Journal of Luminescence 132 (2012) 2341–2344

Contents lists available at SciVerse ScienceDirect

Journal of Luminescence

0022-23

http://d

n Corr

E-m

journal homepage: www.elsevier.com/locate/jlumin

Intense 974 nm emission from ErxYb2�xSi2O7 films through efficient energytransfer up-conversion from Er3þ to Yb3þ for Si solar cell

Jun Zheng n, Yeliao Tao, Wei Wang, Zhihua Ma, Yuhua Zuo, Buwen Cheng, Qiming Wang

State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, People’s Republic of China

a r t i c l e i n f o

Article history:

Received 18 April 2011

Received in revised form

4 April 2012

Accepted 16 April 2012Available online 23 April 2012

Keywords:

Erbium–ytterbium disilicate

Up-conversion

Si solar cell

13/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.jlumin.2012.04.015

esponding author. Tel.: þ86 10 82304522; fa

ail address: zhengjun@semi.ac.cn (J. Zheng).

a b s t r a c t

The optical properties of the ErxYb2�xSi2O7 thin films were investigated by photoluminescence

measurements and the intense 974 nm light emission was observed. The 974 nm emission was mainly

from the transition 2F5/2 to 2F7/2 level of Yb3þ upon exploring energy-transfer via up-conversion at Er3þ

4I13/2 level. Under 972 nm excitation, the lifetime at Er3þ 4I13/2 level reaches up to 4 ms for film

containing 2 at% Er3þ , while decreases to about 20 ms as the film is pumped by 488 nm. This confirmed

that the energy transfer up-conversion process was the dominant transition at Er3þ 4I13/2 level. This

may be of interest to improve the solar cells0 efficiency by placing this film at the rear of cell, converting

the near-infrared photons between 1480 nm and 1580 nm to just above the Si bandgap.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

Nowadays the most widely used materials in solar cells arebased on crystalline Si and the efficiency of best conventionalsingle junction Si solar cell is 25%, [1] close to the theoretic limit.The transmission of sub-band-gap light is one of the major lossmechanisms in Si solar cells. One promising approach to reducethis loss is proposed by placing an up-conversion (UC) layer at therear of Si solar cells [2,3], with no additional changes made on Sisolar cell. Thus, the application of UC layer has great advantagesover multi-junction solar cell, such as avoiding complex epitaxialgrowth, current matching problem. Trupke et al. estimated thatthe Schockley–Queisser limit efficiency for a single junction solarcell could rise to 50% by using up-conversion materials [2].

Up-conversion material used for Si solar cell performs thesequential absorption of two low-energy infrared photons followedby emission of a high energy photon (above the Si bandgap). Er3þ

ions can absorb two infrared photons in 1480 nm–1580 nm andemit one 980 nm photon through up-conversion process at 4I13/2

level. B. Henke et al. have experimentally demonstrated the feasi-bility of this concept by placing the erbium-doped fluorozirconateglass on a Si solar cell [4].

For efficient UC process, a very short distance between donor andacceptor is one of the key points. Therefore, high concentrationof rare earth ions is needed. Usually, the doping concentration isvery low because of the constraint of ‘‘concentration quenching’’.Erbium compounds, like erbium silicate, where the number of

ll rights reserved.

x: þ86 10 82305052.

optically active Er exceeds 1022/cm3, 100 times higher than dopedmaterials, have attracted much interest recently [5–7]. The980 nm emission was easily observed through the UC process inthese materials with the lifetime at 4I13/2 level only several 10 ms,due to the shortened Er–Er distance [8]. Moreover, fabricationmethods for rare earth silicate can be quite diverse, such assol–gel and sputtering [6,7].

Yb3þ has one order larger emission cross-section at 980 nmthan Er3þ . And Er/Yb codoped fiber amplifiers are widely appliedin market due to the efficient energy transfer from Yb3þ to Er3þ

at 980 nm. Here we propose a UC system containing Er3þ andYb3þ , where the Er3þ acts as a sensitizer and Yb3þ acts as anactivator. ErxYb2�xSi2O7 thin films, with high concentration ofrare earth ions, were fabricated by the sputtering method. Intenseand efficient 980 nm emission via energy transfer UC processbetween Er3þ and Yb3þ was achieved.

2. Experimental

The ErxYb2�xSi2O7 thin films were fabricated in a vacuumchamber by using KJLC Lab 18 magnetron sputtering system. Allthe depositions were performed in an Ar/O2 atmosphere at 3 mTorr(the Ar/O2 ratio is 10), by confocal sputtering from Er2O3, Yb2O3

and SiO2 targets. The films were deposited on thermally oxidizedn-Si (1 0 0) substrates heated at 400 1C and rotated at 20 rpm.The components of ErxYb2�xSi2O7 films were varied by applyingdifferent rf power (between 20 W and 100 W) to the target. Afterdeposition, films were thermally treated at 1100 1C for 1 h inO2 ambient atmosphere.

Fig. 2. Visible PL spectra of ErxYb2�xSi2O7 films excited by 488 nm laser.

J. Zheng et al. / Journal of Luminescence 132 (2012) 2341–23442342

The thickness of as-deposited films was between 80 nm and90 nm, measured by ellipsometer, equipped with a 632.8 nmlaser. The components of as-deposited films were carried out byRutherford backscattering spectrometry (RBS) using 2.022 MeV4He ions beam at a scattering angle of 1651 and Auger electronspectroscopy. The sum of rare earth (RE), Si and O atomicconcentration were the same in all as-deposited samples, about17%, 18% and 65%, respectively, corresponding to stoichiometricRE2Si2O7. The Er fraction was evaluated to be 270.2 at%, 3 at%and 4.5 at% for film containing Yb and Er.

Crystalline structures were studied by X-ray diffraction (XRD)measurements with a Cu Ka radiation. Room temperature photo-luminescence (PL) measurements were performed by using488 nm line of an Arþ laser. The pump power was 20 mW witha spot of several 10 mm in diameter. The visible PL spectrum wascollected by a single grating monochromator and a Si detectorworked at �651. The infrared PL signal was processed by anothersingle grating monochromator and a liquid-nitrogen cooled InGaAsdetector. Time-resolved luminescence measurements were carriedout by detecting the modulated luminescence signal with aphotomultiplier tube. The tunable laser was chopped at a fre-quency of 10 Hz, with a diameter of the facula of about 1 mm.

Fig. 3. Near-infrared PL spectra of ErxYb2�xSi2O7 films pumped by 488 nm laser.

The solid star indicates the Yb3þ transitions.

3. Results and discussion

Fig. 1 shows the XRD patterns of the annealed samples. The peakpositions of a-Er2Si2O7 phase at 30.01 [8,9] and (1 2 0) plane ofYb2Si2O7 [10] at 30.21 are well indicated. We can observe similarspectra from all samples. For erbium disilicate film, the main peak isat 30.01, well fitted with others’ works [9,11]. For Er–Yb disilicatefilms, the main peaks are between 30.01 and 30.21 and do notexhibit a doublet. This indicates the formation of a homogeneousEr–Yb solid solution in a single RE2Si2O7 crystalline structure.

The optical properties of ErxYb2�xSi2O7 films between 515 nmand 900 nm have been studied by PL measurements. Fig. 2 showsthe PL spectra of the films with different Er fraction pumped by488 nm Arþ laser, resonant with the Er3þ 4F7/2 level. Three PLemission peaks, centered at 550 nm, 660 nm and 850 nm, can bewell distinguished in the ErxYb2�xSi2O7 films. They are associatedwith the radiative transitions of Er3þ 4S3/2, 4F9/2 states tothe ground state and 4S3/2 state to 4I13/2 state, respectively.For different Er concentrations, the PL spectra exhibiting the same

Fig. 1. XRD spectra of annealed ErxYb2�xSi2O7 films. The full squares and full

triangle indicate the diffraction peaks associated to the Er2Si2O7 and Yb2Si2O7,

respectively.

shape and fine structure are observed, confirming that the crystallinefield around Er3þ ions is similar in all ErxYb2�xSi2O7 films.

As shown in Fig. 2, the PL intensities of all emission peaksincreased with increasing Er3þ amount. In fact, with the increas-ing Er3þ concentration, the Er–Er interaction can be enhanced bythe reduction of Er–Er distance. Therefore, two kinds of nonra-diative decay need to be considered, that is cross relaxation andup-conversion process. The former process is the energy transferbetween one excited Er3þ and another one at the ground state. Asa result, two Er3þ ions are excited to the low energy level. Thus,the intensity of visible emission peaks is reduced with increasingEr3þ concentration. The latter one is the energy transfer betweentwo excited Er3þ and the emission of photons having wavelengthshorter than the incident ones. Therefore, the intensity of visibleemission is enhanced. As shown in Fig. 2, by increasing the Er3þ

concentration, both green and red emission intensities areenhanced, suggesting that up-conversion process may be higherthan cross relaxation process in these high Er compounds, whichwas previously reported in Y2O3:Er3þ ,Yb3þ microspheres [12].

Fig. 3 shows the emission band at 974 nm and 1528 nm (assignedto the 4I13/2-

4I15/2 transition of Er3þ) pumped by 488 nm Arþ laser.The shape of the PL spectrum near 1528 nm is identical for different

J. Zheng et al. / Journal of Luminescence 132 (2012) 2341–2344 2343

Er concentrations; similar to those that appeared in visible region.Moreover, the PL intensities at 1528 nm are much weaker than thoseat 974 nm. It is known that in Er-doped oxide material the emissionfrom 4I11/2 state to the ground state at 974 nm is very weak,compared to that from 4I13/2 state level at 1528 nm, due to the rapidmultiphonon relaxation at 4I11/2 level [13]. And 974 nm emissionoccurs through UC process in high Er3þ concentration host, such asErxY2�xSiO5 nanocrystals with Er concentration above 1.5 at% [14].This indicates that UC process at 4I13/2 state should be responsible forthe abnormal high PL intensity at 974 nm.

However, one may think that energy transfer from Er3þ highenergy levels to Yb3þ through down conversion (DC) processshould not be ignored. Indeed, A. Meijerink et al. reported thatthe efficiency of down-conversion process was very low inNaYF4:Er3þ/Yb3þ due to fast multiphonon relaxation from the4F7/2 to the 4S3/2 via the intermediate 2H11/2 level [15]. Since thephonon energies are much higher in silicate (�1200 cm�1) thanthat in fluoride (�250 cm�1), the excitation of Yb3þ 2F5/2 levelthrough down-conversion of Er3þ under 488 nm excitation doesnot likely happen here.

It is worth noting that the 974 nm PL spectral shape for Er3þ at18 at% is very different from others, as shown in Fig. 3. Thisindicates that 974 nm light emission comes from 2F5/2-

7F7/2

transition of Yb3þ . Hehlen et al. reported that the UC coefficientwas proportional to Er content for the UC process at Er3þ 4I13/2

level [16]. Therefore, if the 974 nm emission is from Er3þ 4I11/2

level transition, the Er2Si2O7 film (Er content 18 at%) should havethe highest intensity. Obviously, the 974 nm intensity is not thestrongest among all samples. Moreover, the emission cross-section of Yb3þ at 980 nm is one order larger than that of Er3þ .So we suggest that the strong 974 nm emission is likely from theYb3þ transition, and it well fitted the 974 nm emission shapechange in PL spectra. Together, we deduce that strong 974 nmemission observed in ErxYb2�xSi2O7 films is mainly from theelectronic transition 2F5/2 to 2F7/2 of Yb3þ through energy-transferup-conversion (ETU) process at Er3þ 4I13/2 level [17]. As shown inFig. 4, two Er3þ ions are excited to the 4I13/2 level. Then one Er3þ

transfers energy to the other one with the result that one ionbecomes excited to the 4I9/2 level and the other ion returns to theground state. Subsequently, electrons nonradiatively relax to theYb3þ 2F5/2 level and excite the Yb3þ ions.

Fig. 4. Scheme of the energy transfer up-conversion (ETU) process in Er3þ/Yb3þ

coupled system. Solid arrows represent radiative transitions and dashed arrows

represent nonradiative transitions.

Luminescence decay curves recorded for the emission fromthe 4I13/2 level provided further insight into the ETU process atdifferent Er3þ concentrations. The decay rate of Er3þ at 1532 nmis expressed by 1/tradþ1/tnonrad

þ Aup. Here, the 1/trad and 1/tnonrad

are radiative and non-radiative decay rate, respectively, constantwith regard to the excitation wavelength, Aup is the up-conversionprocess rate, related with different excitation wavelength. Under488 nm excitation, the ETU process is involved in the Aup. On theother hand, when excited by 972 nm laser, the ETU process isneglectable in the Aup term due to the direct excitation of Yb3þ

2F5/2 level. As shown in Fig. 5(a), for film with Er3þ 4.5 at%, thedecay times are 0.4 ms and 23 ms under 974 nm and 488 nmexcitation, respectively. A similar situation was observed in filmswith Er3þ fraction 3 at% and 2 at%, that was 1.42 ms and 3.89 msunder 972 nm excitation, while all decreased to about 20 msunder 488 nm excitation (Figures not shown here). The greatdecrease of lifetime indicates that the ETU process overwhelmsother transition processes in Er3þ 4I13/2 level in the experimentalrange, consistent with strong 980 nm PL intensity in Fig. 3. Inter-estingly, the lifetimes change little and are about 45 ms in Er2Si2O7

film with regard to excitation wavelength (See Fig. 5(b)). This isreasonable since ETU process does not exist in the Er2Si2O7 film.Furthermore, it seems that the ETU process is saturated for Er3þ

content lower than 4.5 at%, which is likely due to the Yb3þ

concentration quenching in ErxYb2�xSi2O7. However, as shownin Fig. 3, the highest 974 nm intensity is found to be at Er3þ

Fig. 5. Decay curves of the films under 488 nm and 972 nm excitation, respec-

tively. Circles: 488 nm excitation; square: 972 nm excitation. (a) Er content

4.5 at%; (b) Er content 18 at%.

J. Zheng et al. / Journal of Luminescence 132 (2012) 2341–23442344

4.5 at%. This is because at lower Er3þ concentrations, the number ofsensitizer is decreased and Yb3þ concentration quenching may takeplace at high Yb3þ content. Hence the maximal 974 nm emission isthe trade-off between sensitizer (Er3þ) and activator (Yb3þ) centers.

4. Conclusions

In summary, we have studied the optical properties of ErxYb2�x-

Si2O7 films with different Er concentrations. The high concentrationof optically active Er3þ facilitates the up-conversion process andintense 974 nm light emission from Yb3þ 2F5/2 level via UC processat Er3þ 4I13/2 level is evidenced. The luminescence decays curvesrecorded by exploring different excitation wavelengths show thatthe desired ETU process exists and is the main transition process atEr3þ 4I13/2 level in the test region. The ETU process is completed forEr3þ content lower than 4.5 at% and the maximal 974 nm emissionintensity is found for Er3þ 4.5 at% in ErxYb2�xSi2O7. Our resultsshow that it is possible to use the Er3þ as sensitizer and Yb3þ asactivator for achieving intense 980 nm emission through UC processin the ErxYb2�xSi2O7 film. This may have promising applications inSi solar cells by up-converting the 1480 nm–1580 nm photons to Siabsorbable photons.

Acknowledgments

This work was supported by the National Basic ResearchProgram of China (973 Program) (no. 2007- CB613404), National

Natural Science Foundation of China under Grant nos. 60906035,51072194 and by State Key Lab on Integrated Optoelectronics, ISDivision.

References

[1] M.A. Green, Prog. Photovoltaics. 17 (3) (2009) 183.[2] T. Trupke, M. Green, P. Wurfel, J. Appl. Phys. 92 (2002) 4117.[3] A. Shalav, B.S. Richards, M.A. Green, Solar Energy Mater. Solar Cells 91 (2007)

829.[4] B. Henke, B. Ahrens, J.A. Johnson, P.T. Miclea, S. Schweizer, Proc. SPIE 7411

(2009) 74110E.[5] M. Miritelloa, R. Lo Savioa, F. Iaconab, G. Franzo’a, C. Bongiornob, A. Irreraa,

F. Priolo, J. Lumin. 121 (2006) 233.[6] X.X. Wang, J.G. Zhang, B.W. Cheng, J.Z. Yu, Q.M. Wang, J. Cryst. Growth. 289

(2006) 178.[7] J. Zheng, W.C. Ding, C.L. Xue, Y.H. Zuo, B.W. Cheng, J.Z. Yu, Q.M. Wang,

G.L. Wang, H.Q. Guo, J. Lumin. 130 (2010) 411.[8] M. Miritello, R. Lo Savio, F. Iacona, G. Franzo, A. Irrera, A.M. Piro, C. Bongiorno,

F. Priolo, Adv. Mater. 19 (2007) 1582.[9] R. Lo Savio, M. Miritello, A.M. Piro, F. Priolo, F. Iacona, Appl. Phys. Lett. 93

(2008) 021919.[10] JCPDS Card no. 30-1439.[11] X.J. Wang, T. Nakajima, H. Isshiki, T. Kimura, Appl. Phys. Lett. 95 (2009)

041906.[12] H. Guo, Y.M. Qiao, Opt. Mater. 31 (2009) 583.[13] L. Fornasiero, K. Petermann, E. Heumann, G. Huber, Opt. Mater. 10 (1998) 9.[14] K. Suh, J.H. Shin, S.J. Seo, Byeong-Soo Bae, Appl. Phys. Lett. 92 (2008) 121910.[15] L. Aarts, B.M. van der Ende, A. Meijerink, J. Appl. Phys. 106 (2009) 023522.[16] M.P. Hehlen, N.J. Cockroft, T.R. Gosnell, A.J. Bruce, G. Nykolak, J. Shmulovich,

Opt. Lett. 22 (1997) 772.[17] F. Auzel, Chem. Rev. 104 (2004) 139.