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.

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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

charg-

Figure2.I–VcharacteristicsofthebestdevicesfabricatedwithaZnOparticlelayerannealedatvarious

temperatures.Figure3.UPSspectraofZnOnanoparticlelayersannealedatvarioustemper-atures.

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ingduetothephotonfluxoftheUVsourceusedfortheUPSmeasurements,andthemeasurementswerereproduci-bleacrossmultiplesamples.Additionally,spectrawerehighlyreproducibleandnochangeswereobservedduetothelow-intensityUVradiationusedfortheUPSmeasure-mentsortheeffectofstoringthesamplesunderultra-highvacuum,whichsuggeststhatthelowworkfunctionvalueob-servedisnotattributabletothelossofsurfaceoxygenorduetoUVexposure,asreportedpreviously.[20]Also,asallthreesamples(A,B,andC)wereannealedinairbeforetransfer-ringintothevacuumchamber,theexposureofallsamplestohydrocarbonsand/ormoisturewassimilar.Therefore,thepresenceofhydrocarbonsand/orwateronthesurfacecannotbetheonlyreasonforthelowvalueoftheworkfunctionmeasured.

ThevalencebandmaximumforSampleAwasfoundat3.6eVrelativetotheFermilevel,andforsamplesannealedathighertemperaturesthiswasfoundtodecrease(3.5eVforSampleBand3.4eVforbothSamplesCandD).Theob-serveddependenceofthevalancebandpositiononthean-nealingtemperatureisquitesignificant,asitinfluencestheenergylevelalignmentbetweentheZnOandPCBMandthusthechargetransportacrosstheinterface.TheelectronaffinityforSampleAwasfoundtobe3.5Æ0.05eV,whichin-creasedsignificantlyto3.9Æ0.05eV,4.0Æ0.05eV,and3.9Æ0.05eVforSamplesB,C,andD,respectively.

Tobetterunderstandtheinterfacialenergeticsinaninvert-eddevicestructuresuchastheonereportedhere,wemodeltheenergylevelalignmentofZnOwithPCBMtakingintoaccounttheinterfacialdipolebetweentheZnOandPCBM,asreportedelsewhere.[21]TheenergybandalignmentofZnOwiththeBHJintwodifferentscenarios(1508Cand2508CannealingtemperaturesofZnO)isdepictedinFigure4,con-structedusingtheworkfunctionandvalencebandmaxima(VBM)valuesofZnOasmeasuredusingUPS.Itisassumed

thatthebandalignmentinthecaseofbulkheterojunctiondeviceswouldbesameasthatincaseofaninvertedbilayerstructureresultinginFermilevelalignmentbetweentheZnOandPCBM.[21]

TheoffsetbetweentheconductionbandofZnOandtheLUMOofPCBMforDeviceC,wasfoundtobe0.5eV,facil-itatingefficientelectrontransferacrosstheBHJ–ZnOinter-face.However,inthecaseofDeviceA,therewasnooffsetfoundbetweentheconductionbandofZnOandtheLUMOofthePCBM.ThiscouldresultinpoorchargeseparationandtransportacrossBHJ–ITOelectrode,whichisevidentfromtherelativelylowshort-circuitcurrentdensity(Jsc)ob-servedfromtheI–VcharacterizationofDeviceAcomparedtoDevicesBandC.

ThedipoleattheinterfaceusedtoconstructFigure4isbasedontheassumptionthatitisthesameasinDavisetal.[21]ThisassumptionmightbeincorrectbecausetheZnOusedinliterature[21]mightbeslightlydifferentthantheZnOusedinthepresentstudyandthiscouldalsopotentiallybeaffectedbydifferentannealingeffectsattemperaturesof150and2508C.However,whatisimportantforthecorrelationofFigure4withtheI–VcharacteristicsisnottheexactvalueoftheoffsetbetweentheconductionbandofZnOandtheLUMOofthePCBMbutthefactthattheoffsetismorepos-itivefortheZnOsampleannealedat2508Cthanat1508C.Evenforthecasethatourassumptionaboutthedipoleattheinterfaceisincorrect,thisdifferencewillbeaffectedonlytoaminordegree.

X-rayphotoelectronspectroscopy(XPS)wasusedtode-termineiftherearechemicaldifferencesinducedintheZnOlayerduetotheannealingconditionsthatmayhaveaffecteddifferentelectronicproperties,asfoundwithUPS.Figure5andFigure6showthecorelevelXPSspectraofZn2p3/2andO1scorelevelsrespectively,foreachoftheannealingconditions.

TheZn2p3/2peakwasfoundtobesymmetricforallSam-plesA,B,C,andDanditexhibitsashiftof

approximately

Figure4.EnergylevelalignmentofaZnOnanoparticlelayerwiththeacceptor(PCBM)intheBHJinaninvertedOPV.TheenergyoffsetattheZnO–PCBMinterfaceisshowedin(A)forthecaseof1508Cannealingand(B)forthecaseof2508Cannealing.Thevalueforthedipoleusedtoconstructtheenergydiagramisbasedonanassumptiondiscussedinthemainbodyofthe

text.

Figure5.CorelevelXPSoftheZn2p3/2level.

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ofthetotaloxygencorrespondingtothezincoxidematrix.However,astheannealingtemperatureincreased,therela-tivepeakareaofthehigherbindingenergycomponentofoxygendecreasescomparedtothelowerbindingenergycomponent(correspondingtotheZnOmatrix)becoming68%and73%ofthetotaloxygenpresentinthesystemforSamplesCandD,respectively.

TheobservedincreaseintheproportionofoxygenintheZnOmatrix(thelowerbindingenergycomponent)uponthermalannealingcanbeattributedtoeithertheremovalofchemisorbedoxygenspecies(e.g.,water)orthedecomposi-tionofremainingtracesoftheacetateprecursorintheZnOparticlelayer

FromtheUPSofSamplesA,B,andC,threecharacteristicemissionpeaksatapproximately11.5,8.2,and5.2eVwerefound,asseeninFigure7.Thesepeakshavepreviouslybeen

Figure6.CorelevelXPSoftheO1s

level.

0.4eVfrom1021.66to1022eVwiththeincreaseinanneal-ingtemperature,asshowninFigure5.

Thisshifttowardshigherbindingenergyhaspreviouslybeenattributedtoincreasedoxidationofzincandhenceamoreelectronegativeenvironment.[19]Theatomicpercent-ageofZnwasfoundtoincreasefrom33%at1508Canneal-ingto44%and45%fortreatmentsat200and2508C,re-spectively,attheexpenseofthetotaloxygeninthesystem.Theratioofthetotaloxygenrelativetozincandcarbonde-creasesfrom37%to29%withhigherannealingtempera-tures.

Theoxygenpeaksinallsampleswerefoundtobeasym-metricwithtwocomponentsatapproximately530and532eV,asshowninFigure6,andbothofthecomponentsofoxygenwerefoundtoshiftslightlytowardsthehigherbind-ingenergyafterheatingfrom150to2608C.

Thelowerbindingenergycomponentcorrespondstotheoxygeninthezincoxidematrixinstoichiometriccomposi-tion,[22]whereasthehigherbindingenergycomponentat532eVhasearlierbeenreportedtobeduetochemisorbedoxygen,[23]zinchydroxide,[24]orduetothepresenceofre-mainingfragmentsofthezincacetateprecursor.[25]BasedontheUPSspectra,wewillshowbelowthatthepeakat532eVcouldpartiallybeattributedtoOinZnOaswell.

Thepeakratiosofthetwocomponentsoftheoxygenpeakwerefoundtobecloseto1:1forSampleA,withonly51%

Figure7.UPSspectrafromSamplesA,B,C,andDdepictingthethreechar-acteristicpeakemissions.

attributedtotheZn3dband,bondingoftheZn4sand4pwiththeO2pelectrons,andtotheO2pnonbondingelec-trons,respectively.[26,27]

TheZn3dpeakat11.5eVforSamplesBandCwasfoundtobemorepronouncedthanthatofSampleA.Nopeakat8.2eVwasfoundforSampleA,howeverforSamplesBandC,thisfeaturealsobecomesmoreprominent.Thisisconsis-tentwiththeXPSresults,whichshowedincreasedzinc–oxygeninteractionforSamplesBandC.ForallSamplesA,B,C,andD,nosignificantpeakshiftwasobserved.

ForSampleA(annealedat1508C),theO2ppeakwasalmostabsentinitially,butwithfurtherheattreatment(upto2608C),boththeZn3dandO2pfeaturesbecamemoreprominentasseenfromSampleD.Thissuggeststhatthe1508CannealingtreatmentresultsinZnOthatisdeficientinthenonbondingoxygenorbital(lonepair),whichcouldalsohaveimplicationsforitselectronicpropertiesandthusforthedeviceperformance.

Physisorbedwateronthesurfaceshowsanoxygenfeatureatapproximately10eVintheUPS.[28–30]SomeintensityisseeninSampleAinthisregion(aGaussianfunctioncouldbefittedaftersubtractingthebackground);however,itis

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servedtoamuchlowerdegreeinSamplesB,C,andD,sug-gestingthatthe532eVpeakobservedbyXPSinallsamplescannotbeexclusivelyduetochemisorbedoxygenorab-sorbedwaterasthereisnowaterfoundonthesesamplesbyUPSmeasurement.Theoxygencomponentat532eVasseeninthecorelevelXPSofoxygen(Figure6)couldthereforebeduetothepresenceoftheacetateintheZnOparticlelayer,water,andoxygeninZnO.

Allpossibilitiesfortheoriginofthechangesintheoxygenstructuredescribedabovecouldbethereasonforthechangeinworkfunction.Basedontheinformationavailablethroughthisstudy,itcannotbedecidedwhichofthepossibilities(orwhichcombinationofpossibilities)isthereasonforthechangeinworkfunctionuponheating.However,becausetheworkfunctionisameasureforthepolarityoftheinterface,itcanbeassumedthatthepresenceofacetategroupsatthesurfaceisthelikelyreasonforthelowworkfunctionofthesampleannealedat1508C;ofthecompoundsZnO,H2O,andacetate,thelatteristheleastpolarcompound.

Further,theC1sregionoftheZnOparticlelayerforallsamples(Figure8)wasfoundtohavethreecomponentswiththestrongestpeakatapproximately285eVandsmallerpeaksat287and289eV,whichhavepreviouslybeenattrib-utedtotheCÀHorCÀCbonds,CÀO,andC=O/COOÀbonds,respectively.[31]

Althoughtherewasonlyaslightdifferenceof1%ob-servedinthetwocarboncomponentsathigherbindingenergyafterthesampleswereheatedfrom1508C(Sam-pleA)to2508C(SamplesBandC),theatomicconcentra-tionofthecarboncomponentat285eVwasfoundtode-creasefrom22%(SampleA)to15%and13%forSam-plesBandC,respectively.AfterSampleAissubsequentlyheatedat2608Cundervacuum(toformSampleD),thecarboncomponentat285eVreducesfrom22%toonly17%.ThepeakpositionsandatomicconcentrationsofZn,O,andCatdifferenttemperaturesaresummarizedinTable2.

TheXPSresultssuggestingthepresenceofremainingace-tateintheZnOparticlelayerannealedat1508CagreewellwiththeprevioushypothesisestablishedfromtheI–Vchar-acterizationofthesedevicesthatresidualacetateororganicimpuritiescouldexplainthehighseriesresistanceobservedindevicesthatareannealedatsuboptimaltemperatures.Theloweropen-circuitvoltageofDeviceAascomparedtoDevi-

Figure8.CorelevelXPSoftheC1s

level.

cesBandCcanbeattributedtothereducedshuntresistanceinDeviceA.

Conclusions

TheannealingtemperatureoftheZnOparticlelayerdepos-itedontheunderlyingITOelectrodeininvertedOPVsiscriticaltoachieveoptimumdeviceperformance;theefficien-cywasfoundtohaveadirectdependenceontheannealing

temperatureoftheZnObufferlayer.Anefficiencyof2.3%wasachievedfromdevicesin-corporatingaZnOparticlelayerannealedat1508C,whichsignificantlyincreasedto3.6%

withanincreaseintheanneal-ingtemperaturesofupto

2508C,showntobeoptimalforZnOfilmsfromzincacetateprecursors.TheworkfunctionoftheZnOparticlelayertreated

at

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1508Cwasfoundtobe3.2eV,significantlylessthanprevi-ouslyreportedvalues,butitincreasedto3.9eVaftertheZnOlayerwasannealedattemperaturesofupto2508C.Moreimportantly,theelectronaffinityofZnOparticlelayerannealedat1508Cwasfoundtobe3.5eV,comparedto4.0eVforaZnOparticlelayerannealedat2508C.Thesig-nificantlylowerelectronaffinityforlow-temperatureanneal-ingleadstozeroenergyoffsetbetweentheconductionbandofZnOandtheLUMOofPCBMandresultsinpoorchargetransportacrosstheBHJandtheITOelectrode;thisistheproposedmechanismforthepoorperformancefordevicesinwhichtheZnOlayerwasannealedatlowertemperatures.Thechangeinelectronaffinitycanbeattributedtodifferen-cesinthesurfacechemistryintheZnOlayeratdifferentan-nealingtemperatures.ThenatureoftheoxygenspeciesintheZnOchangeswithannealingtemperatureandthiscouldbeduetoeithertheeliminationofremainingprecursorim-puritiesorchangesinthenatureofthezinc–oxygenbonditself;however,itcannotbeduetochangesinphysisorbedwater.Heatingthepartiallyconvertedzincacetateinvacuumto2608CleadstoZnOthatshowsdeficienciesofthenonbondingoxygenorbital.

ThoughZnOparticleinkcanbeusedforflexibleOPVsasalow-temperaturealternativetothesol–gelmethod,thisstudysuggeststhattemperatureslowerthan2008Careinsuf-ficientforachievingZnOthatisoptimalfordeviceper-formance.

TheheterojunctionblendwaspreparedbydissolvingP3HT(45mg,Merck)andPCBM(36mg,fromnanoC)inchloroben-zene(1.5mL).Bothsolutionsweredissolvedseparatelyingloveboxat808Cforonehour,followedbycoolingdown,filtering,andstirringatroomtemperature.TheheterojunctionlayerofP3HT:PCBMwasspincoatedat3000rpmfor30sfollowedbyannealingat1508Cfor10min.

Amolybdenumoxidelayer(20nm)wasthermallyevaporatedastheanodicbufferlayerbeforeevaporatingthesilveranode(100nm).

Electronspectroscopy

TheinvestigationsoftheZnOparticlelayeronITOsubstrateswithXPSwereperformedbyusinganultra-highvacuum(UHV)apparatusbuiltbySPECS(Berlin,Germany)withanonmono-chromaticX-raysourceforMg.ThebasepressureoftheUHVchamberwasontheorderof10À10mbar.High-resolutionXPSspectrawereobtainedtodeterminethechemicalstatesofzincandoxygeninthenear-surfaceregionoftheZnOthinfilms.TheXPSspectrawerereferencedtotheC1speak,whichwasduetotheadventitioushydrocarbonsabsorbedontheZnOsurface.TheC1speakwassetto285eV.

Theapparatuswasfurtherequippedwithultravioletphotoelec-tronspectroscopy(UPS)withatwo-stagecoldcathodegasdis-chargefromMFS(Clausthal-Zellerfeld,Germany)togeneratesimultaneouslymetastableheliumatoms(He*3S1)andlow-in-tensityUVlight(HeIline).Thespectraoftheelectronsemittedfromthesampleswererecordedatapassenergyof10eV,withahemisphericalPhoibos100energyanalyserfromSPECS.Atthepassenergyof10eVtheanalyserhasanenergyresolutionof400meVasevaluatedfromtheFermiedgeofpolycrystallinesilver.Theanglebetweenthelightirradiationsourcesandtheanalyzer(bothHe*/UVandX-ray)wereboth548.TheUPSspectrawereacquiredbyapplyingabiasof10Vtothesampletoclearlyobservethesecondaryelectroncut-off.

InaUPSexperimentthesamplewasirradiatedwithUVphotonsleadingtophotoionizationviathephotoelectriceffect.Theenergyofemittedelectronsisgivenby

ExperimentalSection

ZnOparticleink

TheZnOparticlesweresynthesizedbydissolvingzincacetatedehydrate(0.44g)inethanol(40mL)at608Cfor30min.Thiswasfollowedbydrop-wiseadditionoftetramethylammoniumhy-droxide(2mL,20%inMeOH)inethanol(10mL)tothesolu-tionovertheperiodof5min.TheZnOnanoparticlesolutionwasheatedat608Cfor30mintoattainzincoxidenanoparticlesofapproximately5nminsize.Detailedinformationaboutthesynthesiscanbefoundelsewhere.[32]

EkE¼EðhnÞÀEbinÀ0specð1Þ

Devicefabrication

ITO-coatedglasssubstrates(7W/&)werecleanedusingthestan-dardrecipe—theglasswascleanedindeconexPAneutraldeter-gent(5%solutionsuppliedbyBorer)at908Cfor20min.Thesampleswerethenrinsedindeionized(DI)waterfollowedbysuccessivesonicationfor10mineachinDIwater,acetone,andisopropanol.SubsequentlyUV-ozonecleaningoftheITOsub-strateswasappliedusingaNovascanPDS-UVTUV/ozonecleanerwiththelampintensitybeinggreaterthan36mWcmÀ2atadistanceof100cm,givinganozoneconcentrationgreaterthan50ppmatambientconditions.

TheZnOparticlelayerwasspincoatedat3000rpmfollowedbyannealingonapreheatedhotplateinair.Threedifferentsetofsampleswerepreparedandwereannealedat150,200,and2508C.Fordevicefabrication,allZnOcoatedsamplesweretransferredtothegloveboxafterannealing.WiththeexceptionoftheZnOannealingtemperatures,alldeviceswerepreparedsimilarlyusingthesamebatchofmaterials.

inwhichEkEisthekineticenergyoftheemittedelectron,E(hn)thephotonenergy(21.22eVfortheHeIlineusedhere),Ebinthebindingenergyoftheelectronbeforeexcitation,andfspecthespectrometerworkfunction.UPSspectracanbeusedtodeter-minetheworkfunctionandthedensityofstatesinnear-surfaceregionofamaterial.Theworkfunctionofthesampleswasdeter-minedasthedifferencebetweentheexcitationenergyandthewidthofthespectrum.Thelatterisgivenasthedifferenceofthehighbindingenergycut-offandthecut-offofthespectrumatthelowestbindingenergy.

I–Vcharacterization

Invertedsolarcellswithanactiveareaof0.1cm2werefabricat-ed,withdeviceareabeingdefinedbytheelectrodegeometry.Thecurrent-voltage(J–V)characteristicsofthedevicesweremeasuredinsidetheglovebox,withoutencapsulation,underAM1.5Girradiation(100mWcmÀ2).ThedevicesweretestedbyusingaKeithley2400sourcemetercontrolledbyLabviewsoft-ware.

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Acknowledgements

TheauthorswouldliketoacknowledgeDr.JacekJasieniakofCSIROforfruitfuldiscussionsandforprovidingtheZnOparticleink.A.S.wishestoacknowledgeFlindersUniversityforpostgraduateresearchscholarshipandCSIROforaPhDstudentship.ThisworkhasbeenfundedthroughtheFlexibleElectronicsThemeoftheCSIROFutureManufacturingFlag-shipandbytheSchoolofChemicalandPhysicalSciencesofFlindersUniversity.

Keywords:energylevelalignment·PCBM·photovoltaics·workfunction·zincoxide

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Received:December16,2013Revised:January27,2014

Publishedonlineon&&&&,0000

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

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热处理温度对倒置OPV电池器件的影响

FULLPAPERS

A.Sharma,S.E.Watkins,G.Andersson,*D.A.Lewis*&&–&&

EffectofAnnealingTemperatureofZnOontheEnergyLevelAlignmentinInvertedOrganicPhotovoltaics

(OPVs)

OptimizingZnOforOPVs:Thean-nealingtemperatureofZnOlayersisfoundtohaveasignificantimpactontheefficiencyofinverteddevices.Thedependenceoftheelectronicproper-tiesontheannealingtemperatureisat-tributedtoadeficiencyofelectronscorrespondingtothenonbonding(lonepair)oxygenorbitalsintheZnOmatrixandthepresenceofprecursorimpurities.

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

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