Comparative study of arylene bisimides substituted with imidazole side group for different dielectrics on the OFET application

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ContentslistsavailableatSciVerseScienceDirect

Synthetic

Metals

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

Comparative

study

of

arylene

bisimides

substituted

with

imidazole

side

group

for

different

dielectrics

on

the

OFET

application

Cem

Tozlu

a,∗

_,

_Sule

_Erten-Ela

b

_,

_Th.

_Birendra

_Singh

c

_,

_N.

_Serdar

_Sariciftci

c

_,

_{Sıddık ˙Ic¸}

_li

b,∗ a_Department_of_Energy_System_Engineering,_Faculty_of_Engineering,_{Karamano˘glu}_Mehmetbey_University,₇₀₁₀₀_Karaman,_Turkey

b_Solar_Energy_Institute,_Ege_University,_Bornova,₃₅₁₀₀_Izmir,_Turkey

c_Linz_Institute_for_Organic_Solar_Cells_(LIOS),_Physical_Chemistry,_Johannes_Kepler_University_Linz,_{Altenbergerstr.}_69,_A₄₀₄₀_Linz,_Austria

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received23January2013

Receivedinrevisedform25February2013 Accepted29March2013

Keywords:

Organicﬁeldeffecttransistor Naphthalene

Perylene Benzimidazole

a

b

s

t

r

a

c

t

Weanalyzedtheeffectofpolymericdielectricwithhydroxylandhydroxyl-freegrouponcurrent–voltage

characteristicsoforganicthinﬁlmtransistorbytheuseofbenzimidazole-derivedarylenebisimide

derivatives.Polyvinylalcohol(PVA)withpolargroupandbenzocyclobutene(BCB)withnon-polargroup

wereusedassolutionprocesseddielectricmaterialstocomparewitheachotherinthinﬁlm

transis-torapplication.Thehydroxylgrouphasasigniﬁcanteffectonturn-onvoltageandturn-offcurrentin

depletionregimeduetohydroxylgroup.Itisobservedthatthesurfacemorphologyisinﬂuencedbythe

chemicalstructureofpolymericdielectricconcerningsurfaceenergy.Theelectronﬁeldeffectmobilityof

botharylenebisimidesisenhancedbydecreasedsurfaceenergyofdielectric.Thehighestmobilitywas

obtainedbyemployingnaphthalenebis-benzimidazoleasanactivelayeronbothdielectricscompared

withperylenebis-benzimidazolesemiconductor.Theelectricalbehaviorsofthesesemiconductorsare

discussedinrelationtogatedielectricsurfaceproperties.

1. Introduction

Organicsemiconductorshavebeenattractingmuchattention for electronicdevices becauseof theirpotential applicationsin low-cost,largeareaandflexibledevicessuchasplasticsolarcell, flat-paneldisplaysetc.[1–3].Organicthinfilmtransistors(OTFT) based on highly conjugated p-channel or n-channel active, as organicelectronicdevices,havebeenusedonactiveandpassive matrixdisplays[4,5],radiofrequencyidentificationtags, chem-icalsensors,biological sensors,etc.[6,7].Lowmolecularweight peryleneandnaphthalenebisimidederivatives,arylenebisimides werewidelyinvestigated assmallorganicmoleculeswith vari-ous substituents tocreate advantages ontheirelectrochemical, luminescentandelectronicpropertiesallowingapplicationto con-structorganicbased electronicdevices [8–10]. Thestrong␲−␲ interactions and high electron affinity are special features of these materials to utilize in n-channel organic thin film tran-sistors[11–13].Inrecentyears,electronacceptingmoietieslike halogen substituents [14–16] and polymer containing arylene bisimides [17,18] have been reported as air stable n-channel OTFT. Remarkable studies have been reported employing vari-ous perylene and naphthalenebisimides as modified dielectric

∗ Correspondingauthors.Tel.:+903882265053;fax:+902323882023. E-mailaddresses:tozlu.cem@gmail.com(C.Tozlu),sicli@yahoo.com(S. ˙Ic¸li).

layerwithpoly-methylmethacrylate(PMMA)[19,20],n-octadecyl triethoxysilane (OTS) [21], hexzamethyl disilazane (HDMS)and 3,5-bis(triﬂuoromethyl)thiophenol[22],onbareSiO2[23,24].The

surface modiﬁcation improves drastically the performance of OTFTsincomparedonlywiththebareSiO2.Polymericdielectrics

havebeenusedwidelyingateinsulatorsinrecentyears,dueto theirfavorablechemicalandphysicalpropertiesinsolution pro-cessabledepositiontechnique.Someofthesepolymericmaterials alsocompetewiththebareSiO2insulatorforapplicationin

elec-tronics[25].Theirsurfacepropertiesinﬂuencedirectlymorphology ofsemiconductorsandsemiconductor/dielectricinterfaceinwhich motionofchargecarriersoccurs[26,27].ThestudiesofOTFTbased onperyleneandnaphthaleneorganicsemiconductorsareless com-monfor comparativeofperformancearylenebisimidesonbare polymericdielectricsurfaces.

We nowreporttheOTFTperformanceswithnovel naphtha-leneandperylenebisimidessubstitutedbenzimidazolegroupson hydroxyl and hydroxyl-freepolymeric insulators.The synthesis of thenaphthalene and perylene bis-benzimidazole is reported elsewhere [28,29]. Novelty of application on OTFTs for these molecular structures is the extended conjugation of aromatic rings onbare polymeric dielectrics, compared to the naphtha-lene and perylene diimides studiedin literature. Hydroxyl-free divinyltetramethyldisiloxane-bis(benzocyclobutene)(BCB,called “cyclotene”)and hydroxyl polyvinylalcohol (PVA)werechosen toevaluatetheperformanceoffabricatedtransistorsdependence 0379-6779/$–seefrontmatter © 2013 Elsevier B.V. All rights reserved.

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6 C.Tozluetal./SyntheticMetals172 (2013) 5–10

ondielectricsurfaceproperties.Naphthalenebis-benzimidazole, NBBI,andperylenebis-benzimidazole,PBBI,basedOTFTsarenot usedinthinﬁlmtransistorasanactivelayer.

2. Experimental

Theorganicthinﬁlmtransistors(OTFTs)werefabricatedwith top contact/bottom gate geometry to investigate properties of dielectric/semiconductorinterfacebydepositionoforganic semi-conductor on dielectric layer. The OTFT device geometry and chemical structure of used NBBI and PBBI semiconductors are shownin Fig.1(a–c).Theindium tinoxidecoatedglasseswere patternedbyetchingwithdilutedHClforbottomgateelectrode. Prior to coating of dielectrics, the substrates were cleaned by soakinginultrasonicbathwithdistilledwater,acetoneand iso-propanol, respectively. Benzocyclobutene (BCB) and poly(vinyl alcohol) (PVA)(Mowiol® _40-8) _with _average _molecular_weight

of120,000werepurchased fromDownChemicalCompany and AldrichCompany,respectively.Themolecularstructuresof poly-meric dielectrics are shown in Fig. 1(d–e). The BCB thin ﬁlm polymericdielectricwascoatedonITOsubstratebyspincoating at1500rpmin 45sand curedfor1hat285◦Cin vacuumoven underargonatmosphere.PVAthinﬁlmpolymericdielectricwas coatedonITOsubstratebyspincoatingof10wt/v%PVAsolution at1500rpmin45sanddriedfor24hunderroomtemperaturein vacuum.NBBIandPBBIsemiconductorswiththicknesses100nm werethermallyevaporated undervacuum(10−5mbar)atarate of0.05nm/sonBCBandPVAcoatedsubstratesundersame con-ditions,respectively.LiF/Almetalelectrode(purity99.9%)witha

Fig.1.(a)Cross-sectionalillustrationoffabricatedtopcontactOTFT,(b)chemical structuresofnaphthalenebis-benzimidazole(NBBI),(c)perylenebis-benzimidazole (PBBI), (d) divinyltetramethyldisiloxane-bis (benzocyclobutene) (BCB) and (e) polyvinylalcohol(PVA).

Fig.2. Outputcharacteristics(Ids−Vds)ofNBBIandPBBItransistorsondielectricsofBCBandPVAforvariousVgsindeviceI(a),deviceII(b),deviceIII(c)anddeviceIV(d), respectively.Allmeasurementsweretakenundersameconditions.

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Fig.3. TransfercharacteristicsfordeviceI(a),deviceII(b),deviceIII(c)anddeviceIV(d).AllcurveswereobtainedunderVds=50V.Leftaxisrepresents(logIds−Vgs)curve andrightaxisI1/2_ds −Vgscurve.

thickness0.6nm/60nm wasdeposited onactive layersof NBBI andPBBIwithvacuumevaporatorunder1×10−6mbarbyusing shadowmaskhavingachannellength(L)of47_␮mandchannel width(W)of2mm.Theelectricalcharacterizationsoffabricated devicewerecarriedoutinaninertgasatmosphere.Theoutput (Ids−Vds)andtransfer(Ids−Vgs)characteristicsoffabricatedOFETs

weremeasuredwithAgilentE5273ASource/Measureunit. Sepa-ratesandwichstructuresofmetal–insulator–metal(MIM)withAu electrodeswerepreparedforcapacitance–voltagemeasurements ofdielectricfilms.AHP4842ALCRmeterwasusedforthe geometri-calcapacitancemeasurementsofinsulatorinMIMdevicestructure. Thesurfacemorphologiesofdielectricfilmsandactivelayerswere examinedintappingmodewitha Q-Scope250ScanningProbe MicroscopeAmbiosTechnologyoperatingat roomtemperature. Thethicknessofthedielectricwasdeterminedbyasurface pro-filometer(AmbiosXP-1).

Inthisstudy,devicesarereferredasdeviceIanddeviceIIfor NBBIandPBBIbasedOTFTsontheBCBdielectric,deviceIIIand deviceIVforNBBI andPBBIbasedOTFTsonthePVA dielectric, respectively.

3. Resultsanddiscussion

ThethicknessesofpolymericdielectricﬁlmsofPVA andBCB wereadjustedtoobtaininsameordercapacitancevaluesof insu-latedﬁlms.ThecapacitancevaluesofBCBandPVAinsulatorswere found to be 1.2nF/cm2 _and _1.8_nF/cm2 _with ₂_␮m _and _3.8_␮m

ﬁlmthicknesses,respectively.Thedielectricconstantsofinsulators frommeasureddielectricthicknesseswereestimatedasεPVA=8

andεBCB=2.6,respectively.Thepolymericinsulatedﬁlmsdifferin

theirsurfaceenergydependonmolecularstructures.Thesurface energyofBCBandPVAwere34.3mJ/m2 _and₄₅_mJ/m2 _in

accor-dancewiththeRefs.[30,31],respectively.

AllOTFTswerefabricatedunderthesameconditionsinorder toabletocomparewitheachother.Alldevicespresenttypical n-channeltransistoroutputcharacteristicsasafunctionofVgs.The

outputcharacteristicsofalldevicesfabricatedontopofthe poly-mericgate-insulatorwithLiF/Altop drain-sourcecontactswere showninFig.2(a–d).ThedraincurrentIdsincreaseslinearlywith

Vds, at low voltages, implies that good establishment of ohmic

contactforelectroninjectionbetweensemiconductorsandLiF/Al contacts. When the output characteristics of devices are com-pared withPVA and BCB dielectrics,thewell-deﬁned pinch-off (Vds≈Vgs−Vth)voltagesarenotobservedasshowninFig.2.Inideal

thin ﬁlmtransistor (TFT),the differentialresistancer=

∂

Vds/

∂

Ids

must behighin thesaturationregime(Vds>Vgs).In contrastto

idealTFToutputcharacteristics,deviceswithexceptionofdevice IIIexhibitedalowdifferentialresistanceinoutputcharacteristics abovethepinch-offvoltages.Thehighconductivityinsaturation regimecanbeattributedtothepresenceofinterfacechargesatthe dielectric/semiconductorinterface[32].

Fig. 3(a–d) depicts the transfer characteristics (logIds−Vgs)

and (I_ds1/2−Vgs)of OTFTs at Vth=50V to illustrate turn-off and

turn-onstatebehaviorbysweepinggatevoltage.Themagnitude oftheturn-offcurrentexhibitsadependenceonpolymericgate dielectric,PVAgateinsulateddevicesshowedveryhighoffcurrent comparedwithBCBgateinsulateddevicetransfercharacteristicsin depletionregime(Vgs≤0V).TheOH−groupofPVAhassigniﬁcantly

largedipolemomentthatcreatesinternalelectricfieldinaddition to gate induced field at the semiconductor/dielectric interface. AlthoughcapacitancevalueofPVAgatedielectriciscomparable withBCBdielectric,draincurrentisverylargeinbothaccumulation regimeandalsoindepletionregimeforPVAdielectricduetodipole fieldofgateinsulator.Inadditiontoturn-offcurrent,weevaluated turn-on voltagewhichis definedasthegatevoltagewhere the drain current starts to increase. From transfer characteristic of

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8 C.Tozluetal./SyntheticMetals172 (2013) 5–10

Fig.4. Atomicforcemicroscopy(AFM)imagesofNBBIfilm(a)andPBBIfilm(b)whichweredepositedonBCBlayers;NBBIfilm(c)andPBBIfilm(d)whichweredeposited onPVAdielectric.Allimagesweretakenintappingmode.

devices,thelargenegativeturn-onvoltagewasobservedforPVA gateinsulateddevicescomparedwithBCBdielectric.Thisnegative turn-onvoltageshowspresencesoftrapstatesthatprovidemobile charge in the channel formed at the dielectric/semiconductor interface [33,34] and also molecular dipole field of dielectric influencesturn-onvoltageabsolutelyduetoband-bendingatthe insulator/semiconductorinterface[35,36]. Thehysteresiseffects werealsoobservedespecially in thepresenceof PVA dielectric whichhavehydroxylgroup.Althoughthereareseveralarguments astothemechanismofthehysteresiseffectinOFET,theresults showthatthechargeinjectionandtrappingmechanismaremain sourcesofhysteresismechanisminOFETwhichisproducedonthe dielectricwithOH−[37].Furthermore,forinvestigationofinterface trapsinfluencesontransfercharacteristics,subthresholdswing,S, whichispointedoutthesharpnessoffieldeffectonsetdetermined from inverse slope of the log(Ids) versus Vgs below threshold

voltage, was evaluated. From transfer characteristics of below thresholdvoltage whichis extracted froma slope of(Ids(sat))1/2

versusVgswhereinterceptontheVgsaxis,alldevices exhibited

largesubthresholdswing(S).Actuallynormalizedsub-threshold swing,SN=S×Ci whereCiis thedielectric capacitanceper unit

area,permitstocomparemoreaccuratelydevicesbehaviorbelow thresholddependsondielectric thicknessand varyingdielectric materials.Thenormalized subthresholdswingvalues varyfrom

50nFVdecade−1cm−2 to 120nFVdecade−1cm−2, the largest valuewasobtainedfromPVAinsulateddevices thatshowmore deepinterfacetraps.Thesubthresholdswingdependsonnotonly dielectricthicknessandalsostronglyinterfacetrapspositionnext tointerface[38].Thedeviceparameters extractedfromtransfer characteristicsofOTFTsusingEq.(1)aresummarizedinTable1. Thesaturatedelectronﬁeld-effectmobility,,wasobtainedfrom saturatedregimeofOTFTbyusingfollowingequation[39], (Ids(sat))1/2=

_W

2LCi

1/2

(Vgs−Vth) Vds>Vgs−Vth (1)

whereIdsisthedrain–sourcecurrent,Wisthewidthofchannel,

L is thechannel length, Ci is the capacitance per unit area of

theinsulator.Vgsisthegatevoltage,isthemobilityofcharge

carrierandVthisthethresholdvoltage.Aslopeof(Ids(sat))1/2versus

Vgs determines saturated electron ﬁeld-effect mobility, , and

thresholdvoltagesinwhichinterceptVgsaxis.

The saturated electron mobilities of devices fabricated on BCB dielectrichave highermobility compared withsame semi-conductorsbaseddevicesfabricatedonPVAdielectricalthoughthe permittivityofPVAismuchhigherthanBCB.Thecarriermobilities ofarylenebisimidederivativesbasedOTFTsincreasedasthesurface energyofthegatedielectricdecreased.Asdiscussedforpentacene smallmoleculesinbyYangetal.[40],mobilityvaluesofpentacene

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Table1

SummaryofdeviceparametersforNBBIandPBBIbasedOTFTswithPVAandBCBdielectrics.

Device Semiconductor Dielectric V0(V) Vth(V) e(cm2V−1s−1)

DeviceI NBBI BCB 0 15 1×10−3

DeviceII PBBI BCB −2 18 1.6×10−4

DeviceIII NBBI PVA −12 3.9 8.1×10−4

DeviceIV PBBI PVA −16 2.8 9×10−5

Thevaluesshownhereareanaverageofatleastsixdifferentdevicesfabricatedundersameconditions.

decreasedwithincreasingsurfaceenergyofdielectric.Shinetal.

investigated the effects of polar functional groups of dielectric

resultingfromO2plasmatreatment.Theyfoundthatthemobility

ofpentaceneincreasesasdecreasesurfaceenergyofgatedielectric

withlargepolarfunctionalgroups[41].Gaoetal.demonstrated

similarresultsbyinvestigatingrelationshipbetweenmobilityand surfaceenergyofgatedielectricregardingthesurfaceenergyof activesemiconductor.Itisgenerallyacceptedthatthehigherfield effectmobility is achieved in case identicalsurface energies of dielectricandthesemiconductor[42].Thetransfercharacteristics ofsemiconductorsshowthatnaphthalenebis-benzimidazolehas agood performanceinOTFTonbothdielectricscompared with perylenebis-benzimidazoleonthesamegateinsulatedlayer.This performance difference mayattribute tofacilitation of channel formationandstrong␲–␲interactionatinsulator/NBBIinterface. Inordertoexaminethesurface effectsonthechargecarrier mobility depends on dielectric and semiconductors, an atomic forcemicroscopy(AFM)wasused.Beforecomparisoninsurface morphologiesofsemiconductorsonPVAandBCBinsulating-films, weevaluatedthesmoothnessofbare insulatorfilms.Themean roughnessofPVAwas0.2nmandthatofBCBwas0.43nm, indicat-ingthereisnolargedifferencesinroughnessofinsulating-films. Fig.4(a–b)depictsatomicforcemicroscopy(AFM)imageofvacuum evaporatedNBBIandPBBIfilmsonBCBlayerinthreedimensions. NBBIandPBBIfilmsexhibitroughsurfacewitharoot-meansquare (RMS)roughnessof1.73nmand1.33nm,respectively.Thegrains ofsemiconductorgrowthanislandshapeonBCBdielectricwith averagepeaktovalleydistanceof110nmforNBBIand150nmfor PBBI.Theaveragemaximumheightvalueofgrainsaboveaverage filmthicknessonBCBdielectriclayerwasfoundtobe5.57nmfor NBBIand4.392nmforPBBI,respectively.Thefilmmorphologies ofNBBIandPBBIonPVAdielectriclayerareshowninFig.4(c–d). ThegrainheightsofNBBIdepositedonPVAinsulating-filmwere decreased withtheexception ofsomeisland formationslocally whicharesufficientlyseparatedfromeachother,whilePBBI semi-conductorshowsasamorphous-likefilmswholethesurface.These imagesshowanevidenceoflower mobilitydue tograinshape propertiesonsurface.Althoughvalleysactasacarriertrapsdue to roughness scattering at first or second semiconductor layer formedclosetotheinterface,itisdifficulttoestimateinterface morphologyfromAFMimagesoftopsurface.Hence,wecanmerely makeanassumptionaboutthechargecarriermobilitydependence ongrainsappeared ontopsurfaces.In generally, grains signifi-cantlyinfluencethetransport mechanism ofcharge carrierand itsmobilitydue tohoppingprocess betweenorganicmolecules [43]. The grain size differenceof same semiconductors on dif-ferentinsulating-filmscanbeattributedtotheRMSdifferences andhydrophilic/hydrophobicpropertyofinsulatorfilms.The ary-lenebisiimidessubstitutedwitharomaticsidechainscanchange surfacepropertyfromhydrophilictohydrophobic.Inthecaseof C O and N H terminated perylene and naphthalene tetra car-boxylicdimidesmayshowrelativelymorehydrophilicsurfacein accordancewithdensityofpolargroups.Thegrainsof semiconduc-torgrowthshowmoreorderedaggregationsandbetterelectrical characteristicsinthepresenceofsubstratewiththesamesurface property.

4. Conclusion

Adetailedstudyofn-channelOFETsbasedonnaphthaleneand perylene bis-benzimidazole is presentedby incorporating polar andnon-polarpolymericdielectrics.Wefindthedipolemoment depending onthe polarfunctional groups modifies thesurface potentialoftheinterfacenexttochannelandinducesmobilecharge carriersatturn-offstateofOTFT.Experimentalresultsindicatethat thechoiceofgatedielectricscandominatetheelectrical perfor-manceofOTFTbyinfluencingmorphologyandstructureofactive layer.Thepresenceofhydroxylinpolymericdielectrichasaneffect ontheinterfacialelectrontrappingstatesnexttochannelat insu-lator/semiconductor.ThehighestmobilitywasobtainedfromBCB gate insulatedalthoughitscapacitance valueandpermitivityis lowerthanPVAdielectric.Theresultsshowedthatnaphthalene bis-benzimidazolehasagoodperformanceinOTFTonbothdielectrics compared with perylenebis-benzimidazole. The imidazole side groupinfluencessignificantlychargecarriermobilityandsurface morphologyduetosurfaceenergyoforganicthinfilm.

Acknowledgments

This studywas supportedby European Science Foundation-ExchangeGrantofORGANISOLARandEUFP6OrgaPVNetprojects. We acknowledge Scientiﬁc and Technological Research Council of TURKEY(TUBITAK)and StatePlanningOrganization (DPT)of Turkey.

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