Improvement on the long-term stability of flexible plastic dye-sensitized solar cells

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ContentslistsavailableatScienceDirect

Journal

of

Power

Sources

j ou rn a l h o m e pa g e :w w w . e l s e v i e r . c o m / l o c a t e / j p o w s o u r

Improvement

on

the

long-term

stability

of

ﬂexible

plastic

dye-sensitized

solar

cells

Kun-Mu

Lee

a,∗

,

Wei-Hao

Chiu

b

_,

_Ming-De

_Lu

a

_,

_Wen-Feng

_Hsieh

b,c,∗∗

a_Green_Energy_&_Environment_Research_{Laboratories,}_Industrial_Technology_Research_Institute,_Hsinchu_31040,_Taiwan

b_Department_of_Photonics_&_Institute_of_{Electro-Optical}_Engineering,_National_Chiao_Tung_University,₁₀₀₁_Tahsueh_Road,_Hsinchu_30050,_Taiwan

c_Department_of_Photonics_&_Institute_of_{Electro-Optical}_Science_and_Engineering,_National_Cheng_Kung_University,1_University_Road,_Tainan_701,_Taiwan

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received28January2011

Receivedinrevisedform6May2011

Accepted15June2011

Available online 22 June 2011

Keywords:

Plasticdye-sensitizedsolarcell

Long-termstability

Organiciodide

Flexible

a

b

s

t

r

a

c

t

Weinvestigatethelong-termstabilityofperformanceforplasticdye-sensitizedsolarcells(DSSCs)based onorganiciodides(TBAIorPMII)inmethoxypropionitrile-basedelectrolytes.PlasticDSSCscontaining TBAImaintain96.9%ofbaselineefﬁciencyundermorethan1000hprolongedonesunlightirradiation andthermalstress(60◦C)aging.Thefactorsofdevicelong-termstability,suchastheeffectsoforganic iodides,cell-sealingconditions,andthesheetresistanceofindiumtinoxidecoatedpolyethylene naphtha-latesubstrate(ITO/PEN)arediscussedviausingelectrochemicalimpedancespectroscopyandelectrical resistancemeasurement.

1. Introduction

Researchershavestudiedthephotovoltaicapplicationsof dye-sensitizedsolarcells(DSSCs)duetotheirhighconversionefﬁciency andlowcost[1].ADSSCgeneratesphotocurrentthroughultra-fast injectionofelectronsfromphoto-exciteddyemoleculesintothe conductionbandofasemiconductorsuchasTiO2orZnO.This

pro-cessis followedbydyeregenerationand holetransportationto thecounterelectrode.Currently, oneofDSSC researchhasbeen focusedonthedevelopmentofﬂexibleplasticsubstrates.The low-temperature (<150◦C) treatment of semiconductor materials is acrucialtechnologyinthemanufactureofplasticDSSCsdueto thelowmeltingpointsofthesubstrates.Severallow-temperature techniqueshavebeendevelopedtoaddressthisissue,including chemicalsintering[2],hydrothermalnecking[3,4],microwave sin-tering[5,6],electrophoreticdeposition(EPD)[7],andmechanical compressing[8–11].Recently,wehavemadeahighqualityTiO2

electrodebyamultipleEPDprocesswithbinder-freeTiO2

nanopar-ticles (NPs) solution and applied it to a plastic DSSC withthe conversionefﬁciencyofca.6.6%atanilluminationof100mWcm−2 [12].

∗ Correspondingauthor.Tel.:+88635913135;fax:+88635834389.

∗∗ Correspondingauthor.Tel.:+88635712121x56316;fax:+88635716631.

E-mailaddresses:kunmulee@itri.org.tw(K.-M.Lee),wfhsieh@mail.nctu.edu.tw

(W.-F.Hsieh).

Forcommercialapplications,aDSSCmustpassdurabilitytest inacceleratedconditionssuchas60–80◦Cfor1000hand expo-sure to continuous solar irradiation of 100mWcm−2. In many cases,thedurabilityofDSSCisrelated totheleakageand dete-rioration of the electrolyte due to imperfections in thesealing materialandprocess[13].SeveralDSSCdesignshavebeenmadeto replaceorganicliquidelectrolytewithgelelectrolytesorsolid-state electrolytes [14–17]. However,liquid electrolytes withsolvents having high boiling temperature are still a better solution for DSSCswithmesoporousTiO2electrodes.Researchersarecurrently

investigatingthelong-termstabilityofsmallcellsandlarge-area DSSC modules undersimulated solarlight. Thesedesigns show good long-term stability under continuous irradiation of more than1000h,especiallyintheabsenceofUVlight[18–22]. How-ever,onlyfewstudiesreportedﬂexibleDSSCs[13,23].Therefore, inthisstudywedevelopafabricationmethodofTiO2 electrode

atroomtemperature(RT)combiningtheEPDandcompress pro-cesses for plastic DSSCs. We alsoinvestigate theeffects of ITO propertiesanddifferentiodidesonphotovoltaicperformanceand long-termstability.Finally,wereportelectrochemicalimpedance spectroscopy(EIS)analysisofvariationsintheelectrontransport resistanceoftheTiO2 electrode (Rw), charge-transfer resistance

related to electroncombination (Rk), effective electron lifetime

(eff), electrondiffusion coefﬁcient(Deff)and electron diffusion

length(Ln)ofdevicesduringlight-irradiationandthermalaging

processes.

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A Kethley2400 SourceMeter served as a powersupply for EPDprovidingdifferentcurrentsanddepositiondurationsin con-stantcurrentmode,whichismoreeffectiveandcontrollablethan constant voltagemode[24].Fig.1 presents sketchplots of the electrophoreticcell,thepreparation of TiO2 electrodes, andthe

parametersusedinthisprocess.Inaddition,ascatteredlayermade ofTiO2NPs(approximately100nmindiameter,denotedby100nm

TiO2) was introduced on top of the light-absorbing layer. The

opticallyscattering100nmTiO2NPsweresynthesizedinabasic

solutionusingthesol–gelmethod.Acolloidalsuspensionwasmade bymixing58.6gtitaniumisopropoxidewith290mLdistilledwater. Themixturewasthenﬁlteredandplacedinanautoclave contain-ing20mLtetramethylammoniumhydroxide(TMAH)at250◦Cfor 12h.Finally,thesolutionintheautoclavewaswashedbyDIwater andcentrifugedtoobtainthe100nmTiO2 powder.Afterdrying

TiO2-depositedITO/PENsubstrateatRTandoneatmospheric

pres-sure,apressuretreatmentwasappliedtoimprovetheadhesionof TiO2onthesubstrateandtoenhancethephotovoltaicperformance

ofthedevice.

The mesoporous TiO2 ﬁlm was immersed in a solution of

0.5mM N719 dye (cis-bis(isothiocyanato)bis(2,2-bipyridyl-4,4 -dicarboxylato)-ruthenium(II)bis-tetrabutylammonium,Solaronix, Aubonne, Switzerland) in acetonitrile (AN, HPLC grade, J.T. Baker)/tert-butyl alcohol (t-BuOH) (v/v=1/1) binary solvent at 40◦Cfor4htoabsorbsufﬁcientN719dyeforlightharvesting.The dye-sensitizedTiO2electrodewasthenrinsedwithANtoremove

theremaining dye,and dried under standard atmosphere con-ditionsatRT.A platinum-sputteredITO/PEN ﬁlmserved asthe counterelectrode.Atwo-electrodesandwichcellwasseparatedby a60␮mhot-melttypespacerandﬁlledwiththeelectrolyte.The effectiveworkingareaofthedevicewaslimitedtoapproximately 0.28cm2_by_using_a_black_mask.

The electrolyte used for the high conversion efﬁciency test consistedof0.4MLiI(Merck),0.4Mtetrabutylammoniumiodide (TBAI,Aldrich),0.04MI2and0.5MN-methylbenzimidazole(NMBI,

Aldrich) in AN/methoxypropionitrile (MPN Alfa Aesar) mixture (v/v=1/1).Low-volatilityelectrolyteswithacompositionof0.8M TBAI,0.1MI2(99.8%),and0.5MNMBIinMPN(electrolyteA)or

0.8M1-methyl3-propylimidazoliumiodide(PMII),0.1MI2,and

0.5MNMBIinMPN(electrolyteB)wereusedtotestthe durabil-ityofplasticDSSCs.Theabove-mentionedchemicalswereusedas receivedwithoutfurtherpuriﬁcation.

Thedurabilitytestinthisstudyincluding100mWcm−2 light irradiationwiththerangefromUVtoIRandthermalstress(60◦C) agingofhermeticallysealedcellswasperformedwithasuntest xenonarc lamp(ATLASCi3000xenonFadeometer).Followinga periodofcontinuouslightirradiation,photoelectrochemical mea-surementsweretakenafterthecellscooledtoRT.AnUV-cutﬁlter wascommonlyputontoDSSC toavoid thedegradationofdye, butwasnotusedinDSSCwithPENpolymersubstratesduetothe cut-offincidentlightofabout380nmbyPENsubstrates.

loidparticleswithaporousstructure.TheP-90NPssizeisabout 15nmandtheopticallyscatteringTiO2NPsisabout100nm(also

denotedby100nmTiO2).The100nmTiO2waswell-depositedon

thetransparentﬁlmcomposedbyP-90NPswithoutanylagerpin holesandthesethreekindsofTiO2ﬁlmswerepreparedtodevice

assembling. Fig. 2b shows the typical monochromatic incident photon-to-electronconversionefﬁciency(IPCE)spectra,whichis toscalethespectralresponseofthephotovoltaicdevices.Around thedistinctpeakat540nmattributedtothemetal-to-ligandcharge transferabsorptionbandoftheN719dye,theDSSCswithonlyP-90 NPswereobtainedtheIPCEvalueabout54%whichwasgreaterthan theoneswithonly100nmNPs(IPCEvalueabout47%)becauseof thegreateramountoftheN719dyeonTiO2ﬁlm.TheDSSCs

consist-ingofbothP-90and100nmNPsshowedthegreatestIPCEvalue of60%duetoenoughsurfaceareafordyeabsorptionwithgood lightscattering.TheinsetﬁgureinFig.2bpresentedthenormalized IPCEofthreekindsofDSSCs.TheshapesofthethreeIPCEcurves arealmostthesameinthewavelengthrangeof400–600nmOnthe otherhands,inthewavelengthrangeabove600nm,theIPCEcurves shapeswithDSSCsconsistingof100nmTiO2NPsweregreaterthan

theP-90one,meaningofthelackofthelight-scatteringeffectfor P-90NPs.It suggeststhatthehighIPCEin DSSCsbasedon rel-ativelythinﬁlmscanbeimprovedbyenhancedlightabsorption withenoughsurfaceareafordyeabsorptionandgoodlight scat-teringlayerwithlargersizeNPs,whichwasconsistentwiththe workreportedbyArakawaetal.in2004[25].Ontheotherhand,a goodqualityﬁlmwithnocrackisnecessaryforelectroncollection thatmayalsoleadtoanimprovementinIPCE(Table1).

Althoughthepresstreatmenthasbeenusedonflexible sub-strateDSSCs[8–11],thecorrelationbetweenappliedpressureand deviceperformanceshouldalsobeclarifiedinthiswork.Therefore, adetailedexperimentalwasperformedunderdifferentcompress pressureconditionstoobtaintheperformanceoftheassembled DSSCsdevices, and theresultsareshown in Fig.3a. The thick-nessofelectrophoreticdepositedTiO2 filmis about20␮m,and

reducesto6–8␮mviadifferentappliedpressures.Betweena non-pressedanda 100MPa-pressed TiO2 ﬁlms, JSC increasedrapidly

from1.7to11mAcm−2,respectively.Whentheexternalpressure appliedonTiO2ﬁlmsisfrom100to500MPa,JSCbecamea

con-stant.Itisbelievedthatthehighexternalpressure(over100MPa) notonlymeltthesurfaceofTiO2NPstoformagoodporous

pho-toanodeforelectrontransporting,butalsoincreasetheadhesion strengthbetweentheTiO2 ﬁlmandITO/PENsubstrate[26].The

slightchangesofVOCwithdifferentpressurefromnon-pressedto

400MPawerenotsignificantbutrelatedtothefilmthickness.The fillfactor(FF)wasobservedalmostaconstantabout0.7intherange 0(non-pressed)to400MPa,butFFoftheDSSCswith500MPa pres-suredroppedto0.4duetotheoccurringofcrackintheITOfilm. Consequently,the6%overallconversionefficiencywith100MPa pressuretreatmentwasachieved.Fig.3bshowstherelationship betweenthetimeofthepressureappliedforpressingtheTiO2film

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Fig.1. Sketchplotsoftheelectrophoreticcell,thepreparationofTiO2electrodes,andthesigniﬁcantparametersinthisprocess.

Fig.2. (a)Cross-sectionSEMimageofanEPDﬁlmfor(I)100nmTiO2;(II)P-90TiO2and(III)P-90TiO2and100nmTiO2asdouble-layerﬁlmonITO/PENsubstrate.(b)IPCE

spectraandnormalizedIPCEspectra(insetﬁgure)ofDSSCswiththesethreekindsofnanocrystallineTiO2ﬁlm.

Table1

Physic-chemicalpropertyofTiO2particlesusedinthisstudy.

Material Speciﬁcsurfacearea(BET)(m2_g−1₎ _Average_primary_particle_size_(nm) _Crystal_phase

P-90TiO2 90±20 ca.14 Anatase

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Fig.3.(a)Thecorrelationbetweenthecompressionpressureoftheelectrophoretic depositedTiO2 thinﬁlmanddeviceperformance.Theparametersofthe

non-pressurephotoanodewereplottedatthe0MPaposition.(b)Thecorrelationbetween thecompressiontimeoftheelectrophoreticdepositedTiO2thinﬁlmandthecurrent

densityandconversionefﬁciency,respectively.

andtheperformanceoftheassembledDSSCs.Afterthe100MPa

compressing,thethicknessoftheTiO2photoanodesusedforthis

measurementwere7–8␮m.JSCandtheconversionefﬁciencywere

observedtobelowintheabsenceofpressuretreatment(0s),and

tobealmostconstantforarapidpressuretreatment(1s)toa

con-tinuedpressuretreatment(60s).Itissuggestedthattheelectron

transportinginthephotoanodeandtheadhesionstrengthbetween

theTiO2ﬁlmandITO/PENsubstratecanbesigniﬁcantlyimproved

inarapidtime.Astheresults,itopensupthepossibilityof

develop-ingacontinuousrolltorollprocessformassproductionofﬂexible

DSSCs.

BeforeinvestigatingthedurabilityofplasticDSSC,itisimportant

totestthestabilityofITO/PENinelectrolyteandtocheckthesealing

materialandconditionintheplasticcell.Fig.3ashowsthe

varia-tionofsheetresistanceofITO/PENimmersedintwoelectrolytes for1000hwhichareelectrolyteI(0.5MPMII/0.05MI2 inMPN)

andelectrolyteII(0.5MLiI/0.05MI2inMPN).Thesheetresistance

ofITO/PENimmersedinelectrolyteIincreasesslightlyfrom11.2to 12.2ohmsq−1;however,thatinelectrolyteIIincreasessigniﬁcantly from11.1to18.6ohmsq−1.TheOMimagesofITO/PENsurfaceare alsoshowedinFig.3b,whichclearlyobservedbycracking phe-nomenonofITOﬁlmafterimmersinginelectrolyteIIfor1000h.

after100hasshownin Fig.5.The apparentincrease inJSCand

conversionefﬁciencymaybeduetoimprovementinelectrolyte penetrationintothemesoporousTiO2 ﬁlm,loweringoftheTiO2

conductionbandboundaryandactivationofthePt-coatedcounter electrodeasreportedpreviously[27].Notethatdevicesexhibit dif-ferentconversionefﬁciencyimprovementswithdifferentcation iodides.InFig.5aandb,JSCandconversionefﬁciencyofthedevice

withelectrolyteIincreasedsigniﬁcantlyfrom3.81to6.89mAcm−2 and from 1.85% to 3.14%, respectively, under 100hcontinuous lightirradiation.On theotherhand,thedevicewithelectrolyte IIachievesanefﬁciencyvalueof2.38%fromtheinitialvalueof 1.84%,whichJSConlyslightlyincreasesfrom4.03to4.81mAcm−2

inFig.5b.ThisimprovementresultsfromTBA+ _on_the_TiO

2 ﬁlm

surface thatprotected thevoids in thedye-coatedTiO2 ﬁlm in

turnblockedundesirableinterfacialchargerecombinationand sup-pressedsurfaceprotonation.Thegradualdecreaseinconversion efﬁciencyofcellafter100hinFig.5bsuggeststhat the confor-mation,dyealignment, andintermolecularinteractionsofN719 onthesurfaceofTiO2ﬁlmshouldchangeduringtheaging

pro-cess.

Tofairlyevaluatedevicedurability,thephotovoltaicparameters ofthedevicesatthesteadystateobtainedafter100hagingwere usedasabaseline.Aftercontinuousagingfor1000h,thedevices withelectrolytesIandIIstillmaintained96.9%and72.3%ofthe baselineefﬁciencymeasuredat100h.Thisperformanceisbetter thanthatofpreviouslyreported[13,23].Themajorfactorof degra-dationintheefﬁciencyofthedevicesisduetoadecreaseofVoc

(_∼0.13V)(seeFig.5c)thatiscausedbysurfaceprotonationunder theacceleratedagingtest[28–30].

Tounderstandtheeffectsofdifferentcationiodides,TBAIand PMII,onchargetransportationanddevicedurability,the electro-chemical impedance spectra (EIS) of the devices aged for 100, 500, and 1000h were measured under open-circuit condition and illuminationof 100mWcm−2. Fig.6a–cshows the Nyquist plots of theimpedance data,and the equivalentcircuit model ofDSSC isshowedin Fig.6d [31].TheTiO2 workingelectrodes

arecharacterizedbythechemicalcapacitanceC␮,thetransport resistanceRwandtheTiO2/electrolyteinterfacialcharge-transfer

resistanceRk. Theterms Rs and ZN represent thecharge

trans-portresistanceof ITO(includingexternal circuit)and theﬁnite Warburgimpedanceintheelectrolyte.RPtandCPtrepresentthe

charge-transferresistanceandcapacitanceatthePtsurface;RITO

andCITOrepresentthecharge-transfer resistanceand the

inter-facialcapacitanceattheITO/electrolyteinterface;ﬁnally,RFand

CF representtheresistanceandthecapacitanceat theITO/TiO2

interface.

TheEIS-ﬁttingdatafromthesedevices are listedin Table2. The Rw values increase with light soaking time for electrolyte

containing eitherTBAI or PMII that lowersthe estimated elec-trondiffusioncoefﬁcient(Deff)andshortensthediffusionlength,

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Fig.4.(a)TherelationshipbetweenthesheetresistanceofITO/PENandtheimmersiontimeinelectrolyteat60◦_C_(electrolyte_I:_0.5_M_PMII_and_0.05_M_I₂_in_MPN;_electrolyte

II:0.5MLiIand0.05MI2inMPN).(b)TheOMimagesofITO/PENﬁlmafterimmersioninelectrolyteIIat60◦C.(c)Thesealingtestcellandtheweightlossofcellduring

thermaltreatmentat60◦_C_in_the_dark.

theeffectiveelectronlifetime,eff,increaseswiththelightsoaking

time.Itimpliesthattherecombinationofelectronswithtriiodide attheinterfaceofTiO2NPsandtheelectrolytehasinhibited

dur-ingtheprolongedstabilitytestresultinginastablephotocurrent output.Furthermore,thedevice withelectrolyte Ihasa higher

resistanceofcharge transferatthePt/electrolyteinterfacethan thatwithelectrolyteIIasshowninFig.6.Thismeansthatthe elec-trolytecontainingTBAIhadalowertriiodidereductionrateatthe Pt/electrolyteinterfacethatleadstothelowerFFvalueasshown inFig.5.

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Fig.5.PhotovoltaicparametersforplasticDSSCwithTBAIorPMIIaftervisiblelightsoaking(1sun)at60◦_C._(a)_Short_circuit_current_density,_J_SC_;_(b)_Energy_conversion

efﬁciency,;(c)Opencircuitvoltage,VOCand(d)Fillfactor,FF.

Fig.6.EISresultsofplasticDSSCswithdifferentiodidesafter1sunlightsoakingfor(a)100,(b)500and(c)1000h.(d)Theequivalentcircuitmodelemployedforﬁttingthe

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Table2

ParametersdeterminedfromﬁttingEISdataofplasticDSSCwithelectrolytescontainingTBAIandPMII,respectively.

Iodide Lightagingtime(h) Rw() Rk() Rk/Rw eff(ms) Deff(10−5cm2s−1) Ln(␮m)

TBAI 100 12.9 13.5 1.05 36.8 1.02 6.2 500 12.6 13.2 1.05 44.2 0.85 6.1 1000 18.8 14.4 0.77 48.4 0.57 5.3 PMII 100 28.9 13.2 0.46 16.6 0.99 4.1 500 37.4 14.3 0.38 15.3 0.90 3.7 1000 41.4 14.4 0.35 14.5 0.86 3.5 4. Conclusions

Wehaveinvestigatedthedurabilityofﬂexibledeviceswith

dif-ferentcation iodides.Underprolongedone-sunlight-irradiation

and60◦C-thermalstressaging,ourplasticDSSCdevicesshowed

aninitialimprovement inperformanceof 96.9%followedbyan

extendedsteady-stateperiodofmorethan1000h.Thepresence

ofTBAIintheelectrolyteprovideshigherphotocurrentandbetter

durability.ThisimprovementisaresultofTBA+_on_the_TiO

2ﬁlm

surface,whichstericallyprotectsthevoidsinthedye-coatedTiO2

ﬁlminturnblocksundesirableinterfacialchargerecombinationto

suppresssurfaceprotonation.

Acknowledgements

WegratefullyacknowledgetheﬁnancialsupportofIndustrial

Technology Research Institute (ITRI) and the National Science

Council of Taiwan(NSC-99-2112-M-006-017-MY3 and

NSC-99-2221-E-009-095-MY3).

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