artigo_tio2_corante

JournalofIndustrialandEngineeringChemistry18(2012)18071812

ContentslistsavailableatSciVerseScienceDirect

JournalofIndustrialandEngineeringChemistry

journalhomepage:www.elsevier.com/locate/jiec

Improvedperformanceofdye-sensitizedsolarcellswithcompactTiO2blockinglayerpreparedusinglow-temperaturereactiveICP-assistedDCmagnetronsputteringHyun-JoongKima,Jae-DeokJeonb,DongYoungKimc,Jung-JoongLeea,Seung-YeopKwaka,*aDepartmentofMaterialsScienceandEngineering,SeoulNationalUniversity,599Gwanangno,Gwanak-gu,Seoul151-744,RepublicofKoreabMaterialsandConvergenceResearchDivision,KoreaInstituteofEnergyResearch,152Gajeongno,Yuseong-gu,Daejeon305-343,RepublicofKoreacOptoelectronicMaterialsLab.,KoreaInstituteofScienceandTechnology,Hwarangno14-gil5,Seongbuk-gu,Seoul136-791,RepublicofKorea

ARTICLEINFO

Articlehistory:Received28February2012Accepted9April2012Availableonline17April2012

Keywords:Dye-sensitizedsolarcellTitaniumdioxideInductivelycoupledplasmaassisteddirectcurrentmagnetronsputteringElectrolyteblockinglayer

1.Introduction

ABSTRACT

ThisstudydiscussesaboutcarrierblockingeffectsofacompactTiO2depositedontouorine-dopedtinoxide(FTO)toimprovetheperformanceofdye-sensitizedsolarcells(DSSCs).ThecompactTiO2blockinglayerispreparedbyreactiveinductivelycoupledplasma(ICP)-assistedDCmagnetronsputteringwithoutsubstrateheating.Theresultinglayeryieldsa47%improvementintheenergyconversionefciency,indicatingthattheblockinglayerpreparedbylowtemperatureICP-assistedsputteringeffectivelypreventsphysicalcontactwiththeFTO.ThisprocessmaysuggestanewmethodforfabricatingphotoelectrodeswhichconsistofTiO2blockinglayerforuseinexibleDSSCs.CrownCopyright2012PublishedbyElsevierB.V.onbehalfofTheKoreanSocietyofIndustrialandEngineeringChemistry.Allrightsreserved.

porousstructureoftheTiO2layerpermitselectrolytetopercolateintotheFTOsurface,providingbareFTOconductingsitesatwhich

Sincetherstreportoflow-costdye-sensitizedsolarcells(DSSCs)in1991byOReganandGratzel[1],DSSCshavebeenregardedasthenext-generationsolarcells[28].DSSCsconsistofdyemoleculesactingassensitizers,aporousTiO2layer,auorine-dopedtinoxide(FTO)substrate,anelectrolytechargecarrier,andaPt-coatedcounterelectrode.Unidirectionalchargeowisanessentialprerequisiteforhighenergyconversionefciencies.FoursignicantinterfacesplayaroleintheunidirectionalelectrontransportinDSSCs:porous-TiO2layer/dyemoleculeinterface,FTO/porous-TiO2layerinterface,dyemolecule/electrolyteinterface,andelectrolyte/Pt-coatedcounterelectrodeinterface.Amongthese,chargerecombinationprocessesthatinterferewithunidi-rectionalelectrontransportoccurmainlyattheporousTiO2layer/dyemoleculeandFTO/porousTiO2layerinterfaces.However,therecombinationattheporousTiO2layer/dyemoleculeinterfaceisinsignicant[9,10],becausetheinjectionofphotoelectronsfromdyemoleculesissignicantlyfasterthanthechargetransferfromTiO2totheoxidizeddyemolecules.Ontheotherhand,attheFTO/porousTiO2layerinterface,chargerecombinationtakesplaceduetothephysicalcontactbetweentheelectrolyteandthesurfaceoftheFTOsubstrate.The

*Correspondingauthor.Tel.:+8228808365;fax:+8228851748.E-mailaddress:[email protected](S.-Y.Kwak).

theinjectedelectronscanrecombinewithI3speciesfromtheredoxelectrolyte.Therefore,suppressionofelectronleakageontheFTO/porousTiO2layerinterfaceplaysanimportantroleinimprovingtheenergyconversionefciencyofDSSCs.ApromisingmethodforpreventingrecombinationistheapplicationofacompactmetaloxideblockinglayerbetweentheFTOandtheporousTiO2layer[11,12].Muchresearchhasbeendevotedtothefabricationofcompactmetaloxidelayersthatcanbeusedasblockinglayerstoimpedeelectrolyteinvasion.Variousmetaloxides,includingTiO2[1315],ZnO[16],andNb2O5[17,18],havebeenusedaselectrolyteblockinglayersattheFTO/porous-TiO2interface.Amongthesemetaloxides,TiO2isthemosteffectiveelectrolyteblocker,andhasbeenextensivelystudied.Moreover,enhancedadherencebetweentheFTOsubstrateandtheporous-TiO2layerprovidesagreatnumberofpathwaysforphoto-excitedelectrontransportfromtheporousTiO2totheFTO,whichpromoteelectrontransferandenhancetheelectrontransferefciency.ThecompactTiO2layercanbepreparedusingsolgelprocessing[13],dip-coating[19],spraypyrolysis[12,15,20],layer-by-layerprocessing[10],andothertechniques.Despiteimprovementsintheenergyconversionefciencywiththesetechniques,high-temperatureheattreatmentsanddifcultiesassociatedwithlarge-scalefabricationlimittheutilityofthesemethods.Althoughcompactblockinglayerspreparedatlowtemperatures,usingmicrowaveshasbeenstudied[21],the

1226-086X/$seefrontmatter.CrownCopyright2012PublishedbyElsevierB.V.onbehalfofTheKoreanSocietyofIndustrialandEngineeringChemistry.Allrightsreserved.ttp://dx.doi.org/10.1016/j.jiec.2012.04.008

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H.-J.Kimetal./JournalofIndustrialandEngineeringChemistry18(2012)18071812

resultingcellsyieldedlowerenergyconversionefcienciesthanthosepreparedathightemperatures.Onthebasisofthispoint,reactiveinductivelycoupledplasma(ICP)-assistedDCmagnetronsputteringmayprovideamoreattractiveprocessbecauseitproducesauniformthickness,canbeappliedtolargesizesubstrates,andprovidesgoodcrystallinity.Toourknowledge,thisistherstreportofthepreparationofcompactTiO2blockinglayersvialow-temperaturereactiveICP-assistedDCmagnetronsputteringforDSSCs.Inthisstudy,weinvestigatedthecarrierblockingeffectsofacompactTiO2layerdepositedontoauorine-dopedtinoxide(FTO)substrateasafunctionoftheTiO2layerthickness.ThecompactTiO2blockinglayerwaspreparedbyusinglowtemperaturereactiveICP-assistedDCmagnetronsputteringwithoutintentionalsubstrateheating.Thetemperatureofthesubstratewasmain-tainedbelow1508Cwithwatercooling.TheplacementofthecompactTiO2blockinglayerdepositedbyICP-assistedDCmagnetronsputteringbetweentheFTOandtheporousTiO2layerwasselectedtoreduceelectronrecombinationattheFTO/porousTiO2interface,andtherebyincreasingtheDSSCperformanceintermsoftheopen-circuitvoltage(Voc)andtheshort-circuitcurrent(Jsc).OptimizationoftheTiO2blockinglayerthicknessisreportedinthispaper.

2.Experimental

2.1.PreparationofthecompactTiO2layer

Fig.1showsaschematicdiagramoftheprocessofICP-assistedDCmagnetronsputteringwithtargetvoltagecontrol.Atwo-turnantennashieldedbyaninsulator,whichwasmadeofcoppertoenablewatercooling,wascoupledtoa13.56MHzradiofrequency(RF)powergenerator.TheoppositeendoftheRFcoilwasconnectedtotheground.Thetargetwasa3in.diametermetallicTi(99.9%)platepositioned4in.fromthesubstrate,andwasmountedonthemagnetron.TransparentFTO(Pilkington,TEC-8,8Vcm2)wasusedasthesubstrateandwasnotsubjectedtoadditionalheating.Thechamberwasevacuatedtoabasepressureof5.0105Pa,thenthechamberwaspurgedwithAr.Theworkingpressurewas2.67Pa(20mTorr),andtheRFpowerappliedtothecoilwas400W.AconstantDCpowerof400Wwasappliedtothetarget,andthetargetvoltagewascontrolledbyvaryingtheoxygenowrateusingaproportionalintegralderivative(PID)controller.Inthispaper,BLxdenotesacompactTiO2blockinglayer(BL)ofthicknessxdepositedontheFTOsubstrate.

2.2.CharacterizationofthecompactTiO2layer

ThemicrostructuresofthecompactTiO2layerswereexaminedusingeld-emissionscanningelectronmicroscopy(FE-SEM,JEOLJSM-6330F),andthecrystalstructuresoftheTiO2layerweredeterminedusingwide-angleX-raydiffraction(XRD,MAC/Sci.MXP18XHF-22SRAwithCuKaradiation)atroomtemperature.An

Fig.1.AschematicdiagramofICP-assistedDCmagnetronsputteringandtargetvoltagecontrol.

acceleratingvoltageof50kVandanemissioncurrentof100mAwereused.X-rayphotoelectronspectroscopy(XPS)wascarriedoutusingaPHI5000Versaprobe(UIvac-PHI).Thebackgroundpressurewas6.7108Pa,andtheanodegeneratedanAlKaX-raybeamof15kV(photonenergyof1486.8eV).ThethicknessesoftheTiO2layerweremeasuredusingana-stepproler.TheopticaltransmittancesoftheTiO2blockinglayersweredeterminedbyultravioletvisible(UV/Vis)spectroscopy.

2.3.FabricationofDSSCs

TheFTOsubstrateswithblockinglayerswerecoatedwithnanocrystallineTiO2(NanoxideD,Solaronix,5mm).Forcompari-son,anFTOsubstratewithoutablockinglayer(blanksample)wasalsopreparedusingthesamemethod,andthecellcomponentswerethensintered5008Cfor30min.Theannealedelectrodeswereimmersedinethanolcontaininga0.5mMofdye(N719,Solaronix)solutionfor24h.Electrodescoatedwithadsorbeddyewererinsedwithanhydrousacetonitrile.EachcounterelectrodewasfabricatedbycoatingaPtlayerontoanFTOsubstratebyspin-coatingaH2PtCl6isopropanolsolution,followedbyheatingat4008Cfor20min.TheworkingandcounterelectrodesweresealedwithaSurlyntapeofthickness25mm,andthenimmersedinaredoxelectrolytecontaining0.7M1-butyl-3-methylimidazoliumiodide(BMII),0.03Miodine,0.5Mtert-butylpyridine(tBP),and0.1Mguanidinethiocyanate(GSCN)inacetonitrile/valeronitrile(8.5:1.5byvolumeratio),therebyproducingtheDSSCs.Inthisstudy,D-BLxdenotesaDSSCpreparedusingacompactTiO2blockinglayerofthicknessx.

2.4.PhotovoltaicandelectrochemicalcharacterizationofDSSCs

ThephotovoltaicpropertiesofDSSCsweredeterminedusingaKeithley2400sourcemeasuringunit.AXelamp(Oriel,300W)servedasthelightsourceanditslightintensitywasadjustedusingaSireferenceDSSCequippedwithaKG-5lter(FraunhoferInstitute,Germany)toapproximatelyAM-1.5,whichcorrespondstoasolarelevationof428withrespecttothehorizon.AllDSSCparametersweremeasuredunderstandardconditionsatAM1.5globalradiationwith100mWcm2lightintensity.Cyclicvoltammetrywasperformedinanacetonitrilesolutionatascanrateof100mVs1.ThebareFTOorthecompactTiO2layerontheFTO(BL100)wasusedastheworkingelectrode,aPtfoilprovidedthecounterelectrode,andanAg/Ag+electrodeprovidedthereferenceelectrode.Anacetonitrileelectrolytesolutionwasused.Thesolutioncontained0.1MLiClO4asthesupportingelectrolyteand5mMLiI+I2(LiI:I2=9:1bymolarratio)astheredoxcouple.

3.Resultsanddiscussion

3.1.CharacterizationofthecompactTiO2layer

Fig.2(a)and(b)showsthetargetvoltageandtotalpressurecurvesasfunctionsoftheoxygenowrate,respectively.Thehysteresisloopsinthetargetvoltageandtotalpressurecurvesindicatethatthetargetsurfacechangedfromametallicstatetoanoxidizedstatepoisonedbyanoxidelayerdependingontheoxygenowrate.AsshowninFig.2(a),becausethetransitionbetweenthemetallicandoxidizedstatesisirreversibleoncethesurfaceoftheTitargetwaspoisonedbyanoxidelayer,thedepositionofacompactTiO2layerbyreactivesputteringmustbecontrolledprecisely.Thisresultwasingoodagreementwiththeresultsofourpreviousstudy[22],whichfoundthatahighdepositionrateandTiO2stoichiometry,couldbeobtainedbyconductingdepositionatanoxygenowof2.63.0sccmwiththetargetvoltagexedat


Fig.2.Hysteresisloopsin(a)thetargetvoltageand(b)totalpressurecurvesatvariousoxygenowrates.

450V.Undertheseconditions,thedepositionratewasaround13nmmin1.Asaresult,thethicknessofeachcompactTiO2layercouldbecontrolledaccordingtothesputteringtimeintherangeof030min,andthelayerthicknessvariedfrom0to400nm.TherelationshipbetweentheTiO2layerthicknessandthesputteringtimeisshowninTable1.Fig.3showstheFE-SEMsurfacemorphologiesoftheFTOsubstrateandthecompactTiO2layerontheFTOsubstrate.TheFTOsurfacehadamorphologycharacteristicofatinoxidecrystalwithparticles100200nminsize.Aftersputtering,thesmoothsurfacewasobservedtochangetoaroughsurfacewithrelativelysmallparticles,whichindicatedthattheFTOsubstratewaswell-coveredwithathincompactTiO2layer.XPSmeasurementswereusedtoexaminethevalenceoftheTiandOspecies.ThesampleusedforthisstudywasBL100.TheTi2pandO1sXPSspectraarepresentedinFig.4andthebindingenergiesandatomicconcentrationsaresummarizedinTable2.TheTi2p1/2andTi2p3/2spinorbitalsplittingphotoelectronswerecharacterizedbybindingenergiesof465.12eVand459.25eV,

Table1TherelationshipbetweenthesputteringtimeandthecompactTiO2layerthickness.

Fig.3.FE-SEMmicrographsof(a)thebareFTOsubstrateand(b)theTiO2compactlayerontheFTOsubstrate.

respectively.Apeakseparationof5.87eVagreedwellwiththevaluesreportedelsewhere[23,24].OnlytheTi4+signalwasdetectableonthesurface,indicatingthataTiO2layerformedontheFTOsurface.TheO1sfeatureshowninFig.5(b)includedtwopeaks,oneat530.5eV,whichcorrespondedtooxygenintheTiOgroups,andanothersmallersignalat532.25eV,whichcorre-spondedtooxygenintheOHgroups.Thus,theTispeciesinthecompactTiO2wereTi4+andalmostalloxygenpresentwasboundtoTiwithatitanium-to-oxygenratioof0.53:1,inagreementwiththeexpectedstoichiometry.ThusTiO2wassuccessfullydepositedontheFTOsubstrate.Fig.5showstheXRDpatternsoftheFTOandthecompactTiO2layerontheFTO.ToaccommodatethesensitivityoftheXRDmeasurement,a1mmTiO2layerwaspreparedover75minofreactivesputtering.AcomparisontothebareFTOshowedthatanewpeakappearedat25.48,correspondingtothe(101)planeofanataseTiO2(JCPDSCardNo.21-1272)withouttheuseofheattreatments.Theformationofthiscrystallinephasewasattributedtotheuseofhigh-densityplasma.

Table2BindingenergyandatomicconcentrationsoftheBL100.

SamplenameSputteringtime(min)

CompactTiO2layerthickness(nm)

TiO

WithoutBL00BL50457BL1007.5103BL20015201BL40030412

Locationofbindingenergy(eV)2p1/22p3/2O2OH465.12459.25530.50532.25Atomicconcentration(%)18.6233.08

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Fig.4.High-resolutionXPSspectrafortheBL100;(a)theTi2ppeakand(b)theO1speak.

Fig.6showsthetransmissionspectraofcompactTiO2layersonFTOincomparisonwiththetransmittanceofaconventionalP25(Degussa)layerwiththesamethickness(1mm)intherangefrom400to700nm.ThecompactTiO2layerontheFTOsubstratehadanaveragetransmittanceof80%inthevisiblelightregion,whereastheconventionalP25layerhadarelativelypoortransmittanceof20%inthesamerange.TransparentTiO2layersinDSSCsincreasethelightpathlength(lightscattering)andenhancethelight

Fig.5.XRDdataforthecompactTiO2layerontheFTOandthebareFTOsubstrate.

Fig.6.Opticaltransmittancesof(a)thecompactTiO2layerand(b)aconventionalP25lmwiththesamethickness.

harvestingefciency[25].Therefore,thedepositionoftransparentTiO2blockinglayersonFTOisveryimportant.

3.2.PhotovoltaicandelectrochemicalcharacterizationofDSSCs

Thephotocurrentdensitypotentialcurves(JV)forDSSCswithcompactTiO2layersofvariousthicknessvaluearepresentedinFig.7(a).TherelationshipbetweentheTiO2layerthicknessandtheopen-circuitvoltage,Voc,short-circuitcurrent,Jsc,andllfactor(FF)areillustratedinFig.7(b)(d),respectively.TheelectricalparametersarelistedinTable3.Withoutablockinglayer,thecellwascharacterizedbyJsc=10.16mAcm2,Voc=0.703V,andFF=0.61,correspondingtoaconversionefciencyof4.37%.AfterdepositionofacompactTiO2layerattheFTO/porous-TiO2interface,thedevicesyieldedimprovedproperties,Voc=30mVandJsc=13mAcm2.TheD-BL100cellgavethehighestimprovementsinVocandJsc,resultingina6.42%energyconversionefciency.VocincreasedwiththethicknessoftheTiO2layer,andhighvalueswereobservedfor100nmthicklayers.ThisresultsuggestedthattheelectronrecombinationattheFTOinterfacewaspreventedforlayerthicknessesgreaterthan100nm,asshowninFig.7(b).JscoftheDSSCswasmainlyinuencedbytwofactors:dyeloadingandchargerecombinationattheelectrode[2628].Becausetheblockinglayeritselfcannotadsorbsignicantquantitiesofthedye,thedyeloadingmustnot,therefore,signicantlyinuenceJsc.ThemainfactorthataffectedJscforDSSCsmustbetheelectronbacktransferattheFTO/electrolyteinterfaces.AsshowninFig.7(c),thecompactTiO2layerproducedbyreactiveICP-assistedDCmagnetronsputteringreducedthenumberofrecombinationsites,andchargerecombinationintheDSSCwaseffectivelyinhibited.AnelectrodewiththecompactTiO2blockinglayerdecreasedelectronrecombinationattheFTO/electrolyteandelectronlossattheFTO/porousTiO2,therebyenhancingtheelectroncollectionefciencyattheFTOtoimprovetheoverallconversionefciency.AlongwithanincreaseinthethicknessofthecompactTiO2blockinglayer,D-BL400gavealowFFof0.53witha5.05%energyconversionefciency.Theseresultssuggestedthatthickblockinglayersintroducedanadditionalseriesresistanceintothedevices,whichdecreasedJscandFF,asshowninFig.7(c)and(d),respectively.TheoptimalthicknessofablockinglayerproducedbyICP-assistedreactiveTiO2sputteringwasaround100nm,andthephotovoltaicperformanceofDSSCswassensitivetothethicknessofthecompactTiO2layer.TheenergyconversionefciencyofD-BL100DSSCwaslowerthantheefcienciesofDSSCsreportedcurrently.Itshouldbenotedthatthepurposeofourstudywastodeterminetheeffectofcarrierblocking


Fig.7.(a)JVcurvesofDSSCswiththevariousTiO2layerthicknessandrelationshipsbetweenthesputteringtimeand(b)Voc,(c)Jsc,and(d)FF.

onDSSCperformanceuponintroductionofacompactTiO2layerpreparedbylow-temperaturereactiveICP-assistedDCmagnetronsputtering.Fromthisperspective,itisnoteworthythattheenergyconversionefciencyofaD-BL100DSSCwithacompactTiO2layerwas47%higherthanthatofthereferenceDSSCwithoutacompactTiO2layer.ThegoodperformancearosefromthegoodelectrontransportattheelectrodeinterfacesbecausetheFTOsurfacewaswell-coveredwiththecompactTiO2layer.Ingeneral,itisreportedthattheDSSCperformancedependsonthedyemolecule,theporous-TiO2layer,anFTOsubstrate,

electrontransfer[29,30].Fig.8showsthemeasureddarkcurrentvoltagecharacteristicsofDSSCswithandwithoutblockinglayers.TheonsetofaDSSCwithoutblockinglayeroccurredatalowforwardbias.ThecompactTiO2blockinglayersuppressedthedarkcurrent,andshifteditsonsetbyseveralhundredmillivolts.ThisonsetshiftindicatedthatelectronrecombinationattheexposedregionsoftheFTOsubstratewasresponsibleforthehighdarkcurrentsobservedwiththeporousTiO2layeralone.TheseresultsclearlysuggestthatthecompactTiO2layerdepositedviareactiveICP-assistedDCmagnetronsputteringprovidedaneffective

electrolytes,andaPtcounterelectrode,aswellasthenatureoftheblockinglayer.StudiesareunderwaytofurtherenhancetheperformanceofDSSCsthatincludeacompactTiO2layerusingnewlydevelopedworkingelectrode,andareportwillbeforthcoming.Acomparisonofthedarkcurrentsoftheinvestigatedcellscanprovideusefulinformationregardingthebackelectrontransferprocesses,andDSSCdarkcurrentmeasurementshavebeensuggestedasqualitativemethodformeasuringtheextentofback

Table3PerformanceofDSSCspreparedusingthephotoelectrodewithvariousTiO2layerthickness.SamplenameVoc(V)Jsc(mAcm2)FFh(%)WithoutBL0.70310.160.614.37D-BL500.70311.610.604.90D-BL1000.73313.920.636.42D-BL2000.72913.510.575.57D-BL4000.73212.930.535.05Fig.8.DarkcurrentvoltagecharacteristicsofDSSCswithandwithoutthecompactTiO2blockinglayer.

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ofDSSCs.Insummary,low-temperaturereactiveICP-assistedDCmagnetronsputteringprovidesamethodforfabricatingdye-sensitizedphotoelectrodesbasedoncompactTiO2blockinglayers,whichareexpectedtobegoodcandidateelectrodesinexibleDSSCs.

Acknowledgement

ThisresearchwassupportedbytheBasicScienceResearchProgramthroughtheNationalResearchFoundationofKorea(NRF)fundedbytheMinistryofEducation,ScienceandTechnology(R11-2005-065).References[1]B.ORegan,M.Gratzel,Nature353(1991)737.[2]D.Chen,F.Huang,Y.-B.Cheng,R.A.Caruso,AdvancedMaterials21(2009)2206.[3]K.-M.Lee,V.Suryanarayanan,K.-C.Ho,SolarEnergyMaterialsandSolarCells91

Fig.9.CyclicvoltammogramsofbareFTOandBL100.

blockinglayerthatpreventedelectronrecombinationattheFTOunderdarkconditions.Fig.9showscyclicvoltammetrycurvesoftheferrocene(Fc+/Fc)redoxreactionforthebareFTOandthecompactTiO2layerontheFTO(BL100).TheredoxcurrentofthebareFTOexceededthatoftheFTOcoatedwiththecompactTiO2layerby80mA.AredoxcurrentoffewmicroamperesattheelectrodewiththecompactTiO2layerindicatedthatFTOsubstratewaswell-coveredbythecompactTiO2layer.Tosomeextent,theblockinglayerdepositedbyreactivesputteringwassuperiortotheTiO2blockinglayerpreparedbyspraypyrolysisofTiO2.TheseresultsindicatethatthecompactTiO2blockinglayerdepositedviareactiveICP-assistedsputteringeffectivelysuppressedelectronleakageattheFTO/porousTiO2andFTO/electrolyteinterfaces.

4.Conclusions

ReactiveICP-assistedDCmagnetronsputteringwithoutinten-tionalsubstrateheatingwasusedtodepositacompactTiO2layeronanFTOsubstrate.ThethicknessofthepreparedTiO2layerwascontrolledbyvaryingthesputteringtimeovertherange030min,andthethicknessvariedfrom0to400nm.Thedye-sensitizedphotoelectrodesdisplayedimprovedVocandJscwithoutloweringtheFF,whichimprovedthedeviceperformance.AnoptimumTiO2blockinglayerthicknessof100nmproduceda47%improvementintheenergyconversionefciency(4.376.42%).ThisindicatesthatoptimizingthethicknessofthecompactTiO2layerisimportantforobtaininghigh-performanceDSSCs.ThecompactTiO2layerenhancedJscandimprovedadhesionbetweentheFTOandporousTiO2layer.Vocwasalsoenhanced,indicatingthatthecompactTiO2blockinglayereffectivelysuppressedbackelectrontransferattheFTO/porousTiO2layerinterface.CyclicvoltammetryconrmedthatthecompactTiO2blockinglayerplayedanimportantroleinunidirectionalelectrontransportattheinterfaces

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