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Improving the performance of the organic solar cell and the inorganic heterojunction devices using monodisperse Fe3O4 nanoparticles

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ContentslistsavailableatScienceDirect

Optik

jou rn a l h om ep ag e :w w w . e l s e v i e r . d e / i j l e o

Original

research

article

Improving

the

performance

of

the

organic

solar

cell

and

the

inorganic

heterojunction

devices

using

monodisperse

Fe

3

O

4

nanoparticles

Zakir

C¸aldıran

a

,

Mehmet

Biber

d

,

Önder

Metin

b

,

akir

Aydo˘gan

a,c,∗ aDepartmentofPhysics,FacultyofScience,AtatürkUniversity,25240Erzurum,Turkey

bDepartmentofChemistry,FacultyofScience,AtatürkUniversity,25240Erzurum,Turkey

cDepartmentofEnvironmentalEngineering,FacultyofEngineering,ArdahanUniversity,Ardahan,Turkey dNecmettinErbakanUniversity,ScienceFaculty,BiotechnologyDepartment,Konya,Turkey

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received21February2017 Accepted22May2017 Keywords: ITO/PEDOT:PSS/P3HT:PCBM:Fe3O4solar cell Fe3O4nanoparticles Fe3O4/p-GaAsheterostructure Inhomogeneitybarrier Idealityfactor

a

b

s

t

r

a

c

t

8nm Fe3O4 nanoparticles (NPs) were successfully doped into

poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) to fabricate

ITO/PEDOT:PSS/P3HT:PCBM:Fe3O4/Al solar cell along with a heterojunction device of

Fe3O4/p-GaAsbydepositingthemonp-GaAssubstrates.Theexperimentalresultsrevealed

that the presence of Fe3O4 nanoparticles (NPs) in the ITO/PEDOT:PSS/P3HT:PCBM/Al

solarcellimproveditsperformancewithrespecttotheonewithoutFe3O4.Forexample,

power conversion efficiency was increased from 1.09% to2.22% when doping 5wt% ofFe3O4 NPstoP3HT:PCBM.Thiswasattributedtoincreaseofthelightabsorptionin

thepresence of Fe3O4 NPs doping.Furthermore, the analysis of the current–voltage

(I–V),capacitance–voltage(C–V)andcapacitance–frequency(C–f)characteristicsofthe Fe3O4/p-GaAsheterojunctionhavebeenstudiedsuccessfully.Theexperimentalbarrier

height˚bandidealityfactornweredeterminedas0.80eVand1.53,respectively,from

theexperimental I–V plots. In addition, the valueof the ˚b obtained from the C-V

characteristicswas0.95eV(f=500kHz).Themismatchbetweenbarrierheightsobtained frombothmeasurementswasexplainedbythetwotechniquesarebasedondifferent nature.TheinterfacestatedensityoftheFe3O4/p-GaAsheterojunctionwasdetermined

from5.16×1014cm−2eV-1to1.34×1015cm−2eV−1.

©2017ElsevierGmbH.Allrightsreserved.

1. Introduction

Magnetite(Fe3O4)isamemberofmaterialsreferredasferritesthatformanimportantclassofferromagneticmaterials. Themagneticionsinthesematerialslieintheintersticesofaclosepackedoxygenlatticethatcrystallizesintheface-centered cubic(fcc)withtheinversespinelstructure.Intheinversespinelferrites;thehalfofFe+3ionsoccupiedthetetrahedralholes whileotherhalfofFe+3andFe+2ionsfillstheoctahedralholes[1].Inrecentyears,Fe

3O4nanoparticles(NPs)haveattracted agreatdealofinterestinthelastfewdecadesinthefieldofnanoscienceowingtotheiruniquephysical,chemicaland magneticpropertiescomparedtoothermetaloxideNPs.Therefore,theyhavebeenusedinwidespreadapplicationssuch

∗ Correspondingauthor.

E-mailaddress:saydogan@atauni.edu.tr(S¸.Aydo˘gan).

http://dx.doi.org/10.1016/j.ijleo.2017.05.071

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asnanocatalysis[2,3],insolarcells[4,5],magneticresonanceimaging(MRI)[6],ferrofluids[7],energy/informationstorage [8]andbiosensing[9].

It iswidelyknownthat ametal/semiconductorheterojunctionisoneof theoldestpractical semiconductordevices intheelectronicsindustry.Thesetypeofheterojunctionscanbehaveaseitherrectifyingorohmicjunctionsdepending uponthetypeandchemistryofthecontactmaterials.Ontheotherhand,theSchottkyBarrierDiodes(SBDs)basedona semiconductorarethekeyelementsinhigh-speeddigitalapplications,field-effecttransistors(FETs),microwavediodes, solarcells,manyvaractors,photodetectorsandRFcommunications[10].Forrectifyingjunctions,theunderstandingofthe interfacecangreatlyimprovetheperformanceoftheelectronicdevices.Galliumarsenide(GaAs)hasbeenwidely-used semiconductormaterialinthemicroelectronicindustry,andespeciallyinoptoelectronicdevices.Theelectricalproperties ofGaAshassuperiortothoseofSi.Typicallyitisusedinthedevicesforlowpowerandhighfrequencyapplicationssince ithashighelectronmobility,bigforbiddenbandwidth(1.43eV)andlowchargecarrierconcentration[11–14].Moreover, GaAsbasedheterojunctiondevicesgenerallyofferthelowernoisethatofsiliconbaseddevicesinthesamefrequencyrange [15].Current-voltage(I–V)curvesareperhapsthemostpracticalwaytorevealthetransportmechanismofheterostructure. MostofheterojunctionsbasedonGaAsoftenstartfromn-GaAssubstrates[16,17].

Organicpolymerheterojunctions(OPHJ)arewidelyusedintheconstructionofefficientsolarcells[18].However,the majorityofsemiconductingpolymershavebandgapshigherthan2eV(620nm),whichlimitsthepossibleharvestingofsolar photons[19].Therefore,itisnecessarytoimprovethelightharvestingofOPHJbyfindingaconvenientway.Inthisregard, thegenerationofhybridsolarcellsbythecooperationofmetal/metaloxideNPsintotheOPHJwouldbeapromisingforthe solutionofthisproblembecausemetal/metaloxideNPsexhibitsurfaceplasmonresonances(SPR)thatcouplestronglyto theincidentlightandmoreover,theyhavehighabsorptioncoefficientsandhigherphotoconductivityascomparedtomany organicpoylmersemiconductormaterialsandtherebyincreasetheharvestingofsolarphotonswithintheOPHJmaterials [20,21]Thus,theadditionofmetalNPstoOPHJmaterialsoffersthepossibilityofenhancedabsorptionandcorrespondingly enhancedphotogenerationofmobilecarriers;moderateincreasesinpower-conversionefficiency(PCE)havebeenreported [22–25].

Inthisstudy,wehavefabricatedaFe3O4NPs/p-GaAsheterojunctionandanalyzeditsdetailedelectricalcharacteristics atroomtemperature.Moreover,wehavealsofabricatedaITO/PEDOT:PSS/P3HT:PCBM:Fe3O4/Alsolarcellasanexampleto themetaloxideNPsdopedOPHJandstudiedtheirperformancecomparedtothatofnon-Fe3O4doping.Ourexperimental resultsgaveahigherabsorptionspectrainvolveFe3O4nanoparticlesascomparedtothewithoutthenanoparticles.So,it isexpectedthatthesenanoparticlesshouldincreasethelightabsorption.Inaddition,itisimportantthatthenanoparticles aremonodispersetoosincepolydispersivepropertiesofthenanoparticlescanaffecttheelectronicpropertiesofthedevice.

2. Experimentalprocedure

2.1. SynthesisofmonodisperseFe3O4NPs

Thedetailedprocedureforthesynthesisandstructuralcharacterizationofmonodisperse8nmFe3O4NPswerereported authors’previouspublication[26].Therefore,nocharacterizationdataforthemonodisperseFe3O4NPsispresentedinthe currentmanuscripttoeliminatethereputationethically.

2.2. PreparationofITO/PEDOT:PSS/P3HT:PCBM:Fe3O4NPssolarcell

Firstly,thechemicalcleaningofITOsubstratewascarriedoutandpoly(3,4-ethylenedioxythiophene)polystyrene sul-fonate(PEDOT:PSS)wascoatedonsurfaceoncleanedITObyspincoatingwith4000rpm,60sandthenitwasannealed at453K(180◦C)for2min.AP3HT:PCBM(1:0.8:)layerincludes5%Fe3O4NPsdepositedonPEDOT:PSSusingspin coat-ingwith800rpm,60sandthenitwasannealedat388K(115◦C)for5min.TheSEMimagesofP3HT:PCBMundopedand dopedwithFe3O4areshowninFig.1aandb,respectively.Finally,anAlcontactwith100nmthicknessand2×4mm2area wasevaporatedonP3HT:PCBM:Fe3O4NPslayerat1×10−7Torr.Inaddition,ITO/PEDOT:PSS/P3HT:PCBM/Alheterojunction devicehasalsobeenfabricatedtocomparethesolarcellparametersofthattoITO/PEDOT:PSS/P3HT:PCBM:Fe3O4 NPs/Al solarcell.Thecurrentdensity-Voltage(J-V)measurementsofbothdeviceswerecarriedoutindarkandunderillumination with100mW/cm2.Allexperimentalprocessesandmeasurementwereperformedincleanroom(class1000).Fig.1cdepicts theschematicpresentationofITO/PEDOT:PSS/P3HT:PCBM:Fe3O4NPs/Alsolarcell.

2.3. FabricationofFe3O4NPs/GaAsheterojunction

Insecondpartofthestudy,ap-typeGaAssemiconductorwafer5–10-cmresistivitywithand(100)orientationwas usedforthefabricationofaFe3O4NPs/GaAsheterojunctiondevice.TheGaAswaferwasdippedin(5H2SO4+H2O2+H2O) solutionfor10minandin(H2O+HCl)solutionandthenfollowedbyarinsingstepinde-ionized(DI)waterof18M.For ohmiccontactformation,afterchemicalprocessAuwasevaporatedonthebackofthep-typeGaAsinavacuum-coating systemof10−5Torrandthenitwasannealedat723K(450◦C)for3mininN2ambient.TheFe3O4filmwasgrownonp-GaAs substratebysimpledropcoating.TheFe3O4layerthicknesswascalculatedasapproximately80nmusingtheexperimental capacitance-voltagemeasurements.Toprovidetheelectricalmeasurements,circularAucontacts(areais7.85×10−3cm2)

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Fig.1.(a)TheSEMimageofP3HT:PCBMundopedwithFe3O4.(b)TheSEMimageofP3HT:PCBMundopedwithFe3O4.(c)Schematicpresentationof ITO/PEDOT:PSS/P3HT:PCBM:Fe3O4NPs/Alsolarcelldevice.

weredepositedontopoftheFe3O4filmbyvacuumevaporationatabout10−5Torr.Theelectronicbehaviorofinterfaces betweenGaAsandFe3O4isusuallydominatedbythedensityofinterfacialdanglingbonds.Thecurrenttransportmechanism andelectricalparametersoftheFe3O4/p-GaAsdeviceweredeterminedusingI–VandC-Vcharacteristicsofthedevice.The I–VandC-VcharacteristicsoftheFe3O4/p-GaAsheterojunctionwereperformedatroomtemperature.

3. Resultanddiscussion

3.1. TheperformanceofITO/PEDOT:PSS/P3HT:PCBM:Fe3O4NPssolarcell

TheopticalabsorptionspectraofP3HT:PCBM:Fe3O4NPsandP3HT:PCBMwerestudiedandtheresultsweredepictedin

Fig.2.Thehighabsorptionbandhasbeenobservedat∼500nmwavelengthforthebothmaterials.Moreover,theincrease inthelightabsorptionofFe3O4NPsdopedPHJdeviceisclearlybeseenbyFig.2.Thismightbeduetotheincreasingoptical pathlengthwithinthePHJdevicebythereflectionandscatteringoflightbyFe3O4NPs.Thishigherlightabsorptionofthe inthepresenceofFe3O4NPsisveryimportantforincreasingtheirperformanceinsolarcells.

Fig. 3 shows the experimental current density-voltage (J-V) curves of ITO/PEDOT:PSS/P3HT:PCBM/Al (1) and ITO/PEDOT:PSS/P3HT:PCBM:Fe3O4 NPs/Al (2) solar cells in dark and under illumination. A clear response of ITO/PEDOT:PSS/P3HT:PCBM:Fe3O4NPs/AldevicetoilluminationwithrespecttoITO/PEDOT:PSS/P3HT:PCBM/Alisconcluded bythecurves.Whenasolarcellisunderilluminationandunderopencircuitconditions,measuredmaximumcurrentis calledshortcircuitcurrent(ISC)andunderopencircuitconditionsthemaximumvoltageisnamedopencircuitvoltageand

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400 500 600 700 0.00 0.20 0.40 0.60 0.80 1.00 Absorp ti on (arb. units ) P3HT:PCBM:Fe3O4 P3HT:PCBM

Fig.2.TheopticalabsorptionspectraofP3HT:PCBM:Fe3O4NPsandP3HT:PCBM.

0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00 -0.10 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 14

Current density (mA/cm

2

)

Voltage (V)

1

2

1-)ITO/PEDOT:PSS/P3HT:PCBM/Al and 2-) ITO/PEDOT:PSS/P3HT:PCBM:Fe

3

O

4

/Al

Fig.3. TheexperimentalCurrentdensity-Voltage(J-V)curvesofITO/PEDOT:PSS/P3HT:PCBM/Al(1)andITO/PEDOT:PSS/P3HT:PCBM:Fe3O4NPs/Al(2)solar cellsindarkandunderillumination.

givenbyVOC.Therefore,maximumpowerpointofthecellisdeterminedbyproductofmaximumcurrent(Im)andvoltage (Vm).Inadditiontobothparameters,fillfactor(FF)isalsoanotherimportantparameterandisgivenas:

FF= VmIM VOCISC =

Pmax VOCISC

(1) Thepowerconversionefficiency(PCE)ofasolarcellistheratioofgeneratedelectricitytoincominglightenergyandis givenas:

PCE=FFVOCISC Plight

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Table1

ThepowerconversionparametersofITO/PEDOT:PSS/P3HT:PCBM/AlandITO/PEDOT:PSS/P3HT:PCBM:Fe3O4NPs/Alsolarcells.

Device VOC(V) JSC(mA/cm2) FF(%) PCE(%)

ITO/PEDOT:PSS/P3HT:PCBM/Al 0.32 7.74 44 1.09

ITO/PEDOT:PSS/P3HT:PCBM:Fe3O4NPs/Al 0.56 8.62 46 2.22

Accordingtoourresultstheopencircuitvoltage(VOC),theshortcircuitcurrent(ISC),fillfactor(FF)andpowerconversion

efficiency(PCE)ofthePHJdevicehaveincreasedafterFe3O4NPsdoping.Thevariationoftheseparametersforwithand

withoutFe3O4NPsdevicesaredepictedinTable1.Inthissection,themainnoteworthypointistheincreaseofthePCEfrom

1.09%to2.22%afterusingFe3O4NPsinourPHJsolarcellconfiguration.Fe3O4NPsmayhavearolesuchthattheyactalight scatteringcentersintoP3HT:PCBMlayerortheyenhancetheabsorptionoflight.Thus,theymightincreasethedistance whichthelighttravelsthroughtheactivelayer.Furthermore,Fe3O4NPscanincreasetheabsorptionofthelightandasa resultofthis,theefficiencyofthesolarcellisincreased.

3.2. AheterojunctionapplicationofFe3O4NPs

Inrectifyingdevices,thebarrierheight˚bcanbedeterminedbyI–VandC-Vcharacteristics,aswellasbyballisticelectron emissionmicroscopy(BEEM)andinternalphotoemission.TheI–Vcurvesgivethetransportmechanismsontheinterfaceof rectifyingjunctions[27,28].

InSchottkycontactsmaincurrenttransportisoccurredbymajoritycarriersanditmaybeascribedbythermionicemission (TE)theoryofoverthebarrier.AccordingtoTEtheory,onlysomecarriers,those,withenergyatleastequaltoorlargerthan theconductionbandenergyattheinterfaceofthecontactmaterials,contributetothecurrentflow.Thetheoryacceptsthat thecurrentinjunctionbarrierdiodescanbeexpressedas:

I=I0



exp



qV nkT



−1



, (3)

wherenistheidealityfactorwhichequalsunityforanidealdiodeandI0isthereversecurrent.Anditisgivenby; Io=AA∗T2exp



−q˚b kT



, (4)

Furthermore,Aiseffectivediodearea,A*theRichardsonconstantandequalsto74A/cm2K2forp-typeGaAs,Tisthe temperatureinKelvin,btheeffectivebarrierheightatzerobias,whereqelectroncharge,Vtheforward-biasvoltage,k isBoltzmannconstant.bandnarethefundamentalparametersofSchottkybarrierdiodes(SBDs)[29,30].Experimental valueofnisdeterminedfromtheslopeofthelinearpartoftheforwardbiaslnI-Vcurvesas:

n= q kT

dV

d(nI) (5)

andthevalueofthebisdeterminedas: e˚b=kTln



AA∗T2/I0



(6) Inthepractice,thenon-idealSchottkycontactbehaviourisfrequentlyobservedandtheI–VplotscannotbefitbyEq.(3) forn=1.Thereasonofthenon-idealbehaviourmaybetheSchottkyeffect,interfacestates,seriesresistance,thepresenceof anoxidelayeratthesurfaceonthewafer,theothertransportcurrentmechanismsandinhomogeneityofthebarrierheights andsoon[28].

Fig.4depictsthecurrent-voltage(logI-V)characteristicsoftheAu/Fe3O4/p-GaAsheterostructuredevice.Thedevicehas showedexcellentrectificationcharacteristicswhichhasrelativelylowandsaturatedleakagecurrentof2×10−9Aandthe rectificationratioisabout1×105at±0.8V.Inthisstudy,thevalueofnobtainedfromtheI–VcharacteristicsusingEq.(5)has beenfoundtobe1.53.HighvaluesofncanbeattributedtothepresenceoftheinterfaciallayeratFe3O4andp-GaAsinterface, barrierheightinhomogeneityorseriesresistanceathighbiases.Thevalueofthe˚bobtainedfromtheI–Vcharacteristics usingEq.(6)isfoundtobe0.80eV.Thisvalueisconsideredsignificantlyhighforp-GaAsrectifyingdeviceswithrespect toAu/p-GaAsrectifyingdevices.Forexample,inourpreviousstudy[31]wehavedeterminedthebarrierheightofAu/Pd nanoparticles/p-GaAsas0.68eV.Inanotherexample,H.Saghrounietal.[31]havefoundedthebarrierheightofAu/p-GaAs as0.56eV.

Ahighseriesresistanceofdevicemayresultinperformancedegradation.Forexample,thepresenceofhighresistancemay limittheforwardbiascurrentespeciallyathighvoltages.Inthiscondition,whentheforwardbiasincreasestheresistance alsoincreasesexponentially.Howeverforanidealdiode,inreversebiasshouldbeanextremelyhighresistance.Thus,the reversecurrentsshouldhavebeextremelylowvalues.InanidealdiodethedominantseriesresistanceRstermwillbethe semiconductoronthelightly-dopedsideofthejunctiondeviceandoutsideofthespacechargeregionofthedevice.The valueofRsisinfluencedbythenatureoftheinterfacelayerbetweentheFe3O4andp-GaAsandthehighervalueofRsleads tonon-idealforwardbiasI–Vcurves.UsingforwardbiasI–Vplot,theRs,the˚bandthencanalsobedeterminedusingthe

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-1.20 -0.80 -0.40 0.00 0.40 0.80 1.20

Voltage (V

)

1.0E-10 1.0E-9 1.0E-8 1.0E-7 1.0E-6 1.0E-5 1.0E-4 1.0E-3

Cu

rr

en

t (A

)

Fig.4. Thecurrent-voltage(logI-V)characteristicsoftheFe3O4/GaAsheterostructure.

5.0E-4 1.0E-3 1.5E-3 2.0E-3

Current (A) 0.00 0.20 0.40 0.60 dV /d (l nI ) 0.60 0.80 1.00 1.20 1.40 1.60 H (I ) V H(I)= 200.02 2 X+ 1.2 6961 dV/d(lnI )=200 .395 X + 0.04082 5

Fig.5.Theplotsof dV

d(lnI)vs.IandtheH (I) vs.IcurvesoftheFe3O4/GaAsheterostructure.

methodofCheungandCheung[32].Accordingto[32],whenconsideringtheseriesresistanceeffect,theforwardbiasI–V characteristicsduetotheTEofajunctiondiodecanbemodifiedas;

I=I0exp



q (VIR s) nkT



, (7)

wheretheIRspresentsthebiasdroptermacrossthedevice. dV d (lnI)= nkT q +IRs, (8) H (I)=V−



nkT q



ln



I AA∗T2



, (9)

andH(I)isgivenbyEq.(8):

H (I)=n˚b+IRs, (10)

Aplotof dV

d(lnI) vs.IwillbelinearandtheslopegivesRsand nkT

q asthey-axisinterceptfromEq.(9).Fig.5showsthe plotsof dV

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0.00 0.20 0.40 0.60 0.80 1.00 1.20 Voltage (V) 0.70 0.75 0.80 0.85 0.90 0.95 F (V )

Fig.6.F(V)–VplotoftheFe3O4/GaAsheterostructure.

asn=1.57,Rs=200,respectively.UsingthevalueofnobtainedfromEq.(8),thevalueof˚bisobtainedfromplotofa functionH(I)givenby(10).FromH (I)-Iplots,thevaluesofthe˚bandRShavebeencalculatedas˚b=0.81eV,Rs=200, respectively.ItcanbeobviouslyseenthatthevalueofRsobtainedfromH(I)–Icurveisequalstothevalueobtainedfromthe dV/d(lnI)–Iplot.Inaddition,sincetheseriesresistancevaluesarenothigh,thevaluesoftheidealityfactorandthebarrier heightdeterminedfromCheungfunctionsareveryclosetothelnI-Vplots.

InthedeterminationthevalueoftheRsanalternativemethodwasproposedbyNorde[33].Accordingtothismethod, firstlyaF(V)functionisdeterminedasfollows:

F(V )= V −kTq ln



I(V )

AA∗T2



(11) whereisanintegerwhichisgreaterthann.I(V)iscurrentobtainedfromtheexperimentalI–Vcharacteristics.Afterplotting theFvs.V,thevalueof˚bcanbedeterminedfromEq.(11),whereF(V)istheminimumpointofF(V)andV0isthevoltage whichcorrespondstominimumofF(V).

˚b=F(V )+ V0

 − kT

q (12)

Fig.6depictstheF(V)–Vplotofthestructure.FromNorde’sfunctions,RsvaluecanbedeterminedfromEq.(11). Rs=kT (−n)

qI (13)

UsingtheF–Vplots,thevaluesof˚bandRsfortheFe3O4/p-GaAsstructurehavebeencalculatedas0.79eVand192, respectively.TheexperimentalresultsshowedthatthevaluesoftheRs and˚bobtainedfrombothCheungand Norde methodsareinagreementwitheachother.

Interfacestatesintheband-gapofasemiconductorarehelpfulforthedeterminationofsomeelectricalcharacteristics ofheterojunctions,MOS(orMIS)devices,andSchottkybarrierdiodes.ItmaybeexpectedthatthedepositionoftheFe3O4 layeronGaAswillmodifytheinterfacestatesdensityintheband-gapoftheGaAs.Whentheinterfacestatesinequilibrium withtheGaAs,thenisgivenby[7,34–36];

n(V )= εı i



ε s w +qNss(V )



+1 (14)

whereεiandεsarethepermittivitiesofinterfaciallayerandGaAs,wisthewidthofthedepletionregionandNssisthe densityoftheinterfacestatesunderequilibriumconditions.Forap-typesemiconductors,theenergyoftheinterfacestates Essisgivenas[34];

Ess−Ev=q˚e−qV (15)

WhereEvisthevalence-bandmaximumenergyofGaAs.Theenergydistributioncurvesoftheinterfacestates(Nssvs.Ess-Ev) canbedeterminedfromtheforwardbiasI–Vcurves.Nssvs.Ess-EvplotsoftheFe3O4/p-GaAsheterostructureareshownin

Fig.7.Theexponentialvariationoftheinterfacestatedensitytowardsthemaximumofthevalancebandisclearlyseeninthe figure.Namely,theinterfacestatedensitieshaveshowedanexponentialrisewithbiastowardsthetopofthevalenceband. TheexperimentalNssobtainedfromtheforwardbiasI–Vplotrangesfrom5.16×1014cm−2eV-1to1.34×1015cm−2eV−1. Theseinterfacestatesdensityresultsarerelativelyhighforsuchdevices.Theseresultscanbeexplainedbytheexistence

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0.44 0.46 0.48 0.50 0.52 0.54 0.56 Ess-Ev (eV) 4.0E+14 6.0E+14 8.0E+14 1.0E+15 1.2E+15 1.4E+15 N ss

Fig.7. InterfacestateenergydistributioncurveoftheFe3O4/GaAsheterostructure.

-1.20 -0.80 -0.40 0.00 0.40 0.80 1.20 1.60 2.00 Voltage (V) 0.0E+0 5.0E+2 1.0E+3 1.5E+3 2.0E+3 2.5E+3 3.0E+3 Capacitance ( pF ) f= 500 kHz

Fig.8.TheforwardandreversebiasC-VcharacteristicsoftheFe3O4/GaAsheterostructureat500kHz.

ofathinnativeoxidelayer,theexistenceofaninterfaciallayeranddecreasingoftherecombinationcentresbetween half-metallicFe3O4layerandp-GaAssemiconductor[35–39].ThesechangeshavebeenalsoattributedandbetweentheFe3O4 andp-GaAs.

Figs.8and9showtheC–Vandthereversebias1/C2-VplotsoftheFe

3O4/p-GaAsheterostructure,atroomtemperature. Themeasurementshavebeenperformedatfrequencyof500kHz.Themaincapacitanceofhighfrequenciesisspacecharge capacitance.Thecapacitancevalueshavebeenfoundtobeincreasewithincreasingappliedvoltage.Apeakisseeninthe C-Vplotsat1.8Vduetotheexistenceoftheinterfacestatesandtheseriesresistance.ThediffusionpotentialVdatV=0is determinedfromanextrapolationofthelinearC−2-VplottotheV-axis.TheexperimentalvalueoftheVdfoundtobe0.74V. InMScontactsthedepletionlayercapacitanceisgivenasfollows[11],

C−2= 2 (Vd+V ) εsε0qA2NA

(16) whereεsis12.9forGaAs,ε0isthedielectricconstantofvacuum(=8.85×10−14F/m)andNAisthedensityofionizedacceptors anditiswrittenas:

NA=NVexp(Vp/kT ) (17)

ThevalueofNAhasbeenfoundas6.83×1014cm−3byusing1/C2-Vplot.Using1/C2-Vplotsthevalueofthe˚bcanbe obtainedbytherelation:

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-0.80 -0.40 0.00 0.40 0.80 1.20 1.60 2.00 Voltage (V) 1.0E-6 2.0E-6 3.0E-6 4.0E-6 5.0E-6 C -2(p F) -2 f= 500 kHz

Fig.9.ThereversebiasC−2-VcharacteristicsoftheFe3O4/GaAsheterostructureat500kHz.

1.00E+3 1.00E+4 1.00E+5 1.00E+6 1.00E+7

Frequency (Hz) 1.0E-1 1.0E+0 1.0E+1 1.0E+2 1.0E+3 1.0E+4 Capacitance (pF ) V= 0.00 Volt V= 0.10 Volt V= 0.20 Volt

Fig.10. TheforwardbiasC-fcharacteristicsoftheFe3O4/GaAsatvariousvoltages.

whereˇequalsto1/n.Whenˇequalstounity,thenwesaythatthediodeisideal.Vnisthepotentialdifferencebetween theFermilevelEFandthetopofthevalancebandintheneutralregionofp-GaAs.TheexperimentalvalueofVnforp-type GaAs,usedinthisstudyisfoundas0.21eVusingEq(19):

Vn=kTln( Nv NA

) (19)

whereNv=9.0×1018cm−3isthedensityofstateinthevalancebandforGaAs.FromtheC-Vmeasurementsthevalueof˚b hasbeencalculatedas0.95atf=500kHz.ThisvalueishigherthanthederivedfromtheI–Vmeasurements.Thisdifference maybehappenedinpracticesincetheC-Vmeasuresonlythemeanvalueof˚b.However,I–Vtechniqueconsidersthecharge carrierswhoseenergyexceedstheinhomogeneousbarriers.So,ifthecarriercrossedoverlowbarrier,thenthebarriervalue isdeterminedasloworviceversa.Forourresults,thisdiscrepancywasattributedtotheinhomogeneitiesofthebarrier height,nonuniformityoftheinterfacialFe3O4layerthickness,thepresenceoftheinterfacialoxidelayercomposition,and distributionofinterfacialchargesattheinterfaceofFe3O4andGaAs[40].Ifthebarrierisuniformandideal,bothtechnique givesthesame˚bvalue.

Fig.10showsthecapacitance-frequency(C-f)curvesoftheFe3O4/p-GaAsheterostructureatvariousvoltages.Thecurves showthat thecapacitanceof thestructure decreaseswiththefrequency atlowfrequencies, whereasindependentfor frequencyathighfrequencies.Thehighervaluesofcapacitanceatlowfrequencyhavebeenattributedtotheinterfacestates thatcanfollowthealternatingcurrentA.C.signal,whereasathigherfrequenciestheycannotfollowit.

4. Conclusion

Inthisstudy,wehaveinvestigatedtheinfluenceofFe3O4nanoparticlesonITO/PEDOT:PSS/P3HT:PCBMorganicsolarcell andontheAu/p-GaAsrectifyingdeviceandweshowedthattheinfluenceofFe3O4nanoparticlesonbothdevicesispositive results.Theexperimentalresultsshowedthatthepowerefficiencyoftheorganicsolarcellincreasedabouttwotimesafter dopingofFe3O4nanoparticles.Furthermore,ithasbeenseenthatimprovementofrectifyingcontactshasbeenobtainedby simpledropcoatingofthehalfmetallicFe3O4nanoparticlesonp-GaAssemiconductorsuchthatahighbarrierofAu/Fe3O4

(10)

/p-GaAsdeviceobtained.ThediodeparametersofAu/Fe3O4/p-GaAssuchasseriesresistanceRs,theidealityfactornandthe barrierheight˚bhavebeendeterminedbyperformingdifferentplotsfromtheexperimentalforwardbiascurrent–voltage (I–V)andreversebiascapacitance–voltage(C–V)measurement.Atsufficientlylargevoltages,thedownwardcurvatureof theC-VcurveiscausedbythepresenceoftheeffectofRs,apartfromtheinterfacestateswhichareinequilibriumwith theGaAs.Insummary,theFe3O4/p-GaAsheterostructurecanbeemployedinlightdetectors,high-speedmicroelectronic applications,microwavecommunicationdevices,high-speedcircuitapplicationsandsolarcells.

Acknowledgements

ThisworkwassupportedbyTurkishGovernment(TUBITAK)withresearchprojectnumberof212T012.ÖMthanksto thepartialfinancialsupportbyTurkishAcademyofScienceYoungScientistProgram(TUBA-GEBIP).

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Şekil

Fig. 1. (a) The SEM image of P3HT:PCBM undoped with Fe 3 O 4 . (b) The SEM image of P3HT:PCBM undoped with Fe 3 O 4
Fig. 3. The experimental Current density-Voltage (J-V) curves of ITO/PEDOT:PSS/P3HT:PCBM/Al (1) and ITO/PEDOT:PSS/P3HT:PCBM:Fe 3 O 4 NPs/Al (2) solar cells in dark and under illumination.
Fig. 4. The current-voltage (logI-V) characteristics of the Fe 3 O 4 /GaAs heterostructure.
Fig. 6. F(V)–V plot of the Fe 3 O 4 /GaAs heterostructure.
+3

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