您的当前位置：首页 Electrochemical methods for the characterization and interfacial study of dye-sensitized solar cell

Electrochemical methods for the characterization and interfacial study of dye-sensitized solar cell

来源：乌哈旅游

Sci.Bull.(2015)60(9):850–863DOI10.1007/s11434-015-0769-0www.scibull.comwww.springer.com/scp

ReviewChemistry

Electrochemicalmethodsforthecharacterizationandinterfacialstudyofdye-sensitizedsolarcell

DajiangZheng•MeidanYe•XiaoruWenNanZhang•ChangjianLin

•

Received:1December2014/Accepted:8January2015/Publishedonline:26March2015ÓScienceChinaPressandSpringer-VerlagBerlinHeidelberg2015

AbstractDye-sensitizedsolarcell(DSSC)isoneofthemostrapidlydevelopedsolarcellsinthepast20years.Manycharacterizationmethodshavebeenemployedforfurtherunderstandingtheoperationaldetailsofthephoto-electricconversioninDSSCaswellastheevaluationofcellperformance.Electrochemicalmethodshavebecomepow-erfultoolsforstudyingthechargetransferandinterfacialprocess.Inthisreview,weintroduceandexplainthevariouselectrochemicalmethodsusedtocharacterizeandanalyzeDSSC,includingcurrent–voltage(I–V)scanmeasurement,cyclicvoltammetry,electrochemicalimpedancespec-troscopy,intensity-modulatedphotocurrentspectroscopy,andintensity-modulatedphotovoltagespectroscopy.Inad-dition,someapplicationswereprovidedassamplestoelucidateelectrontransferkinetics,energylevelsandelectrocatalyticactivityofthematerialsusedinDSSC.KeywordsElectrochemicalmethodsÁ

Dye-sensitizedsolarcellsÁCharacterizationÁPerformanceÁMechanism1Introduction

Dye-sensitizedsolarcell(DSSC)isattractiveworldwideasapromisinglow-costsolarcell.Sincetheﬁrstbreakthrough

D.ZhengÁX.WenÁN.ZhangÁC.Lin(&)

StateKeyLaboratoryofPhysicalChemistryofSolidSurfaces,DepartmentofChemistry,CollegeofChemistryandChemicalEngineering,XiamenUniversity,Xiamen361005,Chinae-mail:cjlin@xmu.edu.cn

M.Ye

ResearchInstituteforSoftMatterandBiomimetics,SchoolofPhysics,MechanicalandElectricalEngineering,XiamenUniversity,Xiamen361005,ChinabyO’ReganandGratzel[1]in1991,thisnewtypeofsolarcellwentthrougharapiddevelopmentinthepasttwodecades,reachingaphotoelectricconversionefﬁciencyinexcessof13%in2014[2].Overthisperiod,manynewmaterialshavebeendesignedandsynthesizedtobeusedtoimprovetheefﬁciencyofDSSC.Theseincludesemicon-ductoroxides,organicsensitizers,redoxcouplesandelectrocatalystsforcounterelectrodes,allofwhichhavebeenextensivelyreviewedelsewhere[3–17].Inparticular,varioustheoreticalstudieshavealsobeenconductedtoelucidatethefundamentalprocessestakingplaceinDSSC[18–24].DSSCisactuallyaphotoelectrochemicalsolarcellinvolvedcomplexphotoexcitedreactions,electro-chemicalreactions,electrontransportsatvariousinterfacesinthecell,photocatalyticalreaction.Theinterfacialener-geticsandkineticsarevitallyimportantinDSSC,asitallowsustoknowhowtoreduceenergylossandachieveahighefﬁciency,bothofwhicharestronglydependentontheaccuratedeterminationofenergylevelsandthefullunderstandingofreactionkineticsatcellinterfaces.

Avarietyofcharacterizationmethodshavebeende-velopedtoevaluatetheperformanceofDSSC,aswellastofurtherunderstandtherelevantreactionprocessmechan-ism,suchaselectrontransport,chargetransferandelec-tron–holerecombination[17,24–28],andconsequentlytoreasonablydesignnovelmaterialsforcells[29–31].Elec-trochemicalandphotoelectrochemicalmethodshavebeenfoundtobethemostpowerfultoolsinthedevelopmentofhigh-performancecellsandintheunderstandingofthereactionsmechanisminvolved.Mostphotoelectrochemicalmethodshavebeenusedinthestudyofelectrontransportandchangertransferprocess,appearinginrecentreviews[25,26,28,32,33].However,theprincipleandanalysisoftheelectrochemicalmethodsappliedinDSSCusuallyre-mainsunclearandambiguous.Inthisreview,welook

123

Sci.Bull.(2015)60(9):850–863851

criticallyattheapplicationofelectrochemicalmethodsinthestudyoftheoverallsolarcellandsinglepartofDSSC.Theyincludeconventionalelectrochemicalmethods,suchascurrent–voltage(I–V)dynamicpolarization,cyclicvoltammetry(CV),electrochemicalimpedancespec-troscopy(EIS),andsomenewlydevelopedelectrochemicalfrequency-domainmethods,suchasintensity-modulatedphotocurrentspectroscopy(IMPS)andintensity-modulatedphotovoltagespectroscopy(IMVS).ThesemethodshaveprovedextremelyusefulintheevaluationoftheoverallperformanceofDSSCandtheirseparatedcomponents,helpingustostudyingtheelectrontransferdynamicsandwellastoelucidatetheenergylevelsandelectrocatalyticactivity.

2OverviewofDSSC

DSSCismadeupoffourmaincomponents:asemicon-ductorphotoanode(e.g.,TiO2),adyesensitizer(e.g.,N719),anelectrolytecontainingaredoxcouple(e.g.,I-/I3-),andacounterelectrode(e.g.,Pt-coatedconductiveglass).Figure1showsthebasicstructureandworkingprincipleofatypicalDSSC.Thesemiconductorphotoan-ode,believedtobethemostvitalpartofaDSSC,isusuallyintheformofananoporousﬁlmwithathicknessof10–20lm.Itisusuallymadeupofsemiconductoroxidenanoparticlesonatransparentconductiveglass,whichnotonlyhasaverylargespeciﬁcsurfaceareaforgettingahighadsorptionofdyemolecules,butalsooffersanelectronictransmissionroutefortransportingtheinjectedelectronsfromphotoexciteddyemoleculartotheexternalcircuit.Thesecondcomponentisthedyesensitizer,withmono-layerchemicaladsorptionontothesemiconductornano-porousﬁlm,ismainlyusedtoabsorbsunlightandthenproduceexcitedelectron–holepairs.Theexcitedelectrons

areinjectedintothesemiconductorphotoanodetorealizetheseparationoftheelectron–holepairs.Theholesleftinthegroundstateofthedyemoleculesareeitherreducedbytheoxidizingsubstancesintheelectrolyteortransportedtotheholetransportmaterials.Thethirdcomponentisanelectrolytecontainingredoxcouplesorasolidholetrans-portmaterial,whichhavetheabilitytoregeneratedyemoleculesfromtheirexcitedstatestotheirgroundstatesbyreducingsubstancesintheelectrolyte(suchasI-)orbyprovidingholetransportpaths.Thefourthandﬁnalcom-ponentisthecounterelectrode,whichisusedforelectro-catalyticallyreducingtheoxidizingsubstances(suchasI3-)toformreducingsubstancesintheelectrolyteusingtheelectronsfromexternalelectroniccircuit.Thus,theelectrontransferprocessesarecompletedandthecellop-erationismaintained.TheopencircuitpotentialofaDSSCunderlightispredominantlydeterminedfromthediffer-encebetweenthequasiFermilevelinthesemiconductorandtheredoxpotentialintheelectrolyte.

InthetypicalDSSC,thebasicelectrontransferprocesscanbedescribedasinFig.1.Firstly,electronsaretrans-ferredfromthelowestunoccupiedmolecularorbital(LUMO)tothehighestoccupiedmolecularorbital(HOMO)inthedyewhentheyareilluminated.Theyaretheninjectedintotheconductionbandofthesemicon-ductorandsubsequentlytransportedtothetransparentconductiveglassthroughthephotoanode.Theelectronsthenpassthroughanexternalcircuitandarriveatthecounterelectrode.Here,reductionofoxidizingsubstances(e.g.,I3-)occursatthecounterelectrode/electrolytein-terfaceandtheproduct(e.g.,I-)asreducingsubstances,diffusetothephotoanodetooxidizetheexciteddyeandthuscompletethecycle.

However,inthisprocess,therehastoexistadversebackreactions,preventingtheelectronsharvestbythecollectingelectrode(conductiveglass).Forexample,theinjectedelectronsintheconductionbandofthesemiconductorcouldbecapturedbytheholesintheexciteddyeorbytheoxidizingspeciesintheelectrolyte,andtherecombinationofelectronandholealsooccursatinterfacesinthepho-toanode.Theseprocessesresultinaloweringoftheenergyconversionefﬁciencyandthereforetheoverallperfor-manceoftheDSSC.

3I–Vmeasurement

I–Vcharacteristiccurvesareusedtomeasurethephoto-electricconversionefﬁciencyofsolarcells.FromI–Vcurves,wecangettheopencircuitpotential(Voc),shortcircuitcurrent(Isc),andthemaximumoutputpowerpoint(IVmax)ofasolarcell.Thephotoelectricconversionefﬁ-ciency(g)canthenbecalculatedasfollows:

Fig.1(Coloronline)StructureandoperationprincipleofaDSSC

123

852Sci.Bull.(2015)60(9):850–863

g¼

IVmaxJscVocFF

¼;

PinPinA

ð1Þ

wherePinisthepowerdensityofincidentlight,Aistheactive

surfaceareaofthesolarcell,JscistheshortcircuitcurrentdensityandFFistheﬁllfactor.Usually,theI–Vcurveisobtainedusingalinearsweepwithastepwisechangeinex-ternalpowersupplywitheverystephavingasamplingdelaytime.Whencomparedwithconventionalsolarcells,theI–VcurveofDSSCismoresensitivetothesamplingdelaytimeasaconsequenceofdifferingoperationalprinciples.Thatis,DSSCisaphoto-chemicalsolarcell,whichneedsalongerresponsetimetoachievethesamephotoelectricconversionasaconventionalsolarcell.Hanandco-workers[34,35]in-vestigatedthedependenceofI–Vcurvesondifferentsweepconditions,includingvoltagesweepdirectionandsamplingdelaytime.Theydeﬁnedthevoltagesweepdirectionfromshortcircuittoopencircuitasnormalscanandanotherasreversescan.TheirresultsshowedthatagreaterVocandFFwereobtainedfromthereversescan,whiletheJscremainedthesamewhenacetonitrilewasusedasanelectrolytesolvent.Withanincreaseofsamplingdelaytime,thechangeinVocandFFreduced,andthephotoelectricconversionefﬁciencywasleftalmostunchangedwhenthesamplingdelaytimemorethan40ms.TheeffectofvoltagesweeptimehavealsobeenstudiedbyAokietal.[36]andTakagietal.[37],withsimilaroutcomes.TheypointedoutthatitcanbeindividualfactorsthatinﬂuencetheresponsivetimeofI–Vcurves,includingthicknessofﬁlm,sizeofnanoparticles,viscosityoftheliquidelectrolyte,andthechargediffusioncoefﬁcientofsolidelectrolyte.Whenastableandhighviscosityionicliquidwasusedasanelectrolytesolvent,thesamplingdelaytimewasfoundtoalsoaffectJscvalue.InourI–Vmeasurement,thesameobservationwasmadeandFig.2ashowedtheI–VcurvesforaDSSCwithanionicliquidelectrolytewithnormalscanandreversescanunder100mW/cm2oflightillumination.AlloftheperformanceparametersofDSSCincludingVoc,JscandFFfromnormalscanswerelowerthanthoseforreversescans.Withanincreaseinthesamplingdelaytime,theJscoftheDSSCfrombothnormalandreversescanwasgraduallydecreased,andoneoftheseisshowninFig.2b.Therefore,ithasbeenfoundthatalongersamplingdelaytimeisrequiredforareliableI–VcurvemeasurementinDSSC,whichisusuallymorethan40mswhereanacetonitrile-basedliquidelectrolyteisused.IfahighviscosityionicliquidwasusedinDSSC,thesamplingdelaytimeshouldbelonger,dependingontheviscosityofionicliquid.

4Cyclicvoltammetry

CVisapotentiodynamicelectrochemicalmethod.Itisoftenusedtostudyvariousredoxprocessestodetermine

thereversibilityofareaction,thepresenceofintermediatesinredoxreactions,electrontransferkinetics,thestabilityofreactionmediumsandmanymorereactionparameters.InDSSC,CVtendstobeusedtodeterminethedensityofstates(DOS)ofthenanoporoussemiconductorﬁlms[38,39]andoftheorganicholetransportmaterialﬁlms[40,41],estimatetheenergylevelsofthedyemolecules[2,42–44]andtheredoxcouples[45,46],andevaluatetheelectrocatalyticactivityofthecounterelectrodes[47–49].ItiswellknownthattheDOSdescribesthenumberofstatesavailableforoccupationbyelectronsatanenergylevelwhichwillaffecttheattainablemaximumphoto-voltageandthechargetransferkineticsofelectronsinDSSC.Fabregat-Santiagoetal.[38]developedamodeltoaccountforthefundamentalcharacteristicsofnanoporoussemiconductorelectrodes,whichcanbeidentiﬁedandclassiﬁedbymeansofCV.Accordingtothismodel,theydescribedsomesimplemethodstoquantifytheDOSandabsoluteenergylevelsofnanoporousTiO2electrodesbyﬁttingexperimentalCVdatatotheirowndata.Theycal-culatedtheDOSinthebandedgetailandenergylevelsEcofnanocrystallineTiO2inaqueoussolutionatpH3and11fromCVdata.Theresultscorrelatedwellwiththoseob-´a-Can˜adasetal.[41]tainedusingEIS.Inaddition,Garcı

usedCVtoestablishenergylevelsoftheholeconductorspiro-OMeTADandfoundthatitsenergylevelswerechangedwhenitwasdepositedonindium-dopedtinoxide(ITO)toformaﬁlm.TheirresultsalsoshowedthattheHOMOlevelofoxidizedspiro-OMeTADwaswellcoupledwiththeHOMOleveloftheN719dyeusedintheDSSCandthatthedopinglevelmeantthatthematerialhadahighholeconcentrationallowingittoactasaneffectivesolidelectrolyteinthesolid-stateDSSC.

Conventionally,thedyemoleculesusedinDSSCshouldhaveamorenegativeLUMOlevelthantheconductionbandedgeofthesemiconductorphotoanode(suchasTiO2,Ecb)inordertoensureaneffectiveelectroninjectionpro-cessfromtheexciteddyemoleculestotheconductionbandofthesemiconductorphotoanode.Meanwhile,theHOMOlevelofdyemoleculesshouldbemorepositivethantheredoxpotentialoftheredoxcoupleintheelectrolytetoensureregenerationoftheexciteddyemolecules.Inordertomeasuretheenergylevelsofthedyemolecules,thebandgaps(Eg)aremeasuredusingtheUV–Visabsorptionspectra,whiletheHOMOandLUMOlevelsareestimatedfromCVusinganinternalstandardmethod.Subsequently,theLUMOorHOMOlevelcanbecalculatedbytheequation:ELUMO=EHOMO-EgorEHOMO=ELU-MO?Eg.FortheCVmeasurement,thedyemoleculesareﬁrstlydissolvedinaninertelectrolyte,whichtendstobeanorganicsolution(commonsolventsinclude:acetonitrile,dimethylformamide,dichloromethane,tetrahydrofuranandsoon)combining0.1mol/Ltetrabutylammonium

123

Sci.Bull.(2015)60(9):850–863853

Fig.2(Coloronline)TheI–VcurvesofDSSCwithionicliquidelectrolyte(a)innormalscan(from-0.2to1.0V)andreversescan(from1.0to-0.2V)under100mW/cm2lightillumination,(b)inreversescan(from1.0to-0.2V)withdifferentdelaytime.Theredoxelectrolyteusedinthestudywasanionicliquidcontaining0.60mol/L1-butyl-3-methylimidazoliumiodide(BMIM-I),0.03mol/LI2,0.50mol/L4-tert-butylpyridine(TBP),and0.10mol/Lguanidinthiocyanate(GTC)inamixtureofacetonitrileandvaleronitrile(v:v=85:15)

hexaﬂuorophosphate(TBAPF6),withaferrocene/ferroce-nium(Fc/Fc?)redoxcoupleasaninternalreference.Generally,theworkingelectrodeisaninertelectrode,suchasglassycarbonandgoldelectrode,Ag/Ag?isoftenap-pliedasthereferenceelectrode,andPtisoftenusedasthecounterelectrode,toformathreeelectrodetestingsystem[50–53].BeforetheCVtest,theO2inthesolutionshouldberemovedbybubblingargongas.Figure3isatypicalcyclicvoltammogramofdyemolecules[53].Inthiscyclicvoltammogram,thereisareversibleoxidationwaveonthepositivevoltage.TheHOMOlevelofthisdyecanbecalculatedbytheequation:HOMO=-e(Eox-Eox)Fc=Fcþ-4.77eV(Ag/AgClasreferenceelectrode,theEFc=Fcþ=4.77eV[54]).Onthenegativevoltage,therearetworeversiblereductionwaves,andinthesameway,theLUMOlevelofthisdyecanbecalculatedbytheequation:LUMO=-e(Ered-Ered)-4.77eV,wheretheEredisFc=Fcþ

theﬁrstreversiblereductionwavenearthepositivevoltage.

Theredoxpotentialofthedyeinsolutionisusuallyun-changedbypH.However,bymeansofCV,Zabanetal.[42]showedthatwhenthedyeisadsorbedontothesurfaceofthephotoanode(TiO2),itsredoxpotentialbecomepH-dependent.

CVcanbealsousedtoinvestigatetheelectrontransferkineticsandtocalculatediffusioncoefﬁcientsofredoxcouplesatcontactelectrodes,suchasthePtcounterelectrodeinDSSC.Cameronetal.[45]investigatedtheelectrontransferkineticsoftheredoxmediatorCo(III)/Co(II)(db-bip)2(dbbip=2,6-bis(10-butylbenzimidazol-20-yl)pyridine)inamixedacetonitrile/ethylenecarbonatesolventusingaplatinumandgoldinlaiddiskelectrodeastheworkelectrode.Theyfoundthecyclicvoltammogramhadaquasi-reversiblewavewiththeexchangecurrentdensityatthePtelectrodelowerthantheconventionalI3-/I-couple,indicatingrelativelyslowelectrontransferkinetics.Inaddition,thediffusioncoefﬁcientoftheCo(III)/Co(II)complexwases-timatedbylookingattheoxidationpeakcurrentdensityandthesweepratewasfoundtobeminimal.Bothofthesewerefoundtobelimitedintheirachievementofahigh-perfor-manceDSSCwiththeCo(III)/Co(II)redoxmediator.

ThecommoncounterelectrodeusedinDSSCisPt/FTO,whichexhibitshighphotoelectricconversionefﬁciency.However,Ptisahigh-costnoblemetalandmaynotbeeffectiveforotherredoxcouplesexcepttheI-/I3-couple[48,55,56].Therefore,avarietyofmoreeconomicalcounterelectrodeswithhighelectrocatalyticactivityhavebeendevelopedtotakeplaceoftheconventionalPtelec-trode[47–49,57].OneeffectivemethodtoevaluatetheelectrocatalyticactivityofcounterelectrodesisbyusingCVmeasurement.Figure4givesatypicalcyclicvoltam-mogramoftheoxidationandreductionofanI-/I3-redoxcoupleusingPtandfunctionalizedgraphenesheets(FGS)

123

854Sci.Bull.(2015)60(9):850–863

ascatalyticelectrodes[47].Usually,agoodelectrocatalyticelectrodeforanI-/I3-redoxcouplehastwopairsofredoxpeaks,withthehighpotentialone(Aox/red)attributedtotheredoxreactionofI2/I3-,andtheother(Box/red)attributedtotheredoxreactionofI-/I3-.Theequationisasfollows[58]:

3I2þ2eÀ$2IÀ3;

ÀÀIÀ3þ2e$3I:

ð2Þð3Þ

Inacyclicvoltammogram,thepeaktopeakseparation(DEp)isnegativelycorrelatedwithastandardelectrochemicalreactionrate,whichisrelatedtotheredoxreactionratesofI2/I3-andI-/I3-.Inaddition,theelectrontransferratecanbecalculatedfromDEpbythetheorygivenbyNicholson[59]in1965.ThetheorystatesthatwhenDEpissmaller,theredoxreactionisfaster,indicatingahigherelectrocatalyticactivity[59,60].InFig.4,thePtelectrodeshowedasmallerDEp,correspondingtoabettercatalyticactivityoftheI-/I3-coupleandahigheroverallperformanceoftheDSSC.Inaddition,thepolishedtitaniumelectrodedidnotshowanyredoxpeak,suggestingthatitdidnothaveaproperelectrocatalyticactivityfortheI-/I3-reaction.

Weevaluatedtheelectrochemicalcatalyticactivityof

-Cu2S-basedcounterelectrodesforS2-/S2xsystembyCVanalysis,andtheresultswereshowninFig.5[61].Inthecyclicvoltammogram,thereductionpeaksinthelowpo--tentialrepresentedthereductionofS2toS2-andthexoxidationpeakinthehighpotentialrepresentedthe

oxidationofS2-ionsinthepolysulﬁdeelectrolyte.Here,thereductionpeakcurrentsinthelowpotentialaredirectindicationsoftheelectrocatalyticabilityofthecounter

-electrodeforS2reduction.ThereductionpeakcurrentxdensitiesofCu2SandRGO–Cu2SelectrodeswereevidentlylargerthanthoseofthePtelectrode,indicating

-theyhadbetterelectrocatalyticactivityforS2xreduction.Aboveall,wecanﬁndtheCVmethodiswidelyusedinallpartsofDSSC,asaneffectivemethodfortherapidevaluationofcomponentsandmaterialsforDSSC.5Electrochemicalimpedancespectroscopy

EISisanimportantelectrochemicaltechniquewithwide-spreadapplication.Initially,itwasusedfordeterminingresistanceanddouble-layercapacitance.Now,EISisap-pliedtomorecomplicatedsystem,suchaselectrodepro-cessesandcomplexinterfaces.InDSSC,EIShasbecomeapowerfultoolusedtoinvestigatethekineticsofchargetransportandelectron–holerecombination[62–72]andto

Fig.4Cyclicvoltammogramsobtainedatascanrateof50mV/sfortheoxidationandreductionoftheI2/I3-andI-/I3-redoxcouplesusingpolishedtitaniumrodelectrodesloadedwith12lgofplatinumandFGS13.Thevoltammogramforablankpolishedtitaniumrodisalsoshown.AoxandAredindicatetheoxidativeandreductivepeaksfortheI2/I3-couple,whileBoxandBredindicatetheoxidativeandreductivepeaksfortheI-/I3-couple.ReprintedwithpermissionfromRef.[47].Copyright2010AmericanChemicalSociety

Fig.5(Coloronline)aCyclicvoltammogramofPt,Cu2SandRGO–Cu2SinamixtureofmethanolandDIwater(3:7,v:v)containing0.5mol/LNa2S,0.2mol/LS,and0.2mol/LKCl.bMagniﬁedcyclicvoltammogramofPtcounterelectrodefrom(a).ReprintedwithpermissionfromRef.[61].Copyright2014WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim

123

Sci.Bull.(2015)60(9):850–863855

evaluatetheelectrocatalyticactivityofcounterelectrodes[47–49,73].

Earlyon,EISwasemployedtoanalyzethepotentialdistributioninthenanoporousTiO2electrodes[72,74],andtheresultsshowedthattheappliedpotentialwasunequallydistributedthroughoutthenanoporousTiO2–electrolyteinterface,whichwouldmakesomeimplicationsfortheoperationofDSSC.In2001,aﬁnitesetofpossiblebe-haviorsinthefrequencydomainwithinathinlayerwaspresumedandreportedbyBisquert[68].Thiswasdoneusingadiffusion-recombinationmodelwithfourtypesofboundaryconditions.Sincethen,thisnovelmodelhasbeenwidelyusedtoanalyzeEISdataofDSSC.InFig.6,wecouldseethecompletedgeneraltransmissionlinemodelofDSSC[63].ForanoptimizedDSSC,athinTiO2compactlayerispreparedtoinsulatethetransparentconductiveoxide(TCO)surfaceandelectrolyte,andtheconnectionbetweenTCOandTiO2isextremelycompact.Sothecharge-transferresistanceandthecorrespondingdouble-layercapacitanceattheexposedTCO–electrolyteinterface(RTCO/CTCO)andattheTCO–TiO2contact(RCO/CCO)canbeignored.ThemodelcouldbesimpliﬁedfurtherandatypicalEISspectrumisalsoshowninFig.7[70].InaNyquistplot,thereareasmallsemicircleinthehigh-fre-quencyrange(theleftintheNyquistplot)andalargesemicircleinthelow-frequencyrange(themiddleintheNyquistplot).Occasionally,athirdsemicircleappearsinthelow-frequencyregion(therightintheNyquistplot).Thesmallsemicircleiscorrelatedtotheelectrolyte/Ptcounterelectrodeinterface,andthelargersemicircleisassociatedwiththeporousTiO2/electrolyteandtheTCO/TiO2interface.Inaddition,thelargersemicircleispre-dominantlyassociatedwithchargetransferinthe

TiO2/electrolyteinterface,seenfromfurtheranalysisoftheappliedbiasdependenceoftheresistanceandcapacitanceofthelow-frequency(large)semicirclefromtheEISdata[70].Thethirdsemicircleiscorrelatedwiththediffusionprocessinthecell.Theimpedancefunctioncanthereforebeexpressedas[68,75,76]:

\"󰀂󰀃1=2󰀂󰀃1=2#

1RtRctix

Z¼cothðxk=xdÞ21þ;ð4Þ

xk1þix=xkwhereRtistheelectrontransport(diffusion)resistance,Rctisthechargetransfer(recombination)resistance,xistheangularfrequencyandi=(-1)1/2;xdisthecharacteristicfrequencyofdiffusioninaﬁnitelayer(xd=Dn/L2=1/(RtCl))andxkistherateconstantforrecombination(xk=1/(RctCl))

Forahigh-performanceDSSC,RctismuchlargerthanRt.Inthiscase,theimpedancespectrumwillhaveasmallWarburgpartathighfrequencyandalargesemicircleatlowfrequency,soEq.(4)canbereducedtoexpressedas1RctZ¼Rtþ:

31þix=xk

ð5Þ

Ontheotherhand,foralow-efﬁciencyDSSC,Rt[[Rct,thegeneralimpedancebecomesGerischerimpedanceandtheEq.(4)canbereducedtoexpressedas

󰀂󰀃1=2

RtRct

Z¼:ð6Þ

1þix=xkTherefore,Rt,Rct,andClcouldbeobtainedbyﬁttingtheEISdata.Inaddition,variousimportantevaluationparameters,suchaselectrondiffusioncoefﬁcient(Dn),electronlifetime(sn)andelectrondiffusionlength(Ln),couldalsobecalculatedbytheequations:Dn=L2/(RtCl),sn=RctCl,Ln=(Dnsn)1/2,respectively[63].

Conventionally,theEIStestinDSSCiscarriedoutatopencircuitpotentialorlowpotentials(\\0.8V),inordertoobtaincomparableuniformitytoarealsystem.Wangetal.[71]performedEISmeasurementsonDSSCbothindarkandunderilluminationatopencircuitpotential,andtheyfoundthattheimpedanceofchargetransferintheTiO2/electrolyteinterfacewasfarsmallerunderillumina-tionatopencircuitpotentialthanthatunderthesamebiaspotentialindark,becausetheelectronprocessesoccurringintheDSSCindarkandunderilluminationweredifferent,leadingtoadifferentlocalconcentrationofoxidizingsubstances(I3-).Whenthedevicewasoperatedundertheilluminationoflightatopencircuitpotential,therewasnonetcurrentﬂowingthroughtheDSSC.AlloftheelectronsintheconductionbandoftheTiO2injectedfromtheex-citeddyesarethoughttoberecombinedbytheoxidizingsubstances(I3-)intheelectrolyte,andtheexciteddyesareregeneratedbythereducingsubstances(I-).Inthiscase,

Fig.6(Coloronline)GeneraltransmissionlinemodelofDSSC.rctisthecharge-transferresistanceofthechargerecombinationprocessbetweenelectronsinthemesoscopicTiO2ﬁlmandI3-intheelectrolyte;ClisthechemicalcapacitanceoftheTiO2ﬁlm;rtisthetransportresistanceoftheelectronsintheTiO2ﬁlm;ZdistheWarburgelementshowingtheNernstdiffusionofI3-intheelectrolyte;RPtandCPtarethecharge-transferresistanceanddouble-layercapacitanceatthecounterelectrode(platinizedTCOglass),respectively;RTCOandCTCOarethecharge-transferresistanceandthecorrespondingdouble-layercapacitanceattheexposedTCO–electrolyteinterface,respectively;RCOandCCOaretheresistanceandthecapacitanceattheTCO–TiO2contact,respectively;Rsistheseriesresistance,includingthesheetresistanceoftheTCOglassandthecontactresistanceofthecell.ReprintedwithpermissionfromRef.[63].Copyright2006AmericanChemicalSociety

123

856Sci.Bull.(2015)60(9):850–863

Fig.7aTypicalEISNyquistplotofDSSC.TheinsetisBodeplotforthesamedata.Thecellwasbiasedat0.7Vindark.Theredoxelectrolytecontained0.5mol/LLiI,0.05mol/LI2and0.5mol/LTBPinacetonitrile.bEquivalentcircuitofthecellconsistingofthecounterelectrode(CE)/redoxelectrolyteinterfaceandtheTiO2/redoxelectrolyteinterface,whereRCEandRctarethechargetransferresistances,RsisthesheetresistanceofTCOandPtcounterelectrodeandtheresistanceoftheelectrolyte,andCCEandCaretherespectiveconstantphaseelements

theoxidizingsubstances(I3-)canbeobtaineddirectlybydyeregenerationattheTiO2/electrolyteinterface.How-ever,forDSSCindarkunderabiaspotential,theelectronsarefromtheexternalpowersupplyandtheyareinjectedintotheconductionbandoftheTiO2.Theseelectronsarealsoreactedwithoxidizingsubstances(I3-)intheelec-trolyte,andtheconsumedoxidizingsubstances(I3-)aresuppliedatthecounterelectrode,whichrequiresamasstransferprocessbydiffusion.Fabregat-Santiagoandco-workers[63,76]measuredtheEISofhigh-efﬁciencyDSSCsatdifferentpotentialstostudythetrendsofthemainimpedanceparameters.TheresultsshowedthatbothRtandRctdecreasedwithanincreaseinthebiaspotentialsbyanexponential,whileClwasjusttheopposite.How-ever,thechangeinDnwasstillthesameasthatofRt.Theseresultscanbeexplainedwellbytheclassicalmul-tipletrappingmodel[77].TheyalsoexhibitedthedataunderalightilluminationandpointedoutthattheRtandRctvaluesweresmallerthanthoseindarkatthesomepotential,butthechangingtendsofthemwithdifferentelectrolytesystemsweresimilar.Inordertogetinforma-tiononDSSCintheboundarycondition,ahigherpotentialbeyondtheopencircuitpotentialwasusedforatestsystembyHeetal.[78].TheycarriedoutEIStestsonDSSCathighpotentials(0.925–1.225V).Inthiscase,theFermileveloftheTiO2wouldbeveryhigh,resultinginahighfreeelectrondensityintheconductionband.TheirresultsrevealedthatDnstillexhibitedaslighttendtoincreaseatthepotentialsbelow1.0V,butbothDnandsnmaintainednearlyconstantlevelsinhighpotentialranges.

Usingtheseparametersmentionedabove,researcherscancompareelectrontransportanddiffusionpropertyatdifferentphotoanodesinDSSC,suchasphotoanodesmadeofdifferentmaterials(e.g.,TiO2orcombinedoxides)

[79–87],differentmorphologiesanddifferentstructures(e.g.,nanoparticles,nanotubes,nanorods,orcomplexstructures)[88–96],differentcrystalforms(e.g.,anataseandrutileforTiO2)[97],anddifferentsubstrates(e.g.,rigidorﬂexible)[90,98].Forinstance,weusedEIStocomparetwodifferentmicrospheresphotoanode-basedDSSC,andtheobtainedNyquistplotswereshowninFig.8[99].Intheplots,threesemicirclescanbeclearlyseen,whichwererelatedtothecounterelectrode/electrolytein-terface,TiO2/electrolyteinterfaceandtothediffusionprocessofredoxsubstances,respectively.Theresistancesfromthecounterelectrode/electrolyteinterface(R1)wereclosedfromeachother,duetotheuseofthePtcounterelectrodepreparedinthesameway.Thesmallercontactseriesresistance(Rs)ofwithpolyethyleneglycol(PEG)-basedDSSCshowedabettercontactbetweentheTiO2microspheresandtheFTOsubstrate.Inaddition,thehigherresistancefromtheTiO2/electrolyteinterfacesigniﬁedalowerrecombinationrateofelectronsfromtheTiO2totheI3-intheelectrolyte,comparedtothewithoutPEG-basedDSSC.Finally,thesmallermasstransportresistance(R3)ofwithPEG-basedDSSCimpliedamoresuperiormassdiffusionchannelthanwithoutPEG-basedDSSC.AlloftheseresultsindicatedthatPEG-basedDSSChadabetterelectroncollectionefﬁciencyandledtoahigher-perfor-manceDSSC.

Besides,EISisalsoapowerfultechniqueusedtostudytheinﬂuenceofdifferenttyperedoxcouplesandelec-trolytesontheperformanceofDSSC[66,71,76,100–102].UsingEISmeasurement,Fabregat-Santiagoetal.[76]determinedtheparametersofelectrontransport,ac-cumulationandrecombinationinDSSCwiththreediffer-entelectrolytesandfoundthatelectrolytecompositionscouldaffecttheconductionbandedgebychangingthe

123

Sci.Bull.(2015)60(9):850–863857

Fig.8(Coloronline)Nyquistplot(ﬁttedbasedontheequivalentcircuitshownastheinset)ofrutileTiO2microsphereﬁlmpreparedbyachemicalbathprocessfor12hwithoutandwiththepresenceof0.50gofPEG14000,respectively.Thedataﬁttedfromthemeasurement.EISmeasurementswerecarriedoutinthedarkunderopencircuitpotential.ReprintedwithpermissionfromRef.[99].Copyright2014AmericanChemicalSociety

transportresistanceandchargeaccumulationinTiO2.Meanwhile,theyalsostudiedtheelectrontransportmechanismsinDSSCwithasolid-stateelectrolyte(spiro-OMeTAD)andfoundtheywereverysimilartotheDSSCwithaliquidelectrolyte[66].TheEISdataindicatedthattheelectronrecombinationratewasmuchhigherinthesolid-stateDSSC,resultinginalowperformanceofcelldevices.

Forthehigh-frequencyregion,asmallsemicirclerep-resentsthecharge-transferresistance(RPt)andinterfacialcapacitance(CPt)attheelectrolyte/Ptcounterelectrodeinterface,whichcanbeusedtoevaluatetheelectrocatalyticactivityofcounterelectrodesfortheredoxcouplesintheelectrolyte.FromtheﬁttedEISdata,RPtandCPtcanbeobtained.WherethevalueofRPtwasfoundtobesmaller,theelectrocatalyticactivityofthecounterelectrodeswasbetter.Inaddition,theEIScharacteristicsofthecounterelectrodeswereusuallycarriedoutusingasymmetricalelectrochemicalcell,showninFig.9a,whileFig.9bistherelatedequivalentcircuitmodel[73,103].Rsistheseriesresistance,Rctisthechargetransferresistanceatthecounterelectrodes/electrolyteinterface(therearetwoofthesameinterfacesinthissymmetricalelectrochemicalcell,2Rct),andCCEisthedouble-layercapacitanceatthecounterelectrodes/electrolyteinterface.ThevalueofRctcoulddirectlyindicatetheelectrocatalyticactivityoftheelectrodes,andthesmalleritis,thebetter.HauchandGeorg[73]usedthiscellandmodeltocharacterizetheelectrocatalyticactivityofPt/FTOelectrodesproducedusingdifferentdepositionmethodsinelectrolytescon-tainingI-/I3-redoxcoupleswithdifferentadditivesandsolvents.TheresultsshowedthattheRctofPtislowerthan5X/cm2whereacetonitrilewasusedasasolvent.ThisvaluemeansthatthePtisgoodenoughtobeusedasa

counterelectrodeinacommercialDSSC[73].WhennewcounterelectrodesforDSSCaredeveloped,theirRctvaluesareconventionallytestedbyEISandthenmakeacom-parisonwithPttoevaluatetheircatalyticactivity.Forexample,Wuetal.[48]developedavarietyofeconomicalPt-freecatalystsascounterelectrodematerialsandthencalculatedthecorrespondingRctvaluesfromEISdata.Theyobservedthatthecompositematerial(VC–MC)cat-alystcoatedelectrodehadthelowestRct(2.9X/cm2),considerablylowerthanthatofaPt/counterelectrode(4.7X/cm2)andthuswereusedascounterelectrodesforDSSC,achievingahigherphotoelectricconversionefﬁ-ciency,comparedtothePt-basedDSSC.Inourwork,wealsousedtheEISmethodtoevaluatetheelectrocatalyticactivityofdifferentcounterelectrodesforSx2-reductionbymeasuringtheRct[61,104].Theresultsshowedthatbyusingasulﬁde(e.g.,Cu2S,Co9S8)asacounterelectrode,considerablylowerRctcouldbeachievedwhencomparedtotheuseofaPt/counterelectrode,revealingthepossi-bilityofhighelectrocatalyticactivityinpolysulﬁdeelectrolyte.

Toconclude,EISisanextremelyusefulelectrochemicaltechniqueusedtostudyDSSC.Amultitudeofkeypa-rameterscanbededucedbyestablishingappropriatemodelsandequivalentcircuitstounderstandtheelectronprocessesinDSSC,includingelectrontransport,chargetransferatphotoanodeandcounterelectrode.

6IMPSandIMVS

IMPSandIMVSarephotoelectrochemicaltechniquesbasedonacontrollablemodulationofincidentlightintensity.Whenamodulationoflightisapplied,aphotocurrentor

123

858Sci.Bull.(2015)60(9):850–863

Fig.9(Coloronline)aThesymmetricalelectrochemicalcellusedfortheEISmeasurements.bTherelatedequivalentcircuitmodel

photovoltageresponsecanbemeasuredasafunctionofmodulationfrequency,whichisfairlysimilartotheEISmethod.Theywereprimarilyusedforsinglecrystalsemi-conductorelectrodes[105–107]andnanocrystallinesemi-conductorelectrodes[108,109].Theirusewasthenextendedtothestudyofelectrontransport,chargetransfer,andtherecombinationprocessesinDSSC[70,110–113].Dlocziketal.[110]developedatheoreticalanalysismodelforIMPS,whichoffersinformationonelectrontransport,electrontrapping,andtherecombinationprocessesinDSSC.Fromthetheoreticalmodel,themeasuredIMPSresponseUhasthefollowingform:UðxÞ¼UintðxÞAðxÞ;

ð7Þ

whereA(x)isaRCtimeconstantattenuationfactorgivenbyAðxÞ¼

1ÀjxRC

;

1þx2R2C2ð8Þ

redoxsubstancesintheelectrolytewereconsideredbythistheoreticalmodel.Fromthemodel,avarietyofanalyticalexpressionsforphotovoltageresponsewiththefrequencyatopencircuitvoltagewerederivedtoofferinformationofcharge-recombinationkineticsandbandedgemovementinDSSC.Thetimeconstant(s)forchargerecombinationcanbeobtainedusingthismethodbyﬁttingthephotovoltageresponsewiththefrequencyexpressedas

ÃÃÀM1ÀM2

ReðDVocÞ¼þ;2s21þx2s21þx12ÃÃ

M1xs1M2xs2

ImðDVocÞ¼þ;

1þx2s21þx2s212

ð11Þð12Þ

Uintisthephotocurrentconversionefﬁciencyconsidering

illuminationfromthesubstratesideanddiffusion-controlledlimit,deﬁnedas

ecdÀeÀcdþ2aejphotoaÁUintðxÞ¼¼

aþcqIoecdþeÀcd

Àad

ÀeÀcd

cÀa;ð9Þ

*whereM*1andM2arescalefactors.Ifonlyonesemicircle

*appearsonthefrequencyresponseregion,M*1orM2inEqs.(11)and(12)canbesettozero.

Meanwhile,PeterandWijayantha[114]usedtheelec-trondiffusionlength(Ln)toevaluatethecollectionprop-ertyofelectronsbydiffusionfromthesemiconductor(TiO2)tothesubstratewiththeeffectofbackreactionsofthephoto-injectedelectronsandoxidizingsubstances(I3-)intheelectrolytetakenintoaccount.TheLnisgivenbythefollowing:

pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃLn¼Dnsn;ð13Þ

whereaistheeffectiveabsorptioncoefﬁcient,distheﬁlm

thickness,andcisgivenby

ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃs󰀂󰀃1ixþc¼;ð10ÞDsDwhereDistheelectrondiffusioncoefﬁcientandsistheelectronlifetime.Consideringtheeffectsofelectrontrapping,whentheelectrontrapping/detrappingrateisfast,Dandscanbereplacedbytheeffectiveelectrondiffusioncoefﬁcient(Deff),andtheeffectiveelectronlifetime(seff)respectively.Usingtheseequations,theeffectsofa,D,s,andRConthephotocurrentresponsecanbeexamined.Atthesametime,atheoreticalanalysis

¨rletal.[111].modelofIMVSwasdevelopedbySchlichtho

Electrontransferandchargetrapping/detrappingfromtheconductionbandandsurfacestatesofthesemiconductorto

whereDnistheelectrondiffusioncoefﬁcientandsnistheelectronlifetime.snrepresentsthetimeconstantforchargerecombinationatopencircuitvoltagewhichcanbeob-tainedfromIMVSmeasurementandtheDnisrelatedtosdwhichrepresentsthetimeconstantforthecombinedpro-cessesofchargecollectionandrecombinationatshortcircuitcurrentwhichcanbeobtainedfromIMPSmea-surementinDSSC.Figure10givesthetypicalIMPSandIMVSplotsforDSSC,andthesdandsnvaluescanbeestimatedass=1/xmin=1/2pfminfromIMPSandIMVS,respectively.xministhefrequencyatwhichtheimaginarypartreachesamaximumvalue.AndtheDnvaluecanbeestimatedfromsdasDn=d2/(2.35sd)[115],wheredisthethicknessoftheTiO2ﬁlm.

Fromthetheoreticalanalysismodelandtheresultsmentionedabove,wecandeducethatIMPSandIMVSare

123

Sci.Bull.(2015)60(9):850–863859

usefultoolstostudytheelectrontransportmechanismandchargetransferkineticsinDSSC.Dlocziketal.[110]measuredtheIMPSresponseofahigh-efﬁciencyDSSCandobtainedvaluesforDeffandseffbyﬁttingtheresultstoatheoreticalmodel.ThedatafromIMPSindicatedthattheelectrontransportinthenanoporousﬁlmwasmuchslowerthanthatinthebulkphase.Inaddition,Schlichthorletal.[111]measuredtheIMVSresponseofdye-coatednanocrystallineTiO2electrodeswithorwithouttreatmentbyTBP-basedDSSCs.ThedatafromIMVSrevealedthatthetimeconstantbetweenthephoto-injectionandrecom-binationofelectrons(electronlifetime)wassigniﬁcantlyincreasedaftertheTBPtreatmentofelectrodes,leadingtoanincreaseinelectrondensityintheTiO2conductionbandandthusresultinginahigherphotovoltageofDSSC.Be-sides,thedatafromIMVSalsorevealedsomedetailin-formationabouttherecombination,indicatingthattheelectronrecombinationprocessfromtheconductionbandinTiO2totheredoxintheelectrolytemainlyoccurredthroughthesurfacestatesinTiO2andtheelectrontrap-ping/detrappingprocesswereconsiderablefasterthantherecombinationprocess.PeterandWijayantha[114]alsostudiedelectrontransportandbackreactionprocessesinDSSCunderilluminationwithdifferentlightintensitiesusingIMPSandIMVS.TheresultsshowedthatDnin-creasedwithanincreaseinlightintensity,indicatinganimprovementintheelectrondiffusionrate.Furthermore,sndecreasedwithanincreaseinlightintensity,implyinganincreaseinthebackreactionrate.BothDnandsnaredoublelogarithmiclinearlydependentonlightintensity,whichcanbeexplainedbytheinﬂuenceofthetrapoccu-pancyandsurfacestatesinTiO2.However,forLn,thereisonlyaweaklydependenceonlightintensity,suggestingthebackreactionkineticsofelectronswithI3-maybesecondorderinelectrondensity.

Furthermore,IMPSandIMVSarewidelyusedforevaluatingdifferentphotoanodes(suchasdifferentmate-rials[116],differentcrystalforms[94,117],differentstructures[118–122]),dyes[85,123]andelectrolytes[85,124]inDSSCbycomparingtheirelectrontransportandrecombinationprocess.Parketal.[117]comparedelectrondiffusioncoefﬁcientofrutile-andanatase-basedDSSCusingIMPS,ﬁndingthattheelectrontransportinrutileTiO2ﬁlmwasoneorderofmagnitudeslowerthanthatintheanataseTiO2ﬁlm,whichrevealedtheanataseTiO2nanoparticlesweremoresuitableforDSSC.Quintanaetal.[116]alsousedIMPS,comparingtheelectrondiffusiontimeandlifetimeofZnO-andTiO2-basedDSSCs,ob-servingthattheelectrontransportinZnOandTiO2wasfairlysimilar,butthattheelectronlifetimeintheZnOﬁlmwaslongerthanthatintheTiO2ﬁlm.Thisledtotheconclusionthat,theZnOnanoparticleshadalowerelectronrecombinationrate.Zhuetal.[118]alsoinvestigatedelectrondiffusiontimesandlifetimes,lookingatbothTiO2nanoparticleandTiO2nanotube-basedDSSCs.Theyfoundthattheelectrontransportwassimilarforthetwostruc-tures,butthattheelectronlifetimeintheTiO2nanotubesﬁlmwasoneorderofmagnitudelongerthanthatintheTiO2nanoparticlesﬁlm,revealingthattheTiO2nanotubeshadalowerelectronrecombinationrate,andthenim-provedthechargecollectionefﬁciencyinDSSC.Guillenetal.[85]studiedthreedifferentZnOconﬁguration-basedDSSCconsistingoftwodyesandtwoelectrolytes.Inthiswork,EIS,IMPSandIMVSwereused.Theirresultsshowedthattheionicliquidelectrolyte-basedDSSChadshorterelectronlifetimeandelectrondiffusionlengththantheorganicsolventelectrolyte-basedDSSC,indicatingfasterrecombinationkinetics.Forthedifferentdye-basedDSSC,theyhaddifferentelectronlifetimeandelectrondiffusioncoefﬁcient,buthadacomparableelectrondiffu-sionlength,whichsuggestedthatthemainlimitationforZnO-basedDSSCwasthepoorelectroninjectionproper-tiescomparedtothoseofTiO2-basedDSSC.Krugeretal.[124]usedIMPSandIMVSanddifferentlightintensitiestostudythechargetransportandbackreactioninsolid-stateDSSCusingspiro-MeOTADassolid-stateelectrolyte.

Fig.10TypicalIMPS(a)andIMVS(b)plotsforDSSCunderblueLED(kmax=447nm)illuminationwith100mW/cm2123

860Sci.Bull.(2015)60(9):850–863

Theresultsshowedthattheelectrondiffusioncoefﬁcientincreasedandelectronlifetimedecreasedwithanincreaseinlightintensity,whichwassimilartoobservationsfortheliquidelectrolyte-basedDSSC.However,inthisstudy,theelectrondiffusionlengthremainedrelativelyconstantwiththechangeinlightintensity,suggestingthatthechangesinrateconstantforbothelectrontransportandbackreactionwerethesame.Theelectrontrappingmayhavealsoaf-fectedthechargetransfer,whichmeantthatthebackre-actionratewaslimitedbytheelectrontransportintheTiO2ratherthanbytheholetransportinthespiro-MeOTAD.Italsosuggeststhatagoodphotoanodewithfewerdefectsandsurfacestateswasrequiredforahigh-performancesolid-stateDSSC.

Deﬁnitely,IMPSandIMVSaretwopowerfultech-niquestostudyelectrontransportandchargetransferdy-namicsinDSSC.BytheanalysisofdatafromIMPSandIMVS,wecanﬁndthelimitingfactorsforelectroncol-lectingefﬁciencyandoptimizetherelatedelectrontrans-portandchargetransferprocessestoimprovetheoverallefﬁciencyofDSSC.

morepreciselydeterminedusingCVandUV–Visspec-trometryandthattheelectrocatalyticactivityofmaterialscanbeevaluatedbybothCVandEIS.Thestudyofelec-trontransferdynamicsoftenemploysmanycharacteriza-tiontechniquescombinedtogether,suchasIMPS/IMVS,photoinducedabsorptionspectroscopy,time-resolvedtransientabsorptionspectroscopy,andmanymore.There-fore,bycombiningtheseelectrochemicalmethodswithotheradvancedtechnologies,wecangainfurtherinsightsintothepropertiesofDSSC.Furthermore,electrochemicalmethodscanbeusedforothersystems,suchasquantumdotsensitizedsolarcells,organicsolarcellsandperovskitesolarcells,todeepenourunderstandingofsolarcelltechnologyasawholeandoffermoreusefulinformation.

AcknowledgmentsThisworkwassupportedbytheNationalNat-uralScienceFoundationofChina(51072170,21321062)andtheNationalBasicResearchProgramofChina(2012CB932900).Conﬂictofinterestofinterest.

Theauthorsdeclarethattheyhavenoconﬂict

References

7Conclusionsandoutlooks

ThisreviewintroducedvariousconventionalandnewelectrochemicalmethodsappliedinthestudyofDSSC.IthasbeenshownthatthesemethodsarepowerfulresearchtoolsthatcannotonlybeusedforevaluatingthepropertiesofthecompleteDSSCandtheirseparatedcomponents,butthattheycanalsobeusedtounderstandawidevarietyofthefundamentalprocessesoccurringinDSSC.DSSCtendstohaveseveralcomplicatedelectrontransportandchargetransferprocesses.Oneoftheimportantlimitingfactorstoachieveahighefﬁciencyisthatitisdifﬁculttoﬁndanefﬁcientphotoanode-dye-electrolytesystemtoreducethepotentialdropfromtheelectronrecombinationlossatthedyesensitized-semiconductorphotoanode/electrolytein-terface.Furtherunderstandingoftheseprocessesiscrucialinselectingsuitablematerials,designingproperstructures,andthenassemblingahigh-performanceDSSC.AlthoughtheenergeticsandkineticsinDSSChavebeendeeplystudiedandaquiteclearchargetransferprocesshasbeendescribed,themechanismofelectrontransportandre-combinationreactionisstillcontroversial.Therefore,afurtherchallengewillbetodevelopmorereasonablethe-oriesandmodelsforaccuratelydescribingtheelectronprocessescombinedwiththepresentmeasurementmethodstherebyimprovingtheaccuracyofmeasurement.Inaddi-tion,inordertoobtainmoredetailedinformation,variouskindsofcharacterizationmethodsareusedinconjunctiontooptimizetheirrespectiveadvantages.Previousstudieshaveshownthattheenergylevelsofdyemoleculescanbe

1.O’ReganB,GratzelM(1991)Alow-cost,high-efﬁciencysolarcellbasedondye-sensitizedcolloidalTiO2ﬁlms.Nature353:737–740

2.MathewS,YellaA,GaoPetal(2014)Dye-sensitizedsolarcellswith13%efﬁciencyachievedthroughthemolecularengi-neeringofporphyrinsensitizers.NatChem6:242–247

¨tzelM(2008)CounterelectrodesforDSC:3.MurakamiTN,Gra

applicationoffunctionalmaterialsascatalysts.InorgChimActa361:572–580

4.PoudelP,QiaoQ(2012)Onedimensionalnanostruc-ture/nanoparticlecompositesasphotoanodesfordye-sensitizedsolarcells.Nanoscale4:2826

5.YanagidaS,YuYH,MansekiK(2009)Iodine/iodide-freedye-sensitizedsolarcells.AccChemRes42:1827–1838

6.ZhangQF,CaoGZ(2011)Nanostructuredphotoelectrodesfordye-sensitizedsolarcells.NanoToday6:91–109

7.Roy-MayhewJD,AksayIA(2014)Graphenematerialsandtheiruseindye-sensitizedsolarcells.ChemRev114:6323–63488.LiZD,ZhouY,SunRZetal(2014)NanostructuredSnO2photoanode-baseddye-sensitizedsolarcells.ChinSciBull59:2122–2134

9.SunHF,PanTT,HuGQetal(2014)Rutheniumsensitizersfordye-sensitizedsolarcells.ProgChem26:609–625

10.WangJ,LinZQ(2012)Dye-sensitizedTiO2nanotubesolar

cells:rationalstructuralandsurfaceengineeringonTiO2nan-otubes.ChemAsianJ7:2754–2762

11.LuJF,BaiJ,XuXBetal(2012)Alternateredoxelectrolytesin

dye-sensitizedsolarcells.ChinSciBull57:4131–4142

12.CongJY,YangXC,KlooLetal(2012)Iodine/iodide-freeredox

shuttlesforliquidelectrolyte-baseddye-sensitizedsolarcells.EnergyEnvironSci5:9180–9194

13.WuMX,MaTL(2012)Platinum-freecatalystsascounter

electrodesindye-sensitizedsolarcells.ChemSusChem5:1343–1357

14.ZhangQF,DandeneauCS,ZhouXYetal(2009)ZnOnanos-tructuresfordye-sensitizedsolarcells.AdvMater21:4087–4108

123

Sci.Bull.(2015)60(9):850–863

15.ZakeeruddinSM,GratzelM(2009)Solvent-freeionicliquid

electrolytesformesoscopicdye-sensitizedsolarcells.AdvFunctMater19:2187–2202

16.HaoF,DongP,LuoQetal(2013)Recentadvancesinalter-nativecathodematerialsforiodine-freedye-sensitizedsolarcells.EnergyEnvironSci6:2003–2019

17.HagfeldtA,BoschlooG,SunLetal(2010)Dye-sensitizedsolar

cells.ChemRev110:6595–6663

18.MartsinovichN,TroisiA(2011)Theoreticalstudiesofdye-sensitisedsolarcells:fromelectronicstructuretoelementaryprocesses.EnergyEnvironSci4:4473–4495

19.O’ReganBC,DurrantJR(2009)Kineticandenergeticpara-digmsfordye-sensitizedsolarcells:movingfromtheidealtothereal.AccChemRes42:1799–1808

20.PeterL(2009)‘‘Stickyelectrons’’transportandinterfacial

transferofelectronsinthedye-sensitizedsolarcell.AccChemRes42:1839–1847

21.NelsonJ,ChandlerRE(2004)Randomwalkmodelsofcharge

transferandtransportindyesensitizedsystems.CoordChemRev248:1181–1194

22.DurrantJR,HaqueSA,PalomaresE(2004)Towardsoptimisa-tionofelectrontransferprocessesindyesensitisedsolarcells.CoordChemRev248:1247–1257

23.GreggBA(2004)Interfacialprocessesinthedye-sensitized

solarcell.CoordChemRev248:1215–1224

24.HalmeJ,VahermaaP,MiettunenKetal(2010)Devicephysics

ofdyesolarcells.AdvMater22:E210–E234

25.ListortiA,O’ReganB,DurrantJR(2011)Electrontransfer

dynamicsindye-sensitizedsolarcells.ChemMater23:3381–3399

26.WangYY,CaiJH,SunLetal(2012)Progressofphotoelec-trochemicalperformanceoftitaniananotubearraysforhydrogenproduction.ChinJInorgChem28:647–656

27.BarnesPRF,MiettunenK,LiXEetal(2013)Interpretationof

optoelectronictransientandchargeextractionmeasurementsindye-sensitizedsolarcells.AdvMater25:1881–1922

28.ZhangZ,ZhouBX,GeWJetal(2005)Chargerecombinationin

dye-sensitizednanoporousTiO2solarcell.ChinSciBull50:2408–2412

29.FantacciS,DeAngelisF(2011)Acomputationalapproachto

theelectronicandopticalpropertiesofRu(II)andIr(III)poly-pyridylcomplexes:applicationstoDSC,OLEDandNLO.Co-ordChemRev255:2704–2726

30.MishraA,FischerMKR,BauerleP(2009)Metal-freeorganic

dyesfordye-sensitizedsolarcells:fromstructure:propertyre-lationshipstodesignrules.AngewChemIntEd48:2474–249931.OndersmaJW,HamannTW(2013)Recombinationandredox

couplesindye-sensitizedsolarcells.CoordChemRev257:1533–1543

32.DunnHK,PeterLM(2009)Howefﬁcientiselectroncollection

indye-sensitizedsolarcells?Comparisonofdifferentdynamicmethodsforthedeterminationoftheelectrondiffusionlength.JPhysChemC113:4726–4731

33.LiuWQ,HuLH,HuoZPetal(2009)Developmentandap-plicationofintensitymodulatedphotocurrentspectroscopyandintensitymodulatedphotovoltagespectroscopy.ProgChem21:1085–1093

34.KoideN,HanLY(2004)Measuringmethodsofcellperfor-manceofdye-sensitizedsolarcells.RevSciInstrum75:2828–2831

35.YangXD,YanagidaM,HanLY(2013)Reliableevaluationof

dye-sensitizedsolarcells.EnergyEnvironSci6:54–66

36.AokiD,AokiT,SaitoHetal(2012)Methodsforspectral

responsivitymeasurementsofdye-sensitizedsolarcells.Elec-trochemistry80:640–646

861

37.TakagiK,MagainoS,SaitoHetal(2013)Measurementsand

evaluationofdye-sensitizedsolarcellperformance.JPho-tochemPhotobiolC14:1–12

´I,Garcia-BelmonteGetal38.Fabregat-SantiagoF,Mora-Sero

(2002)Cyclicvoltammetrystudiesofnanoporoussemiconduc-tors.CapacitiveandreactivepropertiesofnanocrystallineTiO2electrodesinaqueouselectrolyte.JPhysChemB107:758–76839.KytinVG,BisquertJ,AbayevIetal(2004)Determinationof

densityofelectronicstatesusingthepotentialdependenceofelectrondensitymeasuredatnonzerotemperatures.PhysRevB70:193304

´a-Can˜adasJ(2004)Effects40.BisquertJ,Garcia-BelmonteG,Garcı

oftheGaussianenergydispersiononthestatisticsofpolaronsandbipolaronsinconductingpolymers.JChemPhys120:6726–6733

´a-Can˜adasJ,Fabregat-SantiagoF,BolinkHJetal(2006)41.Garcı

DeterminationofelectronandholeenergylevelsinmesoporousnanocrystallineTiO2solid-statedyesolarcell.SynthMet156:944–948

42.ZabanA,FerrereS,SpragueJetal(1997)pH-dependentredox

potentialinducedinasensitizingdyebyadsorptionontoTiO2.JPhysChemB101:55–57

43.ZabanA,FerrereS,GreggBA(1998)Relativeenergeticsatthe

semiconductor/sensitizingdye/electrolyteinterface.JPhysChemB102:452–460

44.SumikuraS,MoriS,ShimizuSetal(2008)Photoelectro-chemicalcharacteristicsofcellswithdyedandundyednano-porousp-typesemiconductorCuOelectrodes.JPhotochemPhotobiolA194:143–147

45.CameronPJ,PeterLM,ZakeeruddinSMetal(2004)Electro-chemicalstudiesoftheCo(III)/Co(II)(dbbip)2redoxcoupleasamediatorfordye-sensitizednanocrystallinesolarcells.CoordChemRev248:1447–1453

46.SpokoynyAM,LiTC,FarhaOKetal(2010)Electronictuning

ofnickel-basedbis(dicarbollide)redoxshuttlesindye-sensitizedsolarcells.AngewChemIntEd49:5339–5343

47.Roy-MayhewJD,BozymDJ,PuncktCetal(2010)Function-alizedgrapheneasacatalyticcounterelectrodeindye-sensitizedsolarcells.ACSNano4:6203–6211

48.WuMX,LinX,WangYDetal(2012)EconomicalPt-free

catalystsforcounterelectrodesofdye-sensitizedsolarcells.JAmChemSoc134:3419–3428

49.ZhangTL,ChenHY,SuYetal(2013)AnovelTCO-andPt-freecounterelectrodeforhighefﬁciencydye-sensitizedsolarcells.JMaterChemA1:1724–1730

50.GresserR,HummertM,HartmannHetal(2011)Synthesisand

characterizationofnear-infraredabsorbingbenzannulatedAZA-BODIPYdyes.ChemEurJ17:2939–2947

51.UstaH,RiskoC,WangZetal(2009)Design,synthesis,and

characterizationofladder-typemoleculesandpolymers.Air-stable,solution-processablen-channelandambipolarsemicon-ductorsforthin-ﬁlmtransistorsviaexperimentandtheory.JAmChemSoc131:5586–5608

52.LeeCW,LuHP,LanCMetal(2009)Novelzincporphyrin

sensitizersfordye-sensitizedsolarcells:synthesisandspectral,electrochemical,andphotovoltaicproperties.ChemEurJ15:1403–1412

53.NazeeruddinMK,DeAngelisF,FantacciSetal(2005)Com-binedexperimentalandDFT–TDDFTcomputationalstudyofphotoelectrochemicalcellrutheniumsensitizers.JAmChemSoc127:16835–16847

54.ConnellyNG,GeigerWE(1996)Chemicalredoxagentsfor

organometallicchemistry.ChemRev96:877–910

55.WangM,ChamberlandN,BreauLetal(2010)Anorganic

redoxelectrolytetorivaltriiodide/iodideindye-sensitizedsolarcells.NatChem2:385–389

123

862

56.FeldtSM,GibsonEA,GabrielssonEetal(2010)Designof

organicdyesandcobaltpolypyridineredoxmediatorsforhigh-efﬁciencydye-sensitizedsolarcells.JAmChemSoc132:16714–16724

57.HeBL,MengX,TangQWetal(2014)Low-costCoPtalloy

counterelectrodesforefﬁcientdye-sensitizedsolarcells.JPowerSources260:180–185

58.BoschlooG,HagfeldtA(2009)Characteristicsoftheiodide/

triiodideredoxmediatorindye-sensitizedsolarcells.AccChemRes42:1819–1826

59.NicholsonRS(1965)Theoryandapplicationofcyclicvoltam-metryformeasurementofelectrodereactionkinetics.AnalChem37:1351–1355

60.RamasamyE,LeeJ(2010)Ferrocene-derivatizedorderedme-soporouscarbonashighperformancecounterelectrodesfordye-sensitizedsolarcells.Carbon48:3715–3720

61.YeMD,ChenC,ZhangNetal(2014)Quantum-dotsensitized

solarcellsemployinghierarchicalCu2Smicrosphereswrappedbyreducedgrapheneoxidenanosheetsaseffectivecounterelectrodes.AdvEnergyMater4:1301564

62.AdachiM,SakamotoM,JiuJetal(2006)Determinationof

parametersofelectrontransportindye-sensitizedsolarcellsusingelectrochemicalimpedancespectroscopy.JPhysChemB110:13872–13880

63.WangQ,ItoS,GratzelMetal(2006)Characteristicsofhigh

efﬁciencydye-sensitizedsolarcells.JPhysChemB110:25210–25221

64.Fabregat-SantiagoF,Garcia-BelmonteG,Mora-SeroIetal

(2011)Characterizationofnanostructuredhybridandorganicsolarcellsbyimpedancespectroscopy.PhysChemChemPhys13:9083–9118

65.vandeLagemaatJ,ParkNG,FrankAJ(2000)Inﬂuenceof

electricalpotentialdistribution,chargetransport,andrecombi-nationonthephotopotentialandphotocurrentconversionefﬁ-ciencyofdye-sensitizednanocrystallineTiO2solarcells:astudybyelectricalimpedanceandopticalmodulationtechniques.JPhysChemB104:2044–2052

66.Fabregat-SantiagoF,BisquertJ,CeveyLetal(2008)Electron

transportandrecombinationinsolid-statedyesolarcellwithspiro-OMeTADasholeconductor.JAmChemSoc131:558–562

67.KernR,SastrawanR,FerberJetal(2002)Modelingandin-terpretationofelectricalimpedancespectraofdyesolarcellsoperatedunderopen-circuitconditions.ElectrochimActa47:4213–4225

68.BisquertJ(2002)Theoryoftheimpedanceofelectrondiffu-sionandrecombinationinathinlayer.JPhysChemB106:325–333

69.BisquertJ,ZabanA,SalvadorP(2002)Analysisofthe

mechanismsofelectronrecombinationinnanoporousTiO2dye-sensitizedsolarcells.Nonequilibriumsteady-statestatisticsandinterfacialelectrontransferviasurfacestates.JPhysChemB106:8774–8782

70.vandeLagemaatJ,ParkNG,FrankAJ(2000)Inﬂuenceof

71.WangQ,MoserJE,GratzelM(2005)Electrochemicalimpe-dancespectroscopicanalysisofdye-sensitizedsolarcells.JPhysChemB109:14945–14953

72.CaoF,OskamG,SearsonPCetal(1995)Electricalandoptical

propertiesofporousnanocrystallineTiO2ﬁlms.JPhysChem99:11974–11980

Sci.Bull.(2015)60(9):850–863

73.HauchA,GeorgA(2001)Diffusionintheelectrolyteand

charge-transferreactionattheplatinumelectrodeindye-sensi-tizedsolarcells.ElectrochimActa46:3457–3466

74.ZabanA,MeierA,GreggBA(1997)Electricpotentialdistri-butionandshort-rangescreeninginnanoporousTiO2electrodes.JPhysChemB101:7985–7990

75.PitarchA,Garcia-BelmonteG,Mora-SeroIetal(2004)Elec-trochemicalimpedancespectraforthecompleteequivalentcircuitofdiffusionandreactionundersteady-staterecombina-tioncurrent.PhysChemChemPhys6:2983–2988

76.Fabregat-SantiagoF,BisquertJ,Garcia-BelmonteGetal(2005)

Inﬂuenceofelectrolyteintransportandrecombinationindye-sensitizedsolarcellsstudiedbyimpedancespectroscopy.SolEnergyMaterSolCells87:117–131

77.TiedjeT,RoseA(1981)Aphysicalinterpretationofdispersive

transportindisorderedsemiconductors.SolidStateCommun37:49–52

78.HeC,ZhaoL,ZhengZetal(2008)Determinationofelectron

diffusioncoefﬁcientandlifetimeindye-sensitizedsolarcellsbyelectrochemicalimpedancespectroscopyathighfermilevelconditions.JPhysChemC112:18730–18733

79.WuSJ,HanHW,TaiQDetal(2008)Improvementindye-sensitizedsolarcellswithaZnO-coatedTiO2electrodebyRfmagnetronsputtering.ApplPhysLett92:3

80.WuS,HanHW,TaiQDetal(2008)Enhancementindye-sensitizedsolarcellsbasedonMgO-coatedTiO2electrodesbyreactiveDCmagnetronsputtering.Nanotechnology19:21570481.ChenW,ZhangHF,HsingIMetal(2009)Anewphotoanode

architectureofdyesensitizedsolarcellbasedonZnOnanote-trapodswithnoneedforcalcination.ElectrochemCommun11:1057–1060

82.HeC,ZhengZ,TangHLetal(2009)Electrochemicalimpe-dancespectroscopycharacterizationofelectrontransportandrecombinationinZnOnanoroddye-sensitizedsolarcells.JPhysChemC113:10322–10325

83.GanapathyV,KarunagaranB,RheeSW(2010)Improvedper-formanceofdye-sensitizedsolarcellswithTiO2/aluminacore-shellformationusingatomiclayerdeposition.JPowerSources195:5138–5143

84.OndersmaJW,HamannTW(2010)Impedanceinvestigationof

dye-sensitizedsolarcellsemployingouter-sphereredoxshuttles.JPhysChemC114:638–645

85.GuillenE,PeterLM,AntaJA(2011)Electrontransportand

recombinationinZnO-baseddye-sensitizedsolarcells.JPhysChemC115:22622–22632

86.ZhangJ,DuRG,LinZQetal(2012)HighlyefﬁcientCdSe/CdS

co-sensitizedTiO2nanotubeﬁlmsforphotocathodicprotectionofstainlesssteel.ElectrochimActa83:59–64

87.OmarA,AbdullahH(2014)Electrontransportanalysisinzinc

oxide-baseddye-sensitizedsolarcells:areview.RenewSustainEnergyRev31:149–157

88.LeeKM,SuryanarayananV,HoKC(2006)Theinﬂuenceof

surfacemorphologyofTiO2coatingontheperformanceofdye-sensitizedsolarcells.SolEnergyMaterSolCells90:2398–2404

89.Fabregat-SantiagoF,BareaEM,BisquertJetal(2008)High

carrierdensityandcapacitanceinTiO2nanotubearraysinducedbyelectrochemicaldoping.JAmChemSoc130:11312–1131690.KuangD,BrilletJ,ChenPetal(2008)Applicationofhighly

orderedTiO2nanotubearraysinﬂexibledye-sensitizedsolarcells.ACSNano2:1113–1116

91.LeeKM,SuryanarayananV,HuangJHetal(2009)Enhancing

theperformanceofdye-sensitizedsolarcellsbasedonanor-ganicdyebyincorporatingTiO2nanotubeinaTiO2nanopar-ticleﬁlm.ElectrochimActa54:4123–4130

123

Sci.Bull.(2015)60(9):850–863

92.LeeWJ,RamasamyE,LeeDY(2009)Effectofelectrodege-ometryonthephotovoltaicperformanceofdye-sensitizedsolarcells.SolEnergyMaterSolCells93:1448–1451

93.ChenJG,ChenCY,WuCGetal(2010)Anefﬁcientﬂexible

dye-sensitizedsolarcellwithaphotoanodeconsistingofTiO2nanoparticle-ﬁlledandSrO-coatedTiO2nanotubearrays.JMaterChem20:7201–7207

94.HsiaoPT,TungYL,TengHS(2010)Electrontransportpatterns

inTiO2nanocrystallineﬁlmsofdye-sensitizedsolarcells.JPhysChemC114:6762–6769

95.XuH,TaoX,WangDTetal(2010)Enhancedefﬁciencyindye-sensitizedsolarcellsbasedonTiO2nanocrystal/nanotubedou-ble-layeredﬁlms.ElectrochimActa55:2280–2285

96.YangZZ,XuT,GaoSMetal(2010)Enhancedelectroncol-lectioninTiO2nanoparticle-baseddye-sensitizedsolarcellsbyanarrayofmetalmicropillarsonaplanarﬂuorinatedtinoxideanode.JPhysChemC114:19151–19156

97.DolataM,KedzierzawskiP,AugustynskiJ(1996)Comparative

impedancespectroscopystudyofrutileandanataseTiO2ﬁlmelectrodes.ElectrochimActa41:1287–1293

98.LongoC,NogueiraAF,DePaoliMAetal(2002)Solid-state

andﬂexibledye-sensitizedTiO2solarcells:astudybyelec-trochemicalimpedancespectroscopy.JPhysChemB106:5925–5930

99.YeMD,ZhengDJ,WangMYetal(2014)Hierarchically

structuredmicrospheresforhigh-efﬁciencyrutileTiO2-baseddye-sensitizedsolarcells.ACSApplMaterInterfaces6:2900–2908

100.ShiCW,DaiSY,WangKJetal(2005)Applicationof3-hexyl-1-methylimidazoliumiodidetodye-sensitizedsolarcells.ActaChimSin63:1205–1209

101.ShiCW,DaiSY,WangKJetal(2005)Optimizationof1,2-dimethyl-3-propylimidazoliumiodideconcentrationindye-sensitizedsolarcells.ActaPhysChimSin21:534–538

102.WangM,ChenP,Humphry-BakerRetal(2009)Theinﬂuence

ofchargetransportandrecombinationontheperformanceofdye-sensitizedsolarcells.ChemPhysChem10:290–299

103.BayL,WestK,Winther-JensenBetal(2006)Electrochemical

reactionratesinadye-sensitisedsolarcell-theiodide/tri-iodideredoxsystem.SolEnergyMaterSolCells90:341–351

104.GuoWX,ChenC,YeMDetal(2014)Carbonﬁber/Co9S8nanotubearrayshybridstructuresforﬂexiblequantumdot-sen-sitizedsolarcells.Nanoscale6:3656–3663

105.SchefoldJ(1992)Impedanceandintensitymodulatedpho-tocurrentspectroscopyascomplementarydifferentialmethodsinphotoelectrochemistry.JElectroanalChem341:111–136106.PonomarevEA,PeterLM(1995)Ageneralizedtheoryofin-tensitymodulatedphotocurrentspectroscopy(IMPS).JElec-troanalChem396:219–226

107.PeatR,PeterLM(1987)Intensitymodulatedphotocurrent

spectroscopyofn-GaAs.PhysChemChemPhys91:381–386108.deJonghPE,VanmaekelberghD(1997)Investigationofthe

electronictransportpropertiesofnanocrystallineparticulateTiO2electrodesbyintensity-modulatedphotocurrentspec-troscopy.JPhysChemB101:2716–2722

863

109.GoossensA(1996)Intensity-modulatedphotocurrentspec-troscopyofthinanodicﬁlmsontitanium.SurfSci365:662–671110.DloczikL,IleperumaO,LauermannIetal(1997)Dynamic

responseofdye-sensitizednanocrystallinesolarcells:charac-terizationbyintensity-modulatedphotocurrentspectroscopy.JPhysChemB101:10281–10289

¨rlG,HuangSY,SpragueJetal(1997)Bandedge111.Schlichtho

movementandrecombinationkineticsindye-sensitizednanocrystallineTiO2solarcells:astudybyintensitymodulatedphotovoltagespectroscopy.JPhysChemB101:8141–8155

¨rlG,ParkNG,FrankAJ(1999)Evaluationofthe112.Schlichtho

charge-collectionefﬁciencyofdye-sensitizednanocrystallineTiO2solarcells.JPhysChemB103:782–791

113.PeterLM(2007)Characterizationandmodelingofdye-sensi-tizedsolarcells.JPhysChemC111:6601–6612

114.PeterLM,WijayanthaKGU(1999)Intensitydependenceofthe

electrondiffusionlengthindye-sensitisednanocrystallineTiO2photovoltaiccells.ElectrochemCommun1:576–580

115.vandeLagemaatJ,FrankAJ(2001)Nonthermalizedelectron

transportindye-sensitizednanocrystallineTiO2ﬁlms:transientphotocurrentandrandom-walkmodelingstudies.JPhysChemB105:11194–11205

116.QuintanaM,EdvinssonT,HagfeldtAetal(2007)Comparison

ofdye-sensitizedZnOandTiO2solarcells:studiesofchargetransportandcarrierlifetime.JPhysChemC111:1035–1041117.ParkNG,vandeLagemaatJ,FrankAJ(2000)Comparisonof

dye-sensitizedrutile-andanatase-basedTiO2solarcells.JPhysChemB104:8989–8994

118.ZhuK,NealeN,MiedanerAetal(2007)Enhancedcharge-collectionefﬁcienciesandlightscatteringindye-sensitizedsolarcellsusingorientedTiO2nanotubesarrays.NanoLett7:69–74119.MartinsonABF,McGarrahJE,ParpiaMOKetal(2006)Dy-namicsofchargetransportandrecombinationinZnOnanorodarraydye-sensitizedsolarcells.PhysChemChemPhys8:4655–4659

120.WongDKP,KuCH,ChenYRetal(2009)Enhancingelectron

collectionefﬁciencyandeffectivediffusionlengthindye-sen-sitizedsolarcells.ChemPhysChem10:2698–2702

121.ZhangW,ZhuR,KeLetal(2010)AnatasemesoporousTiO2nanoﬁberswithhighsurfaceareaforsolid-statedye-sensitizedsolarcells.Small6:2176–2182

122.KimGO,RyuKS(2012)Dynamicresponseofchargetransfer

andrecombinationatvariouselectrodesindye-sensitizedsolarcellsinvestigatedusingintensitymodulatedphotocurrentandphotovoltagespectroscopy.BullKoreanChemSoc33:469–472123.WangJ,Mora-SeroI,PanZXetal(2013)Core/shellcolloidal

quantumdotexciplexstatesforthedevelopmentofhighlyef-ﬁcientquantum-dot-sensitizedsolarcells.JAmChemSoc135:15913–15922

124.KrugerJ,PlassR,GratzelMetal(2003)Chargetransportand

backreactioninsolid-statedye-sensitizedsolarcells:astudyusingintensity-modulatedphotovoltageandphotocurrentspec-troscopy.JPhysChemB107:7536–7539

123

因篇幅问题不能全部显示，请点此查看更多更全内容

查看全文