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Applied Surface Science

j o u r n a l ho me p ag e :w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c

Effect of the deposition conditions of NiO anode buffer layers in organic solar cells, on the properties of these cells

D.T. Nguyen

a

, A. Ferrec

a

, J. Keraudy

a,c

, J.C. Bernède

b

, N. Stephant

a

, L. Cattin

a

, P.-Y. Jouan

a,∗

aInstitutdesMatériauxJeanRouxel(IMN),UMR6502,UniversitédeNantes,2ruedelaHoussinière,BP92208,Nantes44000,France

bMOLTECHAnjou,UMR6200,UniversitédeNantes,2ruedelaHoussinière,BP92208,Nantes44000,France

cIRTJulesVerne,CheminduChaffault,44340Bouguenais,France

a r t i c l e i n f o

Articlehistory:

Received3September2013 Receivedinrevisedform4April2014 Accepted5May2014

Availableonline27May2014

Keywords:

Organicphotovoltaiccells Anodebufferlayer NiOthinfilms DCreactivesputtering Formingprocess Annealing

a b s t r a c t

NiOthinfilmsdepositedbyDCreactivesputteringwereusedasanodebufferlayerinorganicphotovoltaic cells(OPVs)basedonCuPc/C60planarheterojunctions.FirstlyweshowthatthepropertiesoftheNiO filmsdependontheO2partialpressureduringdeposition.Thefilmsarefirstconductivebetween0and 2%partialoxygenpressure,thentheyaresemiconductorandp-typebetween2and6%partialoxygen pressure,between6and9%partialoxygenpressuretheconductionisverylowandthefilmsseemtoben- typeandfinally,forapartialoxygenpressurehigherthan9%,theconductionisp-type.Themorphologyof thesefilmsdependsalsoontheO2partialpressure.WhentheNiOfilmsisthickof4nm,itspeaktovalley roughnessis6nm,whenitissputteredwithagascontaining7.4%ofoxygen,whileitismorethandouble, 13.5nm,whenthepartialpressureofoxygenis16.67%.Thisroughnessimpliesthataformingprocess, i.e.adecreaseoftheleakagecurrent,isnecessaryfortheOPVs.Theformingprocessisnotnecessaryif theNiOABListhickof20nm.Inthatcaseitisshownthatoptimumconversionefficiencyisachieved withNiOABLannealed10minat400C.

©2014ElsevierB.V.Allrightsreserved.

1. Introduction

Whatever the organic photovoltaic cell (OPV) families, that basedonbulkheterojunctions(BHJ)orthatbasedonaplanarhet- erojunction(PHJ), thecarriercollectiondepends onthecontact betweentheorganicmaterialsandtheelectrodes,theheterojunc- tionbeingbasedonthecontactbetweenanelectrondonor(ED)and anelectronacceptor(EA).Oneofthemostimportantrequirements forhighpowerconversionefficiencyoftheorganicsolarcellsisthe carrierextractionfromtheorganicdiodeintotheelectrodes.The energybarrierforholeextractiondependsontheenergyleveldif- ferencebetweentheworkfunctionoftheelectrodeandthehighest occupiedmolecularorbital(HOMO)oftheorganicelectrondonor ED.GoodbandmatchingbetweentheanodeandtheEDisnecessary toachievegooddeviceperformance.Howeverthemostsuccessful anode,theindiumtinoxide(ITO)hasdifficultiestoextractholes fromtheED,becauseofitslowworkfunctionrelativetotheHOMO oftheED.Tosolvetheenergylevelmismatch,electrodesmodifica- tionisnecessary[1].Oneofthepossibilitiesistheintroductionof

Correspondingauthor.

E-mailaddress:pierre-yves.jouan@cnrs-imn.fr(P.-Y.Jouan).

ananodebufferlayer(ABL)betweentheITOanodeandtheorganic ED. One successful demonstration is using a PEDOT:PSS layer (poly(ethylenedioxythiophene):polystyrenesulphonate)[2].How- ever,PEDOT:PSShasbeenreportedtopresentsomedisadvantages suchasinconsistentfilmmorphology,inefficientelectron-blocking performance,poorstabilityunderambientconditionsanditiscor- rosivetoITO[3].ThereforeABLhavebeenshowntobesuccessful suchasanultrathingoldfilm[4]oratransitionmetaloxide[5–8].

Amongthetransitionmetaloxide,NiOhasbeenusedwithsuccess [9–13].IthasbeenreportedthatNiOthinfilmsisap-typesemicon- ductorwhichhaveitsvalencebandedgeat5.4eVandaconduction bandedgeat1.8eV[10],sothatabetterenergylevelalignmentis obtainedthanwithITOalone.Inthepresentworkweshowthat theeffectoftheNiOABLonthebehaviouroftheorganicsolarcells dependsstronglyonthedepositionconditionsofNiO.Asamaterof fact,theNiOfilmsbeingdepositedbysputtering,theirproperties dependstronglyonthedepositionconditions

2. Experimental 2.1. Anodepreparation

TheITOcoatedglasssubstratesusedinthisstudywerecom- merciallyobtainedfromtheSOLEMS.Itsthicknessis100nm,its http://dx.doi.org/10.1016/j.apsusc.2014.05.020

0169-4332/©2014ElsevierB.V.Allrightsreserved.

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conductivityisaround25/sq,itsaveragedtransmittanceinthe visibleis93%,itsroomsquaremeanssurfaceis0.8nmwhileitssur- faceworkfunctionis4.7eV[14].Afterscrubbingwithsoap,these ITOsubstrateswererinsedinrunningdeionisedwater.Thenthe substratesweredriedwithanitrogenflowandthenloadedinto avacuumchamberwhereweproceedtothedepositionofNiO.

TheNiOfilmsweredepositedbyDCreactivemagnetronsputtering usingapurenickeltarget(99.99%)andanAr+O2gasmixture.We haveusedatarget2inches(5.08cm)indiameter.Substrateswere placedatapproximately3cmfromthetarget.Thegasesargonand oxygenwereintroducedinthereactorbyamassflowcontrollers;

thedischargecurrentduringdepositionwas110mAand80mA.

2.2. Solarcellsrealisation

Inthepresentstudy,theplanarsolarcellsusedwerebasedon theclassicaljunctioncopperphthalocyanine(CuPc)/fullerene(C60).

CuPcistheelectrondonorandC60theelectronacceptor.Itisknown thatabufferlayer,calledexcitonblockinglayer(EBL),introduced betweentheelectronacceptorandthecathodeincreasesstrongly thecellsperformances(Bathocuproine(BCP)oraluminiumtris(8- hydroxyquinoline).InthepresentworkaluminiumAlq3hasbeen usedasEBLbecauseitallowsthelifetimeoftheOPVs.CuPc,C60 andAlq3 have beendeposited inavacuumof10−4Pa. Thethin filmdepositionratesand thicknesswereestimatedinsitu with a quartz monitor.Thedeposition rateand final thicknesswere 0.05nm/sand35nminthecaseofCuPc,0.05nm/sand40nmin thecaseofC60and0.1and9nmforAlq3.Thesethicknesseshave beenchosenafteroptimisation.Afterorganicthinfilmdeposition, thealuminiumtopelectrodeswerethermallyevaporated,without breakingthevacuum,throughamaskwith1mm×8mmactive areas.TheABLbeingthesputteredNiOfilm,thecellsweretypically:

glass/ITO/NiO/CuPc/C60/BCP(orAlq3)/Al.

2.3. Characterisationtechniques

Thethin filmsstructureswere analysedby X raydiffraction (XRD)byaSiemensD5000diffractometerusingK␣radiationfrom Cu(K=0.15406nm).

Theopticalmeasurementswerecarriedoutatroomtemper- ature usinga Carryspectrometer.The film optical densitywas measuredatwavelengthsof1.2–0.30␮m.

XPSmeasurementswereperformedwithaKratosAxisUltra spectrometerusingasourcemonochromatorAlK␣(1486.6eV).The spectrawererecordedforNiOthinfilmsdepositedatdifferentoxy- genlevelsforsampleswiththicknessof20nm.Kratosneutralise thesystemloadwasusedforallanalyses.Basicunitpressureis 8.10–8Torr.Highresolutionspectrawereobtainedusing20eVpass energy,ananalysisareaof≈500×500␮m.

Themorphologyofthedifferentstructuresusedasanodewas observedthroughscanningelectronmicroscopy(SEM)withaJEOL

7600F at the“centrede microcaractérisation del’Universitéde Nantes”.Imagesinsecondary(SEM)andbackscattering(BEI)mode havebeendone. Thecompositionofthefilmsisdeterminedby electronprobemicroanalysis(EPMA),thescanningelectronmicro- scope beingequipped with a PGT X-raymicroanalysis system;

X-raysweredetectedbyagermaniumcrystal.

AtomicForceMicroscopy(AFM)imagesondifferentsitesofthe filmweretakenexsituatatmosphericpressureandroomtem- perature.Allmeasurementshavebeenperformedintappingmode (NanowizardIII,JPKInstruments).Classicalcantileverswereused (TypePPP-NCHR-50,Nanosensor).Theaverageforceconstantand resonancewereapproximately14N/mand320kHz,respectively.

Thecantileverwasexcitedatitsresonancefrequency.

Themajoritycarriertypehasbeencheckedbythehotprobe technique.Ann-typeconstantanwirewasusedasthereference sample.TheelectricalresistivityandHallmobilityofthefilmswere determinedbyHalleffectmeasurementsinavanderPauwconfig- uration.

Photovoltaic characterisation was performed with an auto- mated I–V tester, in the dark and under sun global AM 1.5 simulatedsolarillumination.Performances of photovoltaiccells weremeasuredusingacalibratedsolarsimulator (Oriel300W) at100mW/cm2lightintensityadjustedwithaPVreferencecell (0.5cm2 CIGSsolarcell,calibratedatNREL,USA).Measurements wereperformedatanambientatmosphere.Alldeviceswereillu- minatedthroughTCOelectrodes.

3. Experimentalresults

3.1. Effectoftheoxygenpartialpressureonthepropertiesofthe NiOsputteredthinfilms

Asdescribedinapreviouspaperwitha1.3inchestarget[15], therearethreebasicfeedbacksignalsforcontrollingthereactive processduringsputterdeposition:massspectrometry[16],optical emissionspectroscopy[17]andcathodevoltage(i.e.dischargevolt- age)[18],whichdependsstronglyonthepropertiesofthematerial whichisformedonthecathodesurfaceduringthesputteringpro- cess[19].Witha2inchestarget,wehavefoundthesamebehaviour ofthecathodevoltage(Fig.1)increasingtheoxygencontentwith fixedvaluesofcurrentandflowofinertgas(10sccm,standardcubic centimetreperminute,ofArgon).Thepartialoxygenpressurehas beendeterminedbytheEq.(1).

%O2= O2flux

O2flux+arflux×100 (1)

AsobservedbyPiersonandRousselotforsilveroxidefilms[20], eachregionexhibitsatypicalX-raydiffractionpatternfrom111 to200preferentialorientationthroughtypicalpowderlikeX-ray diffractionpatternforstoichiometricfilms[15]andalsoatypical electronicbehaviour.Between0and2%oxygenpartialpressure,the

a b

Fig.1.(a)Differentzonesasafunctionofoxygencontent,(b)voltagecurvesV=f(%O2)forI=80,110and150mA.

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Fig.2.XPSdeterminationoftheFermilevelEFofNiOfilmsdepositedwithadischargecurrentof80mAandapartialoxygenpressureof:(a)of5.66%O2(point1inthe Fig.1);(b)of7.4%O2(point2intheFig.1);(c)of16.67%O2(point3intheFig.1).

filmsareconductive(metallicfilms).Between2and6%oxygenpar- tialpressure,thefilmsaresubstoichiometric,thereisO2vacancies andtomaintaintheelectroneutrality,thereissomemetallicnickel.

Indeed,amixturebetweenmetallicnickelandNiOiseasilyformed atlowoxygenambients[21].Therefore,metallicnickeldefectsand nickelvacanciescoexistinaNiOfilmgrownatlowoxygenpartial pressureand,asweincreaseoxygencontentinthegasmixture, metallicnickeldefectsdecreaseandnickelvacanciesincrease[22].

Between6and9%partialoxygenpressurethefilmcomposition isclosetothestoichiometry,theconductionisverylow.

Finally,forpartialoxygenpressurehigherthan9%,thenickel vacanciesincreasesandNi2+issubstitutedbyNi3+inordertomain- taintheelectroneutrality.Typically,forNi0.95Otheconcentration ofnickelvacanciesisabout6.75×1020/cm3,whichwouldmakeit ahighlydopedp-typesemiconductor[22].

AsshownonFig.1,thesefourregimesdependonthedischarge current(i.e.:NisputteredspeciesorNi/Oratio).

Theseresultsarecomfortedbythemeasurementofthediffer- encebetweenthevalencebandmaxima(EV)andtheFermilevel (EF)byXPS.ThebindenergyequaltozerocorrespondstotheFermi energywhichisthereferenceforpotentialifweconsiderthatwe haveagoodthermodynamicalcontactbetweenthesampleandthe spectrometer.Actuallytheresultsofthisdifferenceforthezones1, 2and3arepresentedinFig.2.

Forthezone1(5.66%O2),EV−EF=1.1eV,thefilmsarep-type.

Inthezone2(7.4%O2),ononehand,thefilmshaveaverylow conductivity(oneortwoorderofmagnitudeslowerthanforp-type thinfilms),ontheotherhand,EV−EF=3.5eV.Thatisthereason whywethinkthatthefilmscouldben-typeandthebandgaphigher than4eV.Finally,inthezone 3(16.67%O2)EV−EF=0.4eV,the filmsareclearlyp-type.Theseresults,addedtoonespublishedina previouspaper[15],provethatallcharacteristicsofNiOthinfilms

suchastransmittance,conduction,bandgap...,canbeadjusted withplasmaparameters,especiallywiththeoxygencontentinthe films.

AfterthedeterminationofthemajoritycarriersoftheNiOfilms depositedunderdifferenttypicalpartialoxygenpressure,theseNiO filmswerestudiedbyAFMandSEMcharacterisation.Typicalexam- plesoffilmsobtainedinthesputteringconditionsofzones2and3 arevisualisedinFig.3byAFMandbySEMinFig.4.

AbouttheNiOthinfilmscharacterisationbyAFMandbySEM, itmustbehighlightedthatthemorphologyoftheNiOsputtered filmsdependsontheoxygenpartialpressure.Forinstance,when theNiOfilmsisthickof4nm,itspeaktovalleyroughnessis6nm, whenitissputteredwithagascontaining7.4%(zone2,n-type) ofoxygen,whileitismorethandouble,13.5nm,whenthepartial pressureofoxygenis16.67%(Fig.3).Thesehighpeaksdistributed alloverthesurfaceoftheanodestructure,canbecorrelatedtothe darkpointsvisible intheimagesobtainedinthebackscattering modeoftheSEM(Fig.4b).Actually,thedensityof“blackpoints”

visibleinthebackscatteringmode,issimilartothatofthepeaks visualisedintheAFMimages.Inordertoinvestigatetheoriginof theseblackpoints,wehaveproceededtomicroprobemappingof thevisualisedsurfaces.Unfortunately,Ni,InandOmapsarehomo- geneouswithoutanyfeaturessimilartothatvisualisedbytheSEM, maybetheyaretoosmalltobevisualisedbythistechniquewhich resolutionislimitedbytheeffectofthepearshapedinteraction volume.ThepeakvisualisedbytheAFMbeingverysharp,they mayinduceaneffectofblindnesswhentheyarestudiedbySEM inthebackscatteringmode.Indeed,backscatteredelectronsleave thesampleperpendicularlytoitssurface.Therefore,ifthepeaks areverysharp,theywillappear blackinthephotos,thedetec- torbeingunabletodetectelectronparalleltothesurfaceofthe sample.

Fig.3.AFMimagesofNiOfilmsthickof4nmdepositedinthefollowingconditions:(a)partialoxygenpressure7.4%O2;(b)partialoxygenpressure16.67%O2.

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Fig.4. MicrophotographsofNiOfilmsthickof4nmdepositedinthefollowingconditions:(a)partialoxygenpressure7.4%O2(p-type,zone1);(b)partialoxygenpressure 16.67%O2(p-type,zone3).

Itisknownthatthesurfaceroughnessoftheelectrodecaninflu- encetheefficiencyoftheorganiccells.Asamatteroffact,arough surfacecaninducealargeinterfacebetweentheEDandtheEA, whichcanimprovetheefficiencyofchargeseparationandthere- foretheCurrentoftheOPVs.Ontheotherhandatoohighroughness ofthebottomelectrodemayinduceleakagecurrentinthedevice, whichdecreasestheshuntresistanceofthediodeandtherefore theperformanceoftheOPVsispoor[23].So,sincethemorphology oftheNiOfilmcanbemanagedthroughthedepositionconditions used,wehavesystematicallyusedNiOfilmsdepositedatdifferent O2partialpressuretoprobetheeffectofthemorphologyofthe bottomlayerontheOPVscharacteristics.Aswecanseeinthenext paragraph,wehavealsostudiedtheinfluenceofthethicknessand thepost-depositionannealingoftheNiOfilmsontheperformances oftheOPVs.

3.2. Organicsolarcellscharacterisation

FirstlywehavestudiedtheinfluenceoftheO2partialpressure ontheJ–VcharacteristicsoftheITO/NiO/CuPc/C60/Alq3/AlOPVs.

TypicalcharacteristicsareshowninFig.5(7%O2,NiOn-type)and Fig.6(16.7%O2,NiOp-type)theNiOfilmbeingthickof4nm.Itcan beseenthatthereisanimprovementoftheOPVsperformances withtimeduringthefirstminutesoflightexposureinthecaseofp- typeNiO.Thereissomethinglikea“formingprocess”foreachcycle,

Fig.5.TheJ–VcharacteristicsofthecellswithaNiOABLdepositedinthefollowing conditions:80mAofcurrent,16.67%O2and4nmofthickness.Thedifferentcycles weremeasuredafteracontinuousexposuretoAM1.5lightsources0min( ), 10min( )and1h30min( ).

Fig.6. TheJ–VcharacteristicsofthecellswithaNiOABLdepositedinthefollowing conditions:80mAofcurrent,7.4%O2and4nmofthickness.Thedifferentcycles weremeasuredafteracontinuousexposuretoAM1.5lightsources0min( ) and30min( ).

whichconsistsinadecreaseoftheleakagecurrentofthediode.In thedark,forthefirstcyclethecurrentisohmic.Underlightthereisa commutationformthisohmicbehaviourtoarectifyingeffect.Then, theshuntresistanceincreaseforeachcycleanditstabilisesafter10 ofexposuretolight(AM1.5),thePCEincreasesfrom1.48%to1.70%

(Fig.5,Table1).Itmustbenotedthatiftheformingprocess,i.e.the increaseoftheshuntresistance,isfasterunderlight,itisalsoactive inthedark.InFig.7onecanseethatthereisaslowtransitionfrom theohmictotherectifyingbehaviourcycleaftercycle,themeasure beingdoneinthedark.Inthesameway,whenthemeasuresare madeunderlight,itcanbeseeninFig.8thatwhenthesamplesare storedinthedarkbetweeneachmeasure,theformationoftheOPVs

Table1

FormingprocessunderlightofOPVswithNiOABLthinfilmsthickof4nm,theOPVs beingsubmittedtocontinuousexposuretolight.

t(min) Voc(V) Jsc(mA/cm2) FF(%) (%) Rs() Rsh() NiO,p-type,4nm(Fig.5)

0 0.460 6.21 55 1.48 3 185

10 0.470 6.16 58 1.70 3 620

300 0.490 5.84 58 1.67 4 755

NiO,n-type,4nm(Fig.6)

0 0.45 6.6 53 1.56 9 500

30 0.43 6.7 54 1.55 13 475

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Fig.7.TheJ–VcharacteristicsofthecellswithaNiOABLdepositedinthefollowing conditions:80mAofcurrent,16.67%O2and4nmofthickness.Thesampleissub- mittedtoelectricalcyclesintheobscurity(11times)andthen,1hafter,thesample issubmittedtoanewcycleintheobscurityandunderAM1.5.

Fig.8.TheJ–VcharacteristicsofthecellswithaNiOABLdepositedinthefollowing conditions:80mAofcurrent,16.67%O2and4nmofthickness.Thedifferentcycles weremeasuredafter0min( ),10min( )and90minunderAM1.5.After eachmeasurement,theOPVswereplacedinclosedbox,intheobscurityatroom temperature.

ismoreprogressive,butthestabilisedefficiencyisslightlyhigher thanthatobtainedaftercontinuousillumination(Table2).Inthe caseofn-typeNiOthereisnoformingprocess(Fig.6,Table1),the PCEis1.55%.

Ifformingprocesseshavealreadybeendescribedinorganiclight emittingdiodes(OLEDs)[24]itisnotthecaseforOPVs.Asamater offact,in thecaseof OLEDs,beforethecurrentthreshold volt- age,i.e.thevoltagefromwhichthecurrentincreasessteeply,there arereversiblemicroswitcheffects[25].Thesenegativedifferential resistance(NDR)effectsinducecurrentchangebyseveralordersof magnitude.SuchNDRareusuallyattributedtosmallshortcircuit effectinducedbythesurfaceroughness.Itturnsoutthattheprob- abilityforappearanceofNDRdecreasesmarkedlywithincreasing Table2

Thedifferentcyclesweremeasuredafter0min,10minand1h30minunderAM1.5.

Aftereachmeasurement,thesampleswereplacedinclosedbox,intheobscurityat roomtemperature(Fig.8).

t(min) Voc(V) Jsc(mA/cm2) FF(%) (%) Rs() Rsh()

0 0.25 6.45 32 0.51 3.5 45

10 0.43 6.28 43 1.16 3.5 170

90 0.48 6.03 61 1.75 2.5 930

Fig.9.TheJ–VcharacteristicsofthecellswithaNiOABLdepositedinthefollowing conditions:150mAofcurrent,23.07%O2and20nmofthickness,thedifferentcycles weremeasuredafter0minand30min,6hand24h.

thickness[26].Such areasoningcanapplyhereforourcells.As amatteroffact,whenitisthickofonly4nm,theNiOfilmmay notcompletelycovertheITOfilm.So,thedifferenceofbehaviour ofthecellsusinga4nmthickNiOABLcanberelatedtothedif- ferentmorphologiesofthefilms. Whendepositedunder16.67%

O2 ofoxygen(p-typeNiO) thepeakheightismorethandouble thanthatobtainedwithapartialpressureof7.4ofoxygen(n-type NiO).Therefore,thevacuumdepositionoforganicfilmscouldbe dramaticallyaffectedby“shadoweffect”.Ifthetopoftheanode becomestoorough,throughthepeaksdiscussedabove,itwould beratherdifficulttocovercompletelyuniformlytheanodewith theorganiclayerand,duetodifferentdepositionangle,thecath- odecanbe,inverysmallplaces,indirectcontactwiththeanode, conductivepathscanform,leadingtocurrentleakage.Therefore, theohmiclikeconductionoftheOPVsisduetoultrathinlocalised metalpathsbridgingthetopandbottomelectrodes.Whensubmit- tedtoapotential,i.e.whenacurrentcirculatesintheseultrathin filamentstheyaredestroyedbyJouleeffect.Ofcoursethiseffect isfasterwhenthecurrentishigher,whichisthecasewhenthe OPVsaresubmittedtolight.Moreovertheformingprocessisactive whenNiOisp-type,thatistosaywhenitismoreconductive.Itwas shown,inthecaseofWO3,anothertransitionmetaloxide,that, bychangingtheadditionaloxygencontentoftheoxide,switching insulating-conductivestatesmaybemanaged.Thecommutation isattributedtoconductivepathformation[27].Whenitisn-type, NiOissmoothanditishighlyresistiveandthereforethereisnot formingprocess.

IthasalreadybeenshownthattheNiOABLimprovesstrongly thelifetimeoftheOPVs[12],whatmakespossibletheforming processandtheimprovementwithtime,duringthefirstminutes ofairexposure,ofthePCEoftheOPVs.

ItisknownthatbymodifyingthethicknessoftheNiOABLispos- sibletomodifytheperformancesoftheOPVs.Therefore,inorder toavoidanyformingprocesswehaveincreasedtheNiOthickness.

WhentheNiOanodebufferlayeristhickof20nmtheJ–Vchar- acteristicsarestableasitcanbeseeninFigs.9and10.Thereis nomoreanyformingprocesswhateverthetypeoftheNiO,p-type orn-type(Table3).Thecurvesexhibitsystematicallyaclassical shape,whatevertheO2partialpressureduringNiOdeposition.As amatteroffact,whentheNiOfilmis20nmthickitrecoverscom- pletelytheITOfilm,whichavoidsanydirectcontactbetweenthe anodeandthecathodeevenif“shadoweffects”arepresent,which

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Fig.10.TheJ–VcharacteristicsofthecellswithaNiOABLdepositedinthefollowing conditions:80mAofcurrent,7.4%O2and20nmofthickness.

Table3

OPVsperformanceswithNiOABLdepositedinthefollowingconditions:150mAof current;23.07%O2and20nmofthickness,thedifferentcyclesweremeasuredafter 0min,30min,6hand24h.

t(min) Voc(V) Jsc(mA/cm2) FF(%) (%) Rs() Rsh() NiO,p-type,20nm(Fig.9)

0 0.46 4.19 59 1.15 2.5 1200

30 0.46 4.43 59 1.20 2.5 1200

360 0.47 4.17 59 1.16 3 1150

1840 0.46 3.81 55 0.97 3.5 900

NiO,n-type,20nm(Fig.10)

0 0.42 5.5 56 1.5 29 1600

explainsthataformingprocessisnotnecessarytoachieveclassical behaviour.

Therefore,inordertochecktheeffectofannealingoftheNiO ABLonthePCEoftheOPVs,NiOfilmsthickof20nmhavebeen used.Beforedepositionoftheorganiclayers,theITO/NiOstructures havebeensubmittedtoannealingunderoxygenatmospherefor 10mininatubularoven.ItcanbeseeninFig.11that,upto400C theannealingimprovesthePCEoftheOPVs,forhigherannealing temperatureitdecreasesprogressively(Table4).Thisimprovement isduetotheincreaseofthetransmittance[28].

Forhigherannealingtemperature,theITObottomlayercon- ductivitydecreases,duetothedecreaseofoxygenvacanciesand thereforetheseriesresistanceincreasestothedetrimentofthe PCE.

Fig.11.TheJ–VcharacteristicsofthecellswithaNiOABLdepositedinthefollowing conditions:80mAofcurrent,7.4%O2and20nmofthickness,withoutannealingand after10minofannealingat300C,400C,500Cand600C.

Table4

AnnealingeffectontheperformancesofOPVswithNiOABLdepositedinthefol- lowingconditions:80mAofcurrent,7.4%O2and20nmofthickness(Fig.11).

Annealing temperature(C)

Voc(V) Jsc(mA/cm2) FF(%) (%) Rs() Rsh()

0.47 5.48 56.3 1.45 3 1600

300 0.49 5.96 55.5 1.62 3.5 500

400 0.50 7.71 45.1 1.75 21 540

500 0.51 6.75 46 1.60 47.5 710

600 0.51 6.24 35 1.11 255 560

AboutthethicknessoftheNiOABL,ithasalreadybeenshown, inthecaseofP3HT:PCBMbulkheterojunctionthatwhentheNiO filmisthickof4–8nmtheOPVshavepoorFF,whichisattributed tolargeleakagecurrentanddeficientelectronblockingcapability [11].Thisresultcorroboratesthepresentstudy,whichshowsthat reproducibleandoptimumresultsareachievedwith20nmthick NiOABL.

4. Conclusions

ByvaryingthepartialoxygenpressureduringNiOsputtering depositionitispossibletomanagethepropertiesoftheNiOthin films.Firstmetallicfilmsareobtainedbetween0and2%partial oxygenpressure, then theNiO filmshave a p-type conduction between 2 and 6% partial oxygenpressure, between 6 and 9%

partialoxygenpressuretheirconductionisverylowandn-type.

Finally, for a partialoxygen pressure higher than 9%, the con- ductionoftheNiOfilmsisp-type.Whenthesefilmsareusedas ABLinITO/NiO/CuPc/C60/BCP/AlcellsthepropertiesoftheseOPVs dependontheO2partialpressureduringNiOdepositionandon thethicknessofthisABL.Whenthickof4nm,aformingprocess ispresentduringthefirstelectricalcycles.Thisformingprocess dependsontheNiOsputteringconditions.Whenthefilmsaren- type,O2partialpressure7%,theformingprocessismarginaldue tothefactthattheNiOfilmsareresistiveandsmooth.Whenthe NiOfilmsarep-type,O2partialpressure16.7%,theJ–Vcharacter- isticsvarystronglyduringthefirstcycles,whichisattributedto thedestructionofleakagefilamentpresentintheOPVs.Asamat- teroffact,whendepositedintheseexperimentalconditionsthere isahighdensityofthinpeaksatthesurfaceoftheNiOfilmsand thepresenceofthesepeaksmayinducesultrathinleakagefila- mentsthroughsomeshadowingeffects.Theconductivefilaments aredestroyedbyJouleeffectduringtheformingprocess.Inorderto avoidtheformingprocess,NiOthinfilmshavebeenusedasABL.In thatcaseoptimumefficiencyisachievedwhentheNiOisannealed at400C.

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