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Tuning the degree of oxidation and electron delocalization of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) with solid-electrolyte

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ContentslistsavailableatScienceDirect

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

Full

Length

Article

Tuning

the

degree

of

oxidation

and

electron

delocalization

of

poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)

with

solid-electrolyte

Sesha

Vempati

a,∗

,

Yelda

Ertas

a,b

,

Asli

Celebioglu

a,b

,

Tamer

Uyar

a,b

aUNAM-NationalNanotechnologyResearchCenter,BilkentUniversity,Ankara,06800Turkey bInstituteofMaterialsScience&Nanotechnology,BilkentUniversity,Ankara,06800Turkey

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received31January2017

Receivedinrevisedform3May2017 Accepted4May2017

Availableonline11May2017 Keywords:

XPS

HOMOstructure

Ionicinteractionandintertwinedpolymer network

a

b

s

t

r

a

c

t

WereportontheeffectsofionicinteractionontheelectronicstructureofPEDOT:PSSwheretheoxidation stateofPEDOTisanimportaspectforvariousapplications.Additionalionicinteractionsareintroduced andcontrolledbyvaryingthefractionofpoly(ethyleneoxide)(PEO).Theseinteractionsarebalanced againsttheinherentcohesiveforceswithineachofthepolymersconstitutingintertwinednetworks. Ramanspectraevidencedapeak-shiftashighas∼14cm−1forC Cvibrationalregionwhichsuggested increasingdegreeofoxidationofPEDOTforhigherPEOfractions.Changestothesingleandbipolaronic absorptionbandssupporttheresultsfromtheRamanspectra.ForhighestPEOfractionneutral-PEDOTand loweredbipolarondensityisattributedtolocalizationofPEDOTchainswithinPEOmatrix.Interestingly, forhigherPEOfractionstheelectronicdensityofstates(DOS)ofHOMOandcore-levels(S2p,C1sandO1s) suggestedincreaseddegreeofoxidationandelectronlocalizationonPEDOT.Nearandbelow(∼12eV) Fermilevel,contributiontotheO2pandC2patomicorbitalsdepictedsignificantlydifferentDOS.Alsowe noteenergeticshiftforO2s/C2sandbonding␴CCatomicandmolecularDOS,respectively.Thecorrelation

betweensomesurfaceandbulk-relatedpropertiessuggeststheuniformityoftheblendmaterialwhich mightbevitalfortheapplicationinelectrochemicaldevices.

©2017ElsevierB.V.Allrightsreserved.

1. Introduction

Poly(3,4-ethylenedioxythiophene) poly(styrene sulfonate) (PEDOT+:PSS)isoneofthevariousconductingpolymerswhich

occupies a very special role due to its easy processibility and relatively lower cost. Doping and conduction mechanism of PEDOT:PSSwerestudied[3–6]duetoitstechnologicalimportance [1,2]. PEDOT:PSS forms phase segregated morphology, where PEDOT+ is a hole conductor and surrounded by PSSchains.

Majorlythestrengthoftheelectroniccouplingbetweenadjacent PEDOTchainsdeterminesthemacroscopicelectricalconductivity [3,4]. Within the PSS matrix the ionic conduction may occur throughprotons (orNa[5])whichhop fromoneacidgroupto theother[1].ThismakesthePEDOT:PSSacomplexmaterialfrom chemical,morphologicaland electronicpointofview.The elec-tronicstructure/propertiesofthePEDOTareessentiallygoverned byoxidation stateandorganization ofPEDOTchainswhich are

∗ Correspondingauthor.

E-mailaddress:svempati01@qub.ac.uk(S.Vempati).

controllabletoanextent[3,4,6].Forinstance,inPEDOT:tosylate (Tos) film when exposed to tetrakis (dimethylamino) ethylene (TDAE)vapor,twoelectronsareinjectedintothePEDOTchain[6]. Otherwise,PEDOT:PSSfilm/solution canbesubjected tosolvent processing(ethyleneglycol,ethanol,methanol,isopropanoletc), seeRef. [4].and Refs.1-5 therein.In both approaches [4,6],the electronicstructureaswellas␲-␲stackingofPEDOTarealtered [3], where the solvent or the reducing agent is removed from thePEDOT:PSSfilm.Incontrast,hereweintroduceacondensed phase ionic conductor as a dopantwhich enables a control on theextentofoxidation/dopingandlocalizationofPEDOTchains. PreviouslyreportedblendsofPEDOT:PSSwithpoly(vinyl pyrroli-done)(PVP)haveshownhigherconductivitiesaftercross-linking [2]. Although there wasa clearindication of ionic interactions between PEDOT:PSS and PEO, poly(vinyl alcohol) or PVP none of these blends was investigated for electronic properties [2]. Undeniably,thereisaclearneedforfundamentalunderstandingof theinfluenceofionicinteractionsontheelectronicstructureofthe conductingpolymerblendswithPEO-likesolid-electrolytes.Itis worthmentioningthatDFTstudiesevidenceddistinctionbetween theunoccupieddensityofstates(DOS)forPEDOT0,PEDOT+0.5and http://dx.doi.org/10.1016/j.apsusc.2017.05.049

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PEDOT+1[7].Hencethedegreeofoxidationisanimportantaspect

inconnectiontotheelectronicstructure.

Here we employ surface and bulk sensitive techniques to experimentally investigate the modifications to the degree of oxidation, highlyoccupiedmolecularorbital(HOMO) and core-levelelectronicstructureofPEDOT:PSSwithvaryingPEOfraction. Polymer blends with ionic conductor (PEO) and conducting polymer(PEDOT:PSS)mayhavevastapplicationpotentialin solid-electrochemicaldeviceswheretheintertwinedpolymernetworks exhibitsignificantlyhighelectrolyte-electrodeinterface.Thisstudy elevatestheunderstandingofacomplexPEDOT:PSS-likeorganic conductorsanddrivestheapplicationsinelectrochemicaldevices.

2. Experimental

Poly(ethylene oxide) (PEO, MV=∼1,000,000) and PEDOT:PSS

(1:1.6)dispersion(1.3wt%)inH2Owerepurchasedfrom

Sigma-Aldrichandusedasreceived.AliquotamountofPEOwasaddedto thePEDOT:PSSaqueousdispersionwithoutanyadditionalH2Oand

stirredfor∼48hatroomtemperature.Noprecipitationorphase separationisobservedinthesesolutionswhenleftundisturbedfor months.Wehaveinvestigated10,20,30and40wt%ofPEDOT:PSS (PP)inPEOapartfromtheirpurecounterparts.Thesefour sam-pleswereannotatedrespectivelyinshortasPP10,PP20,PP30,and PP40forconvenience.Spincasted(∼1200rpm)filmsweredriedat 35◦Cinvacuumovenforaminimumof48htoremoveany resid-ualsolvent.XRDpatternswererecordedusingPANalyticalX’Pert ProMPDX-raydiffractometer(CuK␣,␭=1.5418Å).Raman spec-trawererecordedwithWITecinstruments(Alpha300S,532nm laser)within500–1600cm−1.Opticalabsorptionspectrafrom350 to 3000nm were recorded from Varian Cary 5000 UV–vis-NIR spectrophotometer.Scanningelectronmicroscopy(SEM)was per-formedwithFEI–Quanta200FEG.Contactanglesweremeasured with(OCA30,DataphysicsInstruments)forthreedifferentliquids (H2O,CH2I2,andethyleneglycol)of0.1mLeach(dropletmethod).

ThesurfacechemicalnaturewasdeterminedbyX–ray photoelec-tronspectroscopy(XPS,Thermoscientific,AlK␣=1486.6eV)while thepeakfitswithShirleybackgroundwereperformedviaAvantage software.Fermilevel(EF)issettozeroonthebindingenergy(BE)

axis.PeakfitsforXRDpatternsandRamanspectrawereperformed withOrigin8.5wherethenumberofpeaksisfixedandallother parametersareallowedtovaryuntilconvergence.

3. Resultsanddiscussion

3.1. Structuralandelectronicpropertiesofthebulk

Analyses of theXRD patternsunfold changes to the macro-molecularorderingduetoblending.TheXRDpatternfrompristine PEDOT:PSSfilmisshowninFig.1a.Theshoulderat7.110◦ corre-spondsto(200)while(100)isnotexplicitlyobserved.However aclearpeak isidentified at4.789◦.Furthermore,twopeaksare noted at 16.644◦ and 24.366◦ where the latter corresponds to ␲–␲stackingdistanceofthearomaticringsofPEDOT(010)[8]. Intermsoflengthunits,thestackingdistanceisabout3.6Åwhich isslightlyhigherthananearlierreportedvalueof3.5Å[3].This canbeattributedtothedifferenceinratiosofPEDOTtoPSSwhere thepolymerchainsrun paralleltoeach other(Fig. S1) [9].The randomcoilsofPSSdefinethePEDOTpolymergrainsandthe inter-facebetweenthegrainsisfilledbyexcessPSS.AlsotheexcessPSS accumulatesatthesurface(aswewillseefromtheXPSresults) therebydevelopingathinnegativelychargedlayer,inthe back-groundofobviouselectrostaticrepulsion.Hencethegranularity anddisordergoverntheconductionwheredopedPEDOThasshown increasedmolecularorderwhencomparedtotheun-dopedcase

Fig.1.X-raydiffractionpatternsfromthePEDOT:PSS/PEOblendsandpristine coun-terparts.(a)PEDOT:PSS(3–30◦),(b)PEO(12.5–16),(c)PP10(12.5–16)and(d)all

blendsamples(17–25◦).Peaklistforparts(b)and(c)areA-13.529,B-14.699,

C-15.086◦,D-13.258,E-13.693,F-14.791andG-15.250.Theangularpositionsof

selectedpeaksonpart(d)areannotatedintheunitsofdegrees.Env-fitenvelop.

[10].Furthermore,thedifferenceinthestackingdistanceis con-vincingintheviewoftheirconductivities(1S/cmVs2S/cmfrom Ref.[3]).Movingontotheblends,(010)stackingofPEDOTisnot observedduetothepredominantdiffraction/scatteringsignalfrom PEO.Nevertheless,thediffractionpatternfromtheblendisquite interesting,whichestablishesanychangestothecrystallinitydue toionicinteractions.Within12.5–16◦,thepeakscorrespondingto PEOhaveshownanotablereductionintheFWHMvalues(Fig.1b andc)whichindicatethedecreasedcrystallitesizeofPEOinPP10 sample.Interestingly,(200)reflectionfromPEOhasbeensplitinto twopeaksat13.258◦and13.693◦.Theshiftof2␪suggeststhatthe PEOcrystallitesweresubjectedtocompressivestress(Bragg’slaw). However,asthePEDOT:PSSconcentrationisincreasedfurtherthe signaturesofPEOpeakswithin12.5–16◦areextinguished(Fig.S2) duetothelossoflongrange orderofPEOchainsandincreased scatteringfromPEDOT:PSS.DepletedlongrangeorderingofPEO canbeduetotheinteractionwithPEDOT:PSS.AsshowninFig.1d, (120)reflectionfromPEOhasshiftedtohigher2␪◦untilPP20which

againshiftedtolower2␪◦forPP30andPP40.Theshifttohigher

diffractionangle(loweredinterplanarspacing)isduetothe inter-actionbetweenPEOandPEDOT:PSS.Thehighestshiftisnoticed forPP20sampleasaresultofthestrongestinteractionbetween PEDOT:PSSandPEO.ForPP10wecanexpectsomeexcessPEOand hencethecohesiveinteractionwithinPEOchainsispredominant asseeninasmallershift.ForPP30andPP40theconditionis recip-rocatedwhere PEDOT:PSS determinestheresultsofinterplanar spacingsofPEO.Essentially,theeffective/apparentinteractionis

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Fig.2.Ramanspectralresponse(1300–1600cm−1)fromPEDOT:PSS/PEOblends

andpristine counterparts.Thespectrallocationand theshift withrespect to PEDOT:PSSareannotatedintheunitsofcm−1.Env-fitenvelop.

determinedbytheweightedstrengthofeachinteractionswithin theblend(PEDOTVsPSS,PEDOT:PSSVsPEOandPEOVsPEO). Fur-thermore,within23.3and23.6◦PEOdepictstwopeakswhichhave beensplitintomultiplecomponentsintheblends.Thisisalsothe casewith(200)reflectionfromPEO(Fig.1bandc). Althoughit isnottrivialtoassigneachofthecomponents,however,wenote someshifttohigher2␪ingeneral.Theappearanceofmultiplepeaks isattributedtotheformationofintermediatecrystalline/different ordereddomainsundercompressivestress.Interestingly,abroad featurecenteredat∼23◦shiftstohigher2andgetsrelativelymore

intensewithincreasingPEDOT:PSS. For PP40the lesserintense diffractionpatternindicatesahigherdegreeofdisordercausedby increasedinteractionbetweencrystallinedomainsoftwo differ-entmicroscopicorigins.Ontheotherhand,increasedFWHMofthe peaksindicatesmallercrystallitesize.

Raman spectral responses within 1300–1600cm−1 and 500–1300cm−1areshowninFigs.2andS3,respectively.Please refer to Fig. S1 for assignments of chemically distinct carbon atoms in PEDOT. PEDOT:PSS depicted a well-structured vibra-tionalresponsewhichisconsistentwithearlierobservation.[11]. 1300–1600cm−1regionrepresentsC Cstretchvibrationswhich are particularly sensitiveto the presence as well as degree of localized/delocalizedholes.We havedeconvolutedthiscomplex responseintovariouscomponents,vizC Cstretchdeformation (1365cm−1), C␣=C symmetric stretch (1397–aromatic and 1433cm−1–neutral) and asymmetric C=C bend (1496 and 1531cm−1).PEOdidnotdepictanyspectralfeaturesinthisregion (flatresponseinFig.2).However,withrespecttoPEDOT:PSSthe vibrationalbandsfromblendsblue-shifted(∼7–14cm−1,seeFig.2 for specifics) which maybe attributed tothe increaseddegree ofoxidation. WhenPEDOTis doped(additionor subtraction of charge) geometric relaxation can also be expected where the netchargeisredistributed.Thegeometricrelaxationwasclearly evidentfromXRDanalyseswherePEOcrystalliteswereunder com-pressivestress.Consequently,thedegreeofbackbonedeformation producesvibrationalbandsathigherwavenumber.Ontheother hand,thenetchargemayinducepolaronicstates(singlyoccupied

Fig.3.(a)Opticalabsorptionspectra(350–3000nm)fromPEDOT:PSS/PEOblends andpristinecounterpartswheren–neutral,s1,s2–single,b1,b2–bipolaron

transi-tions,and(b)neutral,polaronandbipolaronstructureofPEDOT.Dash-dotcurves representA/A=(APP−APPxx)/APP,wherethesuffixcorrespondtotherespective

sample.Insertofpart(a)showsaschematicofn,s1,s2,b1andb2transitionsin

doped-PEDOT.

molecularstates)self-localizedwithinthebandgapoftheneutral polymer. This is associated with transition between quinoid-andbenzoid-dominatedstructures(Fig.S4)[7,11].Inthecaseof pristinePEDOT:PSSa fractionof chargepermonomeris shifted fromPEDOTtoPSShowever,singleand/orbipolaronisexpected atrelativelyhigheroxidationlevels.Interestingly,blue-shiftsare evidenced inC Cvibrational regionduringoxidative dopingof PEDOTwithelectrochemicalmethod(withoutthepresenceofPSS oranyotherdopant)[12].WhiledeKoketal.[11]employedthe sameprocess,however onPEDOT:PSS. Inclearcontrasttoboth thecases,weincreasedthedopinglevelwithasolid-electrolyte. Itis knownthatPSSstabilizes thePEDOTviaionicinteractions. When PEO is introduced the additional ionic interactions will bebalancedwiththatofPEDOTandPSS.Theionicbondsdueto electrostaticinteractionsareprimarilyresponsibleforthephysical cross-linking and subsequent doping[2].Furthermore the area ratiobetweenthebandscorrespondingtoaromatic(1397cm−1) andqunoidstructures(1433cm−1)isindicativeofthedopinglevel in the polymer chains.Clearly, the quinoid characterincreases withincreasingPEOfraction.Thevibrationalmodes(1 through

6)showninFig.S3originatefromoxyethyleneringdeformation,

symmetric C S C deformation, oxyethylene ring deformation, C O Cdeformation,C C␣’(inter-ring)stretch+C Hbend,and C C␣’(inter-ring)stretch,respectively.Pleaserefertothefigure legend forthefrequency values,which notablymatch withthe literature[12].Nodetectableshiftsintheresonancesareobserved withvaryingPEOcontent.

Opticalabsorptionspectra(350–3000nm)areshowninFig.3a. Variousplausibleopticaltransitions(␲-␲*,n,s1,s2,b1andb2)are

annotatedonthespectra,wheren–neutral,s1,s2–singlepolarons

andb1,b2–bipolarons[13,14].Therelativechangestotheoptical

absorption(A)wereobtainedfromA/A(dottedlinesinFig.3a) definedby(APP−APPxx)/APP,wherethesuffixcorrespondstothe

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theopticaltransitions.Chemicalstructuresofneutral,polaronand bipolaroninPEDOTareshowninFig.3b[15].ForPEDOT:PSSwe notes1,s2,b1,andb2whichisconsistentwiththeobservationfrom

Ramanspectrum.Asexpected,nisnotthestrongestabsorption bandasthePEDOTisinitsdopedform,whileflat␲-␲* transi-tionsuggestsdelocalized␲-electrons.However,XPSevidenceda smallfractionofneutralPEDOTwhichwillbediscussedlater.The increasedlocalizationof␲-electronscausedtheslightincreaseof ␲-␲*absorptionwhilereducingthedensityofs1,s2,b1,andb2

bands.InthecaseofPEDOT:Tos,polaronicandbipolaronicoptical transition(IRregion)areeliminatedafterreductionwithsomenew visibletransitions[6].Herenewabsorptionbandsarenotobserved [16]inthepresenceofPEOapartfromvariationsinA/A.␲-␲* absorptionfromPP10isrelativelyhigherthantheothersamples. TheelectronlocalizationisincreasedinPP10samplecreating neu-tralPEDOT.Furthermore,thedecreasedabsorptionofs1,s2,b1and

b2isattributedtothelowerdensityofpolarons.Asmentioned

ear-lier,withincreasingdopinglevel(0,+0.5to+1)thelocalization ofLUMOshiftsfromPEDOT0 toPEDOT+0.5and thentoPEDOT+1

[7].ForPP10,s2 and b2 aremoreaffectedthans1 andb1 bands

wheretheformerpairisenergeticallyclosetotheHOMO(insert ofFig.3a)[17].Polaronscanbearetransformedintoneutral seg-ments[6],howeverin thepresentcontextRamanspectroscopy evidencedincreaseddoping.Hencetheloweredabsorptionofs2

andb2isattributedtodecreasedtransitionprobabilityortheloss

ofdensityofunoccupiedbands.Absorptionbandsb1ands1from

PP40arestrongerthanothersamplesduetohigherdensityofbi andsinglepolarons.However,thestrongerabsorption(relativeto otherabsorptionbandswithinthespectrum)resultedinmerging ofb1ands1whichappearedtoberelativelyless-featuredband.On

theotherhandPP40hasmoreorlesssustainedtheintensityofthe bandb2.AbsorptionbandsclosetotheHOMOofthepolymerares2

andb2whicharenotsignificantlyaffectedforPP20andPP30.These

variationscanbeexplainedwithinthelinesofearlierdiscussionon PP10.Moreinterestingly,theb1ands1(energeticallyclosetothe

LUMO)havebroadenedafterPEOaddition.

3.2. Morphologicalandelectronicpropertiesofthesurface

Priortothediscussiononthesurfaceelectronic characteris-ticssurfacemorphologiesarebrieflyaddressed.SEMimagesfrom PEDOT:PSSandblendsamplesareshowninFig.S5.Morphology appearstoevolveasthePEOfractionchangesfrom90to10wt%. PEDOT:PSSdepictedgrainymorphologywithoutanycluster for-mation.Incontrast,themorphologyofPP10isratherroughwith relativelybiggergrains.Thismightbeduetotheionicinteractions predominantlydeterminedbythemorphologyofPEO.PP20has shownrelativelysmoothermorphologyinclearcontrasttoPP10. InterestinglyPP30andPP40havedepictedacleartransitionfrom roughertosmoothermorphology.Thesechangeswereclosely asso-ciatedwithanearlierobservationontheblendsofPEDOT:PSSand PVP[2].Notably,grainysurface wasevolvedinPEDOT:PSS and Ptnanoparticlesblendswherethelatterformsapolarbondwith PEDOT[18].Furthermore,thechangesinthemorphologymaybe correlatedtothesurfacefreeenergy.PleaserefertoSFIG5,6and discussionthereinrelatedtosomepreliminaryresults.

XPSsurveyspectra(Fig.S8)unveiledthesurfaceatomic compo-sitionofthesamples.TheanalysesindicatedthepresenceofO,C and/orSasmajorelements(Table1).Nitrogen(1.5at%)ispresent inthecase ofPEDOT:PSSand attributedtocontaminationfrom ambience.WhileNaisabout0.80at%originatingfromtheresidual oxidizingagent(Na2S2O8)duringthepolymerizationofPEDOT.O

andCcontainfractionalcontributionsfromPEDOT,PSS,PSSH,PEO inadditiontoNa2S2O8totheformer.WhileforSandNathe

contri-butionsareasfollows:S(PSS,PSSHandNa2S2O8)andNa(Na2S2O8).

Also,Cpercentagesfromtheblendsamplesfallinbetweenthat

Table1

XPSsurveyanalysesfromthePEDOT:PSS/PEOblendsandpristinecounterpartsare tabulatedinatomicpercent.

Element Samplename

PEDOT:PSS PP10 PP20 PP30 PP40 PEO

O 23.30 31.62 30.38 28.52 30.99 38.00

C 67.20 66.75 66.05 62.44 65.51 62.00

S 7.20 1.63 3.57 9.04 3.50 0.00

Fig.4. S2pcore-levelspectrafromPEDOT:PSS/PEOblendsandpristinePEDOT:PSS. ThespectrallocationofthepeaksandshiftswithrespecttoPEDOT:PSSareannotated intheunitsofeV.Env-fitenvelop.

of pristine counterparts. The surveyanalysis integrates each of theelementsirrespectiveoftheirchemicalenvironments.Hence itshouldbetreatedasafirstglanceatthesurfacewhilethe sur-faceaccumulation(ofPEDOT,PSSand/orPEO)fromblendswillbe identifiedwithcore-levelspectroscopy.

PEDOT:PSS consistsof two chemicallydistinct sulfur groups (majorly)eachbelongtothiopheneringandsulfonategroupof PEDOTandPSS,respectively.S2pcore-levelspectrafromblends areshowninFig.4withpeakBEvaluesannotatedineV.Thepeak valuesofelectronBEofS2pcore-levelfromPEDOT:PSSmatchwith theliterature[6,11,18].ByconsideringthewholeS2pregionboth PEDOTandPSSregionswereinfluencedbythepresenceofthePEO. PEDOT:PSSexhibitedPSSrichsurface(1:1.8at%)whencomparedto thedatafromSigmaAldrich(1:1.2at%).Intheprocessoffilm prepa-rationtheexcessPSSaccumulatesatthesurfacetominimizethe Coulombicrepulsion.AsthePEOfractionincreasesPEDOTtoPSS ratiosare1:1.6,1:1.5,1:1.6and1:1.5at%forPP40toPP10, respec-tively.ThepresenceofPEOdecreasedtheelectrostaticrepulsion thereby reducingtheaccumulationof PSS.Coulombicrepulsion canbescreenedbyintroducingadielectricmaterialhoweverin thepresentcontextitistheionicinteractionwhichpredominantly decreasedtherepulsion.Asmentionedearlier,PVPforms inter-twinednetworkviaionicinteractions[2],whileSinPEDOTforms polarbondwithPtnanoparticle.Essentially thetypeof interac-tion betweenPEDOT:PSS and theguest(PVP or Pt)determines degreeofdopingandpreferentialaccumulationatsurfacewhich are vitalaspects in thecontextof devicefabrication aswellas itsperformance.InthepresenceofPtnanoparticlesPEDOTrich

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surfaceis observedalong withS␦−–Pt␦+ polarbonds(S belongs toPEDOT)[18].ClusterformationbetweenPtnanoparticlesand PEDOT:PSSwasattributedtothePEDOTrichsurface.Asanother example,inP3HT:PCBM blend P3HTis accumulatedatthe sur-faceduetolower surfaceenergywhere thecomponentsofthe blendwereunderweakVanderWaalsinteractions [19].When comparedtothesetwocases[18,19],thepresentblendsdepicted PSSaccumulationdespiteofstrongionicinteractionsresultingin anintertwinedpolymernetwork(seethediscussiononC1sand O1sspectra).S2pspectrumfromPEDOT:PSSdepictedsome neu-tralPEDOT(shadedpeaksat162.72and161.63eV).Thisislimited tothesurfaceastheband‘n’inopticalabsorption(bulksensitive)is notobservedwithinthedetectionlimits.IntheneutralPEDOT low-energyendpointoftheLUMOislocalizedonPEDOT(DFTstudies [7])andhenceitwould,inprincipleshowtheabsorptionif suffi-cientlydense.PP40andPP30samplesdidnotdepictthesignature fromneutralPEDOT(162.72and161.63eVdoublet)dueto dop-ingeffectfromPEOonneutralsegments.Thedisappearanceofthis doubletisconvincingbygiventhefactthattheincreasedb1 and

s1 absorptionforPP40and PP30samples.Apartfromtheeffect

ofdopingoneshouldalsoconsidertheformationofintertwined network.Note that suchnetworks maydecrease(enhance) the electrondelocalization(localization)wherethegeometric struc-tureofthepolymerplaysanimportantrole.Inthepresentcontext theabsenceofneutralPEDOT(atthesurface)andincreased opti-calabsorptionofb1ands1bands(inthebulk)forPP40andPP30

suggestmoredelocalizedelectronsalongthepolymerchain. Inter-estingly,PP20sampledepictedasmallfractionofneutralPEDOT inS2pspectrum(notdeconvolutedforspin-orbitdoublet). Inter-estinglyPP10 alsodepictedthisneutral componenthowever at relativelyhigherBEthanthatofPEDODT:PSSandPP20.Thismight beduetoincreasedlocalization,confinementofneutralsegments inthepredominantPEOmatrix,and/ortheionicstateofPEDOT [7,18].LocalizedpositivechargeonsulfuratomofPEDOTdepicts acomponentatthehigherBEside(filledareainred)[12].This isnoted for allblends,however, theBEin generaldepictedan increasingtrend.ThedelocalizationofelectrononSatomspans severaladjacentrings,meaningaspreadinthevaluesofthe bind-ingenergies.WhilethemagnitudeofthepositivechargeonPEDOT+

ringdependsonthedistancefromPSS– counterionsaswellas

PEO.Notably,PP10sampleexhibitedsmallestwidth/FWHMforthe peakscorrespondingtodopedPEDOT.Theatomicspin–orbit cou-plingsplitsthe2plevelwhichwouldotherwisebedegenerate.This splittingis asmallperturbationhowever,it mayplayrelatively moreimportantroleonthebandsclosetotheEF.Thissplitting

wasfoundtovarybetween0.9eV(PP10)and1.3eV(PEDOT:PSS). InthepresenceofPEO,S2pfromPSSblue-shiftedwhilethe split-tingbetweenS2p1/2and2p3/2variedwithin1.16and1.25eV.The

changesinspin-orbitsplittingcanbeunderstoodfromthe sensi-tivityofsulfurtothechemicalenvironment[20].ThePEDOTchains canbedifferentlydopedrangingfromnomonomertonearlyathird ofthetotal.Thisadditionalchargeisdelocalizedoverseveral adja-centmonomericunitswhichchangestheDOSonthesulfuratoms inPEDOT.ThesechangesshifttheBEofelectronfromS2plevel. AsdescribedearlierPEDOTissurroundedbyPSSchains stabiliz-ingthewaterdispersion.SincePSSandPEOarebothhydrophilic innaturetheysharethesolventinsolutionstate,whichofcourse influencesthemorphologyandchemicalnatureofthefilm dur-ingandafterdrying(Fig.S6).TheinteractionsvizPEOVsPEDOT, PEOVsPSSandPEOVsPEDOT:PSSaffecttheelectronicstructureof PEDOT.Convincingly,theBEofelectronsfromS2phasshown rel-ativelylargershiftsforPSSthanthatofPEDOT.SincePEDOTgrains aresurroundedbyPSS,PEOundergoesionicinteractionwithPSS therebyalsoinfluencingthedegreeofoxidationofPEDOT.Itmight

Fig.5. C1score-levelspectrafromPEDOT:PSS/PEOblendsandpristinecounterparts. TheenergeticshiftsareannotatedintheunitsofeVwithreferencetoPSSorPEO. Env-fitenvelop.

alsobethecasethatPEOreplacessomeofthePSSchainsapartfrom disruptingthebondingbetweenPEDOTandPSS.

C1sspectrafromblendsandpristinecounterpartsareshown inFig.5.PristinePEOdepictedtwocomponentswherethemost intensepeakathigherBEisattributedto C O C ,whilethelower BEpeakisattributedtohydrocarbons(C C C).Hydrocarbonscan beshortchainmonomersand/orcarboncontaminationduringthe samplepreparationandsubsequenttransfer.C1sfromPEDOT:PSS depictedthree componentseach originatefromPSS, C-S(filled region)andPEDOTintheorderofincreasingBE.Clearlythepeak BEvaluescorrespondingtoPEOandPEDOTarespectrallycloseto eachother(C1s/PEO=286.82eVvis-a-visC1s/PEDOT=286.34eV). Onehastokeepthisinmindinthecontextofpeak deconvolu-tionofthespectrafromblendswherePEDOT,PSS andPEOare underthecompetitive–ionicand–cohesiveinteractions.Despite, we identifythree components(refer toFig.S9a and discussion therein)andtwoofwhichbelongtoPEO,vizPEOPP,PEOPSS and

PSSPEOintheviewofthepresenceofexcessPSS.Thecomponent

denotedinthesubscriptindicatesitsinteractionwiththe corre-spondingargument.Themostintensecontributionisattributedto PEOPSSdepictingsomered-shiftwithreferencetopristinePEO.PSS

donateditsprotontoPEDOTandbeingin−vechargestate(–SO3−,

seeFig.S1)itrepelsPEOmoietieswithelectrondonatingnature. TheloweredBEinfacthintssuchaninteractionwherethehighest red-shiftisobservedforPP40withmostPEDOT:PSSfraction.Itis alsoobservedthatthedecreasingPEOPSScontributionisconsistent

withdecreasingPEDOT:PSSfraction(PP40toPP10).Ontheother handPSSPEOdepictedsignificantblue-shift(0.32–0.69eVforPP40

toPP10,respectively)withrespecttopristinePEDOT:PSS.Thisis attributedtotheattractiveinteractionbetween–SO3HandPEO.

Thecorrespondingblue-shiftedPEOcomponentisnotidentified (Fig.S9banddiscussiontherein).Themagnitudeoftheblue-shift increaseswithincreasingPEOfractionwhichisattributedtothe newlyintroducedionicbonds.SincePEDOTandPSSco-exist,the presenceofthelatterisanindicationoftheformer.Pleaserefer

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Fig.6. O1score-levelspectrafromPEDOT:PSS/PEOblendsandpristine counter-parts.Env–fitenvelop.

toFig.S9banddiscussionthereinwhichdealswiththenumberof componentsandreproducibilityofexperimentalDOS.Also,PSSPEO

isrelativelyintenseforPP30samplewhichisconsistentwiththe observationfromS2pspectra(Fig.4,middlepanel).Thiseliminates thepossibilityofPSSPEObeinghydrocarboncomponentfromPEO.

WeattributethePEOPPtothelocalizationofPEOchains.Asthe

PEDOT:PSSfractiondecreases(PP40toPP10)theblue-shiftisalso foundtodecrease.XRDpatternsfromblendsevidenceddecreased crystallitesizeaswellassomesignificantcompressivestresson PEOphase. i.e. thePEOchainsare localizedundercompressive strain.HencePEOPPcomponentisblue-shifted∼0.61eV(PP40and

PP30)withrespecttopristinePEO.ThecohesiveforcewithinPEO increaseswithitsfractionalcontributionanditcanbeexpectedthat thispeakenergeticallymergeswiththatofpristinePEOalongwith thatofPEOPSS.Theblue-shiftofPEOPPdecreaseswithdecreasing

PEDOT:PSSfractionwhichisaclearindicationofPEDOT:PSS influ-enceonPEO.IfPEOPPisarguedtobethecounterpartofPEOPSSthen

relativelyhigher–shiftand–fractionalcontributionareexpected forhigherPEDOT:PSScontent.Thisisnotthecaseastheintensity ratioofPEOPSStoPEOPPare∼4,5.3,6.7and12.5correspondingto

PP10–PP40samples,respectively.Inthecaseofblendsamplesit canbeconcludedthatPEO(PEOPP+PEOPSS)isaccumulatedatthe

surface.

O1sspectrafromblendsandpristinecounterpartsareshownin Fig.6.PristinePEOdepictedtwoO1scomponents.Themostintense peakat533.04eVcorrespondsto C O C ,whiletheminorpeak isassignedto OHendgroupsofPEOnotinganintensityratioof 1:0.12.PEDOT:PSSdepictedtwochemicallydistinctoxygeneous groupscorrespondingtoPEDOTandPSSat532.98eVand531.41eV, respectively.Due todioxyethylenebridgeO1scomponentfrom PEDOToccursathigherBEthanthatof PSS.Itshouldbenoted thatthecomponentfromPSSismoreintensethanthePEDOTdue tosurfaceaccumulation.Thisisconsistentwiththeobservation fromS2pspectra.Alsowenoteahighenergypeakat∼535.6eV (∼2eVofFWHM)whichisattributedtooxygenfromresidualH2O.

It might bethe case that this peak is rather broadto befrom

residualH2O,however,theapparentwidthmayhavesome

contri-butionfromthebackground.Inconnectiontothebackgroundofthe spectrum,wehaveemployedShirley-typefunctionalitywhich pre-sumablyreplicatestheinelasticscatteredelectrons(background). AsinthecaseofC1s,themainO1sspectralfeaturesfromPEOand PEDOTareratherclosetoeach(533.04eVand532.98eV, respec-tively).Inblendsamples,O1scomponentsfromPEDOTandPEO werenotdistinguishableandhencewehaveconsideredasingle peakforthediscussion(PEOPP,seeFig.S10anddiscussiontherein).

Themajorandminorcomponentsfromtheresultofionic interac-tionarerepresentedwithPEOPPandPSSPEO,respectively.PSSPEO

depictednominalblue-shiftmostlyassociated withsomeminor ionicinteractions,incontrasttothatofC1s.TheintensityofPSSPEO

isconsistentwiththeanalysesofS2pspectrafromblends(eg.S2p intensitylevelsfromPP30sample)which make thisattribution consistent.PEOPSSdepictedsomeinitialred-shift(PP40)andthen

blue-shift(PP30-PP10)asthePEOfractionincreases.Bygiventhe factthatthespectralfeaturesfromPEOandPEDOTareenergetically ratherclosetoeachother,theshiftshaveweightedcontribution andhenceshouldbetreatedasfirstapproximation.Thepeakshifts andrelativeintensitiescanbeunderstoodinthelinesofdiscussion givenforC1sspectra.Nevertheless,itisclearthatthesurfaceisPEO richinthecaseofblendsamples.

As mentioned earlier electronic and geometric affects are inevitablewhenanorganicmaterialissubjectedtodopingsuch asadditionorsubtractionofchargemediatedbyaredistribution ofthenetcharge.Thepositivechargeislocalizedonthecarbon atomsin theconjugatedPEDOTchain [7].Doping(counterion) changes a doublebond intoa singlebond acrosscarbonatoms andhencetheC O Cbondisinitschargedstatewherethe oxy-genatomislessnegativelycharged(incompleteoxidationstate) [18].Alsoatheoreticalstudyindicatesthatthebenzoidstructure istransformedintoquinoidwhentheneutralPEDOTisdopedby negativelychargedPSS,seeFig.S4[7].Althoughtheconductivityis improvedduetotheincreasedinterchaincoupling[3,4],the inter-actionbetweenthedipole(originatedduetopolargroup)ofthe dopantwiththedipoles,orpositivechargeofPEDOTisunavoidable [4].Theseinteractionsimposesignificantchangeseitherintermsof chargedelocalization,polymerchainconformationorboth.Thisis thecasewhentheinteractingspecieslikePEOiscontainedwithin thecomposite. Theinteractionbetweenthedipoles (orcharges) causesaflipfrombenzoidtoquinoidstructureinPEDOT[4].

WestartthediscussiononHOMOstructureofPEOwherethe contributionsfromvariousatomicorbitalsareannotatedonFig.7. HOMOedgepositionisestimatedwithatangentextrapolationof theleadingedgeoftheHOMO(Fig.S7a).ForPEO,alowintensity broad feature extends toward theEF. The molecular

confirma-tion(planarzigzag,helix,zigzagIandzigzagII)ofPEOisknown todeterminetheDOS [21].However,it isnottrivialtofindthe molecularconfirmationexperimentallywiththepredictionsfrom theory.PeakAisduetoO2satomicorbital,whileitshigher inten-sitycanbeattributedtorelativelyhigheratomicphotoionization cross-section(Ref[21].andRef.24therein).PeaksB1(␴CCbonding

orbital),B2andB3(␴CC and␴CO antibondingvalencelevels)are

duetoC2sandO2satomicorbitals.Oneshouldnotethattheatomic orbitalsinfactoriginatefrommolecularlevels.Itisinterestingthat B3ismoreintensethanB1andB2,similartoanearlier observa-tion(Ref.[21]andRef.24therein).O2pandC2patomicorbitals depictthepeaksC1andC2whichareratherfeaturelesshowever, specificallyconsistof␴CO,and␴CHcharactersasaresultof

com-binationofC2p,O2p,andH1satomicorbitals.D1isattributedto O2pand C2pwhileD2ismostlyfromlonepairofO2p. In con-nectiontotheHOMOofPEDOT:PSS,PSSunitinjectstwoexcess positivechargesoversixmonomerunitsmakingtheHOMO pop-ulationonPEDOT.HOMOaround5.5eVbelowEFisthebrightest

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Fig.7.HighlyoccupiedmolecularorbitalstructureofPEDOT:PSS/PEOblendsand pristinecounterparts.PeakpositionsareannotatedintheunitsofeVandFWHM ofA1isalsoindicated.Atomicorbitalsshowninsmallerfontindicateminor DOS-contribution.Selectedspectralregionsaremagnifiedandshownasfilledparts.

portionofthespectrumisattributedtoO2p/C2pandO2patomic orbitals.DFTstudiesevidenceddistinctionbetweenthedensityof occupiedstatesforPEDOT0,PEDOT+0.5andPEDOT+1[7].vizPEDOT0

(HOMOonPEDOT),PEDOT+0.5(HOMOispartiallyfilledandmajorly

localizedonPSSandsmallcontributionfromPEDOT)andPEDOT+1

(occupiedpartsofthepartiallyfilledbandsarelocalizedonPSS). Notably,forPEDOT+1,Lenzetal.[7]foundthatHOMOoccupancy

slightlylessthandoublewhileLUMOandLUMO+1arealso par-tiallyoccupied.In thecontextofblend samplestherearesome significantchangestothestructureoftheHOMO.Apartfromthe shiftsofB1,B2,andA1bandstheeffectofionicinteractionisfound tobesignificantonthevalencestateswhereapronounced signa-turefromPEOisobservedfromtheblends.TheFWHMvaluesofA1 increasefromPEOuntilPP20whichthendecrease(Fig.7).However thebroadeningofA1hascontributionfromO2s(major)andC2s atomicorbitalsofPEDOT:PSS.Furthermore,A1isred-shiftedwith respecttothatofPEO.Essentially,thisbroadeningandshiftsuggest a chemicalinteraction includingsomecharge transfer. Further-more,significantchangesoccurredtothestructureoftheHOMO until12eVbelowtheEFduetoPEO.StartingattheedgeofHOMO,

theDOSvarysignificantlywithinthesamples(Fig.S7a).PEDOT:PSS depictedhigherDOSuntil1eVwhich werethenextended until EF.Incontrast,theDOSfromPEOarerelativelyhigher untilEF.

Until3eVblendsampleshavesignificantDOS,howeverlowerthan pristinePEOandPEDOT:PSS(Fig.S7b).Whenexaminedtheregion closertoEF,PP20resemblesthefeaturesasthatofPEOanddepicted

highestDOSamongblendsamples.ThedifferencesintheDOSclose totheEF(composedofO2patomicorbitals)canbeattributedtothe

changesintherelativepopulationsofsingleandbipolaronsonthe PEDOTchain.TheseO2patomicorbitals havecontributionfrom

PEDOT:PSS,PEO,PEOPP,PEOPSSandPSSPEOapartfrom

conforma-tionalchanges[21].However,Ramanandopticalabsorptionprobe thebulkofthesample,instarkcontrasttophotoelectron spec-troscopy.Hence,theresultsfromthebulkmeasurementsmaynot directlyexplaintheDOSobservedintheXPS.Nevertheless,they supporttheexistenceofneutralsegments,singleandbipolaronsof varyingdensities.Furthermore,theelectroniccharacterofthe ␲-systemiscontrolledbyelectrondonatingorwithdrawinggroups whichadjusttheHOMOandLUMOlevels.Thedegreeof␲-overlap via stericinteractiondetermines the bandgapof a conjugated chain.Inconjunctionwiththis,thedegreeofdopingand localiza-tionincreasewithincreasingPEOfraction.Consequentlywemay expectsomeconfinementeffectsonthepolaronsascorroborated bytheincreasingdegreeofoxidation(Raman)andcompressive stressofPEO(XRD-discussion).Na2psignatureoccurredat30.29eV anditsenergeticpositionmatcheswiththatofliterature[22].Apart fromtheDOScontributiontothedeeperHOMO,Nacontributesto theconductivityofPEDOT:PSS[5].

4. Conclusions

Organic/organicinterfacesaregenerallyassociatedwithweak VanderWaalsinteractions.Inclearcontrastweobserveionic inter-actionbetweenPEDOT:PSSandPEO.TheblenddispersionsofPEO andPEDOT:PSSarestableformorethantwomonthswithoutany precipitation.Nophaseseparationisobservedinthefilms. Investi-gationonmacromolecularcrystallinityrevealedcompressivestress onPEOcrystallites.Apartfromhigherdegreeofdisorder, forma-tionof intermediatecrystalline/differentordered domainswere observed.VibrationalspectrarevealedsignificantblueshiftofC C bandswithrespecttoPEDOT:PSS.Thisshiftisassociatedwiththe increaseddegreeofoxidationofPEDOTaswellasgeometric relax-ationofthebackbone.Relativechangestotheabsorptionofsingle andbipolaronbandswereattributedtothechangestothedegreeof oxidationwhicharecorroboratedbyRamanmeasurements. Specif-ically, samplePP10 exhibited relativelyhigher ␲-␲*absorption duetolocalization ofPEDOTchainswithinthePEOmatrix. We foundthatforrelativelylowerPEOfractionsthetransferredcharge fromPEDOT:PSStoPEOisdelocalizedoverseveraladjacent PEDOT-monomersandchangestheelectrondensity.Thischargetransfer interactioncausedtheshiftinBEofcore-levelsincludingS2pand C1s.InvestigationonHOMOofblendsandpristinecounterparts indicatedsignificantchangestotheO2p,C2patomicorbitals.These changeswereattributedtotheconsequenceofinteractionbetween PEDOT:PSS andPEO inaddition totheconformationalchanges. Theresultsofthisinvestigationwouldenhancetheunderstanding ofionicinteractions amongconductingpolymersandionic con-ductorswhileprovidingcrucialinsightsforsurfaceengineeringof solid-electrochemicaldevices.

Acknowledgements

SVwouldliketothankTheScientific&TechnologicalResearch CouncilofTurkey(TUBITAK)(TUBITAK-BIDEB2216-Research Fel-lowshipProgram forForeign Citizens and 2221-Fellowshipsfor VisitingScientists and ScientistsonSabbatical) forpostdoctoral fellowship.

AppendixA. Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound,in theonlineversion,athttp://dx.doi.org/10.1016/j.apsusc.2017.05. 049.

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