光伏材料的合成及其性能研究

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ElectrochimicaActa

journalhomepage:www.elsevier.com/locate/electacta

Highlyasymmetricphthalocyanine-sensitizedsolarcells:Theeffectofcoadsorbentandadsorptiontemperatureofphthalocyanine

LijuanYua,LiLina,XiaohuZhanga,RenjieLia,∗,TianyouPenga,∗,XingguoLib

ab

CollegeofChemistryandMolecularScience,WuhanUniversity,Wuhan430072,China

BeijingNationalLaboratoryforMolecularSciences(BNLMS),CollegeofChemistryandMolecularEngineering,PekingUniversity,Beijing100190,China

article

info

abstract

Articlehistory:

Received14April2013

Receivedinrevisedform6August2013Accepted6August2013Available online xxx

Keywords:

AsymmetriczincphthalocyanineConversionefficiencyDye-sensitizedsolarcellCoadsorbent

Dyeadsorptiontemperature

Highlyasymmetriczincphthalocyaninederivative(Zn-tri-PcNc)containingtribenzonaphtho-condensedporphyrazinewithonecarboxylandthreetert-butyl(t-Bu)substituentgroupsisusedasasensitizertofabricatedye-sensitizedsolarcells(DSSCs),andtheeffectsofchenodeoxycholicacid(CDCA)asacoad-sorbentandthedyeadsorptiontemperatureonthesolarcell’sperformanceareinvestigated.ItisfoundthatCDCAcoadsorptioncanhinderthedyeaggregation,whichisbeneficialforimprovingtheelectroninjectionefficiencyandretardingthechargerecombination,andthusresultingintheenhancementoftheshort-circuitcurrentdensityandopen-circuitphotovoltage.Moreover,thedyeadsorptiontemperatureonelectrodealsoshowsasignificantimpactonthephotovoltaicperformanceofthesolarcell,andanopti-maldyeadsorptionconditionoftheTiO2electrodeisfoundtobe5×10−5MZn-tri-PcNcethanolsolutioncontaining7.5mMCDCAat5◦C,whichcancontributetothemaximumconversionefficiencyof2.%withshort-circuitcurrentdensityof9.42mAcm−2andopen-circuitphotovoltageof0.48V,improvedby47%ascomparedtothesolarcellfabricatedwithTiO2electrodesensitizedbythezincphthalcoyanineinabsenceofCDCA.

© 2013 Elsevier Ltd. All rights reserved.

1.Introduction

Theconversionofsunlighttoelectricitybyusingdye-sensitizedsolarcells(DSSCs)hasbeenresearchedasoneofthemostpromisingmethodsforfuturelow-costpowerproductionfromrenewableenergysourcesinrecentyears[1,2].Atthepresentstage,remarkableconversionefficiencyof12%inthisfieldhavebeenachievedwithmesoscopicTiO2photoanodesensitizedwithRu(II)-polypyridylcomplexes[3].Inspiteofthis,themaindrawbacksofRu-basedsensitizersaretheirlowmolarextinctioncoefficientsandthelimitedresourceofthepreciousrutheniummetalforthepracticalapplications.Organicdyesexhibithighmolarextinctioncoefficientsandsuitablebandgapswhichcanbeeasilytunedbychemicalmodifications,butlackingofabsorptioninthered/near-IRregionofthesolarspectrumisalwaysachallengeformoleculedesigns.

Phthalocyanines(Pcs)arewellknownchromophoresfortheirintenseabsorptionintheUV/blue(Soret-band)andthered/nearIR(Q-band)spectralregions,aswellasfortheirexcellentelectro-chemical,photochemicalandthermalstability.MoreoverPcshaveappropriateredoxpropertiesforthesensitizationofwidebandgapsemiconductorssuchasTiO2,thereforerenderingthemattractive

∗Correspondingauthors.Fax:+862768752237.

E-mailaddresses:lirj@whu.edu.cn(R.Li),typeng@whu.edu.cn(T.Peng).

assensitizersforDSSCs[4–8].Asaconsequence,alargenumberofPcshavebeentestedassensitizersforTiO2electrodeofthesolarcell.Unfortunately,mostoftheconversionefficiencieswereonlyaround1%amongthePc-sensitizedsolarcells.ThegeneralaggregationtendencyandthelackofdirectionalityoftheelectronicorbitalofthesymmetricPcsintheexcitedstateareusuallyrespon-siblefortheabove-mentionedpoorperformance.ForthepurposeofsuppressingtheaggregationofPcs,combinationofperipheralsubstitutionwithlargestericgroupsandaxialcoordinationofthemetalcenterhasbeenemployed[7].Goodelectroniccouplingbetweenthelowestunoccupiedmolecularorbital(LUMO)ofthedyeandTi3dorbitalseemstobeabletoprovidethedirectionalityforallowinganefficientelectrontransferfromtheexciteddyetoTiO2conductionband(CB)[5a,8].Inordertoincorporatetheabovementionedessentialproperties,novelasymmetriczincphthalo-cyaninederivatives(TT1andPCH001)containingthreetert-butyl(t-Bu)groupsandoneortwocarboxylicacidgroupshavedevel-oped[5],whichshowedeffectivered/near-IRlightresponseandyielded3.52%and3.05%ofefficiencies,respectively.Veryrecently,Mori’sgroupandTorres’grouphavereportedbulkyZnPcswithdiphenylphenoxygroupscanpreventtheaggregation,andefficien-ciesof4.6–5.5%wereobtained[6].

Besidessuppressingtheaggregationbychemicalmodifica-tions,coadsorbentssuchas3a,7a-dihydroxy-5b-cholanicacid(chenodeoxycholicacid,CDCA)canalsopreventtheformationofdyeaggregationduringthedyesensitizationprocess,whichis

0013-4686/$–seefrontmatter© 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.electacta.2013.08.031

L.Yuetal./ElectrochimicaActa111 (2013) 344–350

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beneficialforpromotingthedyeregenerationand/ortheelectroninjectionkinetics[9–11].Co-graftingofPcdyeandCDCAcouldresultintheformationofamixedmonolayer,whichismoretightlypackedthanthePcsensitizeradsorbedalone,providingamoreeffectiveinsulatedbarrierforthebackelectrontransferfromtheTiO2conductionbandtothetriiodideionsintheelec-trolyte.Asaresult,thedarkcurrentisreducedandtheelectroninjectionefficiencyisimproved.TheeffectofthedyeaggregationontheTiO2surfaceisanimportantissueinfurtherimprovingtheperformanceofphthalocyanineorporphrin-basedsolarcells.IthasreportedthatCDCAascoadsorbentisalsosuitablefororganicdyesinimprovingtheelectroninjectionefficiencyandretardingthechargerecombination[10].Moreover,thetemperatureduringthedyeadsorptionprocessforthedyemonolayerloadingonthemesoscopicTiO2surfaceshowedrelativelyobviousimpactontheperformanceofDSSCs[12].GrätzelandcoworkershavereportedtheeffectofdyeadsorptiontemperatureontheperformanceofDSSCsfabricatedwithRu(II)-polypyridylcomplex(codedasC101),andfoundasignificantefficiencyenhancementuponloweringthetemperatureappliedduringthedyeuptakefromsolution.How-ever,exploringthiseffectfortheloweradsorptiontemperatureofbelowzeroislimitedbythedecreaseofthedyesolubility.

Recently,wedesignedanddevelopedhighlyasymmetriczincphthalocyaninederivative(Zn-tri-PcNc,Scheme1)containingtribenzonaphtho-condensedporphyrazinewithonecarboxylandthreetert-butyl(t-Bu)substituentgroupsthatcanactas“push”and“pull”groups,respectively[13].Herein,wefocusedontheeffectofchenodeoxycholicacid(CDCA,Scheme1)ascoadsorbentandthedyeadsorptiontemperatureontheperformanceofthesolarcellssensitizedwiththeabovenewlysynthesizedhighlyasymmetricZn-tri-PcNc.DuetothegoodsolubilityofZn-tri-PcNcinethanol,awidertemperaturerangecanbeappliedtochecktheeffectofadsorptiontemperatureontheperformanceofthesolarcells.ItwasfoundthattheoptimumamountofCDCAwas7.5mMin5×10−5Mdyesolutionandtheoptimumadsorbedtemperaturewas5◦C,andthephotoelectrochemicalbehaviorsofthoseelectrodesandtheircorrespondingsolarcellsobtainedfromdifferentdyesensitizationconditionswereinvestigatedindetail.

2.Experimental

2.1.Dyepreparation

Allsolventsandreagentswereofpurequalityandusedasreceivedunlessotherwisestated.Thehighlyasymmetriczincphthalocyaninederivative(Zn-tri-PcNc,Scheme1)containingtribenzonaphtho-condensedporphyrazinewithonecarboxylandthreetert-butyl(t-Bu)substituentgroupswasrecentlysynthesizedbyus,andthedetailedexperimentalproceduresandfullspectro-scopiccharacterizationaregiveninourpreviousreport[13].

2.2.Fabricationofphotoanodes

Thepreparationofphotoelectrodewasperformedbyadoctor-bladetechniqueonaconductingglass(FTO,15–20󰀍sq−1),whichwasrinsedwithdistilledwaterandfullysoakedinisopropanolfor3htogetridofsurfacedirtandincreaseitshydrophilicitybeforeuse.ThepretreatedFTOglass(NipponSheetGlass,4mmthick-ness)wasfirstlytreatedintitaniumtetrachloride(TiCl4,50mM)solutionfollowedbycalcinationat500◦Cfor30min.Apastecomposedof20nmanataseTiO2particleswasspreadontheabovetreatedFTOglassbyadoptingdoctor-bladetechniquetoobtainabout10␮mthicknessfilm,andthenapasteforscatteringlayercontaining200nmanataseparticleswasdepositedbyadoptingdoctor-bladetechniquetoobtainafilmthicknessofabout4␮m

aftercalcinationat500◦Cfor30min.TheTiO2electrodeswerethensinteredat500◦Cfor30min.InordertoenhancethebindingpropertyandconnectivityofTiO2particles,theheatedelectrodeswereimpregnatedwith25mMTiCl4solutioninwatersaturateddesiccatorfor30minat70◦Candwashedwithdistilledwater.TheelectrodecontactareawasprotectedfromtheTiCl4solutionbyadhesivetape.Finally,theTiO2electrodesweresinteredagainat500◦Cfor30minbeforedippingintodyesolution.

2.3.FabricationofDSSC

TheTiO2electrodeswereimmersedintothe5.0×10−5MZn-tri-PcNcethanolsolutionscontainingdifferentCDCAconcentrationsandkeptatdifferenttemperaturesfor12h.Thedye-sensitizedelec-trodewasassembledinaclassicsandwich-typecell.Namely,thePtcounterelectrodewasattachedonthedye-sensitizedphotoanode.Afterinjectionoftheelectrolytesolution,whichconsistsof0.5MLiI,0.05MI2,and0.1M4-tert-butylpyridinein15/85(v/v)mix-tureofvaleronitrileandacetonitrileintotheinterspacebetweenthephotoanodeandthecounterelectrode,thephotoelectroche-micalpropertyofthefabricatedsolarcellwasmeasured.InordertoreducethescatteredlightfromtheedgeoftheglasselectrodesofthedyedTiO2layer,alight-shadingmaskwasusedontothesolarcell,fixingtheactiveareato0.25cm2.

2.4.UV–visabsorptionspectraandphotoelectrochemicalbehaviormeasurements

TheUV–visabsorptionspectraofZn-tri-PcNcdesorbedfromtheTiO2electrodesinthepresenceof0.1MNaOHEtOH-H2O(v/v=1:1)solutionandinsolidadsorbedonTiO2filmwererecordedonaHitachiU-4100spectrophotometer.

Theelectrochemicalimpedancespectroscopy(EIS)measure-mentswerecarriedoutbyapplyingbiasoftheopen-circuitvoltage(VOC)withoutelectriccurrent,andrecordedoverafrequencyrangeof10−2to105Hzwithacamplitudeof10mV.Thesolarcellwasfirstlyilluminatedtoasteadyvoltage,andthentheopen-circuitvoltagedecaycurve(OCVD)wasrecordedoncetheilluminationwasturnedoffbyashutter.AbovemeasurementswerecarriedoutonaCHI-604Celectrochemicalanalyzer(CHInstruments)com-binedwithXe-lampaslightsource.

2.5.DSSCspropertymeasurements

Thesolarcellwasilluminatedbylightwithenergyof100mWcm−2from300WAM1.5Gsimulatedsunlight(Newport,91160).ThelightintensitywasdeterminedusingaSRC-1000-TC-QZ-Nreferencemonocrystalsiliconcell(Oriel),whichwascalibratedbyNationalRenewableEnergyLaboratory,A2LAaccred-itationcertificate2236.01.Acomputer-controlledKeithley2400sourcemeterwasemployedtocollectthephotocurrent–voltage(J–V)curves.Theincidentphoton-to-currentconversionefficiency(IPCE)wasmeasuredasafunctionofwavelengthfrom250to900nmbyusingaModelQE/IPCEsystem(PVMeasurementsInc.).

3.Resultsanddiscussion

3.1.Surveyontheeffectofcoadsorbentonthedye-loading

Fig.1ashowstheUV–visabsorptionspectraofZn-tri-PcNcdesorbedfromtheTiO2electrodesinthepresenceof0.1MNaOHEtOH-H2O(v/v=1:1)solution.Thecorrespondingdye-loadedamountoftheTiO2electrodecanbeestimatedfromtheabsorbencyat700nmofthedesorbedZn-tri-PcNcsolutionbyusingBeer–Lambertlaw.AsshowninTable1,thedye-loadedamountoftheTiO2electrodeshowsaconsecutivedecreasingtrendfrom

346L.Yuetal./ElectrochimicaActa111 (2013) 344–350

Scheme1.Molecularstructuresofhighlyasymmetriczincphthalocyaninederivative(Zn-tri-PcNc)andchenodeoxycholicacid(CDCA).

3.84×10−8to2.25×10−8molcm−2withenhancingtheCDCAcon-centrationsin5.0×10−5MZn-tri-PcNcethanolsolutionfrom0to10mM.Itsuggeststhatthedye-loadedamountoftheTiO2elec-trodeisstronglydependentontheCDCAconcentration,andtheanchoringprocessishighlycompetitivebetweendyeandcoadsor-bent[14].

Fig.1bshowsUV–visspectraofZn-tri-PcNc-sensitizedTiO2filmadsorbedfrom5.0×10−5MZn-tri-PcNcethanolsolutionscontain-ingdifferentCDCAconcentrations.Ascanbeseen,thebroadenedabsorptionspectraandnewpeaksat0nmascomparedtothecorrespondingdyesolutioncanbeobserved.ThebroadenedabsorptionpeakisduetotheaggregationofZn-tri-PcNconTiO2electrodeandthenewabsorptionbandat0nmcanbeassignedtotheH-aggregation[15].BycomparingtheratiooftheopticalabsorptionintensityoftheH-aggregationband(AH0at0nm)andthemonomerband(AQ0at700nm)inTiO2films,theH-aggregationextentofZn-tri-PcNccanbeestimated,andahighAH0/AQ0valueusuallyindicatesasevereaggregationofZn-tri-PcNcontheTiO2electrode.AscanbeobtainedfromFig.1b,theAH0/AQ0valueofZn-tri-PcNcis1.051,1.036,1.022,1.017and1.014whentheCDCAconcentrationis0,2.5,5.0,7.5and10mMinthedyesolution,respectively.TheabovedatasuggestthattheH-aggregationofZn-tri-PcNccanberetardedbyenhancingtheCDCAconcentration.Hence,theCDCAasacoadsorbentinZn-tri-PcNcsolutioncaneffi-cientlydiminishthedyeaggregationontheTiO2electrode.

3.2.PhotovoltaicperformanceofDSSCs

PhotovoltaicperformancesofthesolarcellsfabricatedwithZn-tri-PcNc-sensitizedTiO2electrodesadsorbedfrom5.0×10−5MZn-tri-PcNcethanolsolutionscontainingdifferentCDCAconcen-trationsat5◦CarerecordedinFig.2,andthecorrespondingcalculatedcharacteristicparameterssuchas,open-circuitpoten-tials(VOC),short-circuitcurrents(JSC),fillfactors(FF)andoverallconversionefficiencies(Á)understandardAM1.5Gsunlight,arelistedinTable2.Forevaluatingthephotoelectrochemicalperfor-manceoftheasymmetricZn-tri-PcNcassensitizerforTiO2-basedsolarcells,dyesolutionscontainingdifferentCDCAconcentrationsareusuallyusedforthesensitizationprocesssoastoreducethedyeaggregationandimprovethecellperformance[10,16].Ascan

Table1

Dye-loadedamountofTiO2electrodeadsorbedfrom5.0×10−5MZn-tri-PcNcethanolsolutionscontainingdifferentCDCAconcentrations(ε=1.06×105mol−1cm−1).

CCDCA(mM)02.55.07.510.0

Electrodearea(cm2)

Abs(󰀂max)

Dye-loadedamount(molcm−2)

Fig.1.UV–visabsorptionspectraofZn-tri-PcNcinEtOH-H2Osolution(a)afterdesorbedfromTiO2filmandZn-tri-PcNc-sensitizedTiO2film(b)adsorbedfrom5.0×10−5MZn-tri-PcNcethanolsolutionscontainingdifferentCDCAconcentra-tions.

6.386.386.826.305.72

1.2951.1851.0320.80.682

3.84×10−83.50×10−82.86×10−82.66×10−82.25×10−8

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347

Fig.2.Current–voltage(J–V)curvesofthesolarcellsfabricatedwithZn-tri-PcNc-sensitizedTiO2electrodesadsorbedfrom5.0×10−5MZn-tri-PcNcethanolsolutionscontainingdifferentCDCAconcentrationsat5◦C.

beseenfromFig.2andTable2,underthesamedyeconcentra-tionandadsorptiontemperature,theJSCfirstlyincreasedfrom6.63to9.42mAcm−2withenhancingtheCDCAconcentrationfrom0to7.5mM,andthendecreasedto7.69mAcm−2uponfurtherenhancingtheCDCAconcentrationto10.0mM.Namely,theelec-trodeimmersedinto5.0×10−5MZn-tri-PcNcsolutioncontaining7.5mMCDCAachievedthemaximumJSCvalue(9.42mAcm−2).

ThefirstincreaseinJSCcanbeduetotheadditionofCDCAinthedyesolution,whichcandecreasethedyeaggregationontheelec-trodesurfaceasprovedabove;whereasthefollowedreducedJSCwithfurtherenhancingtheCDCAconcentrationcouldresultfromthelessdye-adsorbedamountontheelectrodeduetothecom-petitiveadsorptionbetweendyeandCDCAontheTiO2electrodesurface.Additionally,theVOCvalueshowedaslightlyincreasingtrendfrom0.44to0.48VuponenhancingtheCDCAconcentrationfrom0to10.0mM.ItiswellknownthattheVOCvalueofDSSCisdeterminedbythedifferencebetweenthepotentialofredoxcou-ple(I−/I3−)intheelectrolyteandthequasi-FermilevelofTiO2.Thequasi-FermilevelisdependentonboththeconductionbandedgeofTiO2andtheelectronconcentrationinTiO2,whichisusuallydeterminedbytheelectronrecombinationrate(oftenrelatedtotheelectronlifetime)[9,17].Itcanbeinferredthatthecoadsorp-tionofCDCAwithZn-tri-PcNcontheTiO2electrodesurfaceinducedabandedgeshiftofTiO2;inaddition,thehighsurfacecoveragebyCDCAcanretardtherecombinationprocess,whichresultedinalongerelectronlifetime.Therefore,itcanbeconcludedthattheincreasedVOCwasmainlycausedbythelongerelectronlifetimeduetothecoadsorptionofCDCA.

Inordertogetsuitablesensitizationtemperature,thephotovoltaicperformancesofthesolarcellsfabricatedwithZn-tri-PcNc-sensitizedTiO2electrodesadsorbedfrom5.0×10−5MZn-tri-PcNcethanolsolutionscontaining7.5mMCDCAatdiffer-enttemperaturesarerecordedinFig.3,andthecorrespondingcalculatedcharacteristicparametersarelistedinTable3.Ascan

Table2

Photoelectrochemicalparametersofthesolarcellsfabricatedwithelectrodessen-sitizedby5.0×10−5MZn-tri-PcNcethanolsolutionscontainingdifferentCDCAconcentrationsat5◦C.

CCDCA(mM)VOC(V)

JSC(mAcm−2)

FF

Á(%)

00.446.630.671.962.50.477.880.692.555.00.478.420.672.607.50.4.420.2.10.0

0.48

7.69

0.69

2.55

Fig.3.Current–voltage(J–V)curvesofthesolarcellsfabricatedwithZn-tri-PcNc-sensitizedTiO2electrodesadsorbedfrom5.0×10−5MZn-tri-PcNcethanolsolutionscontaining7.5mMCDCAatdifferenttemperatures.

beseen,theJSCvalueobtainedis7.22,9.42,7.32and7.53mAcm−2atadsorptiontemperaturesof−15,5,15and25◦C,respectively.TheTiO2electrodesensitizedat5◦CresultedinthemaximumJSCvalue(9.42mAcm−2),whichdecreasesuponfurtherenhancingthedyeadsorptiontemperature.Namely,theoptimumadsorbedtem-peratureis5◦C,whichissimilartothepreviousreport[18].ThereducedJSCvaluescanbepresumablyascribedtothedyecoverageexceedingthemonolayersaturationlimit,andthereforeresultingintheformationofmultilayerordyeaggregationathigheradsorp-tiontemperature[12,18].However,bydecreasingtheadsorptiontemperatureto−15◦C,amuchlowerJSCvalue(7.22mAcm−2)wasobtained,indicatingtheunsaturatedmonolayeradsorptionontheTiO2electrodeduetothelowdyeadsorptionrateatthistempera-ture.TheVOCvaluesstillshowslightlydecreasingtrendfrom0.48to0.46Vuponenhancingtheadsorptiontemperaturefrom−15to25◦C.TheabovedecreaseinVOCvaluemaybeduetotheincreaseddyeaggregationcausedbythemorerapiddyeadsorptionrateatanelevatedadsorptiontemperature.

AcomparisonofthephotovoltaicperformancesofthesolarcellsfabricatedwithTiO2electrodesadsorbedfrom5.0×10−5MZn-tri-PcNcethanolsolutionscontainingdifferentCDCAconcentrationsatdifferentdyeadsorptiontemperaturesislistedinTableS1(ref.theSupportingInformation).Ascanbeseen,theTiO2electrodeimmersinginto5.0×10−5MZn-tri-PcNcethanolsolutionscon-taining7.5mMCDCAat5◦Cfor12histheoptimaldye-sensitizationcondition,andthemaximumconversionefficiency(Á)ofthefab-ricatedsolarcellachieves2.%,improvedby47%ascomparedtothesolarcellfabricatedwithTiO2electrodesensitizedbytheZn-tri-PcNcsolutioninabsenceofCDCA.

Fig.4showsthephotocurrentactionspectraofthesolarcellsfabricatedwithZn-tri-PcNc-sensitizedTiO−52electrodesadsorbedfrom5.0×10MZn-tri-PcNcethanolsolutionswithorwithout7.5mMCDCAat5and25◦C.Ascanbeseen,theincidentmonochro-maticphoton-to-currentconversionefficiency(IPCE)ofTiO2filmsensitizedindyesolutioncontaining7.5mMCDCAismuchhigher

Table3

Photoelectrochemicalparametersofthesolarcellsfabricatedwithelectrodessen-sitizedby5.0×10−5MZn-tri-PcNcethanolsolutionscontaining7.5mMCDCAatdifferenttemperatures.

Temp.(◦C)

VOC(V)

JSC(mAcm−2)

FF

Á(%)

−150.487.220.712.4550.4.420.2.150.477.320.702.4225

0.46

7.53

0.66

2.30

348L.Yuetal./ElectrochimicaActa111 (2013) 344–350

Fig.4.NormalizedIPCEactionspectraofthesolarcellsfabricatedwithZn-tri-PcNc-sensitizedTiO2electrodesadsorbedfrom5.0×10−5MZn-tri-PcNcethanolsolutionswithorwithout7.5mMCDCAat5or25◦C.

thanthatwithoutCDCAatthesameadsorptiontemperature(5◦C).Asmentionedabove,theadditionofCDCAascoadsorbentinthedyesolutioncansignificantlyretardthedyeaggregationontheTiO2electrodesurface.Whereas,thelowerIPCEvaluesattheadsorptiontemperatureof25◦Cascomparedtothatat5◦Ccanbeascribedtothedyeaggregationcausedbymorerapiddyeadsorptionrateatanelevatedadsorptiontemperature.Hence,itcanbeconcludedthatbothCDCAcoadsorbentanddyeadsorptiontemperatureshowsignificantimpactonthephotovoltaicperformanceofthesolarcell.

3.3.Electrochemicalimpedancespectroscopyanalyses

Fig.5showstheBodephase(a)andNyquist(b)plotsofelectro-chemicalimpedancespectra(EIS)ofthesolarcellsfabricatedwithZn-tri-PcNc-sensitizedTiO2electrodesadsorbedfrom5.0×10−5MZn-tri-PcNcethanolsolutionswithorwithout7.5mMCDCAat5or25◦C.Generally,theNyquistplotfeaturesthreesemicirclesthatintheorderofincreasingfrequencyareattributedtotheNernstdiffusionwithintheelectrolyte,theelectrontransferattheoxide/electrolyteinterface,andtheredoxreactionatthePtcounterelectrode.InourEISdiagrams,twosemicirclescanbedetectedintheNyquistplots(Fig.5b).ThesemicircleattributedtotheNernstdiffusionwithintheelectrolyteisfeaturelessduetotherelativelyfastdiffusionoftheelectrolyteintheporousfilms[19].There-fore,asimpleequivalentcircuitmodel,asexhibitedintheinsetofFig.5b,wasusedtosimulatetheDSSCs.Amongthose,Rsrepre-sentstheserialresistance,whichismainlyinfluencedbythesheetresistanceofthesubstrateandelectricalcontactbetweenthecon-ductiveglass/TiO2interfaces,Rct1representstheimpedancewithchargetransferatthePtcounterelectrodeand/orelectricalcon-tactbetweenTiO2nanoparticles,andRct2representstheimpedancerelatedtochargetransferprocessatTiO2/dye/electrolyteinterfaces[20].Rct2wouldbediscussedindetailsbecauseitrelatessignif-icantlytotheusedsensitizationconditionsofdyesolutions.Theelectronlifetime(󰀃n)canbedrawnbythepositionsofthelowfre-quencypeakinFig.5athrough󰀃n=1/2␲f(fmeansthefrequencyofsuperimposedacvoltage).Furthermore,theeffectiverateconstant(keff)fortherecombinationreactioncanbealsoobtainedaccordingtothemethodproposedbyAdachi[21].ThefittedRct2,󰀃n,andkeffvaluesforthoseDSSCsareshowninTable4.

AsshownintheNyquistplots(Fig.5b),thesmallerandlargersemicirclesareattributedtothecharge-transferatthePtcounterelectrodeandtheelectrontransportatTiO2/dye/electrolyteinter-faces,respectively.AscanbeseenfromTable4,thecorrespondingRct2valueofthesolarcellfabricatedwithdye-sensitizedelectrode

Fig.5.Electrochemicalimpedancespectra(EIS)ofthesolarcellsfabricatedwithZn-tri-PcNc-sensitizedTiO2electrodesadsorbedfrom5.0×10−5MZn-tri-PcNcethanolsolutionswithorwithout7.5mMCDCAatdifferenttemperatures.(a)Bodephaseplots;(b)Nyquistplots.

isgreatlydecreasedfrom54.56to36.72󰀍withenhancingtheCDCAconcentrationfrom0to7.5mMat5◦C,indicatingthattheelectronrecombinationresistancecanbereducedbyadditionofCDCA[14].BecauseRct2representstheimpedancerelatedtochargetransferprocessatTiO2/dye/electrolyteinterfaces,itcanbeinferredthatthechargecombinationratebetweentheinjectedelectronandtheelectronacceptor(I3−)intheelectrolytewereloweredbytheadditionofCDCA.AccordingtothefvaluesofthoseBodephaseplotsinFig.5a,theelectronlifetimes(󰀃n)canbecalculatedtobe13.37and6.20msforthesolarcellsfabricatedwithTiO2electrodessensitizedwithandwithout7.5mMCDCAat5◦C.Moreover,theircorrespondingeffectiverateconstant(keff)fortherecombinationreactionsis74.79and161.40s−1.Obviously,theadditionofCDCAinthedyesolutionisbeneficialforpromotingtheelectronlifetimeanddecreasingtheeffectiverateconstantoftherecombinationreaction.ItimpliesthatthebackreactionoftheinjectedelectronwiththeI3−intheelectrolytecanbeefficientsuppressed.

Table4

Kineticparametersoftherecombinationreactionsofthesolarcellsfabricatedelec-trodessensitizedby5.0×10−5MZn-tri-PcNcethanolsolutionswithorwithout7.5mMCDCAatdifferenttemperatures.

Temp.(◦C)

CCDCA(mM)

Rct2(󰀍)

󰀃n(ms)

keff(s−1)5054.566.20161.4057.536.7213.3774.7925

7.5

47.58

9.13

109.52

L.Yuetal./ElectrochimicaActa111 (2013) 344–350

349

Moreover,uponenhancingthedyeadsorptiontemperaturefrom5to25◦Cbutmaintaining7.5mMCDCA,theradiusofthelargersemicirclessignificantlyincreasedasshowninFig.5b,indi-catingadecreaseoftheresistanceforinterfacialelectrontransferfromtheTiO2conductionbandtoI3−ionsintheelectrolyte[18].ThecorrespondingRct2valueincreasesfrom36.72to47.58󰀍,implyingtheaccelerationofthechargerecombination.Thecor-respondingelectronlifetimes(󰀃n)ofthosesolarcellsdecreasedfrom13.37to9.13ms,whiletheeffectiverateconstant(keff)oftherecombinationreactionsincreasefrom74.79to109.52s−1uponenhancingthedyeadsorptiontemperaturefrom5to25◦C.Obviously,thesolarcellsensitizedat5◦Chasthelongerlifetimeandsmallereffectiverateconstantoftherecombinationreaction,andenhancingthedyeadsorptiontemperaturewilllowertheelec-tronlifeandacceleratethechargerecombination,whichmaybeduetothedyeaggregationcausedbytherapiddyeadsorptionrateatanelevatedtemperature.

3.4.Open-circuitphotovoltagedecaycurvesmeasurement

Theopen-circuitphotovoltagedecay(OCVD)curvescanshowthemaininformationoftherecombinationprocessbetweentheinjectedelectronsinTiO2andtheelectrolyteunderthedarkstate

[22].The󰀄lifetime(󰀃,responsetimeofrecombinationreaction)oftheinjectedelectronsn

inTiO2isgivenbythereciprocalofthederivativeofthedecaycurvenormalizedbythethermalvoltageasfollows.

󰀃󰀄OC

1

n

=

kBT󰀁dV󰀂−e

dt

(1)

wherekBistheBoltzmanncoefficient,Tisthetemperature,andeistheelementarycharge.

AsshowninFig.6a,thedecaytrendoftheVOCvaluesforthesolarcellfabricatedwithTiO2electrodesensitizedbydyesolutionwithoutCDCAisfasterthanthatwith7.5mMCDCAatthesamedyeadsorptiontemperature,indicatingtheadditionofCDCAcanretardthechargerecombinationwhichisconsistentwiththeabovecon-clusion.Fig.6bshowstheeffectsofCDCAconcentrationandthedye

adsorptiontemperatureonthe󰀃󰀄ingtoBisquert’sviewpoint[22],then

valueofthesolarcells.Accord-󰀃󰀄n–VOCcurvecanbemarkedoff

asthreevoltage-dependentregions:(1)aconstantlifetimeathighVOCregionmainlydominatedbytheelectrontransferprocessfromtheconductionbandofTiO2totheelectrolyte;(2)anexponentialincreaseregionduetotheinternaltrappingand/ordetrapping;and(3)aninvertedparabolaatlowVOCregioncomingfromtheeffectof

surfacestates.AscanbeseenfromFig.6b,theshapesofthe󰀃󰀄theDSSCfabricatedwithTiOn–VOC

curvesfor2electrodesensitizedbydyesolutionwithandwithoutCDCAatthelowVparabola.The󰀃󰀄OCregionsareinverted

valueforthesolarcellfabricatedwithelectrodesensitizedbydyen

solutionwithoutCDCAisclosetoconstantatthehighVOCregion,indicatingthattheabsenceofCDCAcanacceler-atetheelectrontransferfromtheconductionbandofTiO2tothe

electrolyte.Moreover,theoverall󰀃󰀄n–VOCcurveforthesolarcellfab-ricatedwithelectrodesensitizedbydyesolutioncontainingCDCA

movesupward,implyinglongerlifetimeoftheinjectedelectronsinTiO2byaddingCDCA.ThelongerlifetimeoftheinjectedelectronscausestheenhancementoftheelectroninjectionefficiencyandinturntheimprovementofthephotovoltaiccharacteristicsandIPCEasmentionedabove.

Additionally,thedyeadsorptiontemperaturealsoplaysaveryimportantrolefortherecombinationreactionontheTiO2electrodesurface.Asthedyeadsorptiontemperatureenhancingfrom5◦Cto25◦CwiththesameCDCAconcentrationinthedyesolution,

thedecaytrendoftheVOCvalueisfaster,andtheoverall󰀃󰀄–Vmovesdownward,resultinginfasterchargerecombinationnOC

curveandshorterlifetimeoftheinjectedelectrons.Thus,ahigherdye

Fig.6.Open-circuitvoltagedecaycurves(a)andcorresponding󰀃󰀄

n–VOCcurves(b)ofthesolarcellsfabricatedwithZn-tri-PcNc-sensitizedTiO2electrodesadsorbedfrom5.0×10−5MZn-tri-PcNcethanolsolutionswithorwithout7.5mMCDCAatdifferenttemperatures.

adsorptiontemperaturewillbringfasterchargerecombinationandlowerelectroninjectionefficiency,andthereforeresultinginlowerphotovoltaiccharacteristicsandIPCEvalues.

TheaboveresultsandchangingtrendsshowninEISspectraandOCVDcurvesareconsistenttotheabove-mentionedimprovementsofthephotovoltaiccharacteristicsandIPCEaftertheadditionofCDCAinthedyesolution.Therefore,itcanbeconcludedthatundertheoptimaldyesensitizationconditions(5.0×10−5MZn-tri-PcNcethanolsolutioncontaining7.5mMCDCAat5◦C),andthemaxi-mumefficiency(2.%)ofthefabricatedsolarcellcanbeascribedtotheretardeddyeaggregation,moreefficientelectroninjectionandlowerchargerecombinationascomparedtothesituationofthedyesensitizationwithoutCDCA.

4.Conclusions

Highlyasymmetriczincphthalocyaninederivative(Zn-tri-PcNc)containingtribenzonaphtho-condensedporphyrazinewithonecarboxylandthreetert-butyl(t-Bu)substituentgroupswasusedasasensitizertofabricatedye-sensitizedsolarcells,andtheeffectsofchenodeoxycholicacid(CDCA)asacoadsorbentandthedyeadsorptiontemperatureonthesolarcell’sperformancewereinvestigated.ItwasfoundthattheTiO2electrodeimmersinginto5.0×10−5MZn-tri-PcNcethanolsolutionscontaining7.5mMCDCAat5◦Cfor12histheoptimaldye-sensitizationcondition,andthemaximumconversionefficiency(Á)ofthefabricatedsolarcellachieves2.%,improvedby47%ascomparedtothesolarcellfabri-catedwithTiO2electrodesensitizedbytheZn-tri-PcNcsolutionin

350L.Yuetal./ElectrochimicaActa111 (2013) 344–350

absenceofCDCA.Theco-graftedCDCAonTiO2electrodeefficientlydiminishedthedyeaggregationontheTiO2electrodealthoughitalsoreducedtheloadingamountofZn-tri-PcNc,whichisprovedtobelimitedeffectonthephotocurrentdensity.Advantageously,theadditionofCDCAcanenhancetheelectroninjectionefficiencyandretardthechargerecombination,andthereforeresultingintheimprovedcellefficiencywithenhancedphotocurrentdensityandphotovoltage.Moreover,itwasfoundthattheoptimumamountofCDCAwas7.5mMin5×10−5Mdyesolutionandtheopti-mumadsorbedtemperaturewas5◦C.Theaboveresultsprovideimportantinsightintotheeffectofcoadsorbent(CDCA)anddyeadsorptiontemperatureonthephotovoltaicperformancesofthesolarcellsfabricatedwithphthalocyanine,porphyrinororganicdye-sensitizedTiO2electrode,andwouldalsoguideustofurtherimprovethosesolarcells’conversionefficiencyinthefutureinves-tigations.

Acknowledgements

ThisworkwassupportedbytheNaturalScienceFoundationofChina(21271146,21271144,20871096,20901061),andtheBeijingNationalLaboratoryforMolecularSciences(BNLMS),China.

AppendixA.Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound,intheonlineversion,athttp://dx.doi.org/10.1016/j.electacta.2013.08.031.

References

[1]M.García-Iglesias,J.H.Yum,R.Humphry-Baker,S.M.Zakeeruddin,P.Péchy,P.

Vázquez,E.Palomares,M.Grätzel,M.K.Nazeeruddin,T.Torres,Chem.Sci.2(2011)1145–1150.[2]L.M.Gonc¸alves,V.Z.Bermudez,H.A.Ribeiro,A.M.Mendes,EnergyEnviron.Sci.

1(2008)655–667.

[3]M.Grätzel,Acc.Chem.Res.42(2008)1788–1798.

[4](a)H.Imahori,T.Umeyama,S.Ito,Acc.Chem.Res.42(2009)18091818;

(b)F.D’Souza,O.Ito,Chem.Soc.Rev.41(2012)86–96;

(c)J.N.Clifford,E.Martínez-Ferrero,A.Viterisi,E.Palomares,Chem.Soc.Rev.40(2011)1635–16.

[5]

(a)P.Y.Reddy,L.Giribabu,C.Lyness,H.J.Snaith,C.Vijaykumar,M.Chan-drasekharam,M.Lakshmikantam,J.H.Yum,K.Kalyanasundaram,M.Grätzel,M.K.Nazeeruddin,Angew.Chem.Int.Ed.46(2007)373–376;

(b)J.J.Cid,M.García-Iglesias,J.H.Yum,A.Forneli,J.Albero,E.Martínez-Ferrero,P.Vázquez,M.Grätzel,M.K.Nazeeruddin,E.Palomares,T.Torres,Chem.Eur.J.15(2009)5130–5137.

[6]

(a)S.Mori,M.Nagata,Y.Nakahata,K.Yasuta,R.Goto,M.Kimura,M.Taya,J.Am.Chem.Soc.132(2010)4054–4055;

(b)M.E.Ragoussi,J.J.Cid,J.H.Yum,G.Torre,D.D.Censo,M.Grätzel,M.K.Nazeeruddin,T.Torres,Angew.Chem.Int.Ed.51(2012)43754378;

(c)M.Kimura,H.Nomoto,N.Masaki,S.Mori,Angew.Chem.Int.Ed.51(2012)4371–4374.

[7]

E.Palomares,M.V.Martínez-Díaz,S.A.Haque,T.Torres,J.R.Durrant,Chem.Commun.(2004)2112–2113.

[8]

G.Pozzi,S.Quici,M.C.Raffo,C.A.Bignozzi,S.Caramori,M.Orlandi,J.Phys.Chem.C115(2011)3777–3788.

[9]

N.R.Neale,N.Kopidakis,J.Lagemaat,M.Grätzel,A.J.Frank,J.Phys.Chem.B109(2005)23183–231.

[10]

H.Chen,H.Huang,X.Huang,J.N.Clifford,A.Forneli,E.Palomares,X.Zheng,L.Zheng,X.Wang,P.Shen,B.Zhao,S.Tan,J.Phys.Chem.C114(2010)3280–3286.[11]

J.H.Yum,S.Jang,R.Humphry-Baker,M.Grätzel,J.J.Cid,T.Torres,M.K.Nazeeruddin,Langmuir24(2008)5636–50.

[12]

F.Sauvage,J.D.Decoppet,M.Zhang,S.M.Zakeeruddin,P.Comte,M.Nazeerud-din,P.Wang,M.Grätzel,J.Am.Chem.Soc.133(2011)9304–9310.

[13]

L.J.Yu,X.L.Zhou,Y.H.Yin,Y.W.Liu,R.J.Li,T.Y.Peng,ChemPlusChem77(2012)1022–1027.

[14]J.Lim,Y.S.Kwon,T.Park,Chem.Commun.47(2011)4147–4149.

[15]

(a)J.He,G.Benko,F.Korodi,T.Polivka,R.Lomoth,B.Åkermark,L.Sun,A.Hagfeldt,V.Sundstrom,J.Am.Chem.Soc.124(2002)4922–4932;

(b)J.H.Yum,S.J.Moon,R.Humphry-Baker,P.Walter,T.Geiger,F.Nüesch,M.Grätzel,M.K.Nazeeruddin,Nanotechnology19(2008)424005.

[16]

B.E.Hardin,J.H.Yum,E.T.Hoke,Y.C.Jun,P.Péchy,T.Torres,M.L.Brongersma,M.K.Nazeeruddin,M.Grätzel,M.D.McGehee,NanoLett.10(2010)3077–3083.

[17]

(a)X.Jiang,T.Marinado,E.Gabrielsson,D.P.Hagberg,L.Sun,A.Hagfeldt,J.Phys.Chem.C114(2010)2799–2805;

(b)A.J.Frank,N.Kopidakis,J.Lagemaat,Coord.Chem.Rev.248(2004)1165–1179.

[18]

J.H.Yum,P.Walter,S.Huber,D.Rentsch,T.Geiger,F.Nüesch,F.D.Angelis,M.Grätzel,M.K.Nazeeruddin,J.Am.Chem.Soc.129(2007)10320–10321.

[19]

R.Kern,R.Sastrawan,J.Ferber,R.Stangl,J.Luther,Electrochim.Acta47(2002)42134425.

[20]T.Y.Peng,K.Fan,D.Zhao,J.N.Chen,J.Phys.Chem.C114(2010)2346–22351.[21]

M.Adachi,M.Sakamoto,J.Jiu,Y.K.Ogata,S.J.Isoda,J.Phys.Chem.B110(2006)13872–13880.

[22]

A.Zaban,M.Greenshtein,J.Bisquert,Chem.Phys.Chem.4(2003)859–8.