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,

062109-1©2005AmericanInstituteofPhysicsDownloaded 14 Feb 2007 to 218.94.142.5. Redistribution subject to AIP license or copyright, see /apl/copyright.jsp

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 10mb …… 此处隐藏：6185字，全部文档内容请下载后查看。喜欢就下载吧 ……