Effect of Au nano-particles on TiO2 nanorod electrode in dye-sensitized solar cells

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Electrochimica

Acta

jo u r n al h om ep a ge :w w w . e l s e v i e r . c o m / l o c a t e / e l e c t a c t a

Effect

of

Au

nano-particles

on

TiO

2

nanorod

electrode

in

dye-sensitized

solar

cells

M.

Ghaffari

a,b,∗

_{, M.}

_Burak

_Cosar

c,d

_{, Halil}

_I.

_Yavuz

c,d

_{, M.}

_Ozenbas

c,d

_,

_Ali

_K.

_Okyay

a,b,∗ a_{UNAM–InstituteofMaterialsScienceandNanotechnology,BilkentUniversity,Ankara06800,Turkey}

b_{DepartmentofElectricalandElectronicsEngineering,BilkentUniversity,Ankara06800,Turkey}

c_{DepartmentofMetallurgicalandMaterialsEngineering,MiddleEastTechnicalUniversity,Ankara06800,Turkey} d_{CenterforSolarEnergyResearchandApplications(GUNAM),MiddleEastTechnicalUniversity,Ankara06800,Turkey}

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received9March2012

Receivedinrevisedform4May2012 Accepted17May2012

Available online 26 May 2012 Keywords:

Dye-sensitizedsolarcell Hydrothermal TiO2nanorods

Photoreduction

a

b

s

t

r

a

c

t

Aunanoparticles(NPs)weredepositedonverticallygrownTiO2nanorodarraysonFTOsubstrateby

hydrothermalprocess.MetalnanoparticleswereloadedontothesurfaceofTiO2nanorodsvia

photo-chemicalreductionprocessunderultravioletirradiation.X-raydiffraction(XRD),electronmicroscopy (FESEM),transmissionelectronmicroscopy(TEM)andX-rayphotoelectronspectroscopy(XPS)analysis wereusedtocharacterizetheas-preparedAu/TiO2nanorodcomposites.Currentdensity–voltage(J–V)

measurementswereobtainedfromatwo-electrodesandwichtypecell.ThepresenceofAunanoparticles canhelptheelectron–holeseparationbyattractingphotoelectrons.AdditionofAunanoparticlestothe TiO2nanorodsignificantlyincreasedthefillfactorandJSC(shortcircuitcurrentdensity).Theapplication

ofAuNPsTiO2nanorodsinimprovingtheperformanceofDSSCsispromising.

© 2012 Elsevier Ltd. All rights reserved.

1. Introduction

SincethefirstreportbyGratzeletal.in1991,dyesensitized solarcells (DSSC)haveattractedextraordinaryattention.DSSCs couldbeviable lowcostalternativestosilicon-based solarcells [1,2].InitialdemonstrationsofDSSCswerebasedondye sensitiza-tionofporousnanocrystallinesemiconductorslikeTiO2.Thereare

numerousstudiesonthepotentialsofimprovingtheefficiencyof DSSCsbyusingothersemiconductingmetaloxides,suchasNb2O5

[3],CeO2[4],ZnO[5,6],andSnO2[7]andcompositeoxidematerials

[8,9].

Synthesis of aligned single-crystalline TiO2 nanorods or

nanowires has attracted extensive attention because of their excellentanduniquepotentialapplicationsinelectronics, photo-chemistry,biologyandoptics,aswellastheirapplicationsingas sensors[10],dye-sensitizedsolarcells[11],lithiumionbatteries [12],photovoltaicdevices[2],andasphotocatalysts[13].

Recently,numerousmethodsforfabricatingone-dimensional nanostructuredTiO2havebeenreported,suchastemplate-assisted

synthesis[14],sol–gel[15],chemicalvapordeposition[16], elec-trochemicaletching[17],andhydrothermal[18–20].Amongthese techniques,thehydrothermalmethodofTiO2nanorodarraysisa

promisingtechniqueowingtoitsscalability,simpleprocess,and

∗ Correspondingauthorsat:UNAM–InstituteofMaterialsScienceand Nanotech-nology,BilkentUniversity,Ankara06800,Turkey.Tel.:+905443839429.

E-mailaddresses:ghaffari@unam.bilkent.edu.tr,moha0094@e.ntu.edu.sg (M.Ghaffari),aokyay@ee.bilkent.edu.tr(A.K.Okyay).

low cost.Thepotentialadvantages ofthehydrothermal growth methodmaybepartiallyhinderedbythegrowthofrutilephase TiO2nanorods.InordertoachievehighefficiencyDSSCs,TiO2in

anateseformisdesirable,whichmaybeobtainedbya post-growth-coatingmethodsuchasatomiclayerdepositiontechnique.

DSSCefficienciescouldbeimprovedsignificantlybyusingsingle crystalnanorodsofTiO2insteadofnanoparticleswhichcould

pro-motetheefficienttransportofphotogeneratedelectrons[21–23]. Thecrystal structure ofTiO2 also playsa factorin efficiencyof

DSSCs[24–27].Thedifferencebetweentheconductionband lev-elsofanataseandrutileforms,howeverduetothelowdifference between the conduction bandlevels of anataseand rutile, the higherdiffusioncoefficientofanataseplaymoreimportantrolefor theefficientelectrontransportinanatase[28]andtherefore,favors theuseofpureanataseforDSSCapplications[24].Therearereports, however,thatsynergisticuseofrutileandanatasepolymorphsof TiO2nanoparticlescouldincreaseDSSCefficiency[24,25,27].Such

aneffectisalsoobservedinphotocatalyticactivityofanataseTiO2

nanocrystals,whichsignificantlyincreasesuponmixingwitha por-tionoflesseractiverutilenanocrystals[29,30].Onthecontrary, therearesomereportsclaimingbetterefficiencyDSSCswithpure anataseform[31],however,aconclusivecomparison cannotbe obtainedbecausesizeandmorphologyoftheparticleswerealso variedinthosestudies.

InDSSCs,atthesemiconductor/electrolyteinterface,the recom-binationofaportionoftheelectronsisstillinevitable.Thereare severalstudiestodecreasethisundesiredreactionattheinterface ofsemiconductor/electrolytebyusingcore-shellstructureof var-iousmetaloxides[6,32,33]orbyusingdifferentadditivesinthe 0013-4686/$–seefrontmatter © 2012 Elsevier Ltd. All rights reserved.

electrolytetopassivetheelectrolyte-exposed[34–37].Moreover thereareanumberofearlierreportsshowingthatadditionofmetal nanoparticlesimprovesDSSCenergyconversionefficiency[38–41]. Inthisstudy,AuNPsacttopreventchargecarrierrecombination byforminga Schottkyenergybarrier thatpromotesthe photo-injectedelectronsintothenanorod,awayfromthesurfacewhich improvestheoverallconversionefficiencyoftheDSSCswithAu NPs/TiO2nanorodstructure.ShiiChouetal.[41]havereporteddye

sensitizedsolarcellbasedontheTiO2/nano-metalcomposite

parti-clesusingthedrycoatingprocessinamechano-fusionsystem,and studiedtheeffectofnano-metallicparticlesontheperformanceof thecellandobtainedalowerefficiencyof0.29%.Inourresearch, AuNPsweredepositedonTiO2nanorodarraysbyphotoreduction

beforedyeabsorption,andanoverallconversionefficiencyof0.93% wasobtainedforAuNPs/TiO2structurewhichisathreefoldhigher

relativetodeviceswithoutAuNPs.

Thereareseveralparametersthatcanaffecttheefficiencyof DSSCsbasedonhydrothermal-grownTiO2 nanorod arrayssuch

asgrowthtime, growthtemperature,substrates,initialreactant concentration,acidity,titaniumprecursorsandTiCl4treatment.By

optimizingtheseparameters,itispossibletoobtainhighefficiency DSSCs[22,42].Inthisstudy,wecomparativelystudytheeffectof AuNPsonDSSCefficiencybydefiningabaseline(withnoAuNPs) deviceasreference.

2. Experimentalprocedure

2.1. Chemical

FTOglass substrate (F:SnO2,Tec 15/sq.) witha thickness

of4mm waspurchasedfromPilkington,England. Ethanol, ace-tone, titanium butoxide (Ti(OCH2CH2CH2CH3)4) (97%), HAuCl4

(Mw=339.79), and HCl (36%) werefrom Sigma–Aldrich Co. All chemicalswere of analytical grade and were used asreceived. Hydrothermalgrowthwascarriedoutina45mlteflon-lined auto-clave.Deionizedwaterwasusedtoprepareallthesolutions.

2.2. GrowthofTiO2nanorodarray

ThedimensionofFTOglasssubstratewas10mm× 50mmand wascleanedanddegreasedpriortouse,firstbywashingwith deter-gent anddistilled water, then wereultrasonically cleanedwith ethanolandthenbysonicatinginethanol,acetoneanddeionized waterinsequenceforabout30min,andfinallydriedunder nitro-genflow.20mLconcentratedhydrochloricacid(36%byweight) wasmixedwith20mLdeionizedwaterinateflon-linedstainless steel autoclave (45mL vol.). The mixture was stirred at ambi-entconditionsfor10min,andthen0.8mLtitaniumbutoxidewas addedintotheabovementionedsolutionandstirredfor30min. ConductingsideofFTOsubstratewasfaceddownwithanangle againstthewalloftheteflonvessel.Samplewaskeptat140◦Cfor 4hinanelectricaloven.Thesampleandteflonvesselwasunloaded andlettocooltoroomtemperature.TheFTOsubstraterinsedwith deionizedwaterandethanol,andthendriedinair.

2.3. DepositionofAunanoparticlesonTiO2nanorodsby

photoreduction

20␮L(2.5mmol/L)HAuCl4ofaqueousAusaltsolutionwere

dis-persedwith15mLofdeionizedwaterinapyrexpetridishwith capacityofabout20mL.ThentheTiO2 nanorodsgrownonFTO

substratewerefloated inthepetri dish and exposedunder UV light(254nm)fromaUVPcompany,ELseries(8W,UVLMS-38EL, 376mm×96mm×64mm).Thereactionswerecarriedfor3and

Fig.1. Schematicsketchofassembleddyesensitizedsolarcell.

9h.Thenthesamplewaswashedbydeionizedwaterandethanol. Finally,thesamplesweredriedatroomtemperature.

2.4. Materialscharacterization

Thephasestructuresoftheobtainedsampleswereidentified using a Pananalytical (X’pert Pro MPD) instrument. XRD pat-ternswerecollectedoverthe2 angularrange of10–80◦ using Bragg–Brentanogeometry(CuK␣source,primaryandsecondary Sollerslits,0.1mmdivergenceslits,0.3mmreceivingslit,and sec-ondarygraphitemonochromator).Themorphologyandstructural characteristicsoftheobtainedsampleswerestudiedbyscanning electronmicroscopy(FESEM,FEI–NovaNanosem430),and trans-missionelectronmicroscopy(TEM,FEI–TecnaiG2F30).Thesurface chemicalcompositionoftheAu/TiO2nanorodswasmonitoredby

X-rayphotoelectronspectroscopy(XPS)measurements,performed withaThermo(K-Alpha–MonochromatedHigh-performanceXPS Spectrometer)instrument.

2.5. Electrodefabricationandcharacterizationtechniques

A schematic diagram of assembled dye sensitized solar cell ispresentedinFig.1.Theactiveareaofelectrode was0.25cm2

(i.e. 5mm×5mm). Synthesized TiO2 electrodes were soaked

in 0.5mmol/l ruthenium sensitizer dye [cis-di(thiocyanato)-N-N-bis(2,2-bipyridyl-4-carboxylicacid-4-tetrabutylammonium carboxylate) ruthenium(II)] dye(known asN719,Solaronix)in a t-butanol/acetonitrile(1:1, in volumeratio)solution,for24h. Thesensitizedelectrodeswererinsedwithacetonitrile,driedin roomtemperature,and immediatelyusedfor measuring photo-voltaic properties. The platinum coated FTO glass wasused as counterelectrodesbondedtoTiO2nanorodsasworkingelectrode

by 25-␮m-thick hot-melt spacers made of theionomer Surlyn 1702(Dupont).Theinternalspaceofeachcellwasfilledwitha liquidelectrolyte.Theelectrolytewascomposedof0.1M guani-diniumthiocyanate(GuSCN),0.03MI2,0.5M4-tert-butylpyridine (TBP)and0.6Mbutylmethylimidazoliumiodide(BMII)inthe mix-tureofacetonitrileandvaleronitrile(85:15,invol.%).Thedevices werecharacterizedbyaKeithley2440sourcemeterunder stan-dardAM1.5G-filteredirradiation(100mW/cm2_{)fromaNewport}

91192solarsimulatorequippedwith300Wxenonarc.Thespectral measurementsareobtainedbyamechanicallychopped monochro-mated(NewportCornerstone1301/8m,1200L/mm)white-light sourceand alock-in amplifier(SRS 830).The spectralintensity ofthelightsourceisalsoseparatelycharacterizedandisusedto normalizethemeasuredcurrent.

3. Resultsanddiscussion

To identify the crystalline structure and orientation of the nanorodsgrown,XRDanalysiswasperformedontheobtained sam-ples.TheXRDpatternoftheas-preparedTiO2nanorodarrayand

thestandarddiffractionpatternofrutilestructureofTiO2(JCPDS,

82-0514) is presented in Fig. 2. All diffraction peaks can be indexedtotheFTOsubstrateandrutilephaseofTiO2(tetragonal,

P42/MNM).IncomparisonwiththeICSDstandard XRDpattern, XRDpatternofthealignedrutileTiO2 grownonFTOshowsthe

preferable orientation of the nanostructures along TiO2 [101]

(2∼26.4◦).

The FESEM micrographs (Fig. 3(a) and (b)) presents highly orderedTiO2nanorodsgrownontheFTOsubstrates.Theseresults

confirmthatwell-alignedTiO2 nanorodsuniformlygrewinlarge

area.Cross-sectionalFESEMimageofTiO2 nanorodarrayshown

inFig.3(b) indicatesthatlengthof nanorodsareabout2.1␮m. ThenanoroddiameterdistributionwasobtainedfromtheFESEM images,whichrangesfromabout135to150nm(Fig.3(a)).Fig.3(c) and(d)presentstheTiO2nanorodsampleswithdifferentamount

ofgolddepositionthatclearlyshows theeffectdepositiontime, 3hand9h,respectively.InsetFig.3(c)and(d)showsthattheAu NPspreferentiallydepositonthetipsofrodsandtopactivepartof nanorodsarecoveredbyAuNPs.

Fig.4(a)shows theTEMimageofasingleTiO2 nanorodand

correspondingselectedareaelectrondiffractionpattern(SAED). TheTEMmicrographshows that thediameterof TiO2 nanorod

is 148nm. The SAED and HRTEM results further confirm that eachindividualTiO2 nanorodissinglecrystal.Thelatticefringe

Intensity

(a

u

)

20 15

Intensity

(a.u)

30 25

*

(

*

(110) 40 35 45 R (111) (101)

*

R

*

_R 50 (21

*

R TiO₂nan JCPDS 8

2 Theta

55 60 65 1) (002)

*

R R norods 820514 Sta 75 70 )(112)

*

R R andard Card R= Ru

*

= FT 80

*

utile TO

Fig.2.XRDpatternofTiO2nanorodarraysonFTOsubstrate(*),andthestandard

diffractionpatternofrutilestructureofTiO2(JCPDS-82-0514standardcard).

spacing in the HRTEMimage is 0.32nm which corresponds to theinterspacingofthe(110)planesoftetragonalrutilestructure ofTiO2 andindicatesthatthegrowthoccurredalongthe[101]

direction.

TEMmicrographsoftheAuNPsdepositedonTiO2nanorodsare

showninFig.5(a)and(b)whichconfirmthatAuNPsdeposited

Fig.4.TEMmicrographsofrutileTiO2(110)(a)low-magnificationimage,andcorrespondingselectedareaelectrondiffractionpattern(SAED)and(b)high-resolutionTEM

micrograph.

onthesurfaceofTiO2nanorods.Thisisattributedtoatop

illumi-nationandlowpenetrationofUVlightintotherodsalongtheir growthaxishencemostof thereduction reactionoccurringon thetipareaofnanorods.Fig.5(b)alsoshowsthatthesizeofAu

NPsisabout8–12nm.Fig.5(d)showsthecorrespondingSAEDof AuNPsdepositedonTiO2nanorods.Theringdiffractionpattern

clearlyconfirmedthatstructureofdepositedAunanoparticlesare polycrystalline.

Fig.5.TEMmicrographsofAuNPsdepositedonTiO2nanorods(a)low-magnificationimageof1.54at.%Audeposition,(b)low-magnificationimageof3.4at.%Audeposition,

0 200 400 600 800 1000 1200 C 1S Au 4d Au 4f O 1S Ti 2p

Intensity (a.u)

Binding energ

y (eV)

Au/TiO

2

nanoro

ds

TiO

2

nanor

ods

C 1S

(a)

(a)

Fig.6. WidescansurveyXPSspectrumof(a)TiO2nanorodarrays,and(b)Au/TiO2

nanorodarrays.

Thesurfacecompositionoftheobtainedsampleswas charac-terizedbyX-rayphotoelectronspectroscopy(XPS)technique.XPS survey-scanspectrumofTiO2nanorodsandAu-NPdecoratedTiO2

nanorodsareshowninFig.5(a).AllXPSspectralpeakswerefitted withThermoScientificAvantagesoftware.Asrequiredbytheory, theTi2pandAu4fspectrumconsistoftwopeaks,aspin–orbit dou-bletwhereasO1sandC1sspectrallinesconsistofasinglepeak(a singlet).TheC1sspectrallinewasstandardizedto285.0eVand theO1s,Ti2pandAu4f spectrawereadjustedtothis energy. Thedataanalysisinvolvedcurve-fittingLorentzian–Gaussian(30% Lorentzian)lineshapes,spectranormalization,andShirley back-groundsubtraction[43].InallfitstonarrowscanspectraShirley backgroundswereused[44,45].

Fig.6presentsthesurveyscanofpreparedsamples.TheXPS peakswithbindingenergiesof84.03,458.501and529.96eV cor-respondtoAu4f,Ti2p3/2andO1s,respectively.TheAu4fspectra serveasevidencefortheformationofAuNPsontheTiO2nanorods.

Fig.7showsO1sXPSspectrafortheAu/TiO2nanorods(Fig.7(a))

andTiO2nanorods(Fig.7(b))samples.DeconvolutionoftheO1s

Fig.8.Schematicdiagramoftheprincipleof(a)theconventionalDSSC,and(b) theDSSCwithAuNPsdepositedonTiO2nanorodsDSSC.(Forinterpretationofthe

referencestocolorinthetext,thereaderisreferredtothewebversionofthisarticle.)

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8

(a)

(A) 1.54 (at.%) Au NPs (B) 3.4 (at.%) Au NPs (C) 0.81 (at.%) Au NPs (D) Reference (TiO₂ nanorods)

J (mA/cm 2 ) V(volt) 3.4 1.54 0.81 0 0.59 0.60 0.61 0.62 0.63 0.64 0.65 0.66 0.67 0.68 Voc FF

Au content (atomic percentage) Voc (V)

(b)

0.40 0.45 0.50 0.55 0.60 FF 300 350 400 450 500 550 600 650 700 Current (a.u)

ShortCircuitCurrent measurement

Wavelength

1.54 (at.%) Au NPs

3.5 (at.%) Au NPs

Reference (TiO₂ nanorods)

(d)

0 0.81 1.54 3.4 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 Jsc Efficency (%)

Au content (atomic percentage)

Jsc (mA/cm 2 )

(c)

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 Efficency (%)

Fig.9.(a)Current–voltagecharacteristicsoftheDSSCsassembledusingtheTiO2nanorodarrays,andtheAu/TiO2nanorodwithdifferentgoldcontent;(b)dependenceoffill

factorandVOContheamountofAuNPscontentdeposition;(c)dependenceofcellefficiencyandJSContheamountofAuNPscontentdepositionoftheTiO2nanorodcells;

and(d)shortcircuitcurrentmeasurementvs.wavelength.

spectrayieldsthreeindividualpeaksat529.7,531.58and532.64eV. Regardingtheliteraturetheoxygenpeak locatedat 529.7eV is relatedtotheO1sbandpositionofTiO2 [46,47].Fig.7(c)shows

thedetailed peakscanofAu4fwhich confirmsthepresenceof peakswithabindingenergyof83.35eVand87.14eV correspond-ingtoAu4f5/2andAu4f3/2,respectively[9,45].ShamandLazarus explained that ambient TiO2 samplesare always covered with

physisorbedmoistureandchemisorbedandtheO1sbinding ener-giesforthissurfaceshiftstohigherbindingenergy.ThusO1speak at531.5eVcanbeassigned tooxygenspeciesinH2Omolecules

andTi OHwhilethepeakat533.7eVisattributedtothewater, hydroxideabsorbedonthesurfaceand(C O CorC OHgroups) fromtheoxidizedcarbon species ofadventitious carbon [9,45]. TheXPSresultsrevealthatwithdepositionofAuNPstheamount ofmoistureandcarbonspeciesonsurfaceofsamplesdecreased dramatically.

IntheAu/TiO2nanorodsystem,duetothelargerworkfunction

ofAu(5.1eV)comparedtotheelectronaffinityofTiO2(3.2eV),a

Schottkybarrierexistsattheirinterface.Fig.8showstheschematic diagramofthepossibleelectron-transferpathintheAuNPsTiO2

nanorodsDSSC and formation of theSchottky barrier between theAuNPsandTiO2nanorods.By(after)growingtheAuNPson

thesurfaceofTiO2nanorods,Fermi-leveloftheAu/TiO2nanorods

attainsastablebalanceinequilibriumcondition(orangedashed lineinFig.8(b).Generatedelectronandholepairsasaresultof photoexcitationprocess,createelectriccurrentwhichbenddown theconductionandvalencebandsofAu/TiO2nanorods(thegreen

curveinFig.8(b).ConsequentlytheAu/TiO2nanorodsFermilevel

ispusheddownward(thebluedashedlineinFig.8(b).Moreover

someofexcitedelectronscanalsobedirectlyinjectedfromthedye intotheCBoftheTiO2nanorods.Therefore,duetotheexistence

oftheSchottkybarrierattheAuNPsandTiO2nanorodsinterface,

electronsattheCBofTiO2nanorodscannotreversetheirpath,and

flowtowardstheoxidizeddyemoleculesortheredoxelectrolyte, thusleadingtoanimprovementinthephotocurrent[36,41].

Fig.9(a)compares the TiO2 nanorod solar cell and Au/TiO2

nanorodsolarcells.ItcomparesJ–VdatafromsamplesofAuNPs depositedwithdifferentgoldconcentration.Intheplots aTiO2

nanorodcellisincludedasreference.DepositionofAuNPstoTiO2

nanorodcellscausedsignificantimprovementinefficiency,fill fac-tor,andJSC(Table1).Fig.9(b)presentsthatwithincreasingtheAu

NPs,theopen-circuitvoltage,VOC,ofobtainedsamplesdoesnot

changesignificantly,whichconfirmsthatpreparedcellsarequite stableandnocorrosivereactionoccursbetweenAuNPsandthe electrolyte[41].Onthecontrary,short-circuitcurrentJSCoftheAu

NPsTiO2nanorodsexceedsthatofDSSCwithnoAuNPs(Fig.9(c)).

Table1

Efficiency(),fillfactor(FF),opencircuitvoltage(VOC)andshortcircuitcurrent

density(JSC)ofdye-sensitizedsolarcellsbasedonAuNPsdepositedTiO2structure,

andbareTiO2nanorodarrays. Sample AuNPscontent (atomic percentage) JSC(mA/cm2) VOC(V) FF (%) (A) 1.54 2.75 0.63 0.56 0.94 (B) 3.4 1.97 0.61 0.48 0.56 (C) 0.81 1.50 0.62 0.57 0.46 (D) 0 1.07 0.67 0.43 0.31

ThesedifferencesareattributedtothefactthattheVOCisrelated

tothedifferencebetweentheNernstpotentialoftheredoxandthe

Femi-level[41,48].SincetheTEMandSEMimagesconfirmed,with

furtherAuNPsdeposition,ahigherportionofthetipsurfaceofTiO2

nanorodsiscoveredbyAuNPsandtheactivepartofDSSCsamples andconsequentlyoverallefficiencyofcellsdecreased.Though,due tothefactthattherateofincreaseinJSCisgreaterthanthatof

decreasingVOCintheAuNPsTiO2nanorodsDSSC,efficiencyofAu

NPsTiO2nanorodsDSSCandthefillfactorimproved.

Fig.9(d)showstheeffectofAuNPsdepositiononshortcircuit currentatdifferentwavelengthsoflight.Thisfigureillustratesthat withincreasingtheAuNPstheshortcircuitcurrentincreasedand mostofthecurrentintheobtainedcellisgeneratedbetween325 and425nm.Thephotocurrentincreasecouldbepartiallyattributed toresonantabsorptionduetolocalizedplasmonmodesofAuNPs, however,nodistinctresonantabsorptionpeakisobservedinthe spectralresponse.Inaddition,aslightreductioninthe photocur-rent,henceabsorption,inthe500–550nmbandcouldberelatedto ohmiclossesassociatedwithresonantAuNPabsorption(8–12nm particlesize)[49,50].

Table1indicatesthatthecell with1.54at.%AuNPshasthe maximumefficiency,butwithfurtherAuNPsdepositionbecause ofa decreasingfill factorand JSC overallefficiencyreduced and

VOCshowsonlyslightimprovement.Moreovertheobtainedresults

showthatoverallcellefficiencyforthecellwith1.54at.%AuNPs, jumpedfrom0.31to0.94%whichincreasedbymorethantwice, andthefillfactorincreasedfrom0.43to0.56.

4. Conclusions

VerticallyalignedTiO2nanorodarraysweregrownonFTO

sub-stratesbyhydrothermalmethod.DifferentamountsofAuNPswere depositedontheTiO2nanorodsbyphotoreductionmethod.AuNPs

depositedTiO2nanoroddyesensitizedsolarcellshavebeen

fab-ricatedandcomparedtocellsbuiltfromTiO2 nanorodswithout

AuNPs.TheeffectsofAuNPswereinvestigatedonsolarcell effi-ciency.ResultsshowedthatAuNPsdepositedTiO2havepresented

significantimprovementsinfillfactor andshortcircuitcurrent, resultinginasmuchasdoubledoverallconversionefficiencies.Au NPshelppreventrecombinationbyformingaSchottkyenergy bar-rierthatpreventsphotoinjectedelectronsfromapproachingthe surfaceofnanorodandimprovetheoverallconversionefficiency oftheDSSCs.Theoverallconversionefficiencywasincreasedfrom 0.31%forbareTiO2nanorodarrayto0.94%foranAuNPsdeposited

onTiO2nanorod.Moreover,measuredVOCresultsconfirmedthat

obtainedsamplesarequitestableandnocorrosionoccursbetween metalNPsandtheelectrolyte.Mostimportantly,thisstudy sup-portstheapplicationofAuNPsTiO2 nanorodsinimprovingthe

performanceofaDSSC.

Acknowledgments

Thiswork wassupported byEU FP7Marie Curie IRG Grant 239444, COST NanoTP, TUBITAK EEEAG Grants 108E163 and 109E044andTUBITAKBIDEB.

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