Optical, electrical and electrochemical evaluation of sputtered platinum counter electrodes for dye sensitized solar cells

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ContentslistsavailableatScienceDirect

Applied

Surface

Science

j o ur na l ho me pa g e :w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c

Optical,

electrical

and

electrochemical

evaluation

of

sputtered

platinum

counter

electrodes

for

dye

sensitized

solar

cells

R.S.

Moraes

a

_,

_E.

_Saito

b

_,

_D.M.G.

_Leite

a

_,

_M.

_Massi

a,b

_,

_A.S.

_da

_Silva

_Sobrinho

a

a_{Technological}_Institute_of_Aeronautics_(ITA),_Physics_Department,_São_José_dos_Campos,_SP,_Brazil b_Federal_University_of_São_Paulo_-_ICT,_São_José_dos_Campos,_SP,_Brazil

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received14August2015 Receivedinrevisedform 18November2015 Accepted15December2015 Availableonline17December2015 Keywords:

Energyconversion Solarenergy

DSSCcounterelectrode Platinumcatalyticthinﬁlm

a

b

s

t

r

a

c

t

SinceGrätzelandO’Reganstartedin1991,dye-sensitizedsolarcells(DSSC)havebeenextensivelystudied aroundtheworld.Inadditiontoincreasingefficiency,theircharacteristicssuchaslowcostmaterialsand inexpensivemanufacturingprocessesareattractivefororganicsolarcells.SeveralpartsofDSSCdevices arebeingresearchedsuchassemiconductorengineering,lowcostcounterelectrodes,electrolytes,and dyes.Inthiswork,platinum(Pt)thinfilmsweredepositedbysputteringtechniquetoproducecounter electrodesforDSSC.Thefilmswerecharacterizedbyprofilometry,elipsometry,four-pointprobesheet resistance,spectrophotometry,andelectrochemicalimpedancespectroscopy.Theelectroderesponse wasalsocomparedtothatbuiltfromacommercialplatinumsolution.Theresultsallowustodetermine theminimumPtfilmthicknessnecessarytoachievearelevantreductionofthesheetresistanceand chargetransferresistance,whichpreserveasignificantelectrodetransparency.The22nmand24.8nm thickfilmscombinedlowchargetransferresistanceandgoodtransparency.The122nmPtfilmpresented thelowestchargetransferresistance.

1. Introduction

Theenergydemandisachallengeforthefutureinallcountries. Theprogressanddevelopmentheavilydependonenergy. Photo-voltaiccellsforsolarenergyconversionisasustainablesolutionfor independencefromfossilfuelswithouttheirintrinsicﬁngerprint

[1–3].

Historically,thebeginningofsolarenergyconversionstarted experimentallyinBell’slaboratory[4].Sincethen,thistechnology has been continuouslydeveloped to reach high values of efﬁ-ciency,butitshighmanufacturingcostsandchallengingfabrication processareissuesthataffectthedisseminationofthiskindof tech-nology.However,aconstantincreaseinefﬁciencyofdifferentkinds ofsolarcellshaschangedthisscenario.Inrecentyears,the develop-mentofdyesensitizedsolarcells(DSSC)developedbyGratzeland O’Reganhasbeenanimportantapparatusforenergyconversion

[5].

The DSSC operates through different working mechanisms comparedtoconventionalsiliconsolarcells.Comparing conduc-tionmechanismsinsilicon-based solarcells, thesemiconductor assumesthetaskoflightabsorptionaswellaschargetransport. In dye-sensitizedsolarcells, both thesetasksoccurindifferent materials.Dyemoleculesabsorblightandinjectelectronsintothe conductionbandofthesemiconductor,thattransportschargesto

thecounterelectrode[6,7].TheschematicrepresentationofDSSC solarcellsispresentedinFig.1.

Adye-sensitized solarcellinitiallyoperates bythe photoex-citationofadyeadsorbedonasemiconductor(TiO2,ZnO)[8,9], frequentlycalledphotoanode.Thedecay oftheexcitedstateof thedyeoccursbyachargeinjectioninaconductionbandofthe semiconductor andthisprocessis supportedbyaredoxshuttle (I3/I−)presentinelectrolyte[10].Speciﬁcally,theoxidizeddyeis regeneratedbyiodine,whichresultsindiiodinespecie(I2•−)andat theendadiiodinedisproponation[11].Theredoxshuttleofiodine reactiondirectlyaffectstheoverallperformanceofthecell.This processcanbecatalyzedbyseveralmaterialsdepositedonto trans-parentconductingoxide(TCO)glasssubstrateatcounterelectrode, suchasplatinum[12–19],carbon[20–23],conductingpolymers

[24–31],metalsconjugatedwithpolymers[32,33],andotherlow

costalternatives[34].Therefore,theperformanceofcounter elec-trodelimitsthemaximumperformanceofDSSCandcanbeaffected byseveralfactorssuchastransparency,conductivity,andcharge transferresistance.Sheetresistanceandchargetransferresistance (Rct)haveimportantrolesinelectricconductionmechanismsof DSSCsolarcells.DecreasedsheetresistanceofthePtfilmscauses lower seriesresistanceofthecell, increasingthefillfactor and consequentlyitsefficiency[16–18].Lowvalues ofcharge trans-ferresistanceindicatefaster chargetransferand canberelated

http://dx.doi.org/10.1016/j.apsusc.2015.12.114

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Fig.1.SchematicofDSC.Adaptedfrom[52].

tothecharacteristicofthecounterelectrode-electrolyteinterface

[15].

Sincethen,severalimprovementshavebeenmadeinallparts ofDSSCsuchasinelectrolyte[35],semiconductors[36],catalysts

[34],dyes[37],processing[38],andsealing.

Theseimprovementsincreasetheperformanceandlifetimeof thesecellsinrelationtoallthecomplexparametersrelatedtoDSSC devicesuchasohmicloss[39],masstransfer[40],andphysicaland electrochemicalkinetics[41].

ThecounterelectrodethatcombinesTCOglassandplatinum catalystﬁlmisoneofthemostimportantpartsofdye-sensitized solarcells[42].Platinumﬁlmshavebeenthebestsolutiontothis applicationdue totheirexcellentcatalytic activityandstability

[15].

Aimingtoreducemanufacturingcostsofthecell,magnetron sputteringtechniquecanobtainﬁlmswithreducedthicknessby controllingthedepositionparameters,theeasiestbeingthe depo-sitiontime.Obtainingtheplatinumcatalystﬁlmasthinaspossible withoptimizedcatalyticactivityreducescosts.

Toachievethisgoal,abatchofPtﬁlmswasdepositedbydirect current(DC)magnetronsputteringtechniqueusingdifferent depo-sition timesin order toobtaindifferent ﬁlm thicknesses.Their properties,includingtransmittance,sheetresistance,and electro-chemicalresponses,wereanalyzedtodeterminethebestcondition forapplicationinDSSCcounter electrode.Theresultswerealso comparedwithatypicalcounterelectrodebuiltfromcommercial platinumpaint.

DCmagnetronsputteringisaversatileandlowcostdeposition techniquewhencomparedwithotherthinfilmdeposition tech-nologies(asMOCVDandMBEforexample).ForthePtfilmsand otherpreciousmetalsfilms,thesputteringtechniqueisalsomore cost-effectivethansimpleevaporationduetothelowPt consump-tion.Despite these low cost benefits,DC magnetron sputtering techniquehassomeimportanttechnologicaladvantages,namely: (i)theabilitytodeposit a widerange of materialsontoa vari-etyofsubstratewithextensiveareas;(ii)productionoffilmswith highpurity(>99.999%)andhighhomogeneity;(iii)easycontrolof thefilmpropertiesbysimplyvaryingthedepositionparameters, includingDCpower,depositiontime,plasmacomposition, work-ingpressure,andsubstratetemperature;(iv)reproducibilityand repeatability.AllthesecharacteristicsmakeDCmagnetron sput-teringa goodchoicetoproducePtthin filmsforapplication in DSSC.

Table1

Parametersofsputteringprocess.

Parameter Value

Target/substratedistance 20mm

DCpower 20W

Argonpressure 1.3×10−2_Torr

Depositiontime 3s(Sample1) 6s(Sample2) 10s(Sample3) 30s(Sample4) 150s(Sample5) 300s(Sample6) 2. Experimental

Platinum thin ﬁlms were deposited onto FTO (commercial TCO22-7ﬂuorinedopedtinoxideobtainedfromSolaronix®₎_coated

glasssubstrateusingahomemadeplanarmagnetronsputtering system withcylindrical geometry. The cathode with33.78mm (diameter)consistsofhighpurityplatinumtarget(99.99%)placed 20mm from the substrates.Pure argon atmosphere was used.

Table1summarizestheparametersofthesputteringprocess.The

depositiontimeisanexperimentalparameterthatcanbeeasily controlledinthemagnetronsputteringprocessandhasdirecteffect ontheresultingfilmthickness.Theresultsobtainedby sputter-ingtechniquewerecomparedtosimilarelectrodesproducedfrom commercialplatinumpaint(PlatisolTobtainedfromSolaronix®₎ brushpaintedontoFTO,firedat450◦Catarateof10◦C/min, main-tainingthistemperaturefor10mintoobtainactivatedplatinum film.

The film thicknesses were measured by KLA Tencor P-7 perfilometer and HORIBA UVISEL 2 ellipsometer. To evaluate the relation between thin film properties and electro-catalytic response,theopticalandelectricalpropertieswerecharacterized bytransmittancespectrainthevis–NIRregionbyJASCOV570 spec-trophotometerandbysheetresistancemeasurementsusingJandel RM3000four-probetestunit.

The electro-catalytic response was measured by electro-chemicalimpedance spectroscopy(EIS)performedbycomputer controlled potentiostat (Autolab 302N) with Ag/Ag+ _reference electrodeand apureplatinum wireasacounter electrode.The amplitudeofperturbationwas10mVinafrequencyrangefrom 4.5×105_Hz_to₁₀−1_Hz_in_open_circuit_potential_in_a_faraday_cage. Theimpedance datawereﬁttedby adaptedRandles equivalent circuit[43].

The electrochemical evaluation of the counter electrodes was performed by a standard commercial electrolyte AN-50(Solaronix®₎_composed_by_{1,2-dimethyl-3-propylimidazolium} iodideandiodineinacetonitrilesolvent.

3. Resultsanddiscussions 3.1. Thicknessandsheetresistance

Thethicknessesoffilmswerecontrolledbythedepositiontime of theprocesses. Table2 presentsthe resultsobtainedby pro-filometryandsheetresistance.Itwasnotpossibletomeasurethe thicknessofsamples1,2,and3byprofilometer,becausetheywere toothin. Tosolvethisproblem,thesethinfilmsweredeposited ontosiliconsubstratesandanalyzedbyellipsometry.Samples4,5, and6(thickerfilms)showmirroraspect,blockingmostofthesolar spectrumthroughoutthecounterelectrode.

Table2 andFig.2showthatsheetresistancedecreaseswith

increased ﬁlm thickness,reaching a plateau for higher deposi-tiontime,wherethebehavioroftheﬁlmisclosetotheplatinum bulk. Lowersheet resistance is a desirable property because it

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Table2

Thicknessandsheetresistanceofthesamples.

Depositiontime(s) Thickness(nm) Sheetresistance(/䊐)

3(Sample1) 11.5 54.10 6(Sample2) 17.1 24.83 10(Sample3) 22.0 15.63 30(Sample4) 24.8 2.60 150(Sample5) 122.0 0.78 300(Sample6) 319.0 1.07 Platinumpaint – 7.80

FTO n/a 7.00(solaronixdata)

contributestocollectphotocurrent,reducesseriesresistanceof thecell,andcontributestoincreaseﬁllfactor[16].

Theﬁlmsdeposited formore than30s(24.8nmthick) have verylowsheetresistancevalues,whicharelowerthanthevalue obtainedfromtheplatinum paintedsampleand similarto val-uescomparedintheliterature[18].Thisresponsewasexpected consideringthehighconductivityofmetallicplatinumandthe par-ticledistributionuniformityofthesputteringtechnique.Sample5, depositedfor150s(122.0nmthick),hadthelowestsheet resis-tance.Samples4and6alsohadlowerresistancethanFTOorthe ﬁlmobtainedbyplatinumpaint.

Forthefirstthreesamples(thicknessbelow24nm),thesheet resistancegreatlyincreasedwhenthePtfilmthicknessdecreased. Thisbehaviorwasexpectedandcouldbedirectlyrelatedtotwo mainreasons:(i)shortdepositiontimesaffectedthesheet resis-tance,becauseofthepoisoningeffectinsputteringprocess,wherea thinoxidelayeronthesurfaceofmetallictargetswassputteredand consequentlydepositedontothesubstrateformingthefirstatomic layers,whichaffectedthesheetresistanceofverythinfilms[44]; (ii)thefilmswithdecreasedthicknessaremoresuitablefor dis-continuitythroughthesurface,whichalsogreatlyeffectsthesheet resistancemeasurementswhentheconditionsofsimpleconnected areaofthevanderPauwtheoremisviolated[45].

Theseresultsaboutthicknessandsheetresistancesmustbe cor-relatedtoelectrochemicalresultsinordertoselecttheadequate ﬁlmthickness.

3.2. Transmittance

Transmittancemeasurementswereemployedtomeasurethe level of transparency of the ﬁlms, mainly in visible and near infraredregions,asshowedinFig.3.Animportantissuehereis that conventionalDSSCs mayonly operateunder one-side illu-mination. In this sense, transmittance can desirably contribute tolightaccesstoallphotoanode(andpotencializestheaccessof

Fig.2.Sheetresistancerelationwithﬁlmsthickness.

Fig.3.TransmittanceofPtthinﬁlmsdepositedonFTOsubstrates.

photonstoadsorbeddye).Theresultsindicatedthatﬁlmsdeposited for30–300s(thickerthan24nm)hadlowtransparency,whichcan affectthesolarcellperformance,orevenrestricttheirapplicationas semitransparentwindows.Otherwise,thetransmittanceincreases forsamplesdepositedusingshort-timeprocessesandallowsthe radiationthroughoutthecounterelectrode,increasingthecurrent densityofthecells.

3.3. Electrochemicalimpedancespectroscopy(EIS)

TheEIStechniquewasperformedforallelectrodesandfitted tofindthespecificvalueofchargetransferresistanceofplatinum electrodes.Theelectrochemicalcharacterizationofcounter elec-trodeswasperformedinaconventionalelectrochemicalcellusing Ag/Ag+_reference_electrode_and_a_platinum_mesh_as_counter elec-trode.Thistechniquecanpreciselyfindcriticalcounterelectrode response,suchaschargetransferresistance(Rct)attheelectrode relativetoaredoxcouple.

Asexpectedinthisexperimentalsetup,thehighfrequencylimit allowsthereductionofthecapacitivecontributionand determina-tionoftheseriesresistanceoftheelectrode.Alongthefrequency range,atmediuminterval,thecontributionofdoublelayer capac-itance and charge transfer resistance can be determined. The diffusionalimpedanceresponsescanbeattributedtomobilityof redoxspeciesatlowfrequencies.Attheendofallmeasurements, theconditionoflinearity,causality,andstabilitywereveriﬁedby Kramers-Kronigtransform[46].Therefore,thedataconsistencyof EISmeasurementswasvalidated.

Thesample4(24.8nmthick–30s)andsample5(122.0nmthick –150s)presentedthebestcorrelationofseriesresistance,Rctand sheetresistance,whichwasconﬁrmedbytheliterature[14,17,20]. IntheNyquistplot,presentedinFigs.4and5,showsthe semicir-cle(fromchargetransferresistanceanddoublelayercontribution inparallel)andthediffusionalresponseofredoxspeciesatlow frequencies.

AllexperimentaldatawereﬁttedwithadaptedRandles equiv-alentcircuit(withaconstantphaseelementmodelingthedouble layercapacitance)[43].Consideringthedistributionofplatinum in this non-uniform substrate, it was expected that all elec-trodes would present a deviation fromideal capacitor. Besides thedeviation,allexponentialfactorvaluesstayednearunityand doublelayercapacitanceintheexpectedrange(20×10−6Fcm2₎

[10,47–49].

Thesamplesproducedinaperiodlowerthan30s(thickness lowerthan24nm)presentedhighseriesresistances,evenhigher thanbareFTO.Thisresultagreeswiththehighsheetresistance valuesobtainedbyfourprobemeasurements,anditreinforcesthe hypothesisofthepresenceofapristineoxidelayergrownatthe

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Fig.4.Nyquist(a)andBodeplot(b)ofplatinumelectrodesputteredonFTOofSample5.Theexperimentaldataarerepresentedbysymbolsandtheﬁttedbythecontinuous line.

Fig.5. Nyquist(a)andBodeplot(b)ofplatinumelectrodesputteredonFTOofSample4.Theexperimentaldataarerepresentedbysymbolsandtheﬁtbythecontinuous line.

ﬁrstsecondsofthesputteringprocessduetothetargetpoisoning effect[44].

AsconﬁrmedbyFig.5,sample4hadhigherseriesresistance comparedtosample5,inFig.4.Theseriesresistanceisdetermined byhighfrequencyperturbationbecauseitexcludesthecapacitive responseofelectrode.Table3presentsthecomparisonbetween chargetransferresistance(Rct)andsheetresistanceforsamples 1–6.Itisimportanttonotethesimilarbehaviorbetweenthesetwo electricalproperties,bothdecreasewhenthethinﬁlmthickness increases.

Fig.6presentstheplotofchargetransferresistanceasa func-tionofdepositiontime,obtainedfromﬁttingofelectrochemical impedance response with modiﬁed Randles equivalent circuit

[43,50].Thetechniqueallowsa reliablequantiﬁcation ofcharge

transferresistanceofplatinumelectrodeinrelationtoiodineredox coupletobeobtained.

ComparingtheRct resultswithcommercialplatinum(dashed lineinFig.6)28/cm2_[51]_,_it_is_possible_to_verify_that_even_the_thin

Table3

Sheetresistanceandchargetransferresistance(Rct)results.

Depositiontime (s) Sheetresistance (/䊐) Chargetransfer resistance(/cm2₎ 3(Sample1) 54.10 30.72 6(Sample2) 24.83 26.56 10(Sample3) 15.63 27.92 30(Sample4) 2.60 9.63 150(Sample5) 0.78 2.30 300(Sample6) 1.07 6.96

ﬁlminsample2haslowerchargetransferresistance.Inaddition, theminimumvalueofchargetransfer(2.3/cm2₎_and_the_lowest

sheetresistance(0.78/䊐)arebothfoundforsample5.

Takingintoaccountthesimilarsheetresistance, series resis-tance,andchargetransferresistance,samples4,5,and6presented thebestparameterstoimproverecombinationprocessesatcounter electrode.However,the30ssputteringprocess,whichproducesa 24.8nmﬁlm(sample4),isthebestselectionconsideringplatinum consumptioninthiswork.

Fig.6. Chargetransferresistancecomparedtodepositiontime(dashedline corre-spondstocommercialplatinumchargetransferresistance).

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4. Conclusions

PlatinumcounterelectrodeswereobtainedbyDCmagnetron sputteringofapureplatinumtargetvaryingonlythedeposition time.Inordertoselectbestsputteringparameterstoobtain opti-mized counter electrodes for DSSC application, the filmswere characterized by profilometry and elipsometry to measure the thicknessofthefilms;fourpointelectricalresistancetoevaluate the sheetresistance; spectrophotometry to evaluatethe trans-parency;andelectrochemicalimpedancespectroscopytoevaluate theelectroderesponsetoiodineredoxcouple.Thefilmsresponses werecomparedtocommercialplatinumpaintcommonlyusedas counterelectrodefordyesolarcells.Forsamplesobtainedby sput-teringforlessthan30s(Sample4),thesheetresistancepresented animpeditivevalue(fromapproximately15–50/䊐)aswellas betteropticaltransmittance.Insamplespreparedfor30,150,and 300s(thicknessequalto24.8,122.0,and319.0nm,respectively) thevaluesofsheetresistancewerebelowsubstrateresistanceand revealedgoodchargetransferefficiency.

Consideringthesetofparametersobtainedfromthisstudy,the 24.8nmthickﬁlm(obtainedwith30s)hasreducedsheetresistance andchargetransferresistancecomparedtocommercialPtthinﬁlm (fromthermaldecompositionofsolution),therebyimprovingthe recombinationprocessesatcounterelectrode.Forthenextstepof thiswork,dyesolarcellswillbeassembledwithoptimizedcounter electrodes.

Acknowledgements

TheauthorsacknowledgetheﬁnancialsupportofCAPES/PNPD, CAPES/PVNS,andCNPq(Grant555.686/2010-8)andthePhotonics DivisionofInstituteofAdvancedStudies(EFO-IEAv).

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