CZTS absorber layer for thin film solar cells from electrodeposited metallic stacked precursors (Zn/Cu-Sn)

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ContentslistsavailableatScienceDirect

Applied

Surface

Science

j o ur na l ho me pa g e :w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c

CZTS

absorber

layer

for

thin

ﬁlm

solar

cells

from

electrodeposited

metallic

stacked

precursors

(Zn/Cu-Sn)

M.I.

Khalil

a

_,

_O.

_Atici

a

_,

_A.

_Lucotti

b

_,

_S.

_Binetti

c

_,

_A.

_Le

_Donne

c

_,

_L.

_Magagnin

a,∗

a_Dipartimento_di_Chimica,_Materiali_e_Ing._Chimica_“Giulio_Natta”,_Politecnico_di_Milano,_Via_Mancinelli_7,₂₀₁₃₁_Milano,_Italy

b_Dipartimento_di_Chimica,_Materiali_e_Ing._Chimica_“Giulio_Natta”,_Politecnico_di_Milano,_Piazza_Leonardo_da_Vinci_32,₂₀₁₃₃_Milano,_Italy c_Department_of_Materials_Science_and_Solar_Energy_Research_Centre_(MIB-SOLAR),_University_of_Milano-_Bicocca,_Via_Cozzi_53,₂₀₁₂₅_Milano,_Italy

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received25September2015 Receivedinrevisedform7April2016 Accepted10April2016

Availableonline11April2016 Keywords: Kesterite CZTS Bilayerprecursor Sulfurization Photoluminescenceetc

a

b

s

t

r

a

c

t

Inthepresentwork,Kesterite-Cu2ZnSnS4(CZTS)thinﬁlmsweresuccessfullysynthesizedfromstacked

bilayerprecursor(Zn/Cu-Sn)throughelectrodeposition-annealingroute.Adherentandhomogeneous

Cu-poor,Zn-richstackedmetalCu-Zn-Snprecursorswithdifferentcompositionsweresequentially

elec-trodeposited,intheorderofZn/Cu-SnontoMofoilsubstrates.Subsequently,stackedlayersweresoft

annealedat350◦Cfor20mininﬂowingN2atmosphereinordertoimproveintermixingoftheelements.

Then,sulfurizationwascompletedat585◦_C_for₁₅_min_in_elemental_sulfur_environment_in_a_quartz_tube

furnacewithN2atmosphere.Morphological,compositionalandstructuralpropertiesoftheﬁlmswere

investigatedusingSEM,EDSandXRDmethods.Ramanspectroscopywithtwodifferentexcitationlines

(514.5and785nm),hasbeencarriedoutonthesulfurizedﬁlmsinordertofullycharacterizetheCZTS

phase.Higherexcitationwavelengthshowedmoresecondaryphases,butwithlowintensities.Glow

dis-chargeopticalemissionspectroscopy(GDOES)hasalsobeenperformedonﬁlmsshowingwellformed

KesteriteCZTSalongtheﬁlmthicknessascompositionsoftheelementsdonotchangealongthe

thick-ness.InordertoinvestigatetheelectronicstructureoftheCZTS,Photoluminescence(PL)spectroscopy

hasbeencarriedoutontheﬁlms,whoseresultsmatchedupwiththeliteratures.

1. Introduction

Thin film solar cells have been attracted steady by many researchersduringlast20yearsandconsideredasanalternative technologytoSi-basedsolarcellsduetotheirlowcostandalmost compatibleconversionefficiencies.MarketshareofthinfilmPV solarcellsarearound10%whereCdTeandCuIn(Ga)(Se)2(CIGS) aredominantasabsorberlayers[1].However,In,GaandTearenot earthabundant,whileontheotherhandCdandSearebothtoxic elements.Alternatively,Cu2ZnSnS4(CZTS)isapromisingmaterial duetothefactthatZnandSnarecheap,non-toxicandearth abun-dantelements.Furthermore,CZTSisap-typesemiconductorwith directbandgapenergycloseto1.5eVandalargeabsorption coef-ficientover104_cm−1_as_well_as_theoretical_conversion_efficiency of32.2%whichmakeitsuitableamongallthethinfilmsolarcells [2,3].

∗ Correspondingauthor.

E-mailaddresses:[email protected](M.I.Khalil),

[email protected](L.Magagnin).

Severalmethods have beenusedtoprepareCZTS thin films suchas sputtering, evaporation,spray pyrolysis,solution-based methods,electrodeposition method,etc.Thehighest conversion efficiency was achieved for sulfo-selenide thin films (CZTSSe) (␩=12.6%)byWangetal.usinghydrazine-basedsolutionprocess [4].However,incaseofpuresulfideCZTSthinfilms,highest9.2% powerconversionefficiencywasdemonstratedbyKatoetal.by usingevaporation-annealingroute[5].Amongdifferentdeposition techniques,electrodepositionisoneofthepromisingmethodsfor preparingCZTSabsorberfilmsbecauseofitslowcostequipment’s, largescaleproductionandgoodcontroloverthecompositionand morphology. Generally, electrodeposition-based process can be dividedintotwocategories.Theformeristwo-stepsprocesswhere electrodepositionofCu-Zn-Snmetalprecursorsisfollowedbya sul-furizationprocess.Thelatterissingle-stepco-electrodepositionof Cu,Zn,SnandSdirectlytoformCZTScompound.However,itisvery difficulttodeposithomogenousandgood crystallineCZTSfilms fromsingle-stepelectrodepositionprocessduetothelarge reduc-tionpotentialwindowamongtheelements.Electrodepositionof Cu-Zn-Sncanbedoneintwo ways;(i)simultaneous electrode-positionof Cu-Zn-Sn fromsingle electrolyte and (ii)sequential electrodepositionofCu-Zn-Snfromdifferentelectrolyteswith var-http://dx.doi.org/10.1016/j.apsusc.2016.04.062

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Fig.1.Schematictemperatureproﬁleforsoftannealingandsulfurizationstep.

iousstackingorderslikeSn/Zn/Cu,Zn/Sn/Cu,Zn/Cu-Sn,etc.Again, itisdifﬁculttoobtainhomogenousandcompactCu-Zn-Sn metal-licprecursorsthroughsimultaneouselectrodepositionfromsingle electrolyteduetothelargereductionpotentialgapsamongthe metalions.Forthisreason,sequentialelectrodepositionof Cu-Zn-Snhasbeenpreferredbymanyresearchers.

Several groups attempted to fabricate CZTS thin films over thepastfewyearsbyelectrodeposition-annealingroute.In2008, Scraggetal.firstreportedsequentialelectrodepositionofmetal precursorsintheorderZn/Sn/CuonMo/SLGsubstrate followed bysulfurizationat550◦Cfor2hwith0.8%conversionefficiency [6].In2009,Arakietal.reportedCZTSthinfilmsfrom electrode-posited stacked precursors followed by sulfurization at 600◦C whichexhibited 0.98% conversionefficiency[7]. Againin2009, Arakietal.reportedCZTSthinfilmsfromco-electrodeposited Cu-Zn-Snprecursorsfollowedbysulfurizationat600◦C,showed3.16% conversionefficiency[8].In2009,Ennaouietal.fabricatedCZTS thinfilmsfromco-electrodepositedCu-Zn-Snprecursorsfollowed bysulfurization at550◦C for 2hand achieved 3.4%conversion efficiency[9].Inthisstudy,theyobservedsecondaryphasessuch asZnSinZn-richprecursorand Cu2SnS3 inZn-poorprecursors. In 2010, Scragg et al. reported sequential electrodeposition of Cu/Zn/Sn/CuonMocoatedSLGprecursorsfollowedby sulfuriza-tionat575◦Cfor2handachieved3.2%conversionefficiency[10].

In2012,Ahmedetal.showedsubstantialgrowthonconversion effi-ciencyofCZTSthinfilmssolarcellsbyelectrodeposition–annealing approach [11].Theyhave achieved7.3%conversion efficiencyof CZTSfromsequentialelectrodepositionofmetallicprecursorsin theorderCu/Zn/SnandCu/Sn/Znwhichwerefollowedby sulfur-izationat585◦Cfor5–15min.Moreover,theysoft-annealedthe precursorsbeforesulfurizationat temperaturesbetween210◦C and350◦C(300◦Cwasfoundthebest)inordertoobtainwellmixed andhomogenousCuSnandCuZnalloys.Asaresult,theydecreased thesulfurizationtimefrom2hto5–15minwithrespecttoprevious studies.In2013,Linetal.showedthemechanisticaspectsof soft-annealingeffectsofelectrodepositedmetallicstackedprecursors with5.6%conversionefficiency[12].Recently,Jiangetal. demon-strated fabricationof CZTS through electrodeposition-annealing routefromstackedmetalprecursorsofCu-Zn-Snwith8%power conversionefficiency[13].Thisisthehighestconversionefficiency of CZTSbyelectrodeposition-annealingapproach tothe bestof ourknowledge.Veryrecentlyalsoan8.2%efficientpureselenide kesterite(CZTSe)thinfilmsolarcellsthrough electrodeposition-annealingroutehavebeendemonstratedbyVauche[14].

Inthispaper,weproposeaCZTSthin-ﬁlmfabricationprocess thatinvolvesco-electrodepositionofCu-SnlayeronMofoil sub-strate[15]andsubsequentZnelectrodepositionontheCu-Sn/Mo layerfromadifferentsolution.ThisstackingorderofZn/Cu-Sn/Mo hasbeenchoseninordertominimizeSnlossduringsulfurization [7,21].Later,electrodepositedmetalstacksweresoft-annealedat 350◦C[12] andthensulfurized at585◦Ctoobtainwell-formed kesteriteCZTS.

2. Experimental

Electrodeposition of precursor layers were performed gal-vanostically in a conventional electrochemical cell assembly at room temperature using an AMEL Potentiostat Model-549. Mo foilsubstrate(200␮mthickfromGoodfellows)withexposedarea 1×1cm2 _was_used_as_a _working_electrode,_Titanium_mesh _was usedasacounterinertelectrode.Theelectrolyteforthe electrode-positionofCu-Sncontains0.026MCopper-sulfate(CuSO4·5H2O) (SigmaAldrich);0.15MTin-sulfate(SnSO4)(Fluka),2MMethane

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Fig.3.XRDof(i)asdepositedCu-Snand(ii)asdepositedZn/Cu-SnonMofoil substrate.

sulfonic acid (CH3SO3H) (SigmaAldrich), 0.01MHydroquinone (C6H4(OH)2) (Carlo Erba Reagents) whereas Znwas electrode-posited fromcommercialelectrolyte bath (Technochimicas.r.l). pHoftheCu-SnelectrolyteandZnelectrolytewere1.1 and5.5 respectively.ThedesiredcompositionofCu-Sn(Cu:Sn52:48wt%; ±2%)wasobtainedusingcurrentdensity2.5mA/cm2_._The_desired thicknessoftheﬁlmwas500nm(±20nm)andwasobtainedby settingthetime at10min.On theotherhand,200nmZnlayer wasdepositedonCu-Snlayerusingcurrentdensity2mA/cm2_with 6mindepositiontimes.Different Cu-poor,Zn-richprecursorsof Cu-Zn-SnwereobtainedwiththehelpofdifferentZndeposition times(5.5min,6min,6.5min). Attheend of electrodeposition, 650–750nmCu-Zn-Snstackshavebeenfabricated.

Afterelectroplating,Cu-Zn-Snprecursorsweresoftannealedat 350◦Cwithrampingrate20◦C/minfor20mininaquartztube fur-nacewiththepresenceofverysmallﬂowofN2 (seeFig.1)[12]. Latersoft-annealedCu-Zn-Snstacksweresulfurizedat585◦Cin 25–30mgelementalsulfuratmospherewiththesameconditions asdescribedinthecaseofthesoftannealingstep.Arampingrate of20◦C/minwaschosenaccordingtoourpreviousstudies[16].

ThecrystallographicphaseofasdepositedCu-Sn,Cu-Zn-Sn,soft annealedCu-Zn-Snand sulfurizedfilmswereanalyzedbyX-ray diffraction(XRD),usingaPhilipsX-pertMPDwithCuK␣=1.5406Å radiation.Thesurfacemorphologyandthecompositionalstudies ofthosefilmswereinvestigatedbyScanningElectronMicroscopy (ModelZeissEVO50)equippedforenergydispersiveX-ray spec-troscopy(EDS)(Oxfordinstrument, Model 7060).Here electron energywassetat20KeVbothforimagingandEDSelemental com-positionsmeasurement.Johnsonetal.reportedrecentlythatwith 15KeVelectronenergy,EDSprobedepthisaround2␮mforCZTS film.Soitisassumedherethatwith20KeVelectronenergy,EDS probedepthwillbemorethan2␮mandcompositionsofentire CZTSfilmwillbemeasured[17].Ramanmeasurementswere car-riedoutinairatroomtemperaturewithaLabramHR800Horiba JobinYvonmicro-Ramanspectrometerinbackscattering configu-ration.ThespectrometerisequippedwithaPeltiercooledCCDand employingtwolasersources,asolid-statelaser(LaserXTRA, Top-ticaPhotonics)operatingat785nm,andanargonionlaser(Stabilite 2017,SpectraPhysics)operatingat514.5nm.Thelaserpower den-sitywaschosentogeneratethebestsignaltonoiseratioinorderto avoidsamplelocalheating.Glowdischargeopticalemission spec-troscopy(GDOES)wasalsocarriedoutbyusingSpectrumAnalytic GmbH(ModelGDA750analyzer).Photoluminescence(PL) spec-traofthefilmshavebeenstudiedat12Ktemperaturewithabove bandgapexcitation (␭exc=805nm).AllPL measurementswere

Table1

Metalcontentandcompositionalratiosofmetalstacks.

SampleNo. Cu(at.%) Zn(at.%) Sn(at.%) Cu/(Zn+Sn) Zn/Sn Zn/Cu Sn/Cu C55 45.8 27.87 26.33 0.84 1.06 0.60 0.57 C60 41.84 32.53 25.63 0.72 1.27 0.77 0.61 C65 39.16 38.86 21.98 0.64 1.76 0.99 0.56

performedwithaspectralresolutionof6.6nmusingastandard

lock-intechniqueinconjunctionwithasinglegrating

monochro-matorandanInGaAsdetector.

3. Resultsanddiscussions

DuetothehighconductivityofCuand itscomparativeinert

natureduringdepositionofothermetalsonit,Cuhasbeen

cho-senasanunder-layerforthefabricationofKesteritesbymostof

theresearcherswhenstackingorderofprecursors(Cu-Zn-Sn)are

considered[18].Asaresult,Zn/SnlayerorSn/Znlayerareused

todepositontopofCu.Inordertoovercometheissuesofmetal exchangeandlayerstrippingduringsequentialelectrodeposition, Mo/Cu/Sn/Znsequencehasbeenchosenbyalmostallresearchers forthefabricationofstackedlayerprecursorforKesterite.Highest efﬁcientKesteritebasedthinﬁlmsolarcellshavebeenfabricated fromthissequencealso[18].Ithasbeenobservedthatthelossof Znduringsulfurizationisinevitableasitdoesnotdependwhether Znlayerisdepositedinthemiddleoratthetopofthestacking layer[19].Hencemoreemphasizehasbeengivenheretominimize thelossSnduringsulfurization.Ontheotherhand,morphology ofSnisquiteporouswithrespecttoothermetalswhichleadsto non-homogeneousnucleationofZnifZnisdepositedontopofSn. UniformityoftheZnlayerislargelydependentontheuniformity oftheunderlyingSnlayer.Asaconsequence,inordertoovercome allthose issues binaryCu-Snlayerhasbeenchosen hereasan under-layer.ThepossibilityofSnlossduringsulfurizationisalso minimizedwhenSnisco-depositedwithCuintheunder-layer.

Fig. 2 shows the SEM images of as deposited Cu-Sn (a),as depositedZn/Cu-Sn(b)andsoftannealedZn/Cu-Sn(c)onMofoil substrate,respectively.Itcanbeseenthatboththeasdeposited layersareadherenttothesubstrate,showdensepackagingand homogeneousdistributionof grainsalong thesurface.No peel-ingoffhasbeenobservedduringelectrodepositionofCu-Snand Zn.Aftersoft-annealingalso,Zn/Cu-Sn(c)isfoundadherentand homogeneous.Furthermore,themorphologyofthesoft-annealed Cu-Zn-Snstacks doesnot showanydifferences withrespectto differentelemental ratiosofCu/(Zn+Sn)and Zn/Snof different samples.

The compositions of as deposited Cu-poor and Zn-rich pre-cursorsaftersoft-annealing,weremeasuredbyEDS,asshownin Table1. HereC55,C60 andC65 samplescorrespondto5.5,6.0 and6.5depositiontimesofZnonCu-Snsuccessivelywhile keep-ingconstanttheparameterofCu-Sndeposition(currentdensity: 2.5mA/cm2_,_deposition_time:₁₀_min).

Fig.3 shows the XRD spectraof (i)as deposited Cu-Sn and (ii)asdeposited Zn/Cu-SnonMofoilsubstrate.XRD patternof asdepositedCu-Sn/Mowascomposedofbinary-Cu6Sn5 phase (JCPDScard47-1575),pureSn(JCPDScard86-2264)aswellaslow intenseSnOphase(JCPDScard01-0902).WhileXRDpatternofas depositedZn/Cu-Snwascomposedof-Cu6Sn5,␥-Cu5Zn8(JCPDS card25-1228),pureZn(JCPDScard01-1238),pureSnphasesas wellaslowintensitySnO.Formationof-Cu6Sn5 and␥-Cu5Zn8 alloyisreportedinbothprecursors(co-electrodeposited/stacked layerofCu-Zn-Sn)[20,21].

Fig.4shows theXRDspectraof(iii) soft-annealedCu-Zn-Sn metallicprecursorsand(iv)sulfurizedﬁlms(C60).XRDpatternof soft-annealedprecursorswascomposedofCu6Sn5,␥-Cu5Zn8and

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Fig.4.XRDof(iii)aftersoft-annealing(C60)and(iv)aftersulfurization(C60).

pureSn.Ithasbeenobservedthataftersoft-annealing,pureZn phasecompletelydisappearedandintensityofpureSnphaseshave beenreduced.Atthesametime,theSnOpeaksdisappearedafter soft-annealing,whichisinagreementwiththeliterature[22].As expected,therehasbeennoformationofZn-Snphasesafter elec-trodepositionofZnonCu-Snlayerandsoft-annealingstep.After sulfurizationat 585◦C for 15min,allthethree samples mostly exhibitedpeakscorrespondingtothetypicalkesterite structure ofCZTS in theXRD patterns.Fig. 4(iv)shows thetypical poly-crystallinekesteriteCZTS(JCPDS26-0575)withdominantpeakat diffractionangle2=28.53◦,whichhasbeenobservedbymany others[11,16,21,23].Nosecondaryphaseshavebeenobservedafter sulfurizationinXRDpatternsofallsamples.

ThecompositionofsulfurizedﬁlmsmeasuredbyEDSisreported inTable2.ItshouldberemarkedthattheScontentisabsentsince EDSisnotabletomeasuretherightquantityofsulfurintheﬁlms whenMoispresent.Infact,EDScannotresolvesulfurfrom

molyb-Table2

Metalcontentandcompositionalratiosofsulfurizedﬁlms.

SampleNo. Cu(at.%) Zn(at.%) Sn(at.%) Cu/(Zn+Sn) Zn/Sn Zn/Cu Sn/Cu C55 45.74 24.70 29.25 0.85 0.84 0.54 0.63 C60 44.49 28.40 27.10 1.04 1.04 0.63 0.60 C65 43.79 30.40 25.81 0.78 1.17 0.69 0.58

denum,duetotheoverlappingenergypeak,namelyMoL␣andSK␣

[26].Moreover,MosignalisalsoremovedfromEDSspectrumwhile measuringtheelementalcompositionsofCu,ZnandSnbeforeand aftersulfurization.ItcanbenotedthatallofCZTSfilmsremain Cu-pooraftersulfurization,andtheZncontentdecreased,duetothe volatileeffectofZnduringsulfurization[7,24,25,28].Theintensity ofZnlosscanbeeasilyunderstoodfromtheZn/Curatiobeforeand aftersulfurizationwhichisdemonstratedonTables1and2.This resultisalsoconfirmedbyinductivelycoupledplasma-mass spec-trometry(ICP-MS)analysis.AmorerelevantZnlosswasexpected, astheZnlayerwasdepositedonthetopofthemetalstack.Lossof Sn(intheformofSnS)duringsulfurizationhasalsobeenreportedin manyliteratureworks[7,26]whichwasnotobservedinour sam-ples.Infact,ifwe comparetheratiosofSn/Cubeforeandafter sulfurization,anysignificantlossofSnisnotobservedupon sulfu-rization,whichisfavourableforCZTSfabrication.

Fig.5showstheSEMsurfaceofdifferentsulfurizedfilms: lay-ersareadherentandhomogenousmorphologyintermsofgrain sizealongthefilmswasobserved. Nosubstantialdifferencesin morphologyofthesulfurizedfilmswithrespecttovarious com-positionalratiosofCu-Zn-Snprecursorswereobserved.

OwingtotheverysimilarcrystalstructureofCZTSandsomeof itsmostcommonsecondaryphases(e.g.ZnSandCu2SnS3),Raman characterizationsarerequiredtodemonstratetheirpresence,as noevidencecanbeinferredfromXRDanalyses.Heretwo excita-tionwavelengthshavebeenused(i.e.514.5and785nm)forthe characterizationofCZTSﬁlms.Ithasbeenreportedearlierby Fer-nandesetal.that785nmradiationpenetrates400nm,whereas 514.5nmpenetrates150nmthroughCZTSthinﬁlms[26].Raman spectrahavebeencarriedoutatlowlaserpowerdensityinorder tominimizeanythermaleffectonthesamplethatcancause

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Fig.6. ComparativeRamanspectraofsample(C60)whentwodifferentexcitation wavelengthsareusedaslasersource.

Fig.7. Ramanspectraforthreedifferentsampleswhen785nmexcitationlineis used.

turalchangesintheﬁlms.FromFig.6,itisvisiblethat514.5nm excitationlineshowstypicalRamanpeaksat289cm−1,339cm−1 and376cm−1 related toKesterite-CZTSphase withoutany sec-ondary phases,which is ingood agreementwiththeresultsof previousstudies [11,12,16,26] witha commonly acknowledged highestintensitypeakat337–339cm−1forCZTS.Conversely,with 785nmexcitationlinenewbroadand weakbandsat295cm−1, 306cm−1and358cm−1areobserved.Thesebandscanbepossibly attributedtosecondaryphasesliketetragonalCu2SnS3(295cm−1) andcubic Cu2SnS3 (306cm−1,358cm−1)basedontheprevious studies[26,29]. It shouldbe noted that with785nm laser line theCZTSRamanpeaksat268cm−1,372cm−1and380cm−1have beenobservedwhereastheydidnotappearwith514.5nm excita-tionwavelengths,aspreviouslyobservedintheliterature[26].This effecthasbeenattributedtodifferentpenetrationdepthsof785nm (400nm)and 514.5nm (150nm)laser lines. Change of relative intensityinCZTSRamanpeaksobservedwith785and514.5nm excitationlinesmaybeattributedtodifferentresonanceconditions [26].

Fig.7showsacomparisonbetweentheRamanspectraofthree differentsamplesusingthe785nmexcitationwavelength.Itis vis-iblefromtheﬁgurethatsampleC60haslessintensesecondary

Fig.8. GDOESproﬁleofsulfurizedﬁlm(C60).

Fig.9. SEMCross-sectionalviewofsulfurizedﬁlm(C60).

phaseswithrespecttoothertwo.Sofromthispointofview, pre-cursorcomposition(Cu:41.84,Zn:32.53andSn:25.63)isthebest amongthethreesamples.

SampleC60wasfurthercharacterizedbyGDOEStoinvestigate thedistributionofelementsalongthefilmthickness.Itisclearfrom Fig.8thatcompositionofCZTSfilmdoesnotchangethatmuch alongthefilmthicknessincaseofsampleC60.Whatisinteresting inGDOESprofileofsulfurizedfilmisthatthereisnoevidenceof ZnSsegregationattheinterfaceofCZTS/Mo,asZnintensitydoes notshowanyupwardtrendinthatregionunlike[19]. Segrega-tionofZnSusedtobereportedinotherliteratures[9,27,30].As thereisnosignofformationofZnSattheCZTS/Mointerfaceand concentrationofCudecreasealongthedepthinGDOESprofile, for-mationofsolidSnS2(evenifSnconcentrationisnotincreasedat thatregion)canbeproposedtothisphenomenon.Ontheother hand,theincreaseofMosignalisnotsharpattheCZTS/Mointerface inGDOESprofile.TheformationofMoS2cannotbeclaimed conclu-sivelyonthatregionasnon-uniformCZTSfilmthicknesscanalsobe reasonbehindthatunsharpMosignal[27].Furtherinvestigations areneededinordertofullyunderstandthosephenomenon.

Fig.9showsthecross-sectionimageofsulfurizedCZTSfilm: thethicknessofCZTSfilmisaround2␮m.Italsoshowsthatfilm issomewhatrough.Asaresult,CZTSfilmhasbeencharacterized bylaser profilometer and roughnesswasfoundinthe rangeof 200–300nm.Roughnessofthefilmcanbereducedbyperforming prolongsoft-annealingtreatmentonprecursorbeforesulfurization [12]whichisunderinvestigationatthemoment.

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Fig.10.Photoluminescence(PL)spectraofsamples(C55)and(C60)at15K(␭=6.6nm,P=6W/cm2_).

Inordertoinvestigatethepossiblepresenceof levelsinthe bandgapdetrimentalforbothforoptoelectronicandphotovoltaic applications,someofthesamples(C55,C60)werecharacterized byPLspectroscopyatlowtemperature(15K).Accordingtomany literatureworks,broadPLbandsareobservedbetween1and1.3eV inCZTSlayers[31–34]atlowtemperature.Inourstudies,abroad peakappearsinbothsamplesataround1.15eV(Fig.10),whichwas recentlyreportedin[33].Accordingto[33],thispeakshowspower andtemperaturedependencetypicalofradiativerecombinations knownas quasi-donor-acceptor pair (QDAP) transitions, which involve potentialfluctuations [34].Thisbehavior is wellknown inhighlydefectiveandcompensatedsemiconductors[35],where inverselychargeddefectsandfluctuationsinthedefect concentra-tionleadtoirregularfluctuationsoftheelectrostaticpotentialwith position.Thisrandomdefectpotentialperturbs thebandstates energiesandinducesamodificationofthewell-definedenergy lev-elsintobroadbandstates.Consequently,localizedshallowlevels associatedtochargeddefectslosetheiroriginaldiscrete character-isticsandmergewiththevalenceandconductionbands.Incaseof sampleC55,relativeintensityofindividualpeaks(differentcolour ingraph)wereobservedindifferentregionsofsamplewhereasin caseofsampleC60,novariationofindividualpeakswereobserved withrespecttodifferentregions.Thisbehaviourcanbeattributed tothespatialvariationintheCZTSfilm[33].AccordingtoGershon etal.[35],CZTSlayerswhosePLspectrumconsistsofaQDAP emis-sionwithsaturatedpeakenergypositiontowards1.18eVarevery efficientPVabsorbers.

4. Conclusions

CZTS has been successfully fabricated through

electrodeposition-annealingroutefromelectrodepositedstacked bilayerprecursor(Zn/Cu-Sn/Mo).Metalstackswereadherentto thesubstratewithhomogeneousmorphology.ThoughZnlosses havebeenobserveduponsulfurization,noSnlosseswerenoticed, whichisprobablyconnectedtothestackingorderofprecursors. Ramanspectrarecordedwith785nmlaserlineshowedmore sec-ondaryphaseswithrespecttothoseobservedwiththe514.5nm excitationline.Thiseffectisprobablyduetoadifferentpenetration depthin thecase of785and 514.5nm excitationsrespectively. Photoluminescenceresultsrevealthesuitabilityoftheﬁlmasan absorberlayerforthinﬁlmsolarcells.

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