Effect of Au nano-particles on TiO nanorod electrode in dye-sensitized solar cells Electrochimica Acta

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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

, M. Burak Cosar

, Halil I. Yavuz

, M. Ozenbas

, Ali K. Okyay

aUNAM–InstituteofMaterialsScienceandNanotechnology,BilkentUniversity,Ankara06800,Turkey

bDepartmentofElectricalandElectronicsEngineering,BilkentUniversity,Ankara06800,Turkey

cDepartmentofMetallurgicalandMaterialsEngineering,MiddleEastTechnicalUniversity,Ankara06800,Turkey

dCenterforSolarEnergyResearchandApplications(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.MetalnanoparticleswereloadedontothesurfaceofTiO2nanorodsviaphoto- 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].InitialdemonstrationsofDSSCswerebasedondyesensitiza- 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–InstituteofMaterialsScienceandNanotech- 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,whichmaybeobtainedbyapost-growth- coatingmethodsuchasatomiclayerdepositiontechnique.

DSSCefficienciescouldbeimprovedsignificantlybyusingsingle crystalnanorodsofTiO₂insteadofnanoparticleswhichcouldpro- motetheefficienttransportofphotogeneratedelectrons[21–23].

Thecrystal structure ofTiO2 also playsa factorin efficiencyof DSSCs[24–27].Thedifferencebetweentheconductionbandlev- 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,whichsignificantlyincreasesuponmixingwithapor- tionoflesseractiverutilenanocrystals[29,30].Onthecontrary, therearesomereportsclaimingbetterefficiencyDSSCswithpure anataseform[31],however,aconclusivecomparison cannotbe obtainedbecausesizeandmorphologyoftheparticleswerealso variedinthosestudies.

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

http://dx.doi.org/10.1016/j.electacta.2012.05.058

electrolytetopassivetheelectrolyte-exposed[34–37].Moreover thereareanumberofearlierreportsshowingthatadditionofmetal nanoparticlesimprovesDSSCenergyconversionefficiency[38–41].

Inthisstudy,AuNPsacttopreventchargecarrierrecombination byforminga Schottkyenergybarrier thatpromotesthephoto- injectedelectronsintothenanorod,awayfromthesurfacewhich improvestheoverallconversionefficiencyoftheDSSCswithAu NPs/TiO2nanorodstructure.ShiiChouetal.[41]havereporteddye sensitizedsolarcellbasedontheTiO₂/nano-metalcompositeparti- 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-linedauto- clave.Deionizedwaterwasusedtoprepareallthesolutions.

2.2. GrowthofTiO2nanorodarray

ThedimensionofFTOglasssubstratewas10mm× 50mmand wascleanedanddegreasedpriortouse,firstbywashingwithdeter- gent anddistilled water, then wereultrasonically cleanedwith ethanolandthenbysonicatinginethanol,acetoneanddeionized waterinsequenceforabout30min,andfinallydriedundernitro- 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)HAuCl₄ofaqueousAusaltsolutionweredis- 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,andsec- ondarygraphitemonochromator).Themorphologyandstructural characteristicsoftheobtainedsampleswerestudiedbyscanning electronmicroscopy(FESEM,FEI–NovaNanosem430),andtrans- 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 measuringphoto- 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.Theelectrolytewascomposedof0.1Mguani- diniumthiocyanate(GuSCN),0.03MI2,0.5M4-tert-butylpyridine (TBP)and0.6Mbutylmethylimidazoliumiodide(BMII)inthemix- tureofacetonitrileandvaleronitrile(85:15,invol.%).Thedevices werecharacterizedbyaKeithley2440sourcemeterunderstan- dardAM1.5G-filteredirradiation(100mW/cm²)fromaNewport 91192solarsimulatorequippedwith300Wxenonarc.Thespectral measurementsareobtainedbyamechanicallychoppedmonochro- 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,XRDanalysiswasperformedontheobtainedsam- 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(au)

20 15

Intensity(a.u)

30 25

*

(

*

(110)

40

35 45

R

(111) (101)

*

R

*

50 (21

*

R TiO₂nan

JCPDS 8

2 Theta

55 60 65 1)

(002)

*

RR norods 820514 Sta

75 70 )(112)

*

RR andard Card

R= Ru

*

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.3. SEMmicrographsofwell-alignedTiO2nanorodarrays:(a)topviewand(b)sideview(c)with1.54at%Au(d)with3.4at%Au.

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

onthesurfaceofTiO₂nanorods.Thisisattributedtoatopillumi- 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, (c)high-magnificationimageof3.4at.%Audeposition,and(d)correspondingselectedareaelectrondiffractionpattern(SAED).

0 200 400

600 800

1000 1200

C 1S

Au 4d Au 4f

O 1S Ti 2p

Intensity (a.u)

Binding energy (eV) Au/TiO₂ nanorods

TiO₂ nanorods

C 1S

(a)

(a)

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

nanorodarrays.

Thesurfacecompositionoftheobtainedsampleswascharac- terizedbyX-rayphotoelectronspectroscopy(XPS)technique.XPS survey-scanspectrumofTiO₂nanorodsandAu-NPdecoratedTiO₂ nanorodsareshowninFig.5(a).AllXPSspectralpeakswerefitted withThermoScientificAvantagesoftware.Asrequiredbytheory, theTi2pandAu4fspectrumconsistoftwopeaks,aspin–orbitdou- bletwhereasO1sandC1sspectrallinesconsistofasinglepeak(a singlet).TheC1sspectrallinewasstandardizedto285.0eVand theO1s,Ti2pandAu4f spectrawereadjustedtothis energy.

Thedataanalysisinvolvedcurve-fittingLorentzian–Gaussian(30%

Lorentzian)lineshapes,spectranormalization,andShirleyback- groundsubtraction[43].InallfitstonarrowscanspectraShirley backgroundswereused[44,45].

Fig.6presentsthesurveyscanofpreparedsamples.TheXPS peakswithbindingenergiesof84.03,458.501and529.96eVcor- 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.)

Fig.7. XPSspectrumintheO1sregionfor(a)TiO2nanorodand(b)Au/TiO2nanorod,and(c)narowscanofAu4f.

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/cm2 )

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/cm2 )

(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 peakswithabindingenergyof83.35eVand87.14eVcorrespond- ingtoAu4f5/2andAu4f3/2,respectively[9,45].ShamandLazarus explained that ambient TiO2 samplesare always covered with physisorbedmoistureandchemisorbedandtheO1sbindingener- 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,fillfac- tor,andJ_SC(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

ThesedifferencesareattributedtothefactthattheV_OCisrelated tothedifferencebetweentheNernstpotentialoftheredoxandthe Femi-level[41,48].SincetheTEMandSEMimagesconfirmed,with furtherAuNPsdeposition,ahigherportionofthetipsurfaceofTiO₂ nanorodsiscoveredbyAuNPsandtheactivepartofDSSCsamples andconsequentlyoverallefficiencyofcellsdecreased.Though,due tothefactthattherateofincreaseinJ_SCisgreaterthanthatof decreasingVOCintheAuNPsTiO2nanorodsDSSC,efficiencyofAu NPsTiO₂nanorodsDSSCandthefillfactorimproved.

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

Table1indicatesthatthecell with1.54at.%AuNPshasthe maximumefficiency,butwithfurtherAuNPsdepositionbecause ofa decreasingfill factorand J_SC overallefficiencyreduced and VOCshowsonlyslightimprovement.Moreovertheobtainedresults showthatoverallcellefficiencyforthecellwith1.54at.%AuNPs, jumpedfrom0.31to0.94%whichincreasedbymorethantwice, andthefillfactorincreasedfrom0.43to0.56.

4. Conclusions

VerticallyalignedTiO2nanorodarraysweregrownonFTOsub- stratesbyhydrothermalmethod.DifferentamountsofAuNPswere depositedontheTiO₂nanorodsbyphotoreductionmethod.AuNPs depositedTiO2nanoroddyesensitizedsolarcellshavebeenfab- ricatedandcomparedtocellsbuiltfromTiO₂ nanorodswithout AuNPs.TheeffectsofAuNPswereinvestigatedonsolarcelleffi- ciency.ResultsshowedthatAuNPsdepositedTiO2havepresented significantimprovementsinfillfactor andshortcircuitcurrent, resultinginasmuchasdoubledoverallconversionefficiencies.Au NPshelppreventrecombinationbyformingaSchottkyenergybar- rierthatpreventsphotoinjectedelectronsfromapproachingthe surfaceofnanorodandimprovetheoverallconversionefficiency oftheDSSCs.Theoverallconversionefficiencywasincreasedfrom 0.31%forbareTiO2nanorodarrayto0.94%foranAuNPsdeposited onTiO₂nanorod.Moreover,measuredV_OCresultsconfirmedthat obtainedsamplesarequitestableandnocorrosionoccursbetween metalNPsandtheelectrolyte.Mostimportantly,thisstudysup- portstheapplicationofAuNPsTiO₂ nanorodsinimprovingthe performanceofaDSSC.

Acknowledgments

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

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