Effect of Annealing Temperature of ZnO on the Energy

热处理温度对倒置OPV电池器件的影响

DOI:10.1002/ente.201300186

EffectofAnnealingTemperatureofZnOontheEnergyLevelAlignmentinInvertedOrganicPhotovoltaics(OPVs)

AnirudhSharma,[a]ScottE.Watkins,[b]GuntherAndersson,*[a]andDavidA.Lewis*[a]

Introduction

Organicphotovoltaics(OPVs)arerapidlymaturingasatech-nologyanddeviceswithpromisingefficienciesarebeingre-ported;[1]howevermanychallengesremainbeforetheyarecommerciallyviable.Thepromiseofhigh-speedroll-to-rollprocessinghasthepotentialtosignificantlyreducethecostofproductionandenablelarge-scaleproductionbyusingvar-iousmethods;[2–6]however,processingtemperaturesandin-terlayerstabilityremainsignificantchallenges.

OPVsbasedontheinvertedstructureindiumtinoxide(ITO)/zincoxideparticlelayer(ZnO)/poly(3-hexylthiophene)(P3HT):[6,6]-phenyl-C61-butyricacidmethylester(PCBM)/MoO3/Ag(asshowninFigure1)havethepotentialtoover-comeinterfacialinstabilityatthepoly(3,4-ethylenedioxythio-phene):poly(styrenesulfonate)(PEDOT:PSS)/ITOinter-face,[7,8]whichcouldotherwiseleadtodevicedegradationandshorterlifetimesinconventionalOPVs.[9]ZnOhasbeenwidelyexploredasacathodematerialininverteddevices

andvariousprocessingmethodscompatiblewithflexiblesub-stratesarebeingexplored.[10–12]Mostcommonly,ZnOissyn-thesisedinsitu,whichinvolveshighprocessingtemperaturesofover3008C.[10,13]AlthoughthesemethodsenablethequicksynthesisofZnObufferlayers;theyareincompatiblewithmaterialswithlowglasstransitiontemperatures,suchaspol-yethyleneterephthalate(PET)andpolyethylenenaphthalate(PEN),whicharetheleadingcandidatesforflexiblesub-stratesinroll-to-rollprocessingofOPVs.

TouseZnObufferlayersonflexiblesubstrateswithouttheneedforhigh-temperaturepost-depositionannealing,theZnOnanoparticleshavebeendepositedasthinfilmsbyusingroll-to-rollcompatiblemethodssuchasspincoating[10]andslot–dieprinting.[14]Theparticlelayermuststillbean-nealedafterdeposition,notonlytoconsolidatetheparticles,butalsotopromoteadhesiontothesubstrateandremovetheligands[15–17]aswellasanyremainingorganicfragmentsoftheprecursorthatarepresenttoaidedispersionandavoidaggregation.Ithasbeenshownthattheannealingtempera-turecaninfluencethechemicalcompositionofinsitupro-ducedZnOderivedusingsol–gel,forexample.[12]Therefore,inthecaseofZnOparticlelayers,itisequallyimportanttooptimizeandunderstandtheimpactoftheannealing

temper-

[a]A.Sharma,Prof.G.Andersson,Prof.D.A.LewisFlindersCentreforNanoscaleScienceandTechnology

SchoolofChemicalandPhysicalSciences,FlindersUniversitySturtRoad,BedfordPark,Adelaide,SA5001(Australia)E-mail:gunther.andersson@flinders.edu.au

david.lewis@flinders.edu.au[b]Dr.S.E.Watkins

MaterialScienceandEngineeringCSIRO

BayviewAvenue,Clayton,Victoria,3168(Australia)

Figure1.SchematicofaninvertedOPVincorporatingZnOparticlelayer.

EnergyTechnol.0000,00,1–8 2014Wiley-VCHVerlagGmbH&Co.KGaA,

Weinheim

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Thesearenotthefinalpagenumbers!

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热处理温度对倒置OPV电池器件的影响

G.AnderssonandD.A.Lewisetal.

atureondeviceperformance.A

rangeofpost-processingmeth-odshavebeenemployedon

[18]

ZnOparticlefilms,yettherehasnotbeenanysystematicstudiestodeterminethede-pendenceofprocessconditions

onthechemicalandelectricalpropertiesoftheZnOlayerandtheresultingdeviceproperties.

Inthisstudy,theeffectofthepost-depositionannealingtemperatureonthechemicalandelectronicpropertiesofZnOparticlelayersininvertedOPVsisreported.X-rayandultravioletphotoelectronspectroscopies(XPSandUPS)areusedtostudythesurfacechemistryandelectronicpropertiesoftheZnOparticlelayer.ThemeasurementsofenergylevelpositionsofZnO(annealedatvarioustemperatures)asde-terminedusingUPSwerecorrelatedtotheI–Vmeasure-mentsandusedtounderstandthedifferencesobservedinthedeviceperformance.ThechangesinducedintheenergybandsofZnOasaresultofvariousannealingtemperaturesarerelatedtochargetransportacrosstheZnObufferlayerfromthebulkheterojunction(BHJ)tothecathode.

ResultsandDiscussion

InvertedOPVswerefabricatedusingthreedifferentpost-depositionannealingtemperaturesfortheZnOparticlelayer:DeviceA(annealedat1508C),DeviceB(annealedat2008C),andDeviceC(annealedat2508C).Figure2showstheresultantI–Vcharacteristics.Thephotoconversioneffi-ciencyofDeviceAwasfoundtobe2.3%,andDevicesBandCwerealmostidenticalwithanincreasedefficiencyof3.6%.InbothDevicesBandC,themaximumopen-circuitvoltagewas620mVcomparedwith590mVforDeviceAandtheshort-circuitcurrentdensitywascorrespondinglyhigher,asshowninTable1.AlsointhecaseofDeviceA,

thevariationobservedintheshort-circuitcurrentovermulti-plesampleswaslargeincomparisontoDevicesBandC.Theimprovementinseriesresistancewithhigheranneal-ingtemperatureisreflectedinimprovedfillfactors(FFs)of49%(DeviceB)and48%(DeviceC)comparedto38%inthecaseofDeviceA.Thefillfactorisdrivenbytheseriesandshuntresistanceinthedevices,whichwerecalculatedfromtheinverseslopesofthedarkI–VcurvesatV=1VandV=0V,respectively.TheseriesresistanceofDeviceAwasfoundtobe28Wcm2,anorderofmagnitudehigherthanthoseofDevicesBandC,whereastheshuntresistancewasfoundtobe730Wcm2,almosthalfthevalueofDevicesBandC.ToinvestigatetheoriginoftheobserveddifferencesintheI–VcharacteristicsofDevicesA,B,andC,ultravioletphotoelectronspectroscopy(UPS)wasperformedtodeter-mineanypossiblechangesintheelectronicstructureofdif-ferentlyannealedZnO.Followinginitialspectroscopicmeas-urements,SampleAwasheatedto2608Cintheinstrument(underultra-highvacuum)andisreferredtoasSampleD.UPS(Figure3)showsthattheworkfunctionincreasesfrom3.2Æ0.05eV(SampleA)to3.87Æ0.05eV(SampleC)withincreasedannealingtemperaturesduetothesecondary-electroncut-offmoving0.7eVtowardslowerbindingener-giesforSamplesBandCrelativetoSampleA.ThevaluefortheworkfunctionforSampleAissignificantlylowerthanthecommonlyreportedvalueoftheworkfunctionforZnO,[19]thoughaworkfunctioncloseto3eVforaZnOnanoparticlelayerhasalsobeenreportedbyGutmannetal.[20]Inthespectrum,therewerenovisiblesignsof

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