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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
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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,TunisiabLUNAM,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=300◦Cand
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
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
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
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
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.
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