• Aucun résultat trouvé

Towards anode with low indium content as effective anode in organic solar cells

N/A
N/A
Protected

Academic year: 2021

Partager "Towards anode with low indium content as effective anode in organic solar cells"

Copied!
7
0
0

Texte intégral

(1)

Science Arts & Métiers (SAM)

is an open access repository that collects the work of Arts et Métiers Institute of

Technology researchers and makes it freely available over the web where possible.

This is an author-deposited version published in:

https://sam.ensam.eu

Handle ID: .

http://hdl.handle.net/10985/8483

To cite this version :

Saad TOUIHRI, Linda CATTIN, Duc-Tuong NGUYEN, Mustapha MORSLI, Guy LOUARN, Anne

BOUTEVILLE, Vincent FROGER, Jean-Christian BERNÈDE - Towards anode with low indium

content as effective anode in organic solar cells - Applied Surface Science - Vol. 258, n°7,

p.2844-2849 - 2012

Any correspondence concerning this service should be sent to the repository

Administrator :

archiveouverte@ensam.eu

(2)

Towards

anode

with

low

indium

content

as

effective

anode

in

organic

solar

cells

S.

Touihri

a

, L.

Cattin

b

, D-T.

Nguyen

b

, M.

Morsli

c

, G.

Louarn

b

,

A.

Bouteville

d

,

V.Froger

d

,

J.C.

Bernède

e,∗ aUnitédePhysiquedesDispositifsàSemi-conducteurs,UniversitéElManarFacultédesSciencesdeTunis,CampusUniversitaire2092,Tunisia

bLUNAM,UniversitédeNantes,InstitutJeanRouxel(IMN),UMR6502,2ruedelaHoussinière,BP92208,NantesF-44322,France cLUNAM,UniversitédeNantes,FacultédesSciencesetdesTechniques,2ruedelaHoussinière,BP92208,NantesF-44322,France

dArtsetMétiersParisTechAngers,LaboratoireProcédés-Matériaux-Instrumentation,2,bdduRonceray,BP3525,49035AngersCedex,France eLUNAM,UniversitédeNantes,MoltechAnjou,CNRS,UMR6200,FSTN,2RuedelaHoussinière,BP92208,NantesF-44322,France

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received18June2011

Receivedinrevisedform6October2011 Accepted29October2011

Available online 7 November 2011 Keywords:

Organicsolarcell Reactiveevaporation In2O3

Surfaceroughness Surfaceworkfunction

a

b

s

t

r

a

c

t

In2O3thinfilms(100nmthick)havebeendepositedbyreactiveevaporationofindium,inanoxygen

partialatmosphere.Conductive(=3.5×103S/cm)andtransparentfilmsareobtainedusingthe

follow-ingexperimentalconditions:oxygenpartialpressure=1×10−1Pa,substratetemperature=300Cand

depositionrate=0.02nm/s.LayersofthisIn2O3thickof5nmhavebeenintroducedinAZO/In2O3and

FTO/In2O3multilayeranodestructures.Theperformancesoforganicphotovoltaiccells,basedonthe

coupleCuPc/C60,arestudiedusingtheanodeasparameter.Inadditiontothesebilayers,otherstructures

havebeenusedasanode:AZO,FTO,AZO/In2O3/MoO3,FTO/In2O3/MoO3andFTO/MoO3.Itisshownthat

theuseoftheIn2O3filminthebilayerstructuresimprovessignificantlythecellperformances.However

theopencircuitvoltageisquitesmallwhilebetterefficienciesareachievedwhenMoO3ispresent.These

resultsarediscussedinthelightofsurfaceroughnessandsurfaceworkfunctionofthedifferentanodes. © 2011 Elsevier B.V. All rights reserved.

1. Introduction

Organicsolarcellsareattractingconsiderableinterestforsolar energyconversion.Organicphotovoltaiccells(OPV)arebasedon a couple electron donor (ED)/electron acceptor (EA). Two OPV familieshavebeendevelopeddependingontheED/EAinterface geometry,theplanarheterojunction(PHJ)andthebulk heterojunc-tion(BHJ).PHJarebasedonthesuperpositionofthinfilmsofdonor andacceptormaterials,whileinBHJtheactivelayerconstitutesof interpenetratednetworksofthetwoorganicmaterials(EDandEA). Whateverthecellfamilytypeis,theorganiclayersaresandwiched betweentwoelectrodes,oneofthem,atleast,mustbetransparent [1].

Untilnow,mostOPVincorporatedtindopedindiumoxide(ITO) astransparentelectrode.ITOis highlyconductive(>103S/cm)

withagoodtransparency(T>90%)inthevisiblerange.Moreover itsworkfunction,˚ITO,ishigherthanthatofotherstransparent

conductive oxide(TCO).˚ITO is around4.6±0.2eV after

clean-ingofcommercialITO[2–4].Thislattercanbeincreasedbysome treatment suchasUV–ozone plasma [5]. Also, the˚ITO can be

higherthantheclassicalvalueiftheITOlayerisusedimmediately afterdeposition.Forinstance,when depositedbyRFmagnetron sputtering,˚ITOcanbeashighas5.5eV[6].However,indiumis

∗ Correspondingauthor.

E-mailaddress:jean-christian.bernede@univ-nantes.fr(J.C.Bernède).

scarceandthepriceofITOisincreasingduetoahighdemandin alargevarietyofapplications.Thereforeitwouldbevery attrac-tivetouseothertransparentelectrode[7].IfothersTCO,suchas aluminiumdoped zinc oxide(AZO) or fluorinedoped tinoxide (FTO)canshowelectrical andopticalpropertiesclosetothat of ITO,theirworkfunctionsaresubstantiallysmaller[8,9].Toaddress thequestionofwhetheritcanbepossibletokeepthesurface prop-ertiesofITOthroughsmallerindiumconsumption,wehaveprobed AZO/In2O3andFTO/In2O3bilayerstructures,inthethicknessratio

tIn2O3/tTCO=1/20,asanalternativetoITOanodeinclassicalOPV.

AftercharacterizationofthedifferentTCOthinfilms,theproperties ofthebilayerstructureshavebeenprobedandthenusedasanode inOPV.Forcomparison,differentanodeconfigurationshavealso beenprobed:TCO/MoO3,TCO/In2O3/MoO3.

2. Experimental

2.1. Depositionprocessandcharacterizationtechniquesofthe TCOthinfilms

FTOfilmshavebeenprovidedbySOLEMS,whiletheAZOand In2O3filmshavebeendepositedinthelaboratory.TheFTOfilms

havebeenobtainedbychemicalvapourdeposition,whiletheAZO filmshavebeendepositedbyrfmagnetronsputtering.The deposi-tionprocessofthezincoxidefilmshasbeendescribedinaprevious paper[10].Shortly,theyweredepositedbyacmagnetron sputter-ingonglasssubstratesatroomtemperature.ThecylindricalZnO

0169-4332/$–seefrontmatter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.10.146

(3)

targetcontains2wt%ofAl2O3.Arwasusedassputteringgasfor

depositionoftheAZOfilms.Thesputteringpressurewas main-tainedat0.13Pa.Thepressureofthesputteringsystembeforethe depositionofthesamplefilmswasunder2×10−3Pa.

In2O3 thin films were deposited by reactive evaporation of

indium,inanoxygenpartialatmosphere,ontoglasssubstrate.The filmswereelaboratedinavacuumchamberfromanIndiumsource withresistivelyheatedtungstencruciblesource.During deposi-tionthesubstrateswereheatedat300◦C.Theywereheatedbyan

infraredsource.Thetemperatureofthesubstratewasdetermined byathermocouple(typeK)onthedepositionface.Before deposi-tionthevacuuminthedepositionchamberwas5×10−4Pa,during

deposition,theflowrateofoxygenwascontrolledbyusingaflow meter.Thedepositionpressurewasmaintainedat1×10−1Pa.An

HFquartzoscillatorwasusedtocontroltheevaporationrate.Tobe suretoachievecompleteindiumoxidationat300◦C,the

deposi-tionratewas0.02nm/s.BeforeprobingtheIn2O3inanodebilayers

inOPV,wehavedeposited100nmthickfilmtochecktheoptical andelectricalpropertiesofourIn2O3thinfilms.

The crystallinestructureof thefilms wasanalyzedbyX-ray diffraction(XRD) byaSiemensD5000diffractometerusingKa radiationfromCu(=0.15406nm).Themorphologywasobserved throughscanningelectronmicroscopy(SEM)withaJEOL6400F. Thefilmsthicknesswasmeasuredfromthecrosssection visual-izationusingsimplesoftware.Thecompositionofthefilms was checkedbyelectronmicroprobeanalysis(EMPA)usingaJEOL5800 microscope.

XPSanalyseswereperformedwithaLeyboldLHS-12 appara-tus.Thedatawereobtainedwithamagnesiumsourceofradiation (1253.6eV)operatingat10kVand10mA.Theenergyresolution was 1eV at pass energy of 50eV. High-resolution scans were obtainedintheIn3d,O1sandalsoC1sregionsofthespectrum.The quantitativeXPSstudieswerebasedonthedeterminationofthe In3d5/2andO1speakareaswith3.8and0.61assensitivityfactors,

(thesensitivityfactorsaregivenbythemanufacturer,Leybold). Theopticalmeasurementswerecarriedoutatroom tempera-tureonaCarryspectrometer,from2to0.20mmwavelengths.The four-pointsprobetechniquehasbeenusedtomeasurethe elec-trical conductivity. Hall effect measurements were performed utilizingtheVanderPawarrangement.

AFMimagesondifferentsitesofthefilmsweretakenat atmo-sphericpressureandroomtemperature.Allmeasurementswere performedintappingmode(NanoscopeIIIa,(Veeco,Inc.). Classi-calsiliconcantileverswereused(NCH,nanosensors).Theaverage forceconstantandresonancewere40N/mand300kHz, respec-tively.Thecantileverwasexcitedatitsresonancefrequency.Such apparatuscanalsobemanagedaselectrostaticforcemicroscope, whichallowsusingitassurfaceKelvinprobemicroscope.

AKrussG40ContactAngleMeasuringSystemG40isusedto esti-matethesurfaceenergyofTCOsurfacesthroughthesessiledrop method.Adropofliquidwithknownsurfacetension(water, for-mamide,ethyleneglycolandglycerol)isputontheTCOsurface andaccordingtotheOwens,Wendt,RabelandKaelblemethod, thesurfacetension,splitupintoapolarandadispersefraction,can bededuced.

2.2. Organicsolarcellsrealizationandcharacterization

Thisworkdealswiththeinfluenceoftheanodeconfiguration onorganicsolarcellsperformance.Therefore,analreadyknown multilayersheterojunctionstructurebasedonaCuPc/C60junction

withbathocuproine(BCP)aselectronblockinglayerwaschosen. CuPcdonorandC60acceptororganicmaterialsarecommercially

availableandwerepurifiedbyvacuumsublimation[11].Allthe resultspresentedinfiguresortableshavebeenattainedwithcells issuedfromthesamebathsofproducts.

Fig.1. X-raydiffractiondiagramofaIn2O3thinfilmachievedbyreactive

evapora-tion.

CuPc,C60andBCPweredepositedunder10−4Pavacuum.The

thinfilmdepositionrateandthicknesswereestimatedinsituby thequartzmonitor.Thedepositionrateandfinalthicknesswere respectively0.05nm/sand35nmforCuPcand0.05nm/sand40nm forC60,whiletheywere0.1nm/sand9nmforBCP.

Afterorganicthinfilmdeposition,withoutbreakingthevacuum, analuminiumupperelectrode,throughamaskwith2mm×5mm activearea,andthenanencapsulatinglayerofamorphousselenium (Se-a),thickofabout100nm,werethermallyevaporated.

The selenium coating layerhas been proved to be efficient toprotecttheunderlayersfromoxygenandwatervapour con-tamination [12], at least during the first hours of room air exposure[13].Theusedstructureswereasfollows:glass/anode structure/CuPc/C60/BCP/Al/Se-a.

ThedifferentanodestructureswillbedescribedinSection3. ElectricalcharacterizationwasperformedonanautomatedI–V tester,inthedarkandunder1sunglobalAM1.5calibratedsolar simulator(Oriel300W)at100mW/cm2 light intensityadjusted

withaphotovolaticreferencecell(0.5cm2Cu(InGa)Se

2solarcell,

calibratedatNREL,USA.Measurementswereperformedat nor-malroomatmosphere.AlldeviceswereilluminatedthroughFTO electrodes.

3. Experimentalresultsanddiscussion 3.1. In2O3thinfilmcharacterization

Thethicknessesofthefilmsusedwere100nmforIn2O3thinfilm

characterizationand100nmfortheTCO(AZOorFTO)and5nmfor theIn2O3inthecaseofanodebilayerTCO/In2O3.

AsshownbyXRDinFig.1,thefilmsarepolycrystalline.They arecrystallisedintheexpectedbixbyitestructure.Thecrystallites arepreferentiallyorientedalongthe[222]and[400]directions.A typicalmicrographofthefilmsobtainedispresentedinFig.2a.The filmsaregranular,whichisingoodagreementtotheXRDstudy. Theyarehomogeneousandcompact.Forcomparison,inFig.2band carevisualizedFTOandFTO/In2O3films.ItcanbeseeninFig.2,that,

allthestructuresarepolycrystallinewithsimilaraveragedgrain diameterofaround65nm.Themicroprobeanalysisshowsthatthe filmsarestoichiometric.Thelevelofaccuracyofthemethoddoes notallowputtingdirectlyinevidencetheoxygenvacancydensity. ForXPSstudy,thesampleshadtostayinairfromthe deposit-ingchambertotheanalysisapparatus,thereforetheyhavebeen gentlyetched before analysis.Ifsomesurface contaminationis still present, the quantitative analysis shows that the surface ofthefilms isnearly stoichiometric(around60at.%of oxygen), whileitcanbeseeninFig.3athatthemainO1scontributionis

(4)

Fig.2. MicrophotographsofanITO(a),aSnO2(b)andaSnO2/ITO(3)structure.

situatedat 530.3eV, which correspondsto oxygen bounded to

indium.Thesecondcontribution,only15%,issituatedat532.6eV.

Itcan beattributedtosurface contamination. TheIn3ddoublet

(Fig.3b),withIn3d5/2=444.5eVandIn3d3/2=452eV,corresponds

toindiumboundedtooxygen[14].ThespectruminFig.4shows

thattransmissionofafilminthevisibleandnearinfraredishigher than80%upto95%for≈1100nm.Atroomtemperature,the con-ductivityofthefilmsis3.5×103S/cmwithacarriermobilityof

10.5cm2V−1s−1andacarrierdensityof1.4×1021cm−3.

AllthesestudiesshowthattheIn2O3filmsobtainedbyreactive

evaporationexhibitthepropertiesrequestedforaTCO.Therefore theycanbeintroducedinananodebilayersuchasAZO/In2O3 or

FTO/In2O3.

Fig.3. In2O3XPSspectraofanIn2O3thinfilm,O1s(a)andIn3d(b).

3.2. OrganicphotovoltaiccellsusingAZO/In2O3andFTO/In2O3

anodes

AssaidabovetheOPVcellsstudiedweretypically:glass/anode structure/CuPc/C60/BCP/Al/Se-a.

The anode structures probed were: AZO, AZO/In2O3,

AZO/In2O3/MoO3 and FTO, FTO/In2O3, FTO/In2O3/MoO3. MoO3

(5)

Table1

PhotovoltaicperformancedataunderAM1.5conditionsofOPVcellsusingdifferent anodes. Anode Jsc(mA/cm2) Voc(V) FF(%) (%) AZO 2.85 0.22 15 0.10 AZO/In2O3 5.50 0.28 51 0.77 AZO/In2O3/MoO3 8.70 0.43 53 1.95 FTO 3.50 0.30 15 0.15 FTO/In2O3 6.50 0.25 25 0.40 FTO/In2O3/MoO3 6.60 0.43 50 1.42 FTO/Mo03 6.50 0.45 50 1.45

hasbeenusedasbufferlayerbetweentheanodeandtheorganic electrondonorsinceithasbeenshownthatitisveryefficientin improvingtheholecollectionandOPVcellsperformances[15].

ItcanbeseeninTable1andFig.5that,whatevertheTCOused, AZOorFTO,theOPVcellsperformancesaresignificantlyimproved whentheTCOiscoveredwithathinIn2O3layer,thickof5nm.This

improvementcorrespondstoanincreaseofthefillfactorandshort circuitcurrent.Itshouldalsobenotedthat,inthecaseofbareTCO (FTOorAZO),acurrent-limitingeffect,a“rollover”,isclearlyvisible. ThisrollovervisibleontheI–VcurvesofbareTCOhasdisappeared afterIn2O3depositionabovetheTCO.Howeveritcanbeseenthat

theopencircuitvoltageVocstaysquitesmall.Therefore,inorderto

improvetheVocwehaveintroduceda3nmthickMoO3bufferlayer

attheinterfaceIn2O3/organicmaterial.Fig.5andTable1showthat

Fig.5. (a)TypicalJ–VcharacteristicsofAnode/CuPc/C60/BCP/Alstructure,with

Anode=ZnO (), ZnO/In2O3 (d) and ZnO/In2O3/MoO3 (N), under

illumina-tion of AM1.5solar simulation (100mW/cm2).(b) Typical J–V characteristics

of anode/CuPc/C60/BCP/Al structure, with anode=SnO2 (), SnO2/In2O3 (d),

SnO2/MoO3(N)andSnO2/In2O3/MoO3(H),underilluminationofAM1.5solar

sim-ulation(100mW/cm2).

Table2

Propertiesofthesurfaceoftheanode.Thetotalsurfacetensionandcorresponding components,polforpolarcomponentanddispfordispersioncomponentaregiven withthepolarityp.

Sample Disp(mN/m) Pol(mN/m) Totalsurface tension(mN/m) p AZO 8.8 45 53.8 83.6 AZO/In2O3 7.8 52.5 60.3 87.1 FTO 13.8 35.5 49.3 72 FTO/In2O3 6.2 58.1 64.3 90.4

theMoO3 filmimprovessignificantlyVoc,andthereforethe effi-ciencyoftheOPVcells.WehavealsoprobedFTO/MoO3anodes. ThebestresultsachievedarereportedinFig.5,whileinTable1

theresultscorrespondtoaveragedmeasurementsissuedfrom

dif-ferentbatchesofOPVcells.Ifthebestefficiencyisobtainedwith thetrilayersanodeFTO/In2O3/MoO3,itcanbeseenthat,inthecase

oftheaveragedresultsofTable1,roughlysimilarperformances areachievedwhatevertheanode structure:FTO/In2O3/MoO3 or

FTO/MoO3.ThisresultshowsthatMoO3isnecessaryforachieving

goodanode/electrondonorinterface,eveninthepresenceofafresh In2O3thinfilm.

ThesmallvaluesofVoc,whentheTCOanodeiscoveredbyathin

In2O3 layer,canarisefromsmallshuntresistance,Rsh,valueand

throughtrapsandpoorbandmatchingattheanodeinterface.Small Rshvalue,issuedfromthepresenceofpin-holesand/orpeakeffects

(roughness),whichshortcircuittheOPVcells,inducesstrongdark currentandsmallVocunderillumination.Aboutthebandstructure,

indeed,ithasbeenshownthatthepresenceoftherollovereffect intheI–Vcurvescouldbeattributedtothepresenceofanopposite diodeattheinterfaceanode/organicmaterial,whichdecreasesthe Vocvalue[16].

Inordertounderstandthedifferentbehaviourswiththe

orig-inalanodestructures,wehaveconductedcomplementarystudies

ontheanodes,usingscanningelectronmicroscopy,atomicforce

microscopyandcontactanglemeasurements.

Firstofall,itshouldbenotedthatthepresenceofa rollover

effectintheI–Vcurves,whenbareAZOorFTOareused,should

beattributedtothepresenceofanoppositediodeattheinterface

anode/organicmaterial.Thisbarriercanbesuppressedbybetter

bandadjustment.Therefore,thedisappearanceoftherolloverin

theI–Vcurvescanbeattributedtoabetterbandmatchinginduced bythepresenceofthethinIn2O3 layer,sincetheworkfunction

measuredinroomairbyaKelvinprobeis,atleast,0.2eVlarger thanthatofSnO2.

ResultsofthecomplementarystudiesareshowninFigs.2and6.

TheexampleofFTOmicrophotographieswithandwithoutIn2O3

isshowninFig.2.Ifthereisnostrongmodificationofthe mor-phologyoftheanodeafterIn2O3deposition,smallbrightspotsare

visible,randomlydistributedonthefilmsurface.Asimilarresult hasbeenachievedwithAZO.Theseresultsarecorroboratedbythe AFMstudy(Fig.6),asamatteroffact,thereissomeincreaseof thermsafterIn2O3deposition.Forinstance,thereisanincrease

of35%ofthermsinthecaseofZnO(ZnOrms=1.67toZnO/In2O3

rms=2.27forZnO).

AsseeninTable2,thedepositionofathinIn2O3thinfilmonto

AZOorFTOelectrodeincreasessensiblythesurfaceenergyofthe anode.Ithasalreadybeenreportedthat theincreaseofsurface energywouldprovideabetteradhesionoforganicfilmtothe sub-strate.Moreoverhighersurfaceenergyoftheanodeisbeneficialto asmoothgrowthoftheorganicfilm[17].

From the above studies it can be deduced that the thin

In2O3 layerimprovesthepropertiesoftheinterfaceFTOorAZO

anode/CuPcthrougha betterbandmatchingandhighersurface

energy,whilethereissomedegradationoftheanodesurface mor-phology.HoweverthereisnoobviousreasonforthesmallVocof

(6)

Fig.6.TM-AFMimages(a)ofZnOthinfilmsandof(b)ZnO/In2O3.

thecellsusingTCO/In2O3bilayersasanode.Therefore,itappears

that,moreprobably,therearedifferentcauses,whichconvergeto lowertheperformancesofcells,causeswhichtakenindividually,

wouldnotjustifythelimitedperformancesobtained.Loweringof

VocvaluescanarisefromTCOsurfacepeaks,whichshort-circuitthe

PVcellanddecreasetheRsh.Also,themaximum˚Mvaluepossible

withreactiveevaporationofIn2O3beinginferiorto5eV,theband

matchingimprovementisnotoptimum,sincetheHOMOofCuPc

is5.2eV.

Indeed, we have seen that, the roughness of the bilayer is

slightly,butsignificantlyhigherthanthatoftheTCOalone.Inorder

toestimatetheinfluenceoftheanoderoughnessmodificationon

theshuntresistancevalueofthecellswehavecalculatedtheseries, Rs,andshun,Rsh,resistancesoftheOPVcells.Theequivalentcircuit

commonlyusedtointerpretthecharacteristicsofsolarcellconsists ofcurrentphotogeneratorconnectedinparallelwithadiode,which representstheI–Vcharacteristicsunderdarkconditions,while par-asiticseriesandshuntresistancesarerepresentedbyRsandRsh

respectively[18].Theslopesattheshortcircuitpointandatthe opencircuitvoltageoftheelectricalcellscharacteristicsarethe inversevaluesoftheshuntresistance(Rsh)andtheseriesresistance

(Rs)oftheequivalentcircuitschemeofasolarcellrespectively[19].

Table3

Series(Rs)andshuntresistance(Rsh)ofOPVcellsusingdifferentanodes.

Anode Rsh() Rs()

FTO 526 4960

FTO/In2O3 345 270

FTO/In2O3/MoO3 4215 136

FTO/MoO3 3046 176

Forinstance,usingtheJ–VcharacteristicsofFig.5b,theestimated valuesofRsandRsharepresentedinTable3.InthecaseofFTO,it

canbeseenthatthevalueofRsisveryhigh,whiletheRshvalueis

quitesmall.TheintroductionofathinIn2O3filminducesastrong

decreaseinRsbutalsoasmalldecreaseinRsh,whichexplainsthe

limitedimprovementofthecellsperformancesandthesmallVoc

value.TheintroductionoftheMoO3bufferlayerinducesastrong

decreaseinRsandincreaseinRsh,whichexplainstheimprovement

ofthecellsperformances.TheMoO3filmbeingnoconductiveit

allowstheincreaseofRsh,whileitpermitsabetterbandmatching,

whichdecreasestheseriesresistance.

Therefore,thenecessityofaninsulatingbufferlayercanbe jus-tifiedbytheroughnessincreaseinducedbythethinITOfilm,which canpartlyshortcircuittheOPVcells,andthelimitedbandmatching attheanodeinterface.

4. Conclusion

ItiswellknownthatITOgivesbetterresultsthanAZOorFTO

whenusedasanodeinorganicoptoelectronicdevices.Oneofthe

mainreasonsofthis,isthattheworkfunctionofITOishigherthan

thatofAZOandFTO.Weshowthatthedepositionofathinfilm

(5nm)ofIn2O3ontotheTCOallowsimprovingtheOPVcells

perfor-mances.Howeverthecellsefficiencystayssmallerthanexpected

duetosmallVoc values.Itisshownthat ifthisthinIn2O3 film

increases theanode surfaceenergyandthereforeimproves the

interface band matching,the work function of theIn2O3 stays

smallerthantheHOMOvalueoftheCuPcanditincreasesthe sur-faceroughnessoftheanode,bothpropertieswhichcanjustifythe

smallVoc valuemeasured.ThepresenceofathinMoO3

insulat-ingfilmallowsovercomingthesedifficultiesbygivingarealband matchingattheinterfaceandincreasingtheshuntresistanceofthe cells.

References

[1]J.C.Bernède,Organicphotovoltaiccells:history,principleandtechniques,J. ChileanChem.Soc.53(2008)1549–1564.

[2]Y.Park,V.Choong,Y.Gao,B.R.Hsieh,C.W.Tang,Effectofplasmatreatmenton theperformanceoforganicelectroluminescentedevices,Appl.Phys.Lett.68 (1996)2699–2701.

[3]C.-H.Wang,W.C.H.Choy,Efficientholecollectionbyintroducingultra-thin UV–ozonetreatedAuinpolymersolarcells,Sol.EnergyMater.SolarCell95 (2011)904–908.

[4]J.Park,H.MinKim,D.WookKim,J.SunChoi,Effectofgateelectrodework functiononelectricalcharacteristicsofpentacene-basedfield-effectdevices, Appl.Phys.Lett.97(2010)093301.

[5]C.N.Li,C.Y.Kwong,A.B.Djurisic,P.T.Lai,P.C.Chui,W.K.Chan,S.Y.Liu,Improved performanceofOLEDswithITOsurfacetreatments,ThinSolidFilms477(2005) 57–62.

[6]A.Klein,C.Körber,A.Wachau,F.Säuberlich,Y.Gassenbauer,S.P.Harvey,D.E. Proffit,T.O.Mason,Transparentconductingoxideforphotovoltaic: manipu-lationoffermilevel,workfunctionandenergybandalignment,Materials3 (2010)4892.

[7]T.Minami,Substitutionoftransparentconductingoxidethinfilmsforindium tinoxidetransparent electrodeapplication,Thins SolidFilms516 (2008) 1314–1321.

[8]M.Owen,S.Son,K.-H.Yoo,D.B.Ahn,S.Y.Lee,Organicphotovoltaicdeviceswith Ga-dopedZnOelectrode,Appl.Phys.Lett.90(2007)033512.

[9] Q.Qiao,J.Beck,R.Lumpkin,J.Pretko,J.T.McleskeyJr.,Acomparisonoffluorine tinoxideandindiumtinoxideasthetransparentelectrodeforP3OT/TiO2solar

(7)

[10]M.Morsli,C.Amory,A.Bougrine,L.Cattin,J.C.Bernède,Currentvoltage(I–V) studiesofMo/g-In2Se3/ZnO:Aldiodestructures,J.Phys.D:Appl.Phys.40(2007)

7675–7681.

[11]R.F.Salzman,J.Xue,B.P.Rand,A.Alexander,M.E.Thompson,S.R.Forrest,The effectofcopperphthalocyaninepurityonorganicsolarcellperformance,Org. Electron.6(2005)242–246.

[12]A.Latef,J.C.Bernède,Studyofthethinfilminterfacealuminium–tellurium, Phys.StatusSolidi(a)124(1991)243–252.

[13]Y.Berredjem,N.Karst,A.Boulmokh,A.H.Gheid,A.Drici,J.C.Bernède, Optimisa-tionoftheinterfaceorganicmaterial/aluminiumofCuPc/C60basedphotovoltaic

cells,Eur.Phys.J.:Appl.Phys.40(2007)163–167.

[14] C.D.Wagner,W.M.Riggs,L.E.Davis,J.F.Moulder,G.E.Muilberg,Hnadbookof X-raysPhotoelectronSpectroscopy,PerkinElmer,EdenPrairieMN,1979. [15] L.Cattin,F.Dahou,Y.Lare,M.Morsli,R.Tricot,S.Houari,A.Mokrani,K.Jondo,

A.Khelil,K.Napo,J.C.Bernède,MoO3surfacepassivationofthetransparent

anodeinorganicsolarcellsusingultra-thinfilms,J.Appl.Phys.105(2009) 034507.

[16]B. Kouskoussa,M.Morsli,K.Benchouk,G.Louarn,L. Cattin,A.Khelil,J.C. Bernède,On the improvementofthe anode/organicmaterialinterface in organicsolarcellsbythepresenceofanultra-thingoldlayer,Phys.StatusSolidi (a)206(2009)311–315.

[17]Z.H.Huang,X.T.Zeng,X.Y.Sun,E.T.Kang,J.Y.H.Fuh,L.Lu,Influenceofplasma treatmentofITOsurfaceonthegrowthandpropertiesofholetransportlayer andthedeviceperformanceofOLEDs,Org.Electron.9(2008)51–62. [18] B.Mazhari,Animprovedsolarcellscircuitmodelfororganicsolarcells,Sol.

EnergyMater.SolarCells90(2006)1021–1033.

[19] B.Brousse,B.Ratier,A.Moliton,VapourdepositedsolarcellsbasedonCuPc-C60

singleheterojunction:optimizattionofthedepositionprocess,Synth.Met.147 (2004)293–298.

Figure

Fig. 1. X-ray diffraction diagram of a In 2 O 3 thin film achieved by reactive evapora- evapora-tion.
Fig. 2. Microphotographs of an ITO (a), a SnO 2 (b) and a SnO 2 /ITO (3) structure.
Fig. 6. TM-AFM images (a) of ZnO thin films and of (b) ZnO/In 2 O 3 .

Références

Documents relatifs

delays. Another consequence of the presence of the two delays is that it requires stronger host cells in their competition with tumor cells for nutrients and oxygen since,

Fuel cell characterizations were carried out to study the influence of the catalyst materials (Pt/C or Pd/C) and of the anodic fuel (sodium borohydride or ammonia borane) on the

Accordingly, although all of these results from di ff erent electrical measurements support a reduced net doping concentration near the p/n-junction because of interdi ff usion

After deposition of the planar heterojunction 5T/C 60 , the Alq 3 layer and finally the aluminum cathode were de- posited. All the films were deposited under a vacuum. The thin

and the cathode was an aluminum fi lm. Alq 3 was chosen as the ETL because it has been shown that it allows growing of solar cells with higher lifetime [19]. The thickness of the thin

On one side, we show that, whatever the transparent conductive oxide used as anode (TCO: ITO, ZnO, SnO 2 ), the introduction of a thin metal, oxide or even organic layer at

Therefore, when kept in room air, device performance system- atically decreases more or less rapidly depending on the blocking properties of the EBL whereas, when kept in vacuum to

This means that if in the case of PEDOT:PSS the buffer layer should cover the whole surface of the ITO or ZnO film, in the case of gold, a coverage of only 15% is enough to be active