ThinSolidFilms518(2009)15901594

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ThinSolidFilms

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Glancingangledepositedtitanialmsfordye-sensitizedsolarcellsHsiao-YunYanga,Ming-FuLeea,Chia-HuaHuangb,Yu-ShiLoc,Yi-JiaChena,d,Ming-ShowWonga,d,aInst.ofElectronicsEngineering,NationalDongHwaUniversity,Hualien,TaiwanbDept.ofElectricalEngineering,NationalDongHwaUniversity,Hualien,TaiwancDept.ofChemistry,NationalDongHwaUniversity,Hualien,TaiwandDept.ofMaterialsScienceandEngineering,NationalDongHwaUniversity,Hualien,Taiwan

articleinfoAvailableonline19September2009Keywords:Dye-sensitizedsolarcell(DSSC)TiO2AnatasePorouslmSculpturedlmGlancingangledeposition(GLAD)

1.Introduction

abstractAseriesofsculpturedporousnano-columnartitaniumoxidelmswerepreparedbyglancingangledeposition(GLAD)methodusinganelectron-beamevaporationsystem.ThelmsweredepositedonITOglassesatvariousincidentanglesfrom53to86andusedasphotoanodeinadye-sensitizedsolarcell(DSSC).Theas-depositedTiO2lmsarecomprisedofhelicalnano-columnsandassembledinanorderlymannerwithgapsorporesinbetween.Theporousnanostructuredlmsprovideasynergeticeffectofhighsurfacearea,effectiverouteforelectrontransfer,tightinterfaces,andenhancedlighttrapping,whichareallbenecialforhighercellefciency.TheDSSCsincorporatedwiththeGLADlmsof4mthickexhibitedahighllfactor(FF)upto0.77.TheTiO2lmdepositedatanincidentangleof73providesthelargestinternalsurfaceareaandthelargestamountofdyeabsorptionandresultsinthehighestlightconversionefciencyof2.78%.2009ElsevierB.V.Allrightsreserved.

region,andlimitedaccesstotheentireinternalsurface[46].Thus,itisofinteresttoDSSCtechnologytodevelopnewandimprovedporousTiO2

Dye-sensitizedsolarcell(DSSC)hasbecomeapopularandprom-isingphotovoltaiccellsinceitsintroductionbyM.Grtzelin1991,becauseitismadeoflow-costmaterialsandisrelativelyeasytoprepare[1,2].Alotofprogresshasbeenmadetoincreasethesolarconversionefciency,reliabilityandfabricationcostofthecellsthroughimprove-mentonphotoanode,dye,electrolyteandcounterelectrode[16].Thephotoanodemadeofahigh-bandgapsemiconductorisusedmainlyforabsorptionofdyeandchargeseparationandtransport,andthephotoelectronsareprovidedbythephotosensitivedye.Chargeseparationoccursatthesurfacesbetweenthedye,thesemiconductorandtheelectrolyte.Thephotoanodeisusuallya~1020mthicklayerofsinteredtitaniumdioxide(TiO2)nanoparticles(NP)formingahighlyporousstructurewithanextremelyhighsurfacearea,andservingasascaffoldtoholdalargenumberofthelight-absorbingdyemoleculesina3-Dmatrix.ThephotoanodeisusuallypreparedwithanataseTiO2nanoparticlesabout20nminsizebythedoctor-blademethod[7]followingwithlow-temperaturecalcinationsatabout450C[1].Lightconversionefcienciesupto11.2%werereportedwithDSSCsincorporatingTiO2NPlm[3].Nevertheless,therearestillroomsforimprovementintherandomlyporousnanostructuredTiO2layer,includinglowporosity,lackofmaterialgenerality,tediousparticlesynthesis,lowconductivity,lowspacecharge

Correspondingauthor.DepartmentofMaterialsScienceandEngineering,NationalDongHwaUniversity,Hualien,974Taiwan,ROC.Tel.:+88638634206;fax:+88638634200.E-mailaddress:mswong@mail.ndhu.edu.tw(M.-S.Wong).0040-6090/$seefrontmatter2009ElsevierB.V.Allrightsreserved.doi:10.1016/j.tsf.2009.09.026

nanostructurestofurtherenhancethecellefciency.Recently,physicalvapordepositiontechniquehasbeenusedtodepositporousTiO2thinlmswithlargesurfaceareaforuseinDSSCandachieveddecentconversionefciency[811].Thenewmeth-odologyappliedaso-calledglancingangledepositiontechnique(GLAD)topreparethree-dimensionalnanostructuresofnanostruc-turedcolumnarlmswithcontrolledporosityandgeometry[1217].Byplacingasubstrateaboveamaterialvaporsourceatanobliqueangleandrotatingthesubstrate,variousnanostructurescanbeprepared,suchasporousnano-columnarlm,nanorodarrayswithdifferentshapes,nanospringarrays,andevenmultilayernanostructures.Inthisstudyweusedanelectron-beamevaporationsystemandappliedtheGLADmethodtoprepareaseriesofhighly-ordered,sculpturedporousnano-columnartitaniumoxidelmsonITOglassesandusedthemasphotoanodeinthedye-sensitizedsolarcells(DSSCs).Wevariedtheglancingangleandlmthickness,andfocusedonthestructure,crystallinity,dyeabsorption,andlight-absorptionofthelms,andtheireffectsontheperformanceofDSSCs.Thepreparation,characterization,andimplementationofporoustitaniumoxidelayerinDSSCsarereported.

2.ExperimentalTheTiO2thinlmswerepreparedintheelectron-beamevapora-tionsystemassembledbyBranchyVacuumTechnologyCo.,Ltd(Toayuan,Taiwan)[18].Thedistancebetweentheregularhorizontal

H.-Y.Yangetal./ThinSolidFilms518(2009)15901594

rotationholderandtheelectron-beamevaporationsourcewas550mm.AnadditionalvariableglancinganglesubstrateholderwasinstalledfromthechambersidewallandthedistancebetweentheGLADsubstrateholderandthenormalofe-beamsourceis260mm.Thedepositionuxisincidentontoasubstratewithalargeangle()withrespecttothesurfacenormalandthesubstrateisrotating.GLADproducescolumnarstructuresthroughtheeffectofshadowingduringlmgrowth,whilethesubstraterotationcontrolstheshapeofthecolumns.Inthistechnique,therearethreeparametersthatdeterminethemorphologyofthecolumns:theincidentangle,thedepositionrateandthesubstraterotationrate.TheangleandrotationrateoftheGLADholdercanbeadjustedintherangesof45to90andof0.05to8.6rpm,respectively.Thechamberwasevacuatedbyamechanicalpump(ALCATEL-2033SD)andacryo-pump(CTI-Cryo-Torr8)toabasepressurebelow5.3105Pa.Thesubstratesusedwere1815mm2indium-tinoxide(ITO)oxidecoated,0.5mmthicksodalimeglasswithanas-delivered

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resistivityof7.0/square.TheTiO2lmsweredepositedat350Cmaintainedbyquartzlampsinoxygenatmosphere(4.0103Pa)usingrutileTiO2(99.99%)asasourcematerial.Thelmdepositionratewas2.0nm/sandcontrolledbyaquartzcrystalmonitor.AdenseTiO2lmof200nmthickasbarrierlayerwasrstdepositedusingtheregularsubstrateholder.Then,thesamplesweretransferredtotheGLADholderrotatingataconstantrateof1rpmfordepositionofaporousTiO2layerof4mthickwithoutsubsequentcalcination.Aseriesoftitaniumoxidelmsweredepositedatfourdifferentobliqueanglestothesubstratenormal:53,65,73,and86,andtheywereassignedasFilmsA,B,CandD,respectively.ThelmstructureandcrystallinitywereinvestigatedbyaRigakuD/MAX-2500V18kWlowangleX-raydiffractometer(XRD)operat-ingwithCu-Kradiationat40kVand150mA.Thelmmorphologieswereinvestigatedusingaeldemissionscanningelectronmicroscope(SEM)ofJEOLJSM-7000F.TheopticalabsorptionspectraofthesamplesweremeasuredbyUVvisiblespectrometerofJascoV650inawavelengthregionof200~900nm.DSSCcomprisesadye-coatedTiO2lmonatransparentconduct-ingglasssubstrate,aPt-coatedcounterelectrode,andaredoxelectrolyte.Beforedyesensitization,theTiO2lmswereheatedonahotplateat100Cfor10mintogetridofwatervapor.Thelmswerethenimmersedinadyesolutionof5104MofN719(cis-bis(isothiocyanato)bis(2,2-bipyridyl-4,4-dicarboxylato)-ruthenium(II)bis-tetrabutylammonium)(Ru620-1H3TBA,SolaronixSA,Swit-zerland)inethanolandkeptat70Cfor24h.Theexcessdyeofthesamplewasremovedbyrinsingwithethanolandthesamplewasdriedinsideaventhood.Thedye-coatedTiO2lmwasthenbondedtoacounterelectrodepreparedbysputteringa~100nmofplatinumlmonaglassplate.Aredoxelectrolyteof0.1MI2,1.0MLiIand0.5M4-ter-butylpyridineinacetonitrilewasintroducedintotheinter-electrodespacebycapillaryaction.Aclipwasusedtoholdthesandwichedelectrodestogether.TheDSSCsincorporatedwithFilmsA,B,CandDasphotoanodearedesignatedasCellsA,B,CandD,respectively.ThecellswereilluminatedwithaclassAxenonarcsolarsimulator,whichcontainsa1000WXenonarclamp.Thecurrentvoltagecharacteristicsofdevicesweremeasuredunderthestandardconditionsat25Cwithairmass1.5andpowerdensityof1000W/m2.ThesolarsimulatorwascalibratedwithareferencecellcalibratedbyNationalRenewableEnergyLaboratory[19].Theoverallenergyconversionefciency,,ofasolarcelldeterminesitsperformance.Threeparametersareusedtocharacterizesolarcelloutputsincludingshortcircuitcurrent,Isc,opencircuitvoltage,Voc,andllfactor(FF).TheFFiscalculatedfromtheratioofthemaximumpowerpoint(denedbythesquareoftheIVcurve)dividedbytheproductofIscandVoc.Themaximumcellpoweroutput,Pm,isgivenbyPm=FFIscVoc.TheenergyconversionefciencyisthengivenbyPm/Pin,wherePinisthetotalpowerinputbythelightincidentonthecell[20].

Fig.1.XRDpatternsofTiO2lmsdepositedatvariousincidentanglesfrom53to86.

3.ResultsanddiscussionFig.1showstheXRDpatternsoftheGLADTiO2lmsofFilmsA,B,CandDdepositedatvariousincidentangles.MostofthepeaksinthepatternscouldbeassignedtotheanatasephaseofTiO2inadditiontothepeaksfromtheITOsubstrate.Theintensitiesandpositionsofthepeaksremainaboutthesameexceptthewidthofthepeaks.Thewidthsofthepeaksbecomebroaderasgaugedfromthefullwidthathalfmaximum(FWHM)ofthe(101)peak.TheFWHMsofthe(101)peaksincreasefrom0.64to0.81,whentheincidentangleisincreasedfrom53to86,indicatingareductionofoverallanatasecrystallinity.Fig.2showstheFESEMmicrographsofthetopandcross-sectionalviewoftheGLADTiO2lms.Themicrographsrevealasignicantdifferencebetweenlmsdepositedatdifferentincidentanglesfrom53to86.Undernormallmgrowthbyvapordepositionmethods,thesubstratesurfaceisusuallynormaltotheuxofsourcematerial(=0)andthetypicalresultantlmisofdensecolumnarstructureandwithapackingfactorover90%[17].Withtheincreasingincidentangle,thelmmorphologychangesandthelmporosityincreasesduetotheself-shadowingeffect.FilmAdepositedat=53isstillrelativelydensefromtopview,butcross-sectionalviewrevealsadensely-packedhelicalcolumnarstructure.Forthelmsdepositedat=65andabove,poresareclearlyseenonthetopsurfaceandemptyspacesorgapsbetweenthehelicalcolumnsareobviousfromsideview.Thediametersofthehelicalcolumnsandtheseparation(gap)betweenthemincreasewithincreasingincidentangleofdeposition.ItisevidentfromFig.2thatasthedepositionangleisincreasedfrom65,73to86thediameterofthehelicalcolumnsincreasesandthegapsbetweenthecolumnsincreasesfrom~15nm,~16nmto~56nm,respectively.Thus,theeffectivesurfaceareaalsochangeswithdepositionangle.Thesurfaceareaofthelmcontinuestoincreasewithdepositionangleuntilallthecolumnsbecomeisolated.Thereafter,thesurfaceareadecreasessincethedensityofthenumberofcolumnsdecreasesduetoincreasesincolumnsizeandgap.Itwasreportedthattheeffectivesurfaceareabecomesmaximumat=~70asaresultofthecompetitionduetothecolumnarformationandduetothecolumnarspacingorgap[21,22].Fig.3showstheUVVisibleabsorptionspectraoftheas-depositedTiO2lms(solidline)andofthelmsafterdyeabsorption(dottedline)aswellasoftheabsorbeddyealone(inset).Thespectrashowtwoimportantcharacteristics.Astheincidentangleisincreasedto86,theabsorptionedgesofTiO2lmsred-shiftto450nm,andatthesametime,theabsorptiontailsinvisibleregion(400700cm1)becomesmoreintense.Thisphenomenoncanbeattributedtotheincreasinglmporosity,crystallinityanddefects.Afterthelmswere

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Fig.2.FESEMmicrographsofthetopandcross-sectionalviewoftheTiO2lmsdepositedatdifferentincidentanglesof(a)53,(b)65,(c)73and(d)86from53to86.

impregnatedwiththeN713dye,theabsorptionedgesofallthespectrared-shiftedfurtherwithhigherlevelsofabsorptiontail.ThisisclearlyduetotheabsorptionofdyeontheinternalsurfaceoftheporoussculpturedTiO2lm.Thedyewhichispresentintheformofahighlydispersedcoveringorasashellonthesurfaceoftitaniagrains,isreectedintheUVVisspectraasanintense,broadabsorptioninthevisibleregion.Wehaveintegratedtheabsorptionbetween400nmand700nmofthespectraofeachlmbeforeandafterdye

impregnationandfromthedifferenceofthetwospectraweobtainedthenetabsorptionbytheabsorbeddyealone(insetinFig.3).Theintegratedlightabsorptionbytheas-depositedlms,bythelmsabsorbedwithdyeandbythedyealoneaswellasthenormalizedlightabsorptionbydyealonearetabulatedinTable1.FilmDhasthelargestlightabsorptionofall,butFilmChasthelargestnetlightabsorptionbytheabsorbeddye.FilmAhasthelowestporosityandabsorbedthesmallestamountofdye.

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Fig.3.UVVisibleabsorptionspectraoftheas-depositedTiO2lms(solidline)andofthelmsafterdyeabsorption(dottedline).Theinsetisthenetabsorptionbytheabsorbeddye.

Table2providesthesummarizedresultsfromcellcharacterizationofDSSCsincorporatingtheGLADTiO2lms,andFig.4showsthephotovoltaicmeasurementsusedtogeneratethevaluesinTable2.ThecellperformanceoftheDSSCsmadeoftheGLADTiO2lmsvariedquitealotwiththeincidentangleofdeposition.CellCperformedsignicantlybetterthantherestofthecellsintermsofshortcircuitcurrentdensityJscandtheoverallsolarconversionefciencyofthecells.Theopencircuitvoltage(Voc)isabout0.6Vandthellfactorisabove71%foralltheDSSCs.Themaximumphotovoltageobtainablefromthistypeofsolarcellisdependentontherelativepositionoftheconductionbandedge(VCB)ofTiO2electrodeandtheredoxpotentialoftheelectrolyte.Allthelmsweredepositedatarelativelyhighsubstratetemperatureat350C,resultinginlmsofcomparablecrystallinityandthussimilarVoc.ThesameelectrolytewasusedinalltheDSSCsandshouldpossesssimilarredoxpotential.Therefore,thevaluesofVocforalltheDSSCsareaboutthesame.Theshortcircuitcurrentdensities(Jsc)arequitedifferentamongtheDSSCs.CellChasthehighestJscandfollowedbyCellsD,BandA.TheJscisdirectlylinkedtotheamountoflightabsorbedandconvertedbythecell,soitisinuencedmainlybytheamountofdyeabsorbed.However,theamountoflighttrappedandthequalityofTiO2photoanodethatdeterminestheelectrondiffusionefciencycanalsoaffectJsc.PhotoanodeofTiO2intheanatasephaseofhighcrystallinitywillimprovethetransportofinjectedelectrons,reducerecombinationwiththeelectrolyteandresultinhigherquantumefciencies.ThemaximumJscofCellCmaybeexplainedwithregardtothelargestamountofadsorbeddye,sincetheamountofdyeadsorbedwillbeproportionaltotheaccessiblesurfaceareaandFilmCoftheGLADTiO2layerprovidesthemostaccessibleinternalsurfacearea.TheGLADlmswithconnectedopenporeslikelyimprovetheaccessibilityoftheentirelminternalsurfacetothedyeandtothe

Table1ThecrystallinityoftheGLADtitanialmandtheintegratedlightabsorption(ILA)bythelms,thelmsabsorbedwithdyeandthedyealone.FilmIDABCDIncidentAngle53657386FWHM,An(101)0.650.810.800.81ILAofFilmalone36.44468196.5ILAofFilm+Dye56.0177242.9308.3ILAofDyealone19.6133174.9111.8Normalizeddyeabsorption0.110.7610.64

Table2PhotovoltaiccharacteristicsoftheDSSCsincorporatingtheGLADTiO2lms.Film/CellABCDIncidentAngle53657386Voc(V)0.59(0.02)0.64(0.02)0.62(0.01)0.58(0.03)Jsc(mA/cm2)2.2(0.1)3.9(0.2)6.0(0.1)5.4(0.3)FF%71(2)72(4)74(2)77(3)%0.92(0.09)1.81(0.15)2.78(0.08)2.40(0.1)

electrolyte,leadingtoamoredirectpathfortheinjectedelectrons.Furthermore,thecolumnarnatureoftheGLADlmsprovidesamoredirectandshorterpathforelectrontransfer.ThisresultisconsistentwiththereportbyTagaetal.TheresultsoftheirsimulationandactualpreparationofGLADTiO2lmsindicatedthattheeffectivesurfaceareaisenhancedbyobliquedepositionowingtocolumnarformationandbecomesmaximumatadepositionangleof70.Thelmsdepositedat70possessthemaximumeffectivesurfaceareaandthebestphotocatalyticperformance[21,22].ThellfactorsoftheDSSCsare7177%,muchhigherthanmostofthereportedvaluesinliterature[5,6].AsthellfactorisinuencedbytheinterfacesofthemajorcomponentsinDSSCs.TwooftheinterfacesarefromthecontactsbetweentheTiO2photoanodeandtheconductingelectrodesofITOandPt.SinceGLADproducesawell-adheredandwell-organizedsculpturedstructureontheITOlayerandthetopsurfacesofporousTiO2layerarerelativelyatwhichensuresgoodcontactwiththeotherconductingelectrodeofPt-coatedglass.ThetightinterfacesbetweentheTiO2photoanodelayerandthetwoconductingelectrodescontributepositivelytoahighllfactor.TheoverallefcienciesoftheDSSCsincorporatedwiththeGLADTiO2photoanodeinthisstudyweredecidedlargelybytheirJsc.Assuch,thecellswithTiO2layerannealedathighertemperaturestoenhancecrystallinityandwiththickerTiO2layertoincreasetheamountofdyeabsorptionwilllikelyincreaseJscvaluesandtheoverallefciency.Indeed,wehavepreparedanotherseriesofGLADTiO2lmswithvariousthicknessesfrom410matincidentangleof86.Thelmsweredepositedatambienttemperatureandsubsequentlyannealedat350Ctooptimizetheircrystallinity.ThecellefcienciesincreasemonotonicallywithTiO2lmthicknessupto5.23%forthe10mthicklmandtheresultswillbereportedinanotherpaper.Furtherimprovementincellefciencyshouldbepossiblebyoptimizationofcrystallinity,nanostructures,porosityandlmthicknessoftheGLADTiO2photoanode.

Fig.4.IVphotovoltaicmeasurementsoftheDSSCsusedtogeneratethevaluesinTable2.

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4.Conclusions

Photoanodeofsculpturedporousnano-columnartitaniumoxidelmsweresuccessfullypreparedbyglancingangledeposition(GLAD)methodandintegratedintoDSSCs.Thephotoanodeoforderedporousnano-columnarTiO2layerprovideslargesurfaceareafordyeabsorption,fastelectrontransferpath,enhancedlighttrapping,andtightinterfacestoconductingelectrodesandcontributestoahighllfactorandanoverallcellefciency.AwidevarietyofporousnanostructuresofsemiconductingoxidescreatedbyGLADcanbefurtheroptimizedsystematicallyforDSSCstoachieveevenhigherpowerefciencies.Acknowledgements

FinancialgrantbyTaiwanNationalScienceCouncilunderNo.96-2221-E-259-010-MY3andtheuseofcore-facilityofNanotechnologyCenterinEastTaiwanareacknowledged.

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