Thin-Film Solar Cells Device Measurements and Anal

PROGRESSINPHOTOVOLTAICS:RESEARCHANDAPPLICATIONSProg.Photovolt:Res.Appl.2004;12:155–176(DOI:10.1002/pip.518)Thin-FilmSolarCells:DeviceMeasurementsandAnalysisStevenS.Hegedus*,yandWilliamN.ShafarmanInstituteofEnergyConversion,UniversityofDelaware,Newark,DE19716-3820,USACharacterizationofamorphousSi,CdTe,andCu(InGa)Se2-basedthin-filmsolarcellsisdescribedwithfocusonthedeviationsindevicebehaviorfromstandarddevicemodels.Quantumefficiency(QE),current–voltage(J–V),andadmittancemeasure-mentsarereviewedwithregardtoaspectsofinterpretationuniquetothethin-filmsolarcells.Ineachcase,methodsarepresentedforcharacterizingparasiticeffectscommoninthesesolarcellsinordertoidentifylossmechanismsandrevealfunda-mentaldeviceproperties.Differencesbetweenthesethin-filmsolarcellsandidealizeddevicesarelargelyduetoahighdensityofdefectstatesintheabsorbinglayersandtoparasiticlossesduetothedevicestructureandcontacts.Thereisalsocommonlyavoltage-dependentphotocurrentcollectionwhichaffectsJ–VandQEmeasurements.Thevoltageandlightbiasdependenceofthesemeasurementscanbeusedtodiag-nosespecificlosses.ExamplesofhowtheselossesimpacttheQE,J–V,andadmit-tancecharacterizationareshownforeachtypeofsolarcell.Copyright#2004JohnWiley&Sons,Ltd.keywords:thinfilm;amorphousSi;Cu(InGa)Se2;CdTe;quantumefficiency;admittanceINTRODUCTIONThin-filmsolarcells(TFSCs)haveachievedefficienciesapproaching15–20%.Muchoftheprogresshasoccurredbyempiricalprocessingimprovementsandinspiteofarelativelypoorunderstandingoftheunderlyingmechanismsandelectronicdefectswhichcontrolthedevicebehavior.However,aseachtechnologymatures,amorecompletepictureofthedeviceoperationhasemergedtoenablebothabetterunder-standingofthedevicesandidentificationofpathwaystofurtherimprovements.Inthisarticle,differencesbetweenidealcrystallinesolarcellsandtypicalTFSCswillbeidentified.Itwillbeshownthatthephotovoltaicperformanceofpresentdaya-Si,CdTe,andCu(InGa)Se2-baseddeviceshavemuchincommon,despiteverydifferentprocessing,devicestructures,andabsorberproperties.Experimentalandanalyticalprocedureswillbedescribedtocharacterizethequantumefficiency(QE),current–voltage(J–V),andadmittancebehavior.Ineachcase,theapproachwillbebasedoneliminatingparasiticeffectscommontotheTFSCsinordertorevealthefundamentalpropertiesoftheintrinsicjunctionandabsorber.TheobjectiveofthisarticleistodemonstratemethodologyforanalyzingTFSCswithexamplesfromdifferentdevicesratherthanjusttoreviewpreviouswork,soinmanycasesexcellentworkintheliteraturecouldnotbecited.Copyright#2004JohnWiley&Sons,Ltd.Received2September2003*Correspondenceto:StevenS.Hegedus,InstituteofEnergyConversion,UniversityofDelaware,Newark,DE19716-3820,USAyE-mail:[e-mail]/grantsponsor:NREL;contract/grantnumber:ADJ-1-30630-12.SpecialIssueSolarcelloperation,eithercrystallineorthinfilm,canbedescribedbyidentifyinglossmechanisms.Thesecanbedividedintothreecategories.Firstarerecombinationlosseswhichlimittheopen-circuitvoltageVOC.Secondareparasiticlosses,suchasseriesresistance,shuntconductance,andvoltage-dependentcurrentcollec-tion,whichprimarilyimpactthefillfactor(FF),butcanalsoreduceshortcircuitcurrentJSCandVOC.Finally,thereareopticallosseswhichlimitgenerationofcarriersand,therefore,JSC.WefocusonlosseslargelyuniquetoTFSCs.PhysicalandelectricalpropertiesofTFSCswhichcausethemtohavedifferentlossesfromthestandard‘text-book’crystallineSi(c-Si)cellsinclude:*TFSCabsorberlayershavemuchhigherabsorptioncoefficientsthanc-Sisoalargefractionofthephotogenerationoccursneartheinterfaceandinthehighfieldspacechargeregion(SCR).Thisenableshighcurrents,evenwithrelativelysmallcollectionlengths;*thesemiconductorfilmsoftenhavearangeofshallowanddeepdefectlevelsordefectbandswithinthebandgap.Theseresultfromimperfectcrystallinityoramorphousstructureandfromtheuseoflow-costmaterialsandprocessesoptimizedforhighthroughputandlowcostasmuchasforhighdeviceefficiency.Thiscancreatedifferentrecombinationmechanismsthanradiativeband-to-bandrecombinationcommonlyfoundinidealcrystallinesemiconductordevices;*poorminoritycarrierlifetime,duetotheabovefactors,leadstoincreasedrelianceontheelectricfieldforsufficientminoritycarriercollectionratherthandiffusionalone.Thisoftenresultsinvoltage-dependentcollectionoflight-generatedcurrent;*TFSCsareheterojunctiondevicestructureswithhighdensitiesofdefectstatesatinterfaceswhichcanprovideapathforinterfacerecombination;*thegrainboundariesinpolycrystallineCu(InGa)Se2andCdTedevicesmayactashighrecombinationsur-facesorshuntpaths.Thisleadstolateralfieldnonuniformityandrecombinationvaryingintwodimensions;*non-ohmicorblockingcontactscanlimitcurrentatforwardbias.Devicestobestudiedhereincludea-Sip–i–norn–i–ptypecells,andpolycrystallineCdTe/CdSandCu(InGa)Se2/CdSp–nheterojunctioncells.QEmeasurementswillbeusedtodetermineopticallosses,opticalenhancements,andvoltage-dependentphotocurrentmechanisms.J–Vmeasurementsunderstandardtestconditionswillbeshownandasystematicproceduretoseparatediodeandparasiticeffectswillbepresented.StandardC–VmeasurementsoftenfailtoprovidemuchinformationonTFSCiftheydonottoyieldalinearC
2–Vrelation,sowewill