Highly efficient energy transfer in Pr3�, Yb3� codopedCaF2 for luminescent solar converters

Diana Serrano, Alain Braud,* Jean-Louis Doualan, Patrice Camy, and Richard Moncorgé

Centre de Recherche sur les Ions, les Matériaux et la Photonique (CIMAP), UMR 6252 CNRS-CEA-ENSICAEN-Universitéde Caen, 6 Boulevard Maréchal Juin, 14050 Caen, France

*Corresponding author: alain.braud@ensicaen.fr

Received March 29, 2011; revised May 27, 2011; accepted May 31, 2011;posted June 2, 2011 (Doc. ID 145014); published June 29, 2011

The codoping of CaF2 : Pr3þ with Yb3þ ions is shown to lead to the formation of Pr3þ=Yb3þ clusters, which can beattractive quantum cutting systems to enhance solar cells’ efficiency. Very high Pr3þ to Yb3þ energy transfer effi-ciencies (ETEs) are achieved for low Yb3þ and Pr3þ concentrations (ETE ¼ 97% in CaF2 : 0:5%Pr3þ-1%Yb3þ) con-firming the short distance between Pr3þ and Yb3þ ions within clusters. A low Yb3þ concentration offers theadvantage of drastically limiting the Yb3þ concentration quenching usually observed in other hosts where theYb3þ concentration has to be larger to achieve a high ETE for solar cell applications. © 2011 Optical Societyof America

OCIS codes: 160.5690, 260.2160, 260.3800, 160.2540.

1. INTRODUCTIONThe necessary improvement of silicon solar cell performancehas led to extensive research activity over the past few years[1]. Two types of significant losses limiting Si solar cell effi-ciency can be identified [2]. The first one is carrier thermali-zation following the absorption of above-bandgap photons,and the second type of losses is due to the semiconductortransparency for subbandgap photons. The process knownas quantum cutting, through which a high-energy photon isconverted into two lower energy photons, is a potential solu-tion for reducing carrier thermalization losses in photovoltaicdevices [3]. In this study, we investigated a promising quantumcutting system with CaF2 samples codoped with Pr3þ andYb3þ ions. Praseodymium ions, when excited by blue photonsinto the 3PJ (J ¼ 0, 1, 2), 1I6 levels, relax nonradiatively to the3P0 level and then can interact with Yb3þ ions through twosequential resonant energy transfers [4], the first one fromPr3þ (3P0 →

1G4) to Yb3þ (2F7=2 →2F5=2) and the second one

from Pr3þ (1G4 →3H4) to Yb3þ (2F7=2 →

2F5=2). Figure 1 de-picts the two energy transfer processes. As a result, the ab-sorption of a single high-energy photon in the 3PJ levels canlead, in an ideal case, to the excitation of two Yb3þ ions andeventually to the emission of two near-IR photons. Efficientenergy transfer from Pr3þ to Yb3þ ions for the first step ofthe quantum cutting process have been reported in differenthost lattices [4–6]. However, in most cases, efficiency energytransfer requires a large concentration of Yb3þ ions which, inturn, leads to energy migration among Yb3þ ions and to a con-siderable quenching of the Yb3þ luminescence. We presenthere a very efficient energy transfer from Pr3þ to Yb3þ ionsin CaF2 for a limited concentration of Yb3þ ions. Energy trans-fer efficiency (ETE) as high as 97% is obtained for the firstPr3þ to Yb3þ energy transfer with a Yb3þ concentration of only1 at:%. It is worth noting that this first energy transfer by itselfmakes the Pr3þ, Yb3þ codoping a quantum cutting system,since following this energy transfer one can obtain one ex-

cited Pr3þ ion into the 1G4 level and one excited Yb3þ ion.Therefore, one incident photon will effectively lead to theemission of two IR photons. However, the 1G4 emission spec-trum is primarily located in the IR region, and the correspond-ing photons will not be simply absorbed by a Si solar cell. Thisis the reason why a second energy transfer from Pr3þ to Yb3þ

is required in order to ensure the emission of a second photonthat will match the Si absorption spectrum.

Regarding host materials, fluorides, in general, are interest-ing because of their low maximum phonon energy, which re-duces multiphonon relaxations. In particular, the doping withtrivalent rare-earth ions of fluorides having a fluorite struc-ture, such as CaF2, SrF2, and BaF2 implies a charge compen-sation within the host to maintain the electrical neutrality ofthe system, which leads to the formation of rare-earth clusters[7–9]. The short distance between ions within rare-earth clus-ters, combined with the host’s low phonon energy, enable veryefficient energy transfers [10], and therefore make CaF2 :Pr3þ-Yb3þ an attractive system.

2. EXPERIMENTALA series of CaF2 codoped Pr3þ (0:5 at:%) and Yb3þ (0%; 0.5%;1%; 2%; 4%) bulk crystals of about 1:5 cm diameter and 3 cmlength were grown in our laboratory using a standard Bridg-man technique with RF heating and CF4 þ Ar atmosphere. Amixture of pure CaF2, PrF3, and YbF3 powders was first intro-duced in a graphite crucible within the growth chamber. Areasonable vacuum (<10−5 mbar) was achieved before intro-ducing Ar and CF4 gases to reduce oxygen and water pollu-tion. The crystal growth was carried out with a pulling rate of4–5mm=h, and crystals were finally cooled down to roomtemperature within 24h. The dopant concentrations in thecrystals have been assessed by absorption measurementsusing calibrated reference samples. The real concentrationsare very close to the nominal concentrations in the melt. Thediscrepancy is typically around 0:02 at:% for both Pr3þ and

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0740-3224/11/071760-06$15.00/0 © 2011 Optical Society of America

Yb3þ, meaning that a 1% nominal concentration leads to a realconcentration of 0:98 at:%. The only exception is the 4% Yb3þ

concentration, which is measured to be 3.5%. Powder sampleswere afterwards prepared from bulk crystals for the differentexperiments. Pr3þ emission spectra were recorded under cwlaser diode excitation at 440 nm or Ar laser at 457nm. Thelight emitted by the different samples was dispersed by a0:5m monochromator and detected using a standard lock-in amplifier technique associated with a photomultiplier tubeor an InGaAs photodiode for the signal detection. Emissiondecays and time-resolved spectra were carried out using anoptical parametric oscillator pumped by the third harmonic(355nm) of an Nd:YAG laser. After dispersion by a 0:25mmonochromator, the luminescent transient signals obtainedafter pulsed excitation were fed into a digital oscilloscopeand averaged out to improve the signal-to-noise ratio.

3. RESULTS AND DISCUSSIONRoom temperature Pr3þ emission spectra in the 590–650nmrange were recorded under blue light excitation at 440nm(3H4 →

3P2) for all samples. Pr3þ emissions can be observedaround 605nm and 640 nm in Fig. 2. The peaks around 640 nmcan be ascribed without any doubt to the 3P0 →

3F2 transition.Although the luminescence decays in this spectral region aredifferent from one sample to another, the same luminescencedecay is observed both around 605 nm (from 600 to 610nm)and around 640nm in each sample whether they are codopedwith Pr3þ and Yb3þ ions or singly dopedwith Pr3þ ions (Fig. 3).In consequence, for all samples, the peaks around 605nmclearly correspond to the 3P0 →

3H6 transition and not to the1D2 →

3H4 transition, which can also appear around 600nm.Furthermore, the 1D2 level decay recorded at 828nm (1D2 →3H6) under direct excitation at 590nm (3H4 →

1D2) is clearlydifferent from the 3P0 level recorded at 640 and 605nm as

illustrated in Fig. 3 for CaF2 : 0:5%Pr3þ and CaF2 : 0:5%Pr3þ-4%Yb3þ. The observation of a significant emission from 1D2

under 3P2 excitation around 440nm can be expected inmaterials having a large maximum phonon energy [11](700 cm−1 andmore), in which case the excited Pr3þ ions relaxnonradiatively by multiphonon emission from the 3P0 leveldown to the 1D2 level located around 4000 cm−1 below the3P0 level. In fluoride materials, the maximum phonon energyis rather small (around 450 cm−1), making the multiphononrelaxation from 3P0 to 1D2 very unlikely.

Figure 4 shows the emission of Yb3þ ions under Pr3þ exci-tation at 440 nm, illustrating the energy transfer between Pr3þ

and Yb3þ ions in samples having the same 0.5% Pr3þ concen-tration. A direct comparison of the Yb3þ emission intensityfrom one sample to another can be performed in Fig. 4(a),since the excitation and detection geometry was kept identi-cal for all samples that were prepared as powders with thesame geometry from bulk crystals. The Yb3þ intensity in-creases up to 2% Yb3þ, showing that more and more Yb3þ ionsare excited by means of the Pr3þ → Yb3þ energy transfer.However, the Yb3þ intensity starts to decrease when the Yb3þ

concentration reaches 4%. In most materials, this concentra-tion quenching of the Yb3þ emission can be explained by en-ergy migration among Yb3þ ions and subsequent energytransfer to defects or other impurities in the host, whichact as quenching centers. However, in the specific case ofCaF2, energy migration among Yb3þ ions is rather limiteddue to the formation of Yb3þ clusters instead of a random dis-tribution of Yb3þ ions [12]. The Yb3þ quenching observed inFig. 4 is most likely due to backtransfer from Yb3þ

(2F5=2 →2F7=2) to Pr3þ (3H4 →

1G4). Figure 4(b) presents theemission spectra of the different samples normalized to theshoulder at 1040nm. One can notice that the shape of theYb3þ emission spectrum changes as the main emission peakat 980 nm tends to decrease with the Yb3þ concentration. Thiseffect, which becomes stronger when increasing the Yb3þ con-centration, is also often observed and is due to reabsorption ofemitted photons [9].

Fig. 1. Pr3þ and Yb3þ ion energy level schemes. Quantum cuttingtakes place by two consecutive energy transfers, (1) and (2). Solid,dotted, and curved arrows represent optical transitions, energy trans-fer processes, and nonradiative relaxations, respectively.

Fig. 2. Room temperature Pr3þ emission spectra under blue excita-tion at 440nm (3P2) in CaF2 : 0:5%Pr3þ-x%Yb3þ (x ¼ 0, 0.5, 1, 2, 4).

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The 3P0 luminescence decays under blue excitation are pre-sented in Fig. 5 for all samples having the same 0.5% Pr3þ con-centration. The 3P0 decay becomes shorter and shorter as theYb3þ concentration is raised because of the Pr3þ → Yb3þ en-ergy transfer. The efficiency of the Pr3þ → Yb3þ energy trans-fer appears to be remarkable, since the 3P0 lifetime decreasesby a factor of 60 from 150 μs for a singly Pr3þ-doped CaF2

sample down to 2:5 μs with only 4% Yb3þ in CaF2 : 0:5%Pr3þ-4%Yb3þ. For comparison, a decrease by a factor of 30 of the3P0 lifetime because of Pr3þ → Yb3þ energy transfers [13] isobserved with KY3F10 codoped with 0.5% Pr3þ and 20%Yb3þ, a fluoride material that is characterized, unlike CaF2,

by a random distribution of dopants. Thus, Pr3þ → Yb3þ en-ergy transfers in KY3F10 are very efficient, but they are morethan two times less efficient than in CaF2 : 0:5%Pr3þ-4%Yb3þ.This unusual efficiency of Pr3þ to Yb3þ energy transfer in CaF2

is due to the rare-earth clustering effect taking place in thishost, enabling a very short distance between rare-earth do-pants. The 3P0 decay in CaF2 : 0:5%Pr3þ-4%Yb3þ is exponen-tial (Fig. 5) while the other decays for the Pr3þ, Yb3þ codopedsamples clearly exhibit nonexponential features. As will bediscussed further in the text, this nonexponential characterof the decays is due to the presence of two Pr3þ centers. Thesetwo Pr3þ centers already appear in the monotonic change

Fig. 3. (Color online) 3P0 decay curves recorded at 605nm (3P0 →3H6) and 640nm (3P0 →

3F2) with λexc ¼ 440nm and 1D2 decay curve recordedat 828nm (1D2 →

3H6) with λexc ¼ 590nm : a) CaF2 : 0:5%Pr3þ, and b) CaF2 : 0:5%Pr3þ-4%Yb3þ.

Fig. 4. (Color online) Ytterbium emission (2F5=2 →2F7=2) under Pr3þ excitation at 440nm (3P2) in CaF2 : 0:5%Pr3þ-x%Yb3þ (x ¼ 0, 0.5, 1, 2, 4).

(a) Relative Yb3þ intensities of all samples recorded in the same conditions, (b) normalized emission spectra illustrating the effect of photonreabsorption.

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observed in the shape of the Pr3þ emission spectra with theYb3þ concentration in Fig. 2. The first center, in which theemission spectrum appears for singly doped Pr3þ samples(x ¼ 0% in Fig. 2), corresponds to “isolated” Pr3þ ions, inother words, Pr3þ ions noncoupled to Yb3þ ions. When codop-ing with Yb3þ ions, a change in the emission spectrum isclearly seen for the 3P0 →

3H6 transition, with new emissionpeaks appearing at 600nm and 604nm, and can also be ob-served for the 3P0 →

3F2 transition. This new Pr3þ emissionspectrum emerging in addition to the first spectrum is likelydue to the incorporation of Yb3þ ions in the vicinity of Pr3þ

ions, forming Pr3þ=Yb3þ clusters. As the Yb3þ concentrationincreases, the Pr3þ=Yb3þ cluster emission spectrum becomesmore and more prominent, and, for a Yb3þ concentrationequal to 4 at:%, the Pr3þ=Yb3þ cluster emission spectrum dom-

inates. This result shows that when the Yb3þ concentrationreaches 4 at:%, the emission spectrum related to noncoupledPr3þ ions can no longer be observed, meaning that the major-ity of Pr3þ ions are coupled to Yb3þ ions.

Another piece of evidence that most Pr3þ ions are withinPr3þ=Yb3þ clusters for an Yb3þ concentration of 4% is foundwhen recording the emission from the 1G4 level. Figure 6shows the Pr3þ emission spectrum around 1:3 μm (1G4 →

3H5

transition) obtained under direct Pr3þ excitation at 457nm(3H4 →

3P1) and under Yb3þ excitation at 920 nm in CaF2 :0:5%Pr3þ-0:5%Yb3þ and CaF2 : 0:5%Pr3þ-4%Yb3þ samples.The direct Pr3þ excitation excites all Pr3þ ions whether theyare coupled to Yb3þ ions or not. On the contrary, the Yb3þ

excitation only leads to emission from Pr3þ ions that arecoupled to Yb3þ ions and therefore excited by energy transferfrom Yb3þ to Pr3þ ions. Interestingly, in CaF2 : 0:5%Pr3þ-0:5%Yb3þ, the Pr3þ emission spectrum [Fig. 6(a)] is differentunder direct Pr3þ excitation and under Yb3þ excitation, indi-cating that some Pr3þ ions are effectively coupled to Yb3þ ionswhile some Pr3þ ions remain isolated. In contrast, for CaF2 :0:5%Pr3þ-4%Yb3þ both spectra are identical [Fig. 6(b)], mean-ing that the same Pr3þ ions are excited when directly excitingall Pr3þ ions or going through Yb3þ ions. In other words, thisresult shows again that the majority of Pr3þ ions is coupled toYb3þ ions in CaF2 : 0:5%Pr3þ-4%Yb3þ.

In a remarkable way, time-resolved spectroscopy experi-ments enable us to clearly dissociate contributions from non-coupled Pr3þ ions and Pr3þ=Yb3þ clusters in samples whereboth species coexist. The nonexponential features of the3P0 decays observed in Fig. 5 for codoped samples (exceptCaF2 : 0:5%Pr3þ-4%Yb3þ) consist of two components: a veryfast component dominating in the first microseconds and aslow component. Time-resolved spectra were recorded withinthe corresponding time windows as displayed in Fig. 7 forCaF2 : 0:5%Pr3þ-2%Yb3þ. For the fast component [Fig. 7(a)],the resulting spectrum recorded within the first 10 μs is thePr3þ=Yb3þ cluster emission spectrum observed with x ¼ 4%

Fig. 5. (Color online) 2P0 decay curves recorded at 607nm(3P0 →

3H6) in CaF2 : 0:5%Pr3þ-x%Yb3þ (x ¼ 0, 0.5, 1, 2, 4) under blueexcitation at 440nm (3P2). For singly Pr3þ-doped CaF2, one can noticea slight nonexponential feature at the beginning of the decay, which islikely due to Pr3þ clusters in which cross-relaxation processes amongPr3þ ions are very efficient, thus reducing drastically the 3P0 lifetime.

Fig. 6. (Color online) Room temperature Pr3þ emission spectra around 1:3 μm (1G4 →3H5) under Pr3þ excitation at 457nm (3P1) and under Yb3þ

excitation in (a) CaF2 : 0:5%Pr3þ-0:5%Yb3þ and (b) CaF2 : 0:5%Pr3þ-4%Yb3þ. It is noteworthy that the Pr3þ luminescence under Yb3þ excitationillustrates the existence of a back-transfer from Yb3þ (2F5=2 →

2F7=2) to Pr3þ (3H4 →1G4).

Serrano et al. Vol. 28, No. 7 / July 2011 / J. Opt. Soc. Am. B 1763

in Fig. 2 with no visible contribution from the noncoupled Pr3þ

ions emission spectrum. On the contrary, for the long part ofthe decay [Fig. 7(b)], the noncoupled Pr3þ emission spectrumis solely observed and is identical to x ¼ 0% in Fig. 2. The veryrapid 3P0 decay associated with Pr3þ=Yb3þ clusters indicates avery efficient energy transfer from Pr3þ to Yb3þ ions withinthe clusters, while the long 3P0 decay of noncoupled Pr3þ ionsshows that their coupling with Yb3þ ions is very weak.

Experimental decays are accurately adjusted with a doubleexponential fit to determine the values of the fast and slowdecay components (Fig. 7). The 3P0 lifetime for Pr3þ=Yb3þ

clusters derived from the fast decay component (Table 1)makes it possible to quantify the ETE from Pr3þ (3P0 →1G4) to Yb3þ (2F7=2 →

2F5=2) within the clusters. The ETE isdefined as the ratio between the energy transfer rate andthe total relaxation rate comprising the energy transfer rate,the radiative decay rate, and the multiphonon relaxation rate.The ETE can thus be expressed as a function of the Yb3þ

concentration,

ηx%Yb ¼ τ−1x%Yb − τ−10%Yb

τ−1x%Yb

¼ 1 −τx%Yb

τ0%Yb; ð1Þ

where τ0%Yb and τx%Yb are the 3P0 lifetimes without any Yb3þ

and with x%Yb3þ, respectively.Equation (1) does not take into account a possible back-

transfer from Yb3þ (2F5=2 →2F7=2) to Pr3þ (3H4 →

1G4). This

backtransfer should be taken into consideration in a specificETE calculation for the second step of the quantum cuttingprocess, Pr3þ (1G4 →

3H4) to Yb3þ (2F7=2 →2F5=2).

ETE values obtained with Eq. (1) and displayed in Table 1show a very efficient energy transfer from Pr3þ (3P0) to Yb3þ

(2F5=2) in CaF2 for low Yb3þ concentrations (ETE ¼ 94% forCaF2 : 0:5%Pr3þ-0:5%Yb3þ, for instance) clearly demonstrat-ing the clustering of Pr3þ and Yb3þ ions occurring even at lowYb3þ concentrations. Moreover, the 3P0 lifetime within Pr3þ=Yb3þ clusters decreases with the Yb3þ concentration and, as aresult, the corresponding ETE increases. This result is likelydue to the energy transfer from a given Pr3þ ion to not only theclosest Yb3þ ion within the cluster, but also to Yb3þ ions pos-sibly in the second nearest neighbor shell.

The high ETE values obtained in CaF2 : Pr3þ-Yb3þ can beachieved in other hosts, but a much higher Yb3þ concentrationis required. Using the same calculation as for CaF2, we derivedan ETE value [13] of 97% for KY3F10 : 0:5%Pr3þ-20%Yb3þ,while the same ETE is achieved in CaF2 with only 1% Yb3þ

(Table 1). ETEs have also been calculated in other hosts usingEq. (1). ETE in doubly Pr3þ- and Yb3þ-doped lanthanum bor-ogermanate glasses [5] reaches 65% with 20% Yb3þ. Anotherhost of interest is SrF2, which shares strong similarities withCaF2, both hosts having a fluorite structure and a comparablePr3þ 3P0 lifetime {τðCaF2Þ ¼ 150 μs in Fig. 4 and τðSrF2Þ ¼148 μs in [6]}. The clustering of Pr3þ ions with Yb3þ ions ap-pears to be more pronounced in CaF2, which exhibits strongerETE. An efficiency of 77% was reported [6] for SrF2 :

0:1%Pr3þ-5%Yb3þ, while it reaches 98.5% for a similar Yb3þ

concentration in CaF2 : 0:5%Pr3þ-4%Yb3þ (Table 1). The mainadvantage of this high ETE observed in CaF2 is that it limitsthe need for a high Yb3þ concentration. The well-known con-centration quenching that impairs the Yb3þ luminescence is anefficient process [14] that can be greatly reduced at low Yb3þ

concentrations. This concentration quenching is limited inCaF2, since a rather small Yb3þ concentration is required andalso, as mentioned previously, the energy migration amongYb3þ ions is reduced because of Yb3þ clustering. The mainYb3þ quenching process is the backtransfer from Yb3þ

(2F5=2 →2F7=2) to Pr3þ (3H4 →

1G4). This backtransfer, whichis already revealed in Fig. 6, directly affects the ETE of thesecond step of the quantum cutting process Pr3þ (1G4 →

3H4);Yb3þ (2F7=2 →

2F5=2). The 1G4 level being almost at the sameenergy as the 2F5=2 level, energy transfer will take place inboth directions from Pr3þ to Yb3þ and from Yb3þ to Pr3þ.The backtransfer from Yb3þ to Pr3þ has been investigatedin order to use Yb3þ ions as sensitizers to explore a possibleupconversion avalanche mechanism toward 3P0 [15]. Upcon-version by avalanche makes use not only of energy transferprocesses but, more importantly, of an efficient excited stateabsorption transition, which is not the case for quantum cut-ting. Therefore, it is difficult to compare a quantum cuttingprocess with an avalanche process. To our knowledge, thecoupling between the 1G4 and 2F5=2 levels in the quantum cut-ting effect has not yet be taken into account in detail in theliterature for the determination of Pr3þ, Yb3þ quantum cuttingefficiency. A detailed modelling comprising all the relevant en-ergy transfer between Pr3þ and Yb3þ ions is in progress in thepromising case of CaF2 : Pr3þ-Yb3þ.

Fig. 7. (Color online) 3P0 decay curve recorded at 607nm(3P0 →

3H6) with λexc ¼ 440nm in CaF2 : 0:5%Pr3þ-2%Yb3þ. Solidline, biexponential fit of the decay. Insets: time-resolved spectra for(a) first 10 μs and (b) long part of the decay (t > 10 μs).

Table 1. 3P0 Lifetime and ETE as a Function of Yb3�Concentrationa

CaF2 : 0:5%Pr -x%Yb 0% Yb 0.5% Yb 1% Yb 2% Yb 4% Yb

τ3P0ðμsÞ (lc) 150 120 105 75 -τ3P0ðμsÞ (fc) - 8.5 4.5 3.7 2.4ETE (%) - 94.3 97.0 97.5 98.4

a(lc) and (fc) stand for fast component and long component, respec-tively. The decrease of the decay long component with the Yb3þ con-centration shows that energy transfer, although not very efficient,takes place between “isolated” Pr3þ ions and Yb3þ ions.

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4. CONCLUSIONIn conclusion, the study of CaF2 : Pr3þ-Yb3þ shows very effi-cient energy transfer between Pr3þ and Yb3þ ions under blueexcitation in comparison with other hosts. This strong cou-pling takes place within Pr3þ=Yb3þ clusters, which give riseto ETEs as high as 97% with only 1% Yb3þ in CaF2 : 0:5%Pr3þ

-1%Yb3þ. At low Yb3þ concentration, however, some Yb3þ

ions remain isolated, and they all become coupled toYb3þ ions when the Yb3þ concentration reaches 4 at:%. Thisclustering effect limits the need for a high Yb3þ concentrationand therefore reduces considerably the Yb3þ concentrationquenching usually observed.

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