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Electrochimica
Acta
j o ur na l ho me p ag e :w w w . e l s e v i e r . c o m / l o c a t e / e l e c t a c t a
Dye-sensitized
solar
cells
based
on
agarose
gel
electrolytes
using
allylimidazolium
iodides
and
environmentally
benign
solvents
Hsin-Ling
Hsu, Cheng-Fang
Tien,
Ya-Ting
Yang,
Jihperng
Leu
∗DepartmentofMaterialsScienceandEngineering,NationalChiaoTungUniversity,1001UniversityRoad,Hsinchu30010,Taiwan
a
r
t
i
c
l
e
i
n
f
o
Articlehistory:Received15October2012 Accepted30December2012 Available online 6 January 2013 Keywords:
Allylimidazoliumiodides Ionicliquids
Agarosegelelectrolyte Dye-sensitizedsolarcell Environmentallybenignsolvents
a
b
s
t
r
a
c
t
Novel agarose gel electrolytes are prepared by allylimidazolium iodides-based ionic liquids and environmentally benign co-solvents (propylene carbonate (PC) and dimethyl sulfoxide (DMSO)) for dye-sensitized solar cells (DSSCs). Among 1-allyl-3-ethylimidadolium iodide (AEII), 1-allyl-3-propylimidazolium iodide (APII), 1-3-diallylimidazolium iodide (DAII), and 1-methyl-3-propylimidazoliumiodide(MPII)ionicliquids,theagarosegelelectrolytecontainingAEIIexhibitsthe bestDSSCperformance.TheefficiencyoftheDSSCusingtheagarosegelelectrolytecontaining1.5MAEII and0.65wt%agaroseis5.89%withthehighestI3−diffusioncoefficientof7.7×10−6cm2s−1.The
perfor-manceoftheAEIIionicliquid-basedagarosegelelectrolyteiscomparabletotheliquidelectrolytebased on3-methoxypropionitrile(MPN)(5.84%)underilluminationatAM1.5,100mWcm−2.Moreover,the DSSCperformanceoftheallylimidazoliumiodidesionicliquid-basedagaroseelectrolyteisdetermined bytheinteractionbetweenionicliquidandagarose,whichaffectstherigidityoftheionchannelsand theI3−diffusioncoefficient.
© 2013 Elsevier Ltd. All rights reserved.
1. Introduction
Dye-sensitizedsolarcells(DSSCs)haveattractedgreat atten-tionsincethemajorbreakthroughinconversionefficiencymade by Gräzel and O’Regan [1]. Over the past two decades, DSSCs havebecomeanattractivecandidateforservingasa renewable energysourceduetotheirlow-cost,relativelyhighconversion effi-ciency,andhigherefficienciesathighertemperatures.Themost efficientDSSC hasrecentlybeenreportedtopossessconversion efficiencygreaterthan12%[2].AtraditionalDSSCisconstructedby anelectrodeconsistingofaporousTiO2layerwithdyeadsorbed ontheTiO2surface,organicliquidelectrolytesolutioncontaining anI−/I3−redoxcouple,andaplatinum-coatedcounterelectrode. Yet,liquid electrolyte loss caused byleakage and volatilization hasbeenoneofthemajorproblemslimitingthelong-termuse ofDSSC.Toovercomethisproblem,solidifyingliquidelectrolytes toformgelorquasi-solid-stateelectrolytes[3,4]andaddingionic liquids [5] are the viable solutions to make the sealing pro-cesseasierandtominimizethelossofelectrolytesforenhanced durability.
Ionic liquids have been widely used in lithium batter-ies [6], fuel cells [7], electric double-layer capacitors [8], electrochromic devices [9], and DSSC [10], because of their
∗ Correspondingauthor.Tel.:+88635131420;fax:+88635724727. E-mailaddress:[email protected](J.Leu).
non-flammability,negativevaporpressure,non-volatility,andhigh conductivity.Inparticular,alkylimidazoliumiodides, 1-methyl-3-propylimidazoliumiodide(MPII),arecommonlyusedasanionic liquidinelectrolytes[11,12].IntheelectrolytesusedforDSSC,the viscosityoftheionicliquidiscriticalindeterminingtheion trans-portanddiffusioncoefficients[13].Sofar,mostworkshavefocused onthealkylimidazoliumiodides bymodifyingtheirsubstituent groupstotailortheproperties,suchas meltingpoint, viscosity, conductivity,andthermalstability[14].Ithasbeenreportedthat theviscosityoftheimidazoliumbasedionicliquidscanbereduced usingasymmetriccations[11].Inaddition,incertaincases,allyl groupsattachedtoimidazoliumcationshaveresultedinlower vis-cosityionicliquidsthan1,3-dialkylimidazoliumhalides[15,16]. DSSCsusingelectrolytesbasedonsupercooledallylimidazolium iodides,suchas1-allyl-3-methylimidazolium,AEII,andAPII,have beenfoundtoprovidegoodperformanceandstability[17].
Ionicliquidsalsopossessgoodsolubilityofpolysaccharidesand biomacromoleculessuchasagarose,whichareinsolubleinmost conventionalorganicsolvents[18–20].Inourpreviousstudy,the agarosegelelectrolyteswerecomposedofMPIIandthe environ-mentallybenignco-solvents,PCandDMSO,toimprovetheagarose solubilityandcapacitiesofionicliquidadditivesfordye-sensitized solarcells[21].Inspecific,DMSOwaslabeledasan environmen-tallyfriendlysolvent,duetoitslowtoxicpotential[22].Thus,in thisstudy,wearemotivatedtofurtherimprovethesolubilityof theagarosegelelectrolytebyaddingionicliquids.Inaddition,we plantoincorporateallylgroup(s)ontotheimidazoliumcationin 0013-4686/$–seefrontmatter © 2013 Elsevier Ltd. All rights reserved.
ordertoseeiftheviscosityoftheresultingionicliquidscouldbe furtherreducedandtounderstanditsimpactonDSSCefficiency.
In this study, AEII, APII, and DAII were synthesized and introducedintotheagarosegel electrolytes,tocomparewitha referenceelectrolytebasedonthealkylimidazoliumiodideionic liquid,MPII.Theviscositiesanddiffusioncoefficientsoftheseionic liquidsandtheionicagarosegelsweremeasuredbyrheometer andcyclicvoltammetry,respectively.Moreover,thephotoelectric conversionefficienciesandelectrochemicalpropertiesoftheDSSCs containingallylimidazoliumiodidesandMPIIwerecharacterized. Theinteractionbetweenallylimidazoliumiodidesandagarose,the effectsoftheagarosecontent,andtheirimpactonDSSCefficiency areexaminedanddiscussed.
2. Experimental
2.1. Materials
1-Methylimidazole (97%)and iodine(99.8%) werepurchased fromAcrosOrganics.AgarosetypeVII,propylenecarbonate(99%) and dimethyl sulfoxide (99.7%) were obtainedfrom Sigma and guanidiniumthiocyanate (99%) (GuSCN),1-allylimidazole(99%), iodoethane (98%), 1-iodopropane (98%), allyl iodide (97%) and N-methylbenzimidazole(99%)(NMBI)werepurchasedfromAlfa Aesar.N719(Ruthenium535-bisTBA)wasprocuredfrom UniRe-gionBioTech.Theethylacetate(99.8%;HPLCgrade)wasobtained fromECHO.Allorganicmaterialswereusedas-receivedwithout furtherpurification.
2.2. Synthesisof1-methyl-3-proplylimidazoliumiodide(MPII) ThesynthesisprocedureforMPIIfollowedpreviouslyreported methodology[21,23].Thesynthesisreactionswerecarriedoutin waterbathundersonicationconditionsfor4h.Forourwork,an ultrasoniccleaner(DeltaDC200;DeltaNew InstrumentCo.Ltd., Taiwan)with200Wpowerand40kHzfrequencywasused.The temperatureofthesynthesiswasnotspecificallycontrolledand maybeincreasedto40◦Cundersonicationcondition.Theyieldof 1-methyl-3-proplylimidazoliumiodidewas95%.TheMPIIstructure wasvalidatedby1HNMRspectroscopyandelectrosprayionization massspectroscopy.
AEII,APII,andDAIIweresynthesizedbythesameprocedure, whiletheethylacetatesolventwasusedandmixedwiththe start-ingmaterialsinthesynthesisofAEIIandDAII.
2.3. Preparationofagarosegels
Thesolutionsconsistingof1.5Mionicliquidandagaroseat dif-ferentconcentrations(0–1.0wt%)inPC/DMSOco-solvents(volume ratio, 8:2) were heated to 150◦C until the agarose was com-pletelydissolved.Additivessuchas0.1Mguanidiniumthiocyanate (GuSCN),0.2MN-methylbenzimidazole(NMBI),and0.05MI2were thenmixedintothehotagarosesolutions.Gelelectrolyteswere obtainedafterthesolutionswerecooleddown.
2.4. Rheologicalmeasurements
All rheological data were collected using an AR-G2 stress-controlledrheometer(TAInstruments).Theviscosityoftheionic liquidsandagarosegelsweremeasuredinatemperaturerange from25to150◦C(ramprate:5◦C/min.)andatanangular veloc-ityof0.1rad/s.Agarosegelswerecompressedbetweentwoheated parallelplateswithagapof0.5mm.
2.5. Measurementoftheelectrochemicalpropertiesofelectrolytes Thesandwich-typecellwasfabricatedbytwoplatinum-coated ITO (indium-doped tin oxide) glasses as the electrodes with a gapofabout60m.Thegapwassealedbytheadhesiveonthe edge.Steady-statecurrent-voltagecurvewasmeasuredbyusing SolartronSI1287.Thelimitedcurrentwasdeterminedinthe volt-agerangebetween−0.8Vand0.8Vatascanrateof5mVs−1.The diffusioncoefficientoftriiodide(DI3−)wascalculatedbyEq.(1):
DI−3 = Ilimd
2nFC (1)
whereIlimisthelimitingcurrentdensity,disthecellgap,nisthe numberofelectrons,FistheFaradayconstant,andCistheinitial I2concentration.
2.6. Fabricationofadye-sensitizedsolarcell
Adouble-layerTiO2wascoatedonfluorine-dopedSnO2(FTO) conductingglasselectrodebyscreen-printing.A13m-thickTiO2 filmconsistingofanataseTiO2(particlesize:20nm)actedasthe photoelectrode,whilea4m-thickTiO2 filmof400nmanatase TiO2servedasthelight-scattinglayer.Thedetailsofthe prepara-tionofTiO2pasteforscreen-printinghavebeenreportedelsewhere [24]. Finally,theFTO/TiO2 photo-anodewasannealed at500◦C for15minandthensensitized inanN719dye/ethanolsolution (5×10−4M)atroomtemperaturefor24h. DSSCwasfabricated bysealingthedye-sensitizedTiO2photo-anodeandPt-sputtered cathodearound100◦Cwitha25mhotmeltsealingfoil (SX1170-60,Solaronix).ADSSCcell(activearea0.283cm2)wascompleted upontheinjectionoftheelectrolyteintothecell.
2.7. Photoelectrochemicalmeasurement
AnAM1.5SolarSimulator(Newport3A)wasusedasthelight sourcewiththeincidentlightatsetat1Sun(100mWcm−2)as calibratedbyastandardSisolarcell(ORIEL)toevaluatethe photo-currentconversionefficiency.TheconversionefficiencyofDSSC basedonphotocurrentvs.voltage(I–V)curvewasrecordedwitha Keithley2400sourcemeter.Allmeasurementsinthisstudywere carriedoutatroomtemperature,25◦C.
3. Resultsanddiscussion
ThechemicalstructuresofAEII,APII,DAIIandMPIIasillustrated inScheme1.
Scheme1.MolecularstructuresofMPIIandallylimidazoliumiodides(AEII,APII, andDAII).
Fig.1. (a)DynamicviscosityasfunctionoftemperatureforMPII,AEII,APII,andDAII ionicliquidsand(b)dynamicviscosityasfunctionoftemperaturefortheagarose gelswithMPII,AEII,APII,andDAIIionicliquids.
3.1. Theviscositiesoftheionicliquidsandionicagarosegels Theviscosityoftheionic liquidis animportantmass trans-portpropertyinanelectrolyte.Thetemperaturedependencyof thedynamic viscosityofAEII, APII,DAII,and MPII ionicliquids werefirstmeasuredandshowninFig.1(a).Theviscosityatthe sametemperature (T≤70◦C) decreased in thefollowing order: MPII>APII>DAII≈AEII.Moreover,thetemperaturedependencyof AEII’sdynamicviscositywasaboutthesameasthatofDAII.The viscositiesofthethreeallylimidazoliumiodidesinthisstudywere alllowerthanMPII,theconventionalionicliquid.Theirviscosities werereduceddramaticallybecausetheallylgroupsinhibited crys-tallization[17].Inaddition,adecreaseinalkylchainlengthonthe imidazoliumcationwasfoundtoreducetheionicliquidviscosity, asevidencedbythefindingthattheviscosityofAEIIislowerthan APII.
Next,the temperature-dependentdynamic viscositiesof the agarosegelsconsistingoftheionicliquidsand0.5wt%agarosein PC/DMSOco-solvents,areshowninFig.1(b).Theagarosegelwith AEIIexhibitedthehighestviscosity,whilethegelwithDAIIshowed thelowestviscosity.AlthoughAEIIandDAIIbothexhibitedlow vis-cosityamongtheionicliquidsinFig.1(a),thenotabledifferencein agarosegelsviscosityindicatedthattheinteractionbetweenthe ionicliquidsandagaroseplaysacriticalroleindeterminingthe viscosityofagarosegel.Differentfrommostconventionalorganic solvents,ionicliquidscandissolveagarosebydisruptingthe hydro-genbondingwithin[25]toformhydrogen-bondingsbetweenthe hydroxylgroupsofagaroseandtheionicliquids.Inaddition,both
Fig.2.Steady-statecurrent–voltagecurvesofthegelelectrolytescontaining differ-entionicliquids.
thecationandanionoftheionicliquidcanaffectthesolubilityof agarose[18].
Wethenlookintothegelationmechanismofagaroseinthe co-solventssystem.Doublehelicesareformedfromrandomcoils in the solution through an intermediate state withmixed sin-gleanddoublehelicesuponcooling[26].Morespecifically,phase separationoccurswhenthesolvent-richregionandthe polymer-rich region,i.e.agarosegel appear afterhot agarosesolutionis cooleddownintheco-solvents system[21].Similarly,the gela-tionofagarosesolutioninvolvesconversionfromdisorderedcoils toorderedhelix conformation,leadingtodifferentionchannels intheagarosegelsfor transportingtheredoxcouples, depend-ingontheinteractionbetweentheionicliquidandtheagarose. ForAPIIandDAII,theirlargerimidazoliumcationsmayinhibitthe formationofhelixanddoublehelices[25].Inaddition,the inter-actionbetweendiallylgroupsontheimidazoliumionofDAIIand hydroxylgroupsonagarosemayfurtherhindertheformationof helixanddoublehelices.BothfactorsofDAIIgiverisetothe weak-estagarosegelasshown.Inthiscase,largevoidspacewascreated byfiber-likeagaroseconsistingofmoredisorderedagarosechains andlesshelicesintheDAIIsystem.Incontrast,AEIIhasasmaller imidazoliumcationandrelativelyweakerinteractionwithagarose duetoitssingleallylgroupandanalkylsidegroup.Asaresult, moreorderedhelixconformationwasformed,leadingtoastronger agarosebasedonAEIIionicliquid.
3.2. TheperformanceofDSSCswithdifferentallylimidazolium iodides
Thediffusion-limitedcurrentsoftheagaroseelectrolyteswith various ionic liquids are shown in Fig. 2. The I3− diffusion coefficientsareobtainedfromEq.(1).Thediffusionlimited cur-rentanddiffusioncoefficientfortheagaroseelectrolytebasedon AEII wasthehighest among MPII, AEII, APII and DAII.The dif-fusioncoefficientsofagarosegelelectrolytesforMPII,AEII,APII, andDAIIionicliquidswere1.6×10−6,5.6×10−6,2.3×10−6,and 1.8×10−6cm2s−1,respectively.
Theeffectofvariousionicliquids(MPII,AEII,APII,and DAII) intheagarosegelelectrolyteontheperformanceof DSSCswas furtherexaminedbythecurrentdensityvs.voltage.Thediffusion coefficientsoftheelectrolytes,opencircuitvoltage(Voc), short-circuitcurrentdensity(Jsc),fillfactor,andefficienciesoftheDSSCs containingallylimidazoliumiodides-basedagarosegelelectrolytes aresummarizedinTable1,inwhichtheDSSCcellcontainingthe MPII-basedagarosegelelectrolytewasusedasareference.When 0.5wt%agarosewasdissolvedinthemixturesofPC/DMSO,MPII
Table1
PhotovoltaicperformancesofDSSCbasedongelelectrolytescontaining0.5wt% agaroseanddifferentionicliquidsandthediffusioncoefficientoftheelectrolytes.
Ionicliquid MPIIa MPII AEII APII DAII
DI3−(×10 −6cm2s−1) 1.6 2.8 5.8 2.3 1.8 Voc(V) 0.63 0.70 0.72 0.70 0.70 Jsc(mAcm−2) 12.21 11.73 11.71 11.53 11.84 FF 0.61 0.64 0.65 0.62 0.60 (%) 4.72 5.25 5.45 4.97 4.96
aTheliquidphaseelectrolytewithoutagarose.
andtheadditives,theefficiencywasenhanced11%from4.72%to 5.25%.TheVocwasimprovedfrom0.63Vto0.70Vwiththe addi-tionofagarose.Amongfourionicliquids,thebestefficiencywas achievedbyusingtheAEIIagarosegelelectrolyte.TheAEII sys-temdeliveredthehighestVoc(0.72V)andFF(0.65),andrelatively highJsc(11.71mAcm−2),whichwascomparabletothoseofMPII, APIIandDAII(11.73, 11.53,and11.84mAcm−2).Becauseofthe moreorderedhelixanddouble-helicesconformationandrelatively weaker interactionbetweenAEII and agarose,theI3− diffusion coefficientwasenhancedthroughtherigidandstableionchannels andtheconcentrationofI3−aroundTiO2wasreduced.Moreover, thedarkreactionoccurringfromtheelectronsatTiO2andI3−was reduced,resultinginanincreasedVoc.
In contrast, theDSSC using theDAII agarose gel electrolyte showed the lowest open-circuit photovoltage (Voc), fill factor (FF),andphotoelectricconversionefficiencyamong the allylim-idazolium iodides, i.e. 0.70V, 0.65, and 4.96%, respectively. These correlated well withthe lowest I3− diffusion coefficient (1.8×10−6cm2s−1)intheDAIIagarosegelelectrolyteduetoless orderedhelixconformation,largevoidspaceintheagarosegel, and stronger interaction between the two allyl groups onthe imidazoliumcation and theagarose [27]. Thelowest I3− diffu-sioncoefficientoftheDAII-basedagaroseelectrolyteresultedin higherrecombination,leadingtothelowestefficiencyandFFvalue. Regardingthehigh-viscosityAPIIionicliquid,theDSSCusing APII-based agarose gel electrolyte showed similarefficiency to that usingDAIIbecauseofitscomparablylowagaroseviscosity(Fig.1b) resultingfromalongeralkylchainonimidazoliumcation. 3.3. TheeffectofagaroseconcentrationintheAEIIsystem
TheagarosegelelectrolytecontainingAEII(0.5wt%agarose), demonstrated the best performance, indicating that both the viscosity of the ionic liquid and the interaction between ionic liquidandagarosearecriticalfactors.Thus,wefurtherexamine the effect of agarose concentration on the DSSC cell perfor-manceusingtheAEII-basedagarosegelelectrolyte.Fig.3shows the I3− diffusion-limited currents of the agarose electrolytes withAEIIionicliquidcontainingvariousagaroseconcentrations, as measured by cyclic voltammetry using a symmetric thin layerelectrochemicalcell.Thediffusion-limitedcurrentincreased with increasing agarose content from 0wt% to 0.65wt%, and then decreased to a concentration >0.65wt%. The I3− diffusion coefficients were 2.3×10−6, 5.8×10−6, 7.7×10−6, 6.2×10−6, and1.9×10−6cm2s−1 for0(liquid-phaseelectrolyte),0.5,0.65, 0.85,and1.0wt%agarose,respectively.ThehighestI3−diffusion coefficient (7.7×10−6cm2s−1) was found at the concentra-tion of 0.65wt%, while the lowest I3− diffusion coefficient (1.9×10−6cm2s−1)wasfortheelectrolytewith1.0wt%agarose.
Agarose is readily dissolved in AEII ionic liquid and is an environmentallybenignPC/DMSOco-solventfor forming three-dimensional ion channels. With increasing agarose content up to 0.65wt%, the rigidity and stability of ion channels in AEII and PC/DMSO-based agarose system were improved, resulting ina higher I3− diffusioncoefficient.Abovetheoptimalagarose
Fig.3.Steady-statecurrent–voltagecurvesofthegelelectrolytescontaining differ-entconcentrationsofagarose.
concentration,thevoidspaceoftheionchannelsisreduced,leading toalowerI3−diffusioncoefficient[28].
Under a 100mWcm−2 lightsource,theI–Vcurves and dark I–VcurvesoftheDSSCscontainingAEIIionicliquidareillustrated inFig.4(a)and(b),respectively.Thediffusioncoefficientsofthe electrolytes,Voc,Jsc,fillfactor,and theefficienciesof theDSSCs containingAEII-basedagarosegelelectrolytesatvariousagarose concentrationsaresummarized inTable2.Theelectrolyte with-outagarosegel(0wt%)exhibitedthehighestJsc,butthelowest Voc,FF,andefficiency.Theefficiencyincreasedasweincreasedthe
Fig.4. (a)Lightand(b)darkJ–VcurvesofDSSCsusingAEII-basedagarosegel elec-trolyteswithvariousconcentrationsofagarose.
Table2
PhotovoltaicperformancesofDSSCbasedongelelectrolyteswithvarious concen-trationsandthediffusioncoefficientoftheelectrolytes.
Agarosecontent 0wt% 0.5wt% 0.65wt% 0.8wt% 1.0wt% DI3−(×10−6cm2s−1) 2.3 5.8 7.7 6.2 1.9 Voc(V) 0.66 0.72 0.76 0.75 0.70 Jsc(mAcm−2) 12.62 11.71 11.45 11.82 11.96 FF 0.60 0.65 0.68 0.64 0.63 (%) 4.97 5.45 5.89 5.68 5.31
amountofagaroseupto0.65wt%,andthendroppedastheamount
ofagarosewasraisedto0.8wt%and1.0wt%.Theenhancementof
efficiencyrelativetoanoagarosecase(0wt%)is9.7%,18.5%,14.2%,
and6.8% foragaroseconcentrations at0.5, 0.65, 0.8and 1wt%,
respectively.Anincreaseofagaroseconcentration(upto0.65wt%)
wasaccompaniedbyincreasingVocandFF.TheincreasedVocmay
resultfromthesuppressedreductioninthebackelectrons
trans-ferfromtheconductingbandofTiO2totheI3−intheelectrolyte
[29].
AssummarizedinTable2,AEII-basedagarosegelelectrolytes yieldedthehighesttriiodidediffusioncoefficientsattheoptimal concentrationofagarose(0.65wt%).Thisloweredthe concentra-tionofI3−aroundthedyedTiO2andreducedtherecombination ofoxidativedyeandI3−.TheinjectionofelectronsfromTiO2was acceleratedandtheelectronconcentrationonTiO2wasincreased. Moreover,sterichindranceeffectoccurredasagaroseadsorbedon TiO2,whichreducedthereactionbetweenoxidativedyeandI3−. ThereducedrecombinationledtoanincreaseinVocandFF. Over-all,thebestperformanceoftheagarosegelelectrolytewas5.89% for0.65wt%agarose,whichcorrelatedwiththehighesttriiodide diffusioncoefficient(7.7×10−6cm2s−1).Incomparison,the con-versionefficiencywas5.84%fortheDSSCcellusinganMPN-based liquidelectrolytecontaining0.5MLiI,0.2MNMBI,0.1MGuSCN and0.05MI2inMPN.Thus,theAEIIionicliquid-basedagarosegel electrolytedemonstratedcomparableDSSCperformancetotheone usingMPN-basedliquidelectrolyte.
4. Conclusions
Low-viscosity ionic liquids, allylimidazolium iodides, were introduced to environmentally benign co-solvents (PC/DMSO) basedagarosegelelectrolytesforthefabricationofDSSCs.Among MPII,AEII,APII and DAII,theagarosegelelectrolyte containing AEIIexhibitedthebestDSSCperformance.Tobemorespecific,the efficiencyoftheDSSCusingtheagarosegelelectrolytecontaining 1.5MAEII,0.65wt%agarose,0.1MGuSCN,0.2MNMBI,and0.05M I2was5.89%withanI3−diffusioncoefficientof7.7×10−6cm2s−1. Eventhoughtheenvironmentallybenignsolventandthenatural product,agarose,wereadaptedintheelectrolytes,theconversion efficiency(=5.89%)oftheDSSCcontainingtheagarosegel elec-trolytebasedPC/DMSOandAEIIwascomparablewiththeliquid electrolytebasedonMPN(=5.84%).
In summary, the DSSC performance of the allylimidazolium iodidesionicliquid-basedagaroseelectrolytewasdeterminedby theinteractionbetweenionic liquid andagarose, which affects therigidityofionchannelsandtheI3−diffusioncoefficient.Even thoughAEIIandDAIIexhibitedsimilarviscosity,theirDSSC per-formancewasquitedifferent(5.45%vs.4.96%).AEIIhadasmaller imidazoliumcationandrelativelyweakerinteractionwithagarose duetoitssingleallylgroupandanalkylsidegroup.AEIIpossessed low-viscosityionicliquidandhighsolubilityagarosetoforma low-viscosityagarosegelelectrolyte.Moreorderedagarosehelixand doubleheliceswereformedwithrigidandstableionchannelsin theagarosegel electrolyte,leadingtothereleaseof moreionic
liquidintothesolventwithhighdiffusivityintheelectrolyteand enhancedDSSCperformance.
Acknowledgements
TheauthorsthanktheNationalScienceCouncilofTaiwanfor thefinancialsupportundergrantnos:NSC 101-3113-E-007-001-andNSC101-2112-M-009-016-MY2.
References
[1] M.Grätzel,B.O’Regan,Alow-cost,high-efficiencysolarcellbasedon dye-sensitizedcolloidalTiO2films,Nature353(1991)737.
[2] A.Yella,H.-W.Lee,H.N.Tsao,C.Yi,A.K.Chandiran,M.K.Nazeeruddin, E.W.-G.Diau,C.-Y.Yeh,S.M.Zakeeruddin,M.Grätzel,Porphyrin-sensitizedsolar cellswithcobalt(II/III)–basedredoxelectrolyteexceed12percentefficiency, Science334(2011)629.
[3]J.E.Benedetti,A.D.Gonc¸alves,A.L.B.Formiga,M.A.DePaoli,X.Li,J.R.Durrant, A.F.Nogueira,Apolymergelelectrolytecomposedofapoly(ethyleneoxide) copolymerandtheinfluenceofitscompositiononthedynamicsand per-formanceofdye-sensitizedsolarcells,JournalofPowerSources195(2010) 1246.
[4]T.M.W.J.Bandara,M.A.K.L.Dissanayake,B.E.Mellander,Dyesensitizedsolar cellswithpoly(acrylonitrile)basedplasticizedelectrolytecontainingMgI2,
ElectrochimicaActa55(2010)2044.
[5]D.Kuang,P.Wang,S.Ito,S.M.Zakeeruddin,M.Grätzel,Stablemesoscopic dye-sensitizedsolarcellsbasedontetracyanoborateionicliquidelectrolyte,Journal oftheAmericanChemicalSociety128(2006)7732.
[6]L.Niedzicki,G.Z. ˙Zukowska,M.Bukowska,P.Szczeci ´nski,S.Grugeon,S.Laruelle, M.Armand,S.Panero,B.Scrosati,M.Marcinek,W.Wieczorek,Newtypeof imid-azolebasedsaltsdesignedspecificallyforlithiumionbatteries,Electrochimica Acta55(2010)1450.
[7]R.F. de Souza, J.C.Padilha, R.S. Gonc¸alves, J. Dupont,Room temperature dialkylimidazoliumionicliquid-basedfuelcells,Electrochemistry Communi-cations5(2003)728.
[8] A.Orita,K.Kamijima,M.Yoshida,Allyl-functionalizedionicliquidsas elec-trolytesforelectricdouble-layercapacitors,JournalofPowerSources195 (2010)7471.
[9]R.Marcilla,F.Alcaide,H.Sardon,J.A.Pomposo,C.Pozo-Gonzalo,D.Mecerreyes, Tailor-madepolymerelectrolytesbaseduponionicliquidsandtheir applica-tioninall-plasticelectrochromicdevices,ElectrochemistryCommunications8 (2006)482.
[10]P.Wang,S.M.Zakeeruddin,J.-E.Moser,M.Grätzel,Anewionicliquidelectrolyte enhancestheconversionefficiencyofdye-sensitizedsolarcells,TheJournalof PhysicalChemistryB107(2003)13280.
[11]D.-W.Seo,S.Sarker,N.C.D.Nath,S.-W.Choi,A.J.S.Ahammad,J.-J.Lee,W.-G. Kim,Synthesisofanovelimidazolium-basedelectrolytesandapplicationfor dye-sensitizedsolarcells,ElectrochimicaActa55(2010)1483.
[12]M.Berginc,U.OparaKraˇsovec,M.Jankovec,M.Topiˇc,Theeffectoftemperature ontheperformanceofdye-sensitizedsolarcellsbasedona propyl-methyl-imidazoliumiodideelectrolyte,SolarEnergyMaterialsandSolarCells91(2007) 821.
[13]A.Ejigu,K.R.J.Lovelock,P.Licence,D.A.Walsh,Iodide/triiodide electrochem-istryinionicliquids:Effectofviscosityonmasstransport,voltammetryand scanningelectrochemicalmicroscopy,ElectrochimicaActa56(2011)10313. [14]S.Keskin,D.Kayrak-Talay,U.Akman,Ö.Hortac¸su,Areviewofionicliquids
towardssupercriticalfluidapplications,TheJournalofSupercriticalFluids43 (2007)150.
[15]T.Mizumo,E.Marwanta,N.Matsumi,H.Ohno,Allylimidazoliumhalidesas novelroomtemperatureionicliquids,ChemistryLetters33(2004)1360. [16]D.Zhao,Z.Fei,W.Ang,P.J.Dyson,Sulfonium-basedIonicliquidsincorporating
theallylfunctionality,InternationalJournalofMolecularSciences8(2007)304. [17]Z.Fei,D.Kuang,D.Zhao,C.Klein,W.H.Ang,S.M.Zakeeruddin,M.Grätzel, P.J.Dyson,Asupercooledimidazoliumiodideionicliquidasalow-viscosity electrolytefordye-sensitizedsolarcells,InorganicChemistry45(2006)10407. [18]R.P.Swatloski,S.K.Spear,J.D.Holbrey,R.D.Rogers,Dissolutionofcellosewith
ionicliquids,JournaloftheAmericanChemicalSociety124(2002)4974. [19]H.Xie,T.Shi,Liquefactionofwood(Metasequoiaglyptostroboides)inallylalkyl
imidazoliumionicliquids,WoodScienceandTechnology44(2010)119. [20]Y.Fukaya,A.Sugimoto,H.Ohno,Superiorsolubilityofpolysaccharidesinlow
viscosity,polar,andhalogen-free1,3-dialkylimidazoliumformates, Biomacro-molecules7(2006)3295.
[21]H.-L.Hsu,W.-T.Hsu,J.Leu,Effectsofenvironmentallybenignsolventsinthe agarosegelelectrolytesondye-sensitizedsolarcells,ElectrochimicaActa56 (2011)5904.
[22]I.Soroko,Y.Bhole,A.G.Livingston,Environmentallyfriendlyrouteforthe preparationofsolventresistantpolyimidenanofiltrationmembranes,Green Chemistry13(2011)162.
[23]V.V.Namboodiri,R.S.Varma,Solvent-freesonochemicalpreparationofionic liquids,OrganicLetters4(2002)3161.
[24] S.Ito,T.N.Murakami,P.Comte,P.Liska,C.Grätzel,M.K.Nazeeruddin,M.Grätzel, Fabricationofthinfilmdyesensitizedsolarcellswithsolartoelectricpower conversionefficiencyover10%,ThinSolidFilms516(2008)4613.
[25]T.Singh,T.J.Trivedi,A.Kumar,Dissolution,regenerationandion-gelformation ofagaroseinroom-temperatureionicliquids,GreenChemistry12(2010)1029. [26]J.M.Guenet,C.Rochas,Agarosesolsandgelsrevisited,Macromolecular
Sympo-sia242(2006)65.
[27]A.W.Baker,A.T.Shulgin,IntramolecularHydrogenBondsto-Electronsand OtherWeaklyBasicGroups,JournaloftheAmericanChemicalSociety80(1958) 5358.
[28]N.Fatin-Rouge,A.Milon,J.Buffle,R.R.Goulet,A.Tessier,Diffusionand parti-tioningofsolutesinagarosehydrogels:Therelativeinfluenceofelectrostatic andspecificinteractions,TheJournalofPhysicalChemistryB107(2003)12126. [29]H.W. Han, W. Liu, J. Zhang, X.Z. Zhao, A hybrid poly(ethylene oxide)/ poly(vinylidenefluoride)/TiO2nanoparticlesolid-stateredoxelectrolytefor
dye-sensitizednanocrystallinesolarcells,AdvancedFunctionalMaterials15 (2005)1940.
