tb3+

5D4 level, rNR3 is the non-radiative energy transfer rate tothe Tb–Yb pair, rCharT is the Tb3+ (4f75d1) → Yb3+(2F5/2)

charge transfer rate and ηTb and ηYb are the emission quantumefficiencies of Tb3+ and Yb3+, respectively. From equation(3), it is obvious that increasing the values of α, rex, ηCoopT,ηTb and ηYb and decreasing the value of rNR3 and rCharT arebeneficial for obtaining a high ηext value. In the case of Tb3+,Yb3+ codoped Y2O3, ηext is far from 137% due to two facts:

rex is much smaller than (rNR3 + rCharT) and ηYb is far from100% at high Yb concentrations [17]. In designing an efficientTb–Yb codoped downconversion luminescent material, anappropriate sensitizer of Tb3+ (5D4) and host should fulfilthe following requirements: firstly, the sensitizer should havestrong absorption in the UV/blue region and only transferenergy to Tb3+ rather than to Yb3+ ions; secondly, rare earth(Y, La, Gd or Lu) should be the constituent of the host for highconcentration doping of Tb3+ and Yb3+; thirdly, Yb3+ ions inthe host should have a weak concentration quenching effect.Clearly, RE3+ (RE = Ce, Pr, Tb) are not a good choice assensitizers due to their inevitable luminescence quenching andRE3+ (5d) → Yb3+ (2F5/2) energy loss in RE–Yb pairs.

4. Conclusion

In summary, in the downconversion phosphor of Tb3+,Yb3+ codoped Y2O3, two energy transfer mechanismsof Tb3+ → Yb3+ in the UV/blue region were studiedby photoluminescence excitation spectra and time-resolvedluminescence techniques. Tb3+ ions can be effectively excitedup to its 4f75d1 state and relaxed down to the 5D4 level,from which the energy is transferred cooperatively to twoneighbouring Yb3+. However, the cooperative energy transferis weak compared with the competing charge transfer energytransfer mechanism from Tb3+ in the 4f75d1 state directlyto Yb3+. In designing a Tb–Yb codoped downconversionluminescent convertor for crystalline Si solar cells, a bettersensitizer of Tb3+ should be found so that energy cannot betransferred to Yb3+ ions and the concentration quenching effectof Yb3+ should be reduced to enhance Yb3+ emission.

Acknowledgments

This work was supported by the Key Project (50332050) fromthe NNSF of China, the Hundred Talents Program from theChinese Academy of Sciences, the Fund for Young LeadingResearchers from Shanghai municipal government and the973 Project (2007CB936704) for the Ministry of Science andTechnology of China.

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