Development+of+thin-film+Cu(In,Ga)Se2+and+CdTe+solar+cells(2)

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.

K.Ramanathan,M.A.Contreras,C.L.Perkins,S.Asher,F.S.Hasoon,J.Keane,D.Young,M.Romero,W.Metzger,R.Nou ,J.Ward,andA.Duda,Prog.Photovoltaics11,225 2003 .2

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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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