Cu2ZnSnS4薄膜制备

SolarEnergyMaterials&SolarCells93(2009)1230–1237ContentslistsavailableatScienceDirectSolarEnergyMaterials&SolarCellsjournalhomepage:www.elsevier.com/locate/solmatPreparationandcharacterizationofspray-depositedCu2ZnSnS4thinfilmsY.B.KishoreKumar,G.SureshBabu,P.UdayBhaskar,V.SundaraRajaÃSolarEnergyLaboratory,DepartmentofPhysics,SriVenkateswaraUniversity,Tirupati517502,IndiaarticleinfoArticlehistory:Received5April2008Receivedinrevisedform30December2008Accepted18January2009Availableonline3March2009Keywords:Cu2ZnSnS4ThinfilmsSpraypyrolysisCharacterizationSolarcellabsorberabstractThinfilmsofCu2ZnSnS4(CZTS),apotentialcandidateforabsorberlayerinthinfilmheterojunctionsolarcell,havebeensuccessfullydepositedbyspraypyrolysistechniqueonsoda-limeglasssubstrates.TheeffectofsubstratetemperatureonthegrowthofCZTSfilmsisinvestigated.X-raydiffractionstudiesrevealthatpolycrystallineCZTSfilmswithbettercrystallinitycouldbeobtainedforsubstratetemperaturesintherange643–683K.Thelatticeparametersarefoundtobea¼0.542andc¼1.085nm.Theopticalbandgapoffilmsdepositedatvarioussubstratetemperaturesisfoundtoliebetween1.40and1.45eV.Theaverageopticalabsorptioncoefficientisfoundtobe4104cmÀ1.&2009ElsevierB.V.Allrightsreserved.1.Introduction

CuInSe2-basedchalcopyritesemiconductorshaveprovedtobesuccessfulcandidatesforterrestrialphotovoltaics(PV).Cu(In,-Ga)Se2(CIGSe)thinfilmsolarcellsexhibitedarecordconversionefficiencyof19.9%[1].Thesecellsutilizeexpensiveandscarceelementslikeindium,whichaffectslarge-scaleproduction.Toachievethegoalofcost-effectivephotovoltaictechnology,itisnecessarytoexplorenewmaterialslikeCu2ZnSnSe4,Cu2ZnSnS4andotherquaternariesofthesechalcopyrite-likesemiconductors.TheelementszincandtininthesecompoundsemiconductorsarerelativelycheapandabundantcomparedtoindiumandgalliumusedinCIGSthinfilmsolarcells.Cu2ZnSnS4(CZTS),withadirectbandgap(1.45eV)closetooptimumvalueforPVapplications,highopticalabsorptioncoefficient($104cmÀ1)andp-typeconductivityisapromisingmaterialforabsorberlayerinthinfilmsolarcells.Athoroughunderstandingofmaterialpropertiesisverymuchessentialforthesuccessfulutilizationofthiscompoundinsolarcells.OnlylimitedworkisdoneonthegrowthandcharacterizationofCZTSbulk[2–10]andthinfilms[11–33].Nitscheetal.[2]hadsuccessfullygrownCu2ZnSnS4singlecrystal¨ferandNitsche[3]in1967byvapour-phaseiodinetransport.SchalaterreportedthestructuralpropertiesofmanyCu2–II–IV–S4(Se4)singlecrystals.Halletal.[4]investigatedthestructuraldifferencebetweenCu2(Fe,Zn)SnS4andCu2(Zn,Fe)SnS4usingsinglecrystalX-raydiffractionmethods.Bernardinietal.[5]carriedoutEPRandSQUIDmagnetometrystudiesonnaturalandsyntheticCu2FeSnS4ÃCorrespondingauthor.Tel.:+918772289472;fax:+918772248485.E-mailaddress:sundararajav@rediffmail.com(V.SundaraRaja).0927-0248/$-seefrontmatter&2009ElsevierB.V.Allrightsreserved.doi:10.1016/j.solmat.2009.01.011(stannite)andCu2ZnSnS4(kesterite)crystals.Structuralinvestiga-tionofsyntheticCu2FeSnS4–Cu2ZnSnS4singlecrystalswas¨ssbauerstudyonstannite–kes-reportedbyBonazzietal.[6].MoteritesolidsolutionswasinvestigatedbyBenedettoetal.[7].Matsushitaetal.[8]carriedoutX-raydiffraction(XRD)anddifferentialthermalanalysis(DTA)studiesonCu2ZnSnS4andotherrelatedcompoundsemiconductorssynthesizedfromele-mentalmixture.Tanakaetal.[9]reportedforthefirsttimethephotoluminescencestudiesofCZTSbulkcrystalsgrownbyiodinetransportmethod.RecentlySchorretal.[10]reportedstructuralcharacterizationresultsonstannite–kesteritesolidsolutionseriesbyneutrondiffractionstudies.ItoandNakazawa[11]reportedforthefirsttimeaCZTS/CdSnOheterojunctionusingCZTSfilmsdepositedbysputtering.Friedlmeieretal.[12]investigatedthepropertiesofthermally-evaporatedCZTSfilmsandreportedaCZTS/CdS/ZnOheterojunctionwithanefficiencyof2.3%.Katagiriandco-workershaveextensivelyinvestigatedCZTSfilmspreparedbysulphurisationofelectronbeamevaporated[13–20]aswellasRFsputtered[21]precursors.CZTScellswithanefficiencyof5.74%,usingvapour-phasesulphurizedRFsputteredprecursors,havebeenreportedin2007[22].Seoletal.[23]investigatedelectricalandopticalpropertiesofRFmagnetronsputteredCZTSfilm.Hybridsputtering[24],co-evaporation[25],pulsedlaserdeposition[26]andsulphurisationofmetallic/compoundpre-cursorsdepositedusingvacuumevaporation[27],ionbeamsputtering[28]andsputteringandsequentialevaporation[29]havebeenusedinrecentyearstodepositCZTSfilms.ApartfromthephysicalvapourdepositionmethodscitedaboveforthedepositionofCZTSfilms,chemicaldepositionmethodslikephoto-chemicaldeposition[30],sol–gel[31]andspraypyrolysis[32–34]havebeenusedtodepositthesefilms.Y.B.KishoreKumaretal./SolarEnergyMaterials&SolarCells93(2009)1230–12371231Table1ElementalcompositionoftheCZTSfilmsdepositedatdifferentTs.SubstrateCompositionofelementsinatomic%Cu/temperature(Zn+Sn)(Ts)(K)CuZnSnS56327.4919.4311.1641.910.9060326.8618.6413.5340.960.8464327.2018.9611.3142.530.9268328.7816.9914.3739.860.9272328.5819.9815.4336.010.81Fig.1.XRDpatternsofCZTSfilmsdepositedatdifferentTs’s.Fig.2.ExpandedviewofXRDpeaksofCZTSfilmsdepositedat(a)563,(b)603,(c)643,(d)683and(e)723K.Spraypyrolysisisaversatileandlow-costtechnique,whichisextensivelyusedtodepositselenide,sulphideandoxidesemi-conductorfilms.NakayamaandIto[32]studiedtheeffectofethanolandzincconcentrationinthestartingsolutiononthepropertiesofspray-depositedCZTSfilmsusingN2asthecarriergas.Filmsgrownfromaqueoussolutionarehighlysulphur-deficient(28–30%)andnearstoichiometricCZTSfilmswereobtainedwithasolutioncontaining30%ethanol.Subsequentannealingofthefilmsinsulphurambientwasfoundtobenecessary.Madara´szetal.[33]depositedCZTSfilmsbyspraypyrolysisusingthioureacomplexesbutopticalandmorphologicalstudieswerenotcarriedout.RecentlyKamounetal.[34]investigatedtheeffectofsubstratetemperature(553–633K)andthesprayduration(30,60min)onthegrowthofCZTSfilms.XRDstudiesofthesefilmsshowedthatthefilmsaremulti-phase.Wehaveinvestigatedthegrowthandpropertiesofspray-depositedCZTSfilmsatvarioussubstratetemperaturesintherange563–723Kinordertooptimizesubstratetemperaturetoobtainsingle-phaseCZTSfilms.Theresultsoftheseinvestigationsarepresentedinthispaper.2.Experimental

Cu2ZnSnS4thinfilmsweredepositedbyspraypyrolysistechniquestartingwithanaqueoussolutioncontainingcupricchloride(0.01M),zincacetate(0.005M),stannicchloride(0.005M)andthiourea(0.04M).Excessthioureawastakentocompensatethelossofsulphurduringpyrolysis.Thesolutionwas1232Y.B.KishoreKumaretal./SolarEnergyMaterials&SolarCells93(2009)1230–1237Fig.3.Crystalstructureof(a)mineralstannite(Cu2FeSnS4)and(b)mineralkesterite(Cu2ZnSnS4).Table2Atomiccoordinatespositionsofelementsinstannite(Cu2FeSnS4)andkesterite(Cu2ZnSnS4)structures[4].¯2m)Stannite(I4AtomsFeSnCuSWyckoffnotation2a2b4d8iPosition(000)1ð1220Þ1ð0124Þ(xxz)¯)Kesterite(I4AtomsCuSnCuZnSWyckoffnotation2a2b2c2d8gPosition(000)1ð1220Þ1ð0124Þ1ð1204Þ(xyz)morphologywereobservedusingaJEOLscanningelectronmicroscope(SEM).Theelementalcompositionwasdeterminedusinganenergydispersivespectrometer(EDS)systemattachedtoJEOLSEM.ElectricalresistivityofthefilmsatroomtemperaturewasdeterminedwithaKeithley617programmableelectrometer.3.Resultsanddiscussion3.1.CompositionTheelementalcompositionofCu2ZnSnS4thinfilmsdeter-minedfromEDSanalysisforfilmsdepositedatvarioussubstratetemperaturesisshowninTable1.Thereisaconsiderabledeviationfromstoichiometry.Thefilmsaremostlycopperrich,zincrichbutsulphurdeficient.Sulphurdeficiencyissignificantlyhigherathighersubstratetemperaturessincesulphurismorevolatile.Spray-depositedCZTSfilmsobtainedfrompureaqueoussolutionbyNakayamaandIto[32]inthesubstratetemperaturerange553–633Karealsosulphur-deficient(28–38at%).WehavetakenCuCl2,whichissolubleinwaterasthesourceofcopperinsteadofCuClusedbythem,andthereissomeimprovementinthesulphurcontentinthefilm.Yetthefilmsaresulphur-deficient.3.2.Structuralcharacterization3.2.1.X-raydiffractionstudiesTheXRDpatternsofCZTSfilmsdepositedatdifferentsubstratetemperatures(563,603,643,683and723K)areshowninFig.1.Fig.2showstheexpandedviewofthemostintensepeaktoknowsprayed,usingapneumaticallycontrolledair-atomizingspraynozzle(14JAU,SprayingsystemsCo.,USA),ontoheatedglasssubstratesheldatvarioussubstratetemperatures.Compressedairwasusedasthecarriergasandthesprayratewas12ml/min.Experimentswereconductedatvarioussubstratetemperaturesintherange563–723Ktoinvestigatetheeffectofsubstratetemperature(Ts)onthegrowthofthefilms.Thesubstratetemperaturecouldbemaintainedtoanaccuracyof75Kusingadigitaltemperaturecontroller.Filmswereanalyzedbystudyingtheircomposition,structural,opticalandelectricalproperties.X-raydiffractometer(SEIFERTModel3003TT)wasusedinBragg–BrentanomodetorecordX-raydiffraction(XRD)patterns.Cu-Karadiation(l¼0.15406nm)wasusedtorecordthespectrainthe2yrange10–601withastepsizeof0.031.Spectraltransmittanceandreflectancewererecordedinthewavelengthrange300–2000nmonJASCOUV–VIS–NIRdoublebeamspectrophotometer.ThemicrostructureandthesurfaceY.B.KishoreKumaretal./SolarEnergyMaterials&SolarCells93(2009)1230–12371233Table3CalculatedandexperimentalXRDpeakintensitiesforCZTSthinfilms.S.No.1234567891011121314151617181920a(hkl)(002)(101)(110)(112)(103)(200)(004)(202)(211)(114)(105)(220)(204)(312)(116)(303)(224)(314)(008)(332)d-spacing(nm)0.54210.48690.38470.31260.30080.27130.27140.24260.23680.22120.20130.19190.19190.16360.16360.16180.15650.14500.13560.1245Calculatednormalizedintensity(I/I0)07210025511112040141304235Minctrystinformationcard[35]04.55–100210.55.22––––18.0135.9024.7312.30–––––Experimentalintensity–20–100–––––––45a452525a–––––(220)/(204)and(312)/(116)arenotresolvableexperimentallyasc$2a.Fig.4.SEMmicrographsofCZTSthinfilmsdepositedat(a)563,(b)603,(c)643,(d)683and(e)723K.whetherpeakscorrespondingtoanysecondaryphase(Cu2SnS3,ZnS)arepresent.FromFig.1,itisobservedthatfilmsdepositedatTs¼563Karemulti-phasecontainingCu2SnS3(JCPDSCardno.27-0198),Cu2ZnSnS4andCuxS(JCPDSCardno.20-0365)ofwhichCu2SnS3(CTS)isthedominantsecondaryphaseasindicatedby¯1¯1)peak.CZTSfilmsdepositedatTs¼603Karetheintense(2foundtocontainveryweakpeakscorrespondingtoCuxS(Fig.1).XRDpatternsoffilmsdepositedatTs¼643and683Kseemtoindicatethatthefilmsaresinglephasesincenopeakcorrespond-ingtoanysecondaryphaseisseen.ButthelowintensityofXRDpatternsandconsiderabledeviationfromthestoichiometryofthesefilms(Table1)suggestthepossibilityofamorphousphasesbeingpresentinthesefilms.Thesamplesbeingzinc-rich(Cu/Zn+Sn¼0.92,Zn/Sn¼1.68and1.18),ZnSmightbepresentinamorphousform.ThispropositionmightbejustifiedbythefactthatfilmsdepositedatTs¼723KarefoundtocontainZnSasdominantphasepossiblyduetotheimprovementincrystallinityathigherTs(Figs.1and2).ThelatticeparametersofCZTSfilmsdeterminedfromtheobservedd(112)andd(220)spacingsarea¼0.54270.001andc¼1.08570.001nm.Theseareingoodagreementwiththereportedsinglecrystaldata,a¼0.5427andc¼1.0848nm[3].ThereareconflictingreportswithregardtothestructureofCZTSfilms.AfewgroupsreportedthestructureofCZTSfilmstobestannite[11–13,27–30],whilesomeothergroupsreportedkesteritestructureforCZTSfilms[16,22–26].Stannite(Cu2FeSnS4)andkesterite(Cu2ZnSnS4)belongtotetragonalsystembutthe¯2mspacegroupsaredifferent.StannitestructurespacegroupisI4¯(Fig.3).ThecoordinatewhilekesteritestructurespacegroupisI4positionsareshowninTable2[4].However,basedonneutron1234Y.B.KishoreKumaretal./SolarEnergyMaterials&SolarCells93(2009)1230–1237diffractionstudiesofstannite–kesteritesolidsolutionseries(Cu2Fe1ÀxZnxSnS4),Schorretal.[10]recentlyreportedthatthestructureofCu2ZnSnS4(x¼1intheseries)isunambiguously¯).kesterite(I4Wehavecalculatedtheintensitiesofpossiblediffractionpeaks(Ihkl)usingtheatomicscatteringfactors,coordinatepositionsandexperimentallydeterminedlatticeconstantsaandcvaluesusingtheformula[36].Ihkl󰀃󰀄󰀂󰀂21þcos22yÀ2M󰀂󰀂e¼FhklPsin2ycosy(1)arethecoordinatesofeachatom.ThecalculatedpeakintensitiesforkesteritestructurearecomparedwiththenormalizedexperimentalpeakintensitiesinTable3alongwithMincryst¯).Theagreementisinformationcarddata[35]forkesterite(I4reasonablysatisfactory.Thepeakatd¼0.1919nmisassignedto(220)/(204).TheXRDpeaks(220)and(204)arenotresolvableexperimentallyasc$2a.Similarly,thepeaks(312)and(116)arealsonotresolvable.Hencetheobservedpeakatd¼0.1636nmisassignedto(312)/(116).Thecrystallitesize(L)inthesefilmsisdeterminedusingScherer’sformula[36]L¼sl=BCosywheresistheScherer’sconstant,Bisthefull-widthathalfmaximum(FWHM)andyistheBraggangle.Thecalculatedcrystallitesizevaluesarefoundtolieintherange20–45nmbasedonthesubstratetemperature.Thecrystallitesizeisfoundtoincreasewithincreaseinsubstratetemperature.3.2.2.MicrostructureFig.4showsSEMmicrographsofCZTSfilmsdepositedatdifferentsubstratetemperatures.Asthesubstratetemperatureincreases,thereisanimprovementingrainmorphologyandsize.FilmsdepositedatTs¼563Kdonotexhibitwell-definedgrainsandtheappearanceissmeary.AsTsincreases,themorphologyisfoundtoimproveanddistinctgrainsareseenforfilmsdepositedatTs¼683and723K.ThegrainsizeoffilmsdepositedforTsintherange643–723Kisfoundtobe$1mm.ItmaybeworthmentioningherethatScherer’sformulagivesthecrystallitesizenormaltotheX-raybeamdirectionanddoesnotgivelateraldimension[37].3.3.OpticalpropertiesHereFhklisstructurefactor,Pismultiplicityfactor,MisthetemperaturefactorandyistheBraggangle.Intheaboveequation,trigonometrictermrelatestoLorentz-polarizationfactor.ThestructurefactorisgivenbytheequationFhkl¼P2pi(hu+kv+lw)wherefnisatomicscatteringfactor,(u,v,w)fne2x105Absorption coefficient (cm-1)1x105

5x104

1.0

2.03.0

Photon energy (eV)

4.0

Fig.5.OpticalabsorptioncoefficientofCZTSthinfilmsdepositedatTs¼643K.Theopticalabsorptioncoefficient(a)wasdeterminedfromthemeasuredspectraltransmittance(Tl)andreflectance(Rl)usingtheformula[38]\"#ð1ÀRlÞ2(2)at¼lnTl3.0

20

(αhν)2 x 109 (eV cm-1)2(αhν)2 x 109 (eV cm-1)20.9

1.1hν (eV)

1.3

15

2.0

10

1.0

5.0

0.00.0

1.4

1.6hν (eV)

1.8

2.0

Fig.6.(ahn)2asafunctionofphotonenergy(hn)forfilmsdepositedatTs¼563Kfortwodifferentphotonenergyregions.Y.B.KishoreKumaretal./SolarEnergyMaterials&SolarCells93(2009)1230–1237123516

5.0

(αhν)2 x 1010 (eV cm-1)21.4

1.61.8hν (eV)

2.0

(αhν)2 x 109 (eV cm-1)212

4.0

3.0

8.0

2.0

4.0

1.0

0.00.0

1.8

2.0

2.2hν (eV)

2.4

2.6

Fig.7.(ahn)2asafunctionofphotonenergy(hn)forfilmsdepositedatTs¼603Kfortwodifferentphotonenergyregions.1616

12(αhν)2 x 109 (eV cm-1)2(αhν)2 x 109 (eV cm-1)212

8.0

8.04.0

4.00.0

1.4

1.6hν (eV)

Fig.8.(ahn)2asafunctionofphotonenergy(hn)forfilmsdepositedatTs¼643K.0.01.82.0

1.41.6hν (eV)1.82.0Fig.9.(ahn)2asafunctionofphotonenergy(hn)forfilmsdepositedatTs¼683K.wheretisthefilmthickness.Atypicalplotofopticalabsorptioncoefficientversusphotonenergy(hn)forfilmsdepositedatTs¼643KisshowninFig.5.Thenatureoftheopticaltransition,whetherdirectorindirectandtheopticalbandgap(Eg)ofeachfilm,isobtainedfromtheequation[38]a¼AðhnÀEgÞn=hn(3)3whereAisaconstant.Theexponent‘n’cantakevalues12,/2or2basedonwhethertheopticaltransitionisdirect-allowed,direct-allowedorindirect-allowed,respectively.Inthepresentinvestigation,valuesofaarefoundtoobeyEq.(3)forn¼12indicatingthattheopticaltransitionisdirect-allowedinnature.Fig.6(a)and(b)showtheplotsof(ahn)2versushnfortwodifferentphotonenergyregionsforfilmsdepositedatTs¼563K.Theopticalbandgapvaluesarefoundtobe0.92and1.40eV,respectively.Theuncertaintyinthedeterminationofbandgapis70.02eV.Thedirectopticalbandgapof0.92eVisattributedtoCu2SnS3and1.40eVisattributedtoCZTSfilm[11,14].This1236Y.B.KishoreKumaretal./SolarEnergyMaterials&SolarCells93(2009)1230–1237observationthatthefilmscontainthesecondaryphaseCu2SnS3isinagreementwiththeconclusionfromXRDpatternofthefilmsdepositedatTs¼563K.Figs.7(a)and(b)showthe(ahn)2versusplotsfortwodifferentphotonenergyregionsforthefilmsdepositedatTs¼603K.Thedirectopticalbandgapvaluesarefoundtobe1.39and1.90eV.TheformerisattributedtodirectopticalbandgapofCZTSandthelattertoCuxS.ThedirectopticalbandgapofCuxSreportedearlierintheliteratureisfoundtovaryfrom1.7to2.16eVbasedonthevalueofx[39].XRDanalysisofthefilmsdepositedatTs¼603KconfirmedthepresenceofCZTSandCuxS.The(ahn)2versushnplotsforfilmsdepositedatTs¼643and683KareshowninFigs.8and9.Thedirectopticalbandgapsarefoundtobe1.45and1.40eV,respectivelyandareincloseagreementwiththereportedvalueofCZTS[11,14].ThedifferenceindirectopticalbandgapvaluesofCZTSfilmsdepositedatvarioussubstratetemperaturesmightbeduetothedifferenceinthefilmcomposition.NootheropticaltransitioncorrespondingtoanysecondaryphasecouldbenoticedinthespectraltransmittancecurvesforthesefilmsdepositedatTs¼643and683K,indicatingthatthefilmsaresingle-phaseCZTS.Howeverasstatedearlier,considerabledeviationofthesefilms(Table1)fromstoichiometrysuggeststhepossibilityofamorphousphasesbeingpresent.Thesamplebeingzinc-rich,ZnSmightbepresentinthefilm.However,thecorrespondingopticaltransitioncouldnotbeobservedinthetransmittancespectrumowingtothetoolowspectralintensitybelowthe500nmregionafterstrongabsorptionbyCZTS.3.4.ElectricalpropertiesRoom-temperatureelectricalresistivityofCZTSfilmsdepos-itedatdifferentsubstratetemperatureswasdeterminedusingvanderPauwtechnique.Thefilmswerefoundtobep-typeandtheresistivitywasfoundtovaryfrom0.02to2.00Ocm.4.Conclusions

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