Development+of+thin-film+Cu(In,Ga)Se2+and+CdTe+solar+cells

CIGS 2006年高影响因子文章

APPLIEDPHYSICSLETTERS86,062109 2005

BandalignmentattheCdS/Cu In,Ga…S2interfaceinthin- lmsolarcells

L.Weinhardt,a O.Fuchs,D.Groß,G.Storch,andE.Umbach

ExperimentellePhysikII,UniversitätWürzburg,AmHubland,97074Würzburg,Germany

N.G.Dhere,A.A.Kadam,andS.S.Kulkarni

FloridaSolarEnergyCenter,1679ClearlakeRoad,Cocoa,Florida32922-5703

C.Heskeb

DepartmentofChemistry,UniversityofNevada,LasVegas,Nevada89154-4003

Received18August2004;accepted7December2004;publishedonline2February2005 ThebandalignmentattheCdS/Cu In,Ga S2interfaceinthin- lmsolarcellsonastainlesssteelsubstratewasinvestigatedusingphotoelectronspectroscopyandinversephotoemission.Bycombiningbothtechniques,theconductionandvalencebandoffsetswereindependentlydetermined.We ndanunfavorableconductionbandoffsetof 0.45 0.15 eV,accountingforthegenerallyobservedlowopen-circuitvoltageandindicatingthegreatimportanceofthebuffer/absorberconductionbandoffsetforsuchdevices.ThesurfacebandgapoftheCu In,Ga S2absorberis1.76 0.15 eV,beingincreasedwithrespecttotheexpectedbulkvaluebyacopperdepletionnearthesurface.©2005AmericanInstituteofPhysics. DOI:10.1063/1.1861958

WithaCu In,Ga Se2absorber,thin- lmsolarcellshavereachedef cienciesofupto19.2%.1Inordertoachieveanoptimaloverlapwiththesolarspectrum,Cu In,Ga S2 CIGS wouldbeevenmorefavorableduetoitshigherbandgapof atleast 1.5eV,therebyalsopromisingpotentiallyhighervoltages.However,ef cienciesofcellsbasedonCIGSarepresentlylimitedtobelow13%.2Themainreasonisthattheexpectedlineargainintheopen-circuitvoltage VOC comparedtoCIGSehadnotyetbeenachieved.Theoriginofthisbehaviorisnotunderstood.Severalmodelssuggestthatthisiscausedbyanonidealconductionbandoffset CBO attheinterfacebetweentheCIGSabsorberandthecommonlyusedCdSbufferlayer.3–5Speci cally,acliffintheconductionbandispostulated CBO 0;i.e.,thecon-ductionbandminimum CBM ofCIGSlieshigherthanthatofCdS ,whichreducestheVOCandincreasestheinterface-dominatedrecombination.3,5Todate,onlyafewpublicationsdealwiththedeterminationofthebandalignmentattheCdS/CIGSinterface.6–8Furthermore,inallpreviousstudiestheconductionbandoffsetcouldonlybededucedfromthevalencebandoffset VBO ,assumingthatthebandgapsforthetwojunctionpartnersareknown.Scheeretal.haveusedphotoelectronspectroscopy PES todeterminetheVBOattherealinterfacebetweenapolycrystallineCuInS2absorberandtheCdSbufferlayerdepositedbyachemicalbathdepo-sition CBD .6Bycorrelatingthecore-levelpositionsoftherealsystemwiththevalencebandmaximum VBM ofacleavedCuInS2singlecrystalandavacuum-evaporatedCdS lm,theydeterminedaVBOof 1.5 0.3 eV.Assumingbulkbandgapvaluesattheinterface,acliffof 0.6 0.3 eVwasdeduced.Hashimotoetal.appliedx-rayPES XPS topolycrystallineCuInS2andCBDCdS;theyobtainedavalueof 1.18 0.10 eVfortheVBOanddeducedaCBOof 0.05 0.15 eVbyusingopticallymeasuredbulkbandgap.7APESinvestigationofthebandalignmentbetweenaCuInS2singlecyrstalandstepwise-evaporatedCdSlayers

a

Authortowhomcorrespondenceshouldbeaddressed;electronicmail:lothar.weinhardt@physik.uni-wuerzburg.deb

Electronicmail:heske@unlv.nevada.edu

wasconductedbyKleinetal.,8whoobtainedavalueof 0.6 0.1 eVfortheVBO.Thisismuchsmallerthanthatoftherealsystem,whichwasattributedtoadifferentcom-positionand/ororientationofthedifferentCuInS2surfaces.Infact,thetypicalsurfaceoftheCI G Sabsorberstronglydeviatesfromasingle-crystalCuInS2surface.Forinstance,itisnecessarytoprepareaCu-richabsorber lminordertoobtainagoodabsorberqualitywithlargegrains.9However,thispreparationresultsintheformationofCuxSbinaryphasesattheabsorbersurface,whichhavetoberemovedbyanetchingprocess usuallyinKCN .Theresulting lmshavebeenfoundtobeCupooratthesurface.10,11Sincethishasdirectimpactonthebandgap s andthebandalignmentattheinterface,itisveryimportanttoinvestigatetherealinterfaceandtodeterminetheVBOandCBOdirectlyandindependently,whichrequiresacombinationofPESandin-versephotoemission IPES .12–14

WehavethereforeinvestigatedtherealCdS/Cu In,Ga S2interface,asitisusedinthin- lmsolarcellsforspaceapplications15withPESandIPES.Gawasaddedherefortworeasons.First,itresultsinahigherbandgap,whichisparticularlyimportantforspaceapplicationssincethesolarspectruminspacehasmorecontributionsonthehigh-energysidethanthatonearth.Second,ithasbeenshownthattheadditionofGaresultsinanoverproportionalincreaseinVOC,comparedtotheincreaseinbandgap,andinconsequenceimprovesthecellef ciency.2Toreducetheweightofthecells,astainlesssteelsubstrateinsteadofthecommonlyusedsoda-limeglasswastaken.Withthisap-proach,ef cienciesofupto10.4%havebeenreached.15

Theinvestigatedabsorberswerepreparedinatwo-stageprocess.15Inthe rststep,CuGa/Inlayersweresputter-depositedonMo-coatedstainlesssteelfoils.Inthesecondstep,themetalliclayersweresulfurizedduringarapidther-malannealinginanH2Satmosphere.TheCuxSsurfacelayerwasthenremovedbyaKCNtreatmentandafterwardsoxi-dizedinaH2O2/H2SO4bath,whichimprovestheef ciencyofthecells.TheabsorbersurfaceisfoundtobecopperpooraftertheremovaloftheCuxSphasesbytheKCNtreatment,whichdoesnotsigni cantlychangeaftertheH2O2/H2SO4

0003-6951/2005/86 6 /062109/3/$22.5086,

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CIGS 2006年高影响因子文章

062109-2Weinhardtetal.Appl.Phys.Lett.86,062109 2005

bath theeffectsofthetreatmentswillbedescribedindetailinRef.16 .TheCdSbufferlayerwasthendepositedbyCBD.Overall,threesampleswithvaryingCdSthicknesswereprepared,rangingfromabout1nmtoa lmthicknessasusedinrealdevices 50nm .Allsampleswereinvesti-gatedbyultravioletPES UPS withHeIandHeIIexcita-tion,byXPSwithaMgK x-raysource,andbyIPES.ThePESspectrawererecordedwithaVGCLAM4electronanalyzer.FortheIPESexperiments,aCicacci-typeelectrongunandaDose-typedetectorwithSrF2windowandAr:I2 llingwereused.Allexperimentswereperformedinultra-highvacuumwithabasepressurebelow5 10 10mbar.

Duetotheairexposureduringandaftertheproductionprocess,allsamplesshowcontaminationswithC-andO-containingspeciesontheirsurface.Moreover,theH2O2/H2SO4treatmentalsoslightlyoxidizesthesurface.16Be-causetheCBDprocessappliedfortheCdSdepositionre-movesthesecontaminants,itisimportantforthedetermina-tionofthebandalignmenttomeasurea“clean”CIGSsurface,whichwasachievedbyremovingtheadsorbateswithmildAr+sputtering,asjudgedfromthecompleteUPSvalencebandspectraandtheXPSanalysis notshown .Tominimizesputter-induceddamage,wehaveusedverylowionenergies 50eV andlowcurrents 50nA/cm2 .Fur-thermore,wehavecloselymonitoredanychangesatthesur-faceaftereachsputterstepanddidnot ndevidenceforastoichiometryvariation.Whileitiswellknownthatpro-longedsputteringofCuInSe2with500eVArionsleadstotheformationofmetallicphasesatthesurface,13,17,18nosuchphasesarefoundwhensputteringwith50eVions.

InFig.1,theUPSspectraofthevalencebandandtheIPESspectraoftheconductionbandaftereachsputterstepareshownforboth,theCIGSabsorber Figs.1 a –1 d andthethickCdSbuffer Figs.1 e –1 g .Thevalenceandcon-ductionbandsoftheuntreatedsurface 0min aredominatedbytheadsorbates,whichleadtoarti ciallyhighvaluesforthebandgap seeFigs.1 a and1 e ,asdeterminedfromthedistancesofthelinearlyextrapolatedVBMandCBM.ConcomitantwiththeremovaloftheOandCcontaminantsduringthesputtertreatment,asseeninourXPSspectra notshownhere ,theCIGSvalencebandappears,dominatedbyCu3d-likestates2eVbelowtheVBM.After30minofsputtering,nearlyallcontaminantsareremoved,andtheval-uesforVBMandCBMnolongerchangesigni cantlyasafunctionofsputtertime.Wethusobtainabandgapvalueof1.76 0.15 eVforthecleanCIGSsurface.Thisvalueisreasonable,takingintoaccountthatthebulkbandgapvalueofCuInS2of1.5eVshouldbeincreasedinourcase,bothbythesubstitutionofsomeInwithGa ourXPSmeasurementsshowaratherhighGa/Inratioof 0.2 aswellasbyacopperdepletionattheabsorbersurface,whichwe ndbyaquantitativeevaluationofourXPSmeasurements.Theposi-tionoftheFermilevelisfoundtobeinamidgapposition,indicatingabandbendingfromthep-typebulkoftheab-sorbertowardsitssurface,whichiscommonlyfoundforCIGSSeabsorbers.19

FortheCdSoverlayer,thecharacteristicfeaturesoftheCdSvalencebandemergeafterthe rstsputterstep.NeitherthespectralshapenorthederivedvaluesfortheVBMandCBMchangesigni cantlyafteradditionalsputtersteps.Wethusderiveasurfacebandgapof2.47 0.15 eV,similartopreviousstudiesandclosetothebulkbandgapofCdS 2.42eV .20

FIG.1.HeIandHeIIUPS left andIPES right spectraofCIGS a – d andCdS/CIGS e – g ofthepristinesamplesandaftersubsequentAr+sputtersteps 50nA/cm2,50eV,sputtertimesaregivenattherightcoor-dinate .Thebandextremaweredeterminedbylinearextrapolationoftheleadingedges.Theresultingsurfacebandgapsaregivenforeachpairofspectra.

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Thedeterminationofthebandalignmentisnowdoneinatwo-stepprocess.Inthe rststep,theCBM VBM valuesoftheCIGSsurfacearecomparedwiththoseoftheCdSsurface.Inasecondstep,weconsiderchangesofthebandbendinginthesubstrate CIGS duetotheinterfaceforma-tionprocess i.e.,duetochangesoftheinterfacedipole aswellasbandbendingtowardstheCdSsurface together,henceforthcalled“interface-inducedbandbending” .ThisrequiresatleastoneadditionalsamplewithaverythinCdS paringthecore-levellinepositionsofthecleanCIGSsurface,theCIGSinterface,thethinCdSoverlayer,andthethickCdS lmsurface,wecancomputethecorrectionsfortheinterface-inducedbandbend-ing.Thisprocedurewouldalsocorrectanyarti cialchangesinbandbendingduetothesputtercleaningofthesamples.Notethatthesurfacebandgapsjustderivedarenotrequiredfortheoffsetdetermination,butratherconstituteadditionalinformationextractedfromourmeasurements.

Inthe rst approximative stepwecomparetheCBMofCIGSandthatofCdS,whichare0.86 0.10 eVand0.46 0.10 eV,respectively,indicatingacliffofabout0.4eVintheconductionband.Inthevalencebandwe ndaVBMof 0.90 0.10 eVforCIGSand 2.01 0.10 eVforCdS.Theseapproximativeresultsstillneedtobecorrectedfortheinterface-inducedbandbendinginthesecondstep.Forthatpurpose,wehaveuseddifferentcombinationsofcore-levellinesoftheCIGSabsorber Ga2p,Cu2p,In3d andtheCdSbuffer Cd3d,S2p .Intotal,12differentvaluesofbandbendinghavebeenobtained,whichareshowninFig.

2.

CIGS 2006年高影响因子文章

062109-3Weinhardtetal.Appl.Phys.Lett.86,062109 2005

recombinationandlimitstheVOC.Incontrast,werecentlyfoundthatthebandalignmentattheCdS/CIGSeinterfaceis at,12,13beingamuchmorefavorablecon guration.Thecombined ndingsthusindicatethegreatimportanceoftheinterfacealignmentfortheperformanceofCIGSSesolarcells.FuturedevelopmentoftheCIGSsystemshouldfocusonincreasingthebandbendingwithintheCIGSabsorber e.g.,byintentionaldopinginthesurface-nearregion toachievea atconductionbandalignmentor,evenbetter,oninsertingasuitablydesignedthininterlayerwhichallowsoneto“tailor”theinterfacedipoleforanoptimizedelectronicstructureofCIGS-basedthin- lmsolarcells.

WegratefullyacknowledgefundingbytheGermanBMWA FKZ0329218C .WearealsothankfultotheNa-tionalRenewableEnergyLaboratory.

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.

1

FIG.2.Correctionsfortheinterface-inducedbandbendingdeterminedbycombiningthecore-levellinepositionsofthecleanabsorber,aCdSbufferlayeroffullthickness,andtwodifferentthinCdS

lms.

Themeanvalueofthesecombinationsis 0.05 0.10 eV,indicatingthattheinterface-inducedbandbendingcorrec-tion,inthiscase,isverysmall.Includingthecorrection,weobtainvaluesof 0.45 0.15 eVfortheCBOand 1.06 0.15 eVfortheVBO.

TheresultingbandalignmentattheCdS/CIGSinterfaceisshowninFig.3.Themain ndingisthattheconductionbandshowsasigni cantcliff 0.45eV .Furthermore,we ndthatthebandgapoftheCIGSsurfaceof1.76 0.15 eVisincreasedcomparedtotheCISbulkbandgapbecauseoftheGaincorporationandtheCudepletionattheabsorbersurface.Correlatingthe ndingofacliffintheconductionbandwiththemodelsmentionedabove,3–5thisisanonidealalignmentattheinterface,asitenhancespossibleinterface

FIG.3.SchematicdiagramofthebandalignmentattheCdS/CIGSinter-face.ThebandextremaoftheCIGSandCdS lms asdeterminedbyUPSandIPES areshownontheleftandright,respectively.Thecentershowsthebandalignmentattheinterfaceaftertakingtheinterface-inducedbandbendingintoaccount.

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