epvfinal.docx

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epvfinal

ENERGYPHOTOVOLTAICS

Thin-FilmPhotovoltaicsPartnershipProgram

AdvancedCIGSPhotovoltaicTechnology

FinalTechnicalReport

coveringtheperiod

November15,2001–February13,2005

SubcontractNo.ZDJ-2-30630-21

under

PrimeContractNo.DE-AC36-99-GO10337

Preparedby:

A.E.DelahoyandL.Chen

EnergyPhotovoltaics,Inc.

P.O.Box7456,Princeton,NJ08543

Submittedto:

H.S.Ullal,ContractTechnicalMonitor

NationalRenewableEnergyLaboratory

1617ColeBoulevard

Golden,CO80401-3393

February28,2005

Preface

Technically,thin-filmPVtechnologieshaveadvancedconsiderablyinthelastfewyears.Theleadingthinfilmtechnologiesare,broadly,a-Si（andvariations）,CdTe,CIGS,c-Sifilm,anddye-sensitized.Atthetimeofwriting,theleadingcommercially-availablethinfilmtechnologieshavedemonstratedthefollowingrecordapertureareaefficienciesandpowersforlargeareamodules:

CIGSS13.1%64.8WShellSolarGmbH（glass）

CIGS13.0%84.6WWürthSolar（glass）

CdTe10.2%67.4WFirstSolar（glass）

a-Si/c-Si10.0%38.0WKaneka（glass）

CIGS10.1%71.2W*GlobalSolar（ss）

a-Si/a-SiGe/a-SiGe7.6%70.8W*UnitedSolar（ss）

CdTe7.3%52.3WAntecSolar（glass）

a-Si6.4%100WMitsubishiHeavyIndustries（glass）

a-Si/a-Si6.1%33.3WRWE（glass）

a-Si6.0%48.6WKaneka（glass）

a-Si/a-si5.8%43.3WEnergyPhotovoltaics（glass）

*assemblyofcells（notmonolithic）

Evenhighermoduleefficiencieshavebeendemonstratedbysomecompaniesthatcurrentlydonotusethetechnologycommercially,e.g.13.4%forCIGSbyShowaShell,and11%forCdTebyBPSolarandMatsushita.Withineachthin-filmsemiconductortechnologycategory,variousdepositionmethodshavebeendevised,andmanyarerepresentedintheabovetable.Whileahighmoduleefficiencyisdesirable,moduleefficiencyfiguresdonottellthewholestory.Thelongtermcommercialsuccessofthevariousapproachesisnotautomaticallyassured,butisdependentonacombinationofmoduleefficiency,manufacturingcostandmarketniche.Forexample,themanufactureofthin-filmc-Simodulesintheefficiencyrange8-10%canbeapproachedbydepositingandrecrystallizinga-Si:

H.Indeed,prototypemoduleshavebeenproducedandtheprocessusedhasbeendescribedintheliterature.Itisinstructivetoanalyzethisthin-filmc-Siprocess,andtocompareittoCIGSprocessing.TheanalysisrevealsmorecomplexprocessingthanisrequiredforCIGS（about20stepsversus12,exclusiveofencapsulation）,ahighcostfortheborosilicateglass,highcapitalcosts,andextensiveuseofindirectmaterialsinmultipleetchingprocesses.Itisnotclearthatitoffersaviablepathwaytocost-effectivemanufacturing.Amorphoussilicon,ontheotherhand,isoflowerefficiency,butcanbemanufacturedwithhighyieldandatthelowest$/Wofallthetechnologiesmentioned.CIGScontinuestoholdtheefficiencyrecord,butthetechnology,althoughhavingenteredtherealmofmanufacturing,isarguablynotyetsufficientlyevolvedtobecost-competitiveforproductionofstandardpowermodules.

OnefurtherfactorthatwilleventuallyemergeasastrongdriverofsuccessforPVtechnologiesintheenergymarketisthespecificenergyformoduleproduction.ForPVtocontinuegrowingat30%peryearforthenext30yearssothatitcantakeitsplaceasasignificantenergysourceontheworldstage,moduleswillhavetobemadeinamoreenergy-efficientmanner.Atthisgrowthrate,foranewPVfactorytogenerateapositiveenergyreturninlessthan10years,thespecificenergyformoduleproductionmustbelessthan18MJ/Wp[1].IfaparticularPVtechnologycannotmeetthiscondition,itmaybequestionedwhetherlargequantitiesofenergywillinpracticebeexpendedtomanufacturemodulesusingsuchatechnology.Thepublishedrangeoftotalenergyrequirementstoproducewafer-basedmodulesis20-100MJ/Wp.Fora-Sithefigureis12-15MJ/Wp（EPV）,whileforCIGSthefigureis11MJ/Wp（ShellSolar）.

FromtheabovediscussionweseethatthedrivingforcesforCIGSarecompelling:

potentiallyhighefficiencyandlowspecificenergyforproduction.Tothesewemayaddthebroadlyadvantageouspropertiesofmostthin-filmPVprocessesrelativetowafer-basedPV:

monolithicdesignandlargesubstrates（leadingtoreducedpartshandling）,lowconsumptionofbothdirectandindirectmaterials,andfewerprocesssteps.

EnergyPhotovoltaics,Inc.（“EPV”）isasolarenergycompanythatprimarilydesigns,develops,manufactures,andmarketsthin-filmphotovoltaic（PV）modulesandIntegratedManufacturingSystemstoservethegrowinginternationalPVmarketplace.ThestrategybeingpursuedbyEPVispremisedonafundamentalbeliefthat,forPVtobesuccessfulasmorethanaspecialtysourceofelectricity（withgrowthstimulatedbygovernmentincentiveprograms）,itmustdeliverelectricityatthelowestpossiblecost.Inthisvein,EPVcontinuestoshipitsEPV-40tandemjunction,amorphoussiliconPVmodulesmanufacturedatitsheadquartersinLawrenceville,NJ.ThemodulesareUL-listedandhaveIEC61646certification.Theproductionisfullysoldoutfor2005.OntheIMSfront,EPVcompleteda2.5MWa-SimodulemanufacturingplantfortheTianjinJinnengSolarCellCorp.inChinainApril2004.Theplantmetallproductionrateandqualitydeliverablesandisinfullproduction.EPVisalsosupplyingana-SiIMSratedat11MWtoHeliodomi,S.A.ofThessaloniki,Greece.

Inparallelwitha-Siproduction,EnergyPhotovoltaics,Inc.isalsodevelopingtechnologytobeabletocost-effectivelymanufacturemuchhigherefficiencyCIGSmodules.EPVhasconsistentlypursuedavacuum-basedapproachtoCIGSproduction,andhasdevelopednovellinearthermalsourcetechnologytosupplymaterialstoheated,moving,soda-limeglasssubstrates.Ithasalsodeliberatelychosentodevelopprocessingmethodswithworkersafetyinmind.Thesestrategically-importantchoicesofferalow-costsubstrate,controloverlayerhomogeneityandpurity,andproductionwithoutsignificanthazards.AlthoughsuchapproacheshelptominimizetheprocessingcostsofCIGS,furtheradvancesappearednecessaryinordertoimprovebotheaseofproductionandyield.Onesuchadvance,theintroductionofCusputtering,wasaccomplishedundertheNRELThin-FilmPhotovoltaicsPartnershipsProgram（TFPPP）.

TofacilitatethedevelopmentofCIGS,CdTe,andSi-basedthin-filmtechnologies,NRELoperatestheThin-FilmPhotovoltaicsPartnershipsProgram.Thelong-termobjectiveoftheTFPPPistodemonstratecommercial,low-cost,reproduciblemodulesof15%aperture-areaefficiency[2].AsaTechnologyPartnerwithinthisprogram,EPVhasperformedresearchunderathree-phase,cost-sharedsubcontractentitled“AdvancedCIGSPhotovoltaicTechnology”andparticipatesintheNationalCISTeamMeetings.Oneofthemainobjectivesofthissubcontract（RDJ-2-30630-21）wasforEPVtodemonstrateitscapabilitytoproducereasonablyefficientCIGSmodulesatasubstratesizeof4300cm2.Theprocessingalsoneededtobereproduciblewithgoodcontrollability.ThisgoalwassuccessfullyaccomplishedbythedevelopmentandutilizationofanewhybridprocessforCIGSgrowthduringthethreeyearcontractperiod.Thisfinaltechnicalreportmentionshighlightsofthefirstandsecondphasesofthesubcontract,anddescribesindetailresultsobtainedduringthethirdphaseofthesubcontract.

ThebenefitsaccruingfromthehybridprocessraisestheobviousquestionofwhetherprocessandequipmentdevelopmentdoesmoretoadvancePVtechnologythanthemoretraditionalresearchintomaterialsanddevices.

CONTENTS

Preface……………………………………………………………………………………ii

TableofContents…………………………………………………………………………v

ListofTables……………………………………………………………………………..vi

ListofFigures…………………………………………………………………………….vi

1.0Introduction………………………………..………………………………………..1

2.0HighlightsofPhasesIandII………………………………………………………..2

3.0CIGSOptimizationandDeviceResults……………………………………………3

3.1CompositionandCurichregime………………………………………………3

3.2Selenizationtemperature………………………………………………………4

3.3Gadistributionandbandgapprofile…………………………………………...5

3.4Otherparameters……………………………………………………………….5

3.5Deviceperformance……………………………………………………………5

4.0CIGSFilmAnalysis………………………………………………………………….6

4.1Scanningelectronmicroscopy………………………………………………...6

4.2DepthprofilingbyAugerelectronspectroscopy……………………………..7

4.3X-raydiffraction……………………………………………………………….8

4.4Activationenergy……………………………………………………………...9

5.0FullSizeModuleProcessandPerformance……………………………………….9

5.1CBDCdSmadeinfullsizetank……………………………………………….9

5.2InvestigationofcontactresistancebetweenZnOandMo…...………………...10

5.3Uniformityimprovementforlargeareaplates………………………………...11

5.4Moduleperformance…………………………………………………………..15

5.5Long-termstabilityofEPVmodule…………………………………………...16

5.6Processcontrolanddatabase…………………………………………………..17

6.0SurfaceTreatment…………………………………………………………………...17

7.0DevelopmentofNewTCOWindowLayers………………………………………..18

8.0FutureWork…………………………………………………………………………..19

9.0PhaseIIISummary…………………………………………………………………..20

Acknowledgments………………………………………………………………………….21

References………………………………………………………………………………….22

TABLES

Table3.1.Depositionconditionsandresultingcompositionanddeviceperformance

Table5.1.DeviceFFmappingtocomparebeakerandtankCBDprocess

Table5.2.Transmissiondistribution（at420nm）ofCTOglasscoatedwithCdSfilm

Table5.3.PerformanceofmodulesmadeduringtheFinalPhase

Table6.0.Deviceperformancewithvarioussurfacetreatments

Table7.0.PropertiesofTCOfilmsmadebyRE-HCS

Table7.1.J