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Electrochimica
Acta
jo u r n al h om ep a ge :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
Effects
of
environmentally
benign
solvents
in
the
agarose
gel
electrolytes
on
dye-sensitized
solar
cells
Hsin-Ling
Hsu,
Wan-Ting
Hsu,
Jihperng
Leu
∗DepartmentofMaterialsScienceandEngineering,NationalChiaoTungUniversity,1001UniversityRoad,Hsinchu30010,Taiwan
a
r
t
i
c
l
e
i
n
f
o
Articlehistory:
Received21January2011
Receivedinrevisedform24April2011 Accepted30April2011
Available online 7 May 2011 Keywords:
Agarose Gelelectrolyte Dye-sensitizedsolarcell Environmentalbenignsolvent
a
b
s
t
r
a
c
t
Theionicagarosegelelectrolytesarepreparedbyusingenvironmentalbenignsolventsandco-solvents toimprovetheagarosesolubilityandcapacitiesoftheadditivesfordye-sensitizedsolarcells.Theeffects ofsinglesolvents(dimethylsulfoxide(DMSO),propylenecarbonate(PC),propyleneglycol,triethylene glycol,andtetraethyleneglycol)andDMSO-basedco-solventsareexaminedontheconductivities, diffu-sioncoefficientsoftriiodide,andenergyconversionefficiencies.Thehighestconductivity,14.2mScm−1,
andthehighestdiffusioncoefficientoftriiodide,2.7×10−6cm2s−1,areachievedfortheelectrolyte
con-tainingtheco-solventof80vol.%PCand20vol.%DMSO.Theenvironmentalbenignco-solventsuchas DMSO/PCcansignificantlyincreasetheconversionefficiencyto3.4%withagarosecomparedtopure MPIIwithagarose(1.4%),whileretaining∼80%oftheenergyconversionefficienciesofthereferencecell withoutagaroseundertheilluminationatAM1.5,100mWcm−2.
© 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Dye-sensitizedsolarcells(DSSCs)haveattractedgreatattention sincethemajorbreakthroughintheconversionefficiencymade byO’ReganandGräzel[1].Atraditional DSSCisconstructedby anelectrodeconsistingofaporousTiO2layerwithdyeadsorbed
ontheTiO2surface,organicliquidelectrolytesolutioncontaining
aI−/I3− redoxcouple,andaplatinum-coatedcounterelectrode.
Overthepasttwodecades,DSSCshavebeendevelopedassolar cellcandidatesdue totheirlow-cost,relativelyhighconversion efficiency,andhigherefficienciesathighertemperatures[2].
Themost efficientDSSC is reportedtopossess high conver-sion efficiency greater than 11% [3], which shows promise as a low-cost renewable energy source. However, electrolyte loss causedbyleakageandvolatilizationofliquidelectrolyteshasbeen oneofthemajorproblemslimitingthelong-termuseofDSSCs. Inordertoovercomethisproblem, solidifyingliquidelectrolyte forminggelorquasi-solid-stateelectrolyteshasbeentheprimary solutiontomakethesealingprocesseasierandtominimizethe lossof electrolytesfor enhanced durability.Typically, polymers [4–7], low molecularweight gelators [8] and nanoparticles [9] aregelledwithliquidelectrolytestoformquasi-solid-state elec-trolytes.Quasi-solid-stateDSSCsmadebypolyacrylonitrile(PAN) [4],poly(vinylidenefluoride-co-hexafluoropropylene)(PVDF/HFP) [5],poly(ethyleneoxide)(PEO)[6]andtheircopolymer[7] main-tainover90%oftheefficienciesofDSSCsusingliquidelectrolytes.
∗ Correspondingauthor.Tel.:+88635131420;fax:+88635724727. E-mailaddress:[email protected](J.Leu).
Anotherapproachusesnaturalmaterialssuchas polysaccha-rides to gel the liquid electrolytes of DSSCs, which prevents environmentalpollutionbynotformingwastesrequiringdisposal oradditionaltreatment.Agarose,alinearpolymerofcarbohydrates fromseaweed,canactasthegelatorofaqueouselectrolytesand formaporousmatrixwhenthehotsolutionwith0.5–5wt.%agarose iscooleddown.Inrecentyears,agarosehasbeenusedasthematrix ofthegelelectrolyteforDSSCs.Ingeneral,therearetwo meth-odstoobtainagarosegelelectrolytes.Oneistodissolveagarose inpurehotwater,pourthesolutiononaFTO/TiO2/dyeelectrode,
andthensoaktheelectrodeinthesolutioncontainingaredox elec-trolytetoexchangewater[10,11].Theothermethodistodissolve agaroseinionicliquiddirectly[12,13].Thepreparationofthe for-meris complicatedand theconcentrations oftheadditivesand waterarehardtodetermine.Inthelattermethod,thecapacityofLiI intheionicliquidwasfoundtodecrease,resultingina10% reduc-tioninefficiency[12].Tomakeitworse,manyadditivessuchas LiI[14],4-tert-butylpyridine(t-TBP)[15],1-methylbenzimidazole (NMBI)[16] andguanidiumthiocyanate(GuSCN)[17],are com-monlyusedtoimprovetheperformanceofDSSCs,whichfurther decreasesthecapacitiesofionicliquids.Thus,thelowsolubilityof agaroseinsomeionicliquidsandthedecreasedcapacitiesofionic liquidsduetoadditivesarethechallengingissuesfortheuseof polysaccharides.Therefore,thisstudyattemptstochoose appropri-atesolventsforagaroseionicgeltoincreasetheamountofadditives andimprovetheefficiencyofDSSCs.
Therequirementsofsolventsfor ionicagarosegelforDSSCs includehighboilingpoint,lowvaporpressure,andcompatibility withagarose.Solventswithlowtoxicityarealsoconsideredasan importantprerequisiteforourintegralpursuitofenvironmentally
0013-4686/$–seefrontmatter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.04.117
benignchemistryaswellastheuseofnon-pollutingagarose.In thisstudy,ourobjectiveistoreduceoreliminatethesolventsand matrixinthegelelectrolytesthatarehazardoustohumanhealth ortheenvironment.Specifically,theionicgelelectrolytesinthis studyconsistedof1-methyl-3-propylimidazoliumiodide(MPII), effectivesolvents,andagarose.SinglesolventsandDMSO-based co-solventswithhighboilingpoint,lowvaporpressure,andlow toxicitywerestudiedfortheircapacitiesintheionicagarosegel andtheirimpacts onelectrochemicalpropertiesandconversion efficiencyforDSSCs.Theelectrochemicalpropertiessuchas con-ductivityand diffusioncoefficientof theionic agarosegelwith singlesolventsandco-solventswerestudiedbyACimpedanceand cyclicvoltammetry.Moreover,thephotoelectricconversion effi-cienciesoftheDSSCsweremeasured.Theroleoftheco-solventin ionicagaroseelectrolytesforDSSCsarediscussedandelucidated. 2. Experimental
2.1. Materials
1-Methylimidazole(97%),propyliodide(98%),iodine (99.8%) and4-tert-butylpyridine(96%)werepurchasedfromAcros Organ-ics.AgarosetypeVII,triethyleneglycol(99%),tetraethyleneglycol (99%), propylene glycol, propylene carbonate (99%), dimethyl sulfoxide(99.7%)wereobtainedfromSigmaandguanidinium thio-cyanate(99%)(GuSCN)andN-methylbenzimidazole(99%)(NMBI) werepurchasedfromAlfa Aesar.N719(ruthenium535-bisTBA) wasprocuredfromUniRegionBioTech.Allorganicmaterialsand TiO2 (P25, Degussa AG)were used as-receivedwithoutfurther
purification.
2.2. Synthesisof1-methyl-3-proplylimidazoliumiodide(MPII) ThesynthesisprocedureforMPIIfollowedpreviouslyreported methodology[18].1-Methlylimidazoleand propyliodide(molar ratio of 1:1.1) were first mixed in the vessel, and were then placedintoanultrasonicbathfor4h.Ayellowliquidwasobtained andwashedbyacetoneandethylacetate3times.Afterthe sol-vents were removed by a rotary evaporator, the product was driedunder avacuumfor 12hat 50◦C. Theyield of 1 methyl-3-proplylimidazolium iodide was 93%. The MPII structure was validated by 1H NMR spectroscopy and electrosprayionization
massspectroscopy.
2.3. Preparationofagarosegels
The solutions consisting of 1.8M MPII, 5wt% DIwater, and 0.5wt%agarosein varioussolvents wereheatedto150◦C until theagarose was completely dissolved. Additives suchas 0.1M guanidiniumthiocyanate(GuSCN),0.5MN-methylbenzimidazole (NMBI),and0.1MI2werethenmixedintothehotagarose
solu-tions.Gelelectrolyteswereobtainedafterthesolutionswerecooled down.
2.4. Measurementoftheelectrochemicalpropertiesofelectrolytes Thesandwichtypecellwasfabricatedbytwoplatinum-coated ITO (indium-doped tin oxide) glasses as the electrodes with a gapof about25m, which wassealed by theadhesive onthe edge.Electrolytesweretheninjectedintothecellthroughthegap. Steady-statecurrent–voltagecurveandconductivity()were mea-suredbyusingSolartronSI1287andHP4194A.Theconductivity wascalculatedbythefollowingEq.(1):
= L ARb
(1)
HereListhegapofsymmetriccellandAistheareaofthe elec-trode.Rbisthebulkresistanceofthegelelectrolytemeasuredby
impedanceanalyzerat 0.1MHz.The limitedcurrent was deter-minedinthevoltagerangebetween−0.8Vand0.8Vatascanrate of5mVs−1.Thediffusioncoefficientoftriiodidewascalculatedby thefollowingEq.(2):
DI3−=
Ilimd
2nFC (2)
whereDI3−isthediffusionconstantoftriiodide,Ilimisthelimiting
currentdensity,disthecellgap,nisthenumberoftheelectrons,F istheFaradayconstantandCistheinitialI2concentration.
2.5. Fabricationofadye-sensitizedsolarcell
ITO-coated glass (7/square) was cleaned in acetone and ethanol using an ultrasonic cleaner prior to use. Commercial nanocrystallineTiO2(P25)andP123[poly(ethylene
glycol)-block-poly(propyleneglycol)-block-poly(ethyleneglycol))weremixed in n-butanol to form TiO2 colloidal suspension. A TiO2 layer
(∼10m) was subsequently prepared by a doctor-blade coat-ing technique onto the ITO/glass, which was then sintered at 400◦Cfor1h.TheTiO2/ITO/glassanodewasimmersedina0.1M
TiCl4 aqueous solutionfor30minin anice bath,and then
sin-tered at 400◦C again to make a good binding between TiO2
particlesandattheinterfacebetweenTiO2particlesandthe
con-ducting glass. Finally,the ITO/TiO2 photo-anodewas sensitized
in a N719 dye/ethanol solution(3×10−4M) at room tempera-turefor24h. DSSCwasfabricatedbysealingthedye-sensitized TiO2 photo-anodeand Pt-sputteredcathodearound100◦C with
a 60m hot melt sealing foil (SX1170-60, SOLARONIX), which also served as a spacer. A DSSC cell (active area 0.25cm2)
was completed upon the injection of the electrolyte into the cell.
2.6. Photoelectrochemicalmeasurement
An AM1.5 Solar Simulator (Newport 3A) was used as the light source with the incident light at 1 sun (100mWcm−2) calibrated by a standard Si solar cell (ORIEL), to evaluate the photo-current conversionefficiency. I–Vcurves were measured byscanningtheDSSCsfromtheshortcurrent condition(Jsc)to
the open circuit voltage (Voc) of the cell. The conversion
effi-ciency of DSSC based on photocurrent vs. voltage (I–V) curve wasrecorded witha Keithley2400sourcemeter. All measure-ments in this study were carried out at room temperature, 25◦C.
3. Resultsanddiscussion
3.1. Selectionofenvironmentallybenignsolventsforagarosegel Agaroseis a polysaccharide extracted from seaweedwith a repeating structure of 1,3-linked -d-galactopyranose and 1,4-linked3,6-anhydro-␣-l-galactopyranose.Thegelationmechanism ofagarosehasbeenreportedasillustratedinFig.1[19],inwhich doublehelices(GelII)areformedfromrandomcoilsinthe solu-tionviaanintermediatestate(GelI)withmixedsingleanddouble helicesupon cooling.However,thesolubilityofagaroseinMPII islowandthecapacityofadditivesintheagarosegelisscanty. Ingeneral,theadditionofadditivescouldrestrainthedarkcurrent [15,16]andimprovethephotoelectricconversionefficiency. There-fore,appropriatesolventsarerequiredtoincreasethesolubilityof thecommonlyusedadditivessuchasLiI,4-tert-butylpyridine(TBP), guanidinium thiocyanate (GuSCN), and N-methylbenzimidazole (NMBI) in the agarose gel. Typically, agarose can dissolve in
Fig.1. Gelationmechanismofagarose:startingfromrandomcoilsinthehot solu-tiontomixedrandomcoils,singleanddoublehelicesintheintermediatestage(Gel I),thentotheformationofdoublehelicesinthecoolsolution(GelII).
AdaptedfromRef.[19].
dimethylsulfoxide(DMSO),dimethylformamide(DMF), dimethy-lacetamide(DMAc),andboilingwater[20].Amongthesesolvents, LD50,whichisthedosageofchemicalgiven allatonce,causing
thedeathof50%oftestanimals,forDMFandDMAcare2.8and 5.1gkg−1,respectively.SuchanLD50levelisnotbenigntoanimals
andhumanbeings.Currently,manysolventsandplasticizerssuch asacetonitrile(ACN)[21],3-methoxypropionitrile(MPN)[22],and gamma-butyrolactone(GBL)[23]havebeenutilizedinDSSCsfor achievinghighefficiencies.However,thesesolventswithLD50in
therangeof1.5–4.0gkg−1exhibitacuteoraltoxicity.ACNshows highperformanceinDSSCs,butexhibitstoxicitytohumanorgans [24].Thus,lowtoxicityofsolventsisconsideredasanimportant prerequisiteinourefforttodevelopenvironmentallybenign chem-istryaswellastheuseofnon-pollutingagaroseforDSSCs.
Inthisstudy,theLD50levelsoftheenvironmentallybenign
sol-ventswerechosentobehigherthan14gkg−1.Inaddition,high boilingpointandlowvaporpressurewerepreferred.Specifically, DMSO, PC, propylene glycol (PG), triethylene glycol (3EG), and tetraethyleneglycol(4EG)wereselected.Table1summarizestheir keyproperties,includingboilingpoint,meltingpoint,vapor pres-sure,viscosity,anddielectricconstant.Commonsolvents,ACNand MPN,arealsolistedinTable1forthesakeofcomparison.The boil-ingpointsoftheseenvironmentallybenignsolventsarehigherthan 180◦C,whiletheirvaporpressuresarelowerthan0.6hPaat20◦C. Suchcharacteristicsmakethesesolvents(DMSO,PC,PG,3EG,and 4EG)excellentcandidatesforDSSCdevices.
3.2. Influenceofthesolventsonthediffusivityandphotovoltaic performance
Thechoiceofsolventsforagarosewaslimitedbecauseofthe poorsolubilityof agarosein mostorganicsolvents. Amongthe
Fig.2. Steady-statecurrent–voltagecurvesofthegelelectrolytescontainingvarious solvents.
DMSO,3EG,4EG,PGandPC,DMSOisagoodsolventforagarose, while3EG,4EG,PG,andPCarethepoorsolventsofagarosedue totheir poorsolubility.In addition, agarosewas foundto pre-cipitatewhenPCorPGwasaddedinagarose/MPIIhotsolution. Agarose/MPIIdissolvedwellafteradding3EG,4EG,orDMSO.Also, when3EGor4EGwasusedassolvent,theratioofMPIIto3EGor 4EGhadtobehigherthan3:10(1.2Minsolvent)inorderto dis-solveagarosecompletely.Asaresult,theconcentrationof1.8M MPIIwasappliedtoalltheagarosegelelectrolytesinthe subse-quentsectionsbecausethehighestefficiencieswererevealedafter comparingwith1.2and2.0Min3EGand4EG.
Theagarosegelelectrolyteswerethenpreparedusingvarious solvents (DMSO,3EG,and 4EG)atdifferentconcentrations.The solubilityofagaroseinDMSOwasfoundtobemuchhigherthan thatin3EGor4EG.Also,gelationdidnotoccurwhentheamount ofDMSO wasmore than20vol.% intheagarosegelelectrolyte. Itis believedthatthereisa stronginteractionbetweenagarose and DMSO[25],makingthechainsofagarosewellextendedin DMSOandthushardtoaggregateandformagarosewithDMSOat >20vol.%.Thediffusion-limitedcurrentsoftheagaroseelectrolytes withvarioussolventsweremeasuredbycyclicvoltammetryusing asymmetricthinlayerelectrochemicalcell,asshowninFig.2.This showedthatthediffusionlimitedcurrentsforelectrolytesbasedon DMSOwerehigherthanthosebasedon100vol.%for3EGand4EG. Thediffusioncoefficientsoftriiodideionsintheinitialstate(no organicsolvent)for3EGand4EGsystemswere6.1×10−8cm2s−1,
2.7×10−7cm2s−1and2.2×10−7cm2s−1,respectively.Incontrast,
ahigherdiffusioncoefficientof4.8×10−7cm2s−1 wasachieved
usinganelectrolytecontainingonly20vol.%DMSO.
Theeffectofthesolvents(DMSO,3EGand4EG)onthe perfor-manceofDSSCswasexaminedbythecurrentdensityvs.voltage. The additionof thesolventsin theweight percentagerange of 0–100improvednotonlytheopencircuitvoltage(Voc)butalsothe
Table1
KeypropertiesandLD50ofcommonandselectedsolventsfortheagarosegelelectrolytes.
DMSO PC PG 3EG 4EG ACN MPN
Meltingpoint(◦C) 18.4 −49 −59 −7 −6 −48 −50
Boilingpoint(◦C) 189 242 188 285 314 82 165
Vaporpressure(hPaat20◦C) 0.56 0.04 0.11 <0.001 <0.001 91.7 2.29
Viscosity(mPas) 2 2.8 49 48 62 0.37 1.6
Dielectricconstant 46.5 64.4 32 23.69 20.44 38 40
LD50Orl-rat(gkg−1) 14 29 20 15–17 28–34 2.5 4.4
Table2
PhotovoltaicperformancesofDSSCbasedongelelectrolytesusingdifferentsolvents.
Solvent Initial 3EG 4EG DMSO
vol.%ofsolvent 0 50 100 50 100 10 20
Voc(V) 0.45 0.60 0.61 0.59 0.61 0.53 0.58
Jsc(mAcm−2) 2.69 2.58 3.22 2.35 3.20 3.05 3.69
FF 0.55 0.57 0.57 0.60 0.55 0.48 0.54
(%) 0.68 0.88 1.13 0.83 1.08 0.78 1.15
short-circuitcurrentdensity(Jsc),assummarizedinTable2.When
50%3EGand4EGsolventswereaddedintotheinitialgelelectrolyte (withoutorganicsolvent),theirJscvalues(2.58and2.35mAcm−2)
weresmallerthanthatoftheinitialgelelectrolyte(2.69mAcm−2). Whenmoreglycolupto100vol.%wasadded,Jsc wasincreased
to3.22and3.20mAcm−2for3EGand4EG,respectively, presum-ablyduetoa260%increaseindiffusioncoefficient.Vocwasagain
increasedasmoreglycolwasadded.TheefficiencyoftheDSSCs increasedwithincreasingconcentrationsof3EG,4EG,andDMSO asillustratedinFig.3.TheefficiencyoftheDSSCwasraisedfrom 0.68%fortheinitialelectrolyte(withoutorganicsolvent)to1.15% forDMSO(at20vol.%),1.13%for3EG(at100vol.%),and1.08%for 4EG(at100vol.%).Inspecific,DMSO(at≤20vol.%)exhibitedmuch betterperformancethantheinitialelectrolyte.
Inordertoexpandourchoiceofsolventsforagarosegel elec-trolytesandobtainagoodbalancebetweensolubilityandagarose gel,wemixedgood andpoorsolventsin thisstudytoimprove theperformanceoftheagarosegelelectrolytesfortheDSSCs.In theco-solvents,DMSOservesasagoodsolventforagarose.For themixtureofgood(DMSO)andpoorsolvents(3EG,4EG,PC,or PG),wefoundthatagarosecannotbecompletelydissolvedifthe volumeratioofgood topoorsolventislowerthan 1:9.Onthe otherhand,whenthevolumeratioishigherthan3:7,the gela-tionof agarosecannotoccureither. Asaresult, a volumeratio of2:8wasemployedforthesubsequentsectionsinthis paper. Thediffusion-limitedcurrentsoftheelectrolytescontaining vari-ousco-solvents(DMSO/3EG,DMSO/4EG,DMSO/PC,andDMSO/PG) weremeasuredbycyclicvoltammetry,asillustratedinFig.4.The diffusion-limitedcurrent forthe electrolyteusing DMSO/PC co-solventwasthehighest. Thecurrentdidnotreachitsdiffusion limitupto0.8Vascomparedto0.2Vfortheotherco-solvents. TheI3−diffusioncoefficientandconductivityofDMSO-based
co-solvents,andtheirelectriccharacteristics(Voc,Jsc,andconversion
efficiency)aresummarizedinTable3.Thehighestionic conduc-tivity(14.2mScm−1)anddiffusioncoefficient(2.7×10−6cm2s−1)
Fig.3. Theoverallenergyconversionefficienciesasafunctionofsolventsand sol-ventcontent.
Fig.4. Steady-statecurrent–voltagecurvesofthegelelectrolytescontainingvarious DMSOco-solvents.
wereobservedinthegelelectrolyteusingaDMSO/PCco-solvent. Incontrast,thelowestvaluesofconductivity(4.4mScm−1)and triiodidediffusioncoefficient(3.0× 10−7cm2s−1)werefoundin
theDMSO/4EGco-solvent.Theconductivityandtriiodidediffusion coefficientoftheDMSO-basedco-solvents decreasedinthe fol-lowingorder:PC>PG>3EG>4EG.Thedielectricconstantsofthese DMSO-basedco-solventsshowedthesametrend,whiletheir vis-cositiesshowedtheoppositetrendwiththelowestviscosityfor theDMSO/PC co-solvent. High dielectric constant and low vis-cosityofco-solventslikethoseofDMSO/PCpresumablyincrease thedissociation ofthesolutesinthegel electrolytesand allow theions to movefaster in theagarose matrix, leading to high conductivityandhightriiodidediffusivity.Fig.5showsthe effi-cienciesofDSSCswithelectrolytescontainingvariousDMSO-based co-solvents.Thephotoelectricconversionefficiencywasenhanced withtheadditionof organicsolvents.The DSSCwithDMSO/PC co-solventintheagarosegelshowedthehighestVoc,0.73V,
high-estJsc,4.65mAcm−2,andhighestefficiency(=1.97%),whichwas
62%,73%,and190%enhancementovertheinitialgelsystem (with-outorganicsolvent).TheperformanceofPG/DMSOwastheworst, yetitsefficiency(1.06%)wasstill∼1.6timeshigherthanthatof theinitialgelsystem(=0.68%).TheefficienciesofDMSO-based co-solventsdecreasedinthefollowingorder:PC>3EG≈4EG>PG. Furthermore,theefficienciesofelectrolytesbasedonco-solvents
Fig.5.J–VcurvesofDSSCsbasedongelelectrolytesusingbasedongelelectrolytes usingdifferentDMSOco-solvents.
Table3
Summaryoftheefficiencies,diffusioncoefficients,conductivity,andDNofthegelelectrolytesusingdifferentDMSOco-solvents.
Solvent DMSO/PC DMSO/4EG DMSO/3EG DMSO/PG PureDMSO(20vol.%)
Conductivity(mScm−1) 14.2 4.4 4.6 6.2 5.0 DI3−(×10 −7cm2s−1) 27.3 3.0 4.1 9.7 4.8 Voc(V) 0.73 0.65 0.61 0.58 0.58 Jsc(mAcm−2) 4.65 3.69 4.01 3.21 3.69 FF 0.58 0.57 0.57 0.57 0.54 (%) 1.97 1.38 1.39 1.06 1.15
(excludingDMSO/PG)(=1.38–1.97%)arebetterthanthatusinga singlesolvent,DMSO(at20vol.%)(=1.15%).
When no organic solvent or only 20wt.% DMSO was used, theI3− diffusioncoefficientswerelowat0.6×10−7cm2s−1 and
4.8×10−7cm2s−1,comparedto2.7×10−6cm2s−1usingDMSO/PC
co-solvent.ThisimpliedthattherewashighI3−ionconcentration
aroundtheTiO2nanoparticlesintheDSSC.ThehigherI3−ion
con-centrationmightincreasethechargerecombinationbetweenthe injectedelectronsandI3−atTiO2/electrolyteinterface.
As a result, the Voc and Jsc were increased by the addition
of organic solvents because the high I3− diffusion coefficient
decreasedtheI3−ionconcentrationaroundtheTiO2,thenreduced
thedarkcurrent[26].
Thediffusionlimitedcurrentfortheelectrolyteand conduc-tivityusingDMSO/PGco-solventwashigherthanthosebasedon DMSO/3EG and DMSO/4EG co-solvents. Unexpectedly, the con-version efficiencyof DSSCcontaining DMSO/PGwasnot higher thanDMSO/3EGorDMSO/4EG.Inordertoelucidatethe mecha-nismfortheeffectofdifferentco-solventsonDSSCsperformance, thedesorbeddyelevelsinDMSO/PC,DMSO/3EG,DMSO/4EG,and DMSO/PGwerethenstudied.Thiswascarriedoutbyimmersingthe dyedTiO2intotheco-solventsfor1htocollectthedesorbeddye,
whoseamountwasmeasuredbytheabsorbanceusingaUV–vis spectrometer.Thedesorbeddyeamountcouldbejudgedbythe intensityofabsorptionpeakaround310nm.Itwasdeterminedthat moredyesweredesorbedfromTiO2whenpureDMSO,3EG,4EG,
andPCwereaddedintheDMSO-basedco-solventsascomparedto PG.PGdesorbedmoredyefromTiO2 thanPC,4EGand3EG.This
wasreasonwhytheconversionefficiencyofDSSCusingDMSO/PG waslowestamongtheseco-solvents.
Sofar,thebestefficienciesremainedlow,forexample,0.68%for thereferencecellusingMPII/agaroseand1.97%forthecellusingthe newelectrolyte,MPII/agarosewithDMSO/PCco-solvent. Neverthe-less,thereareconcernsaboutthelowefficienciesandthemeritof environmentallybenignelectrolytes.Toaddresstheconcernoflow efficiencies,wemadeafewchangesinthecellfabrication,i.e.(1) replacingITOglassbyfluorine-dopedtinoxide(FTO)(8/square), (2)increasingthesurfaceareaintheTiO2layerbyusingalarger
amountofpolyethyleneglycol(PEG)withhighersurfacearea (sur-face area: 57.5m2g−1), and (3) adding a 4m light scattering
layeron13mTiO2electrode.Higherconversionefficiencieswere
achievedafterimplementingafore-mentionedchangesinthe fab-ricationoftheDSSCs.For instance,theefficiencyof areference cellusing3-methoxy-propionitrile(MPN)-basedliquidelectrolyte consistingof0.5MLiI,0.05MI2,0.1MGuSCNand0.2MNMBIin
MPNis4.83%. Asa result,wefurtherusethis improvedcellto assessthemeritofenvironmentallybenignionagaroseelectrolytes. WhenanenvironmentalbenignDMSO/PCco-solventwasusedto replaceMPN,theconversionefficiencywasdeterminedtobe4.63%. ThisshowedenvironmentallybenignDMSO/PCco-solventoffered comparableconversionefficiencywitha mere∼4%reductionas comparedtoMPNsolvent.
Subsequently,theimpactofagaroseontheconversionefficiency ofDSSCswasevaluated.Becauseofthesolubilityissue,the addi-tivesforagarosegelelectrolyte(1.8MMPII,0.05MI2,0.1MGuSCN
and0.5MNMBI)wereslightlydifferentfromMPN-basedliquid electrolytein theconcentrationsofMPII andNMBI.Whenpure MPII wasused,theuseof agarosereduced theefficiency∼12% from=1.6%to=1.4%.Whileusingslightlydifferentadditives,a conversionefficiencyof=4.3%wasobtainedforDMSO/PC-based DSSCwithoutagaroseand3.4%withagarose.A∼21%dropin effi-ciencyto=3.4%wasfoundwhenagarosewasadded.Therefore, theDSSCmaintainsabout80%oftheconversionefficiencywhen 0.5wt%agaroseisaddedtotheenvironmentalbenignDMSO/PC electrolyte.
4. Conclusions
Theionicagarosegelelectrolyteshavebeenpreparedbyusing environmentallybenignsolventsandco-solventstoimprovethe agarosesolubilityandcapacitiesofionicliquidsadditivesforthe dye-sensitizedsolarcells.Thehighestconductivity,14.2mScm−1, andhighestdiffusioncoefficient,2.7×10−6cm2s−1,areachieved
for the electrolyte containing the co-solvents with 80vol.% PC and 20vol.% DMSO. Such electrolyte yields a conversion effi-ciency of 3.4% based on improved cell fabrication, which is about2.4timeshigherthantheelectrolyteswithoutorganic sol-vents (pure MPII with agarose, =1.4%). The environmentally benign solvents withlowviscosity andhighdielectric constant increase the ionic conductivity and I3− diffusion coefficients,
resulting in a reduction of the charge recombination between electrolyte and dye/TiO2 interface and improved conversion
efficiency.
Theuseofenvironmentalbenignsolventsslightlyreduces(∼4% drop)theconversionefficiencyascomparedtoconventional sol-vent,MPN.However,theuseofagarosereducestheconversion efficiencyabout21%from=4.3%to=3.4%forDMSO/PC-based DSSC.Therefore,theDSSCmaintainsabout80%oftheconversion efficiency when0.5wt%agarose is addedto theenvironmental benignDMSO/PCelectrolyte.
Acknowledgements
TheauthorsappreciatethefinancialsupportinpartbyNational ScienceCouncilofROCunderContractNos.NSC 99-2662-E-006-010-CC2andNSC100-3113-E-007-002.
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