Chracterization of ti o2 towards higher of incident photon to-current efficiency in dss-cs

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Dye sensitized solar cells

Dye sensitized solar cells

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  • 1. Physicalia (2008)3 pp.149-.j60 l,4as.30 CHARACTERIZATION TiO2TOWARDSHIGHER OF INCIDENTPHOTON-TO-CURRENT EFFICIENCY DYE IN SENSITIZED SOLARCELLS* W. Moonsl,K. Vandewall,P. Robaeysl,G. Krishnal and J. Mancal2 1lnstituutvoor Materiaalonderzoek, universiteitHasselt, Wete nschapspark1, 3590Diepen eek. b z IMEC-IMOMEC vzw, Wetenschapspark 3Sg0Diepenbeek 1, Summaryof the work, awardedwith the prize ot the best poster presentation Wouter Moons at by the General Scienfific Meetingofthe BPS at the Universit6 Librede Bruxelles lyav 200g on 21,Abstract Worldenergyprovision probably is one of the most challenging engagements the 2.1"t of century. Amongihe novelgeneration solarcell concepts, so called"GrAEel ce " or the Dye Sensitized SolarCell is likelyone of the most promising. is basedon the spectral lt sensitization a thin ceramic mesoporousmembraneby suitabletransitionmetal of complexes (=dye). The film consists nanometer-sized of colloidal titaniumdioxideparticles sinteredtogether allow for chargecarrierlransport. focusing the fundamenial, to By on thoughmostlyignored, processes like sintering the anataseTiO, porouslayer and dye soaking, incfeasein IPCE is observed. an Duringthe brief heatingof the mdso-porous film,calcination the TiO, particles of takes place.This calcination monitored is in-situby electrical measurements. tuningthe dye coverage porousTiO2,light absorption By of is enhanced,and suTfacetraps are decreased.In this work the adsorptionkjneticsof ruthenium-based havebeenstudiedby measuring optical dyes the absofbance spectraof the sensitized filmsusingUV-Vis spectroscopy. resulting The effective aDsoroance curyes as functionof time were successfully fltted as pseudosecond-order chemicalsorption processes.Both presentedcharacterization techniquesresultedto be efficienttools towards betterunderstanding further a and improvements dye sensitized of solarcells 149
  • 2. lntroduction effect by Becquerel[1] researchers andEver since the discoveryof the photoelectric sunlightcouldbeengineers have been enthralled the idea that freelyavailable withcaptured converted electrical and into power thatgenerates photovoltage a byThe mostcommonly known solarcells,i€ a device The semiconductorshining lightuponit, incorporate pn-junction a semiconductor a in solarspectrum Dependent the onmaterial to be ableto absorb largepartof the has a moreor lesscloseabsorption properties the material lightis absorbed a region ol the in pairs are generated and ifto the surfiace. When photonsare absorbedelectronholetheir recombination prevented is they can reachthe junctionwherethey are separated absorbing semiconductors silicon like mostcaniers by an electric field.Evenfor weakly are generated nearthe surfaceThe pn-junction whichseparates emitterand base the a high collection probability free for layer is very closeto the surfacein orderto have carriers.. operating The principtes beendescribed manypublications [3] have in [2] Sofar,thescienceofso|arce||shasbeendominatedbydevicesinwhichthejuncti betweeninorganicsolid-statemateria|s,usua|lydopedformsofcrySta||ineoram silicon.lnrecentyears,novelclassesofphotovoltaicmaterialsystemshaveimer solarcellscuFently become broader of interest suchas, nano-crystalline organic and roughly molecular polymdr into and organicsolarcellsor into Thesecellscanbe divided flallayer systems bulkheterojunctionsl4] and cheapto fabricate(theexpensive solarcells,whichare relatively Thesethird generation high-temperature high-vacuum and processes neededfor the and energy-intensive on flexiblesubstratesand can be traditionaldevicescan be avoided),can be used shapedortintedtosuitdomesticdevicesorarchitecturalordecorativeapp|icationsl5 developed molecular a system solarlight photovoltraic for ln 1991GreEeland Oregan of basedon the speclralsensitization harvesting Gonversion electricitylG] lt is and to [4 metalcomplexes Jilmconsists transition The of a thin ceramicmembrane suitable Dy nanometer-sizedcol|oida|titaniumdioxideparticlessinteredtogethertoa||owforch chromophore thesemembranes give Whenderivatized a suitable transport carrier with l 150
  • 3. extraordinaryefficienciesfor the conversionof incident photons into electric current(IPCE).These cells are referred as "Gratzel to cells"or .Dye Sensitized Solar Cells"(DSSC). operation the Dye-Sensitized Figure 1: Schematic of SolarCell presentation the operationA schematic of principle givenin Figure The heartof the is 1. oxidefilm (seedeviceis a mesoporous Figure typically 2), 10pmthick,whichis madeof tiny nano-crystals, interconnected allow electronic to conduction take place.Oxides tosuchas TiOzZnO,SnOz suchas CdSe,are the preferred and Nb2O5or chalcogenides ,compounds[8]. till now, titanium Up dioxidehas been the material choicefor these ofapplications. becauseof its large band gap (3_3.2eV), TiO, intrinsicly Unfortunately, part of the solar emissionand so has low conversionabsorbsonly the ultravioletefficiencies. solutionlies in the separation the opticalabsorption The of and charge- 151
  • 4. generating using an electron functions, absorbing the visible transfersensitizer in gap Photo- region injectchargecaniersintoa substrate a widebandspectral to withexcitalionofthesensitizer(S)resu|tsintheinjectionofane|ectroninthecond Theoriginalstateofthe dyeis subbequently restored eleclron by donationbandof TiOz.from the eleclrolyte, impregnated the porous in -TiO2usuallyan organicsolventcontaining redoxsystem, coupleis usedThusresults a suchas the iodide/triiodide inthree dimensional networkwith an enormouscontactarea betweenthe two types ofmaterials, wherecharge tansportis bestdescribed a random by walkmodel[9] The intercepts recapture the conduction the of band regeneration the sensitizer iodide of by electron the oxidized by dye.A dense TiOzlayer(seeFigure is sputtered a hole- 3) as in blocking layerto reducerecombination the electrode iodideis regenerated at The typically withPlatinum coated tum by the reduction triiodide the counter-electrode of at migration through external the load The The circuitis beingcompleted electron via generated underillumination conesponds the difference to between Fermi the voltage overallthe levelof the electron the solidand the redoxpotential the electrolyte in of devicegenerates electricpowerfrom lightwithoutsuffering permanent any chemicaltransformation[10] efficiency this Dyesensitized overall of AM15(Air solarcell is 7% underThe reported mass1.5)f6l [7t. TheAMl.Sconversionpowerefficiency1lAMl.sofaphotovoltaicdeviceisgivenb n no*"=P-ffm=FFv** where -- v.i,"" - ,-l=- v*J* wherePo,uristheoutpute|ectricalpowerofthedeviceunderil|umination,Prnist incident the deviceas measured a calibrated on by reference cell %" is the intensity cunent denslty;m is the spectral ooen circuitvoftale, and Js" is the short-circuit mismatch factorthat accounts deviations the spectraloutputof the solarsimulator for in 152
  • 5. with respectto the standardAM1.5 spectrumand deviationsin the spectralresponseofthe deviceunder measurewith respectto that of the referencecell;FF is the fill factor. novel deviceconceptshave beenGreat effort has been made to improvethe efficiency:developed,new materialshas been tested, different layer techniqueswhere applied.Theseeffortsresulted an efficiency approximately [8] [11][12] lt appears in of 11% thatfor one decade now the efficiency of nano-structuredcells has been essentiallyslagnating researchprogress.[13] despitesignificant thoughWe investigatedwith an alternativeapproach,the effects of the fundamental,mostlyignored,processeslike sinteringand soakingthe anataseTiO2porouslayer, onthe IPCE. printed Screen Figure lresoporous 2: TiO, Figure3: Dense Ti02 sputteredExperimentalFor the productionof substrates,we distinguishtwo types: the first substrates areregular microscopeglasses,the second type are patternedFluorinedoped Tin Oxide ConductiveOxide (TCO),which will act as a(FTO) glasses.The latter is a Transparent 153
  • 6. cathode,respectively anode in the DSSC.TWopattemswere etched uslng anIithography, with inter{igital structures otherwlth planesufaces All substrate-s one the -de-ionizeAwere lhoroughly cleaned in soap water, rinsed Mth water, ultrasonicallyrinsedIn Acetoneand boiledin lsopropanol, this to ensurea fat anddustfree substratefor good adhesion.The FTO substtateswere sputtered in a home made sputterinstallation establish denseTio2-antase to a layer(seeFigure of approximately00nm 3) 1thick and will act as a holeblocklnglayerAll subsfates werethen screenprinted a with@mmerdalTiO2paste,purchased from Solaronix. The pasteis dried and sinteredin afumaceto ensurepercolation the TiO2nanoparticles of 0.2nE Fusg+lbl€Aa a B tr|drdrtrd Ru-dyes Rr535 (C)Ru-s3s.bis Figure 4: (A)Absorbance (B) TBA The obtainedsampleswere then dividedin seven groupssach group containing ten samoles. w€re immersed an The substrates in gthanolsoluiioncontalning commercial a Ruthenium bas€ddye (seeFigure4). Foralt groups,concentration temperature the or of solution changed. was pure Afrer time periods,variatingfrom th to 196h, all substrateswere rinsed wllh ethanolto withdrawthe excEssof dye.The absorbance was measured usinga W-Vis meter. The cathodelayers(FTO)were sputtered with a thin (3onm)Platinum layer,whichwill layerfor elecirondonation act as a catalyzing to the elecfolyte The anodeandcathode
  • 7. are assembled with a hot meltspacer.The complete together cellswere vacuumfilledwith an electrolyte, consistedof 50m[.4tri-iodidein acetonitrile, sealed and ready to betested.Results and DiscussionAnnealingFluorinedoped Tin Oxide (FTO) substrates were patterned that 25pm interdigital sostructureswere obtained.These substraleswere screen-printedwith a TiO2paste andannealed whilein-situresistance wereconducted. measurements profiles,see Figure 5 and can beThis resultedin time vs. temperature/currentinterpretedin such way that the ideal temperatureprofile can be produced to getsufficient percolationwithout destroying the meso-porousnetwork. The ideal profile incorporate differenttemperature will the usedin the Ti02 pasteand the materials thesechemicaltimeto burn-out binders without creating cracksin the layer.Cracksdueto a rapidheatingwill clearlybe shownin the measurement a suddendecrease as inconduction. when an increase conduction followedPercolation the nanocrystals observed of is in isby a slow decreasewherethe crystalsgrow. best results are obtainedwhen the surface area availablefor dyeTheoretically,chemisorption maximum is while still havinga continues network allowfor electron toconduction.While plotting currentin function temperature, the of plols can be made-By Arrheniusfittingthe activation modelcan be derived. this caseconduction energya conduction lnis governed thermal by hopping activated [14][15][16]. 155
  • 8. * T€mperd$.re pllltb o Offialr€flt g , g E I 6 g for annealing TiO2 Figure5: Timevs.Temperature/Current of ? l {r {av} 10Vand 100V) Figura6: Arrhenius for annealing TiO2at several.voltage.s-(1v line ths plot of and the thin solid where the thick solid r,n" t"pt"""ni" the measurement fit. ArrheniusDye soaking pholons should beLightabsorption a key factorof solarcellssinceabsorbed isconverted electrons. Sensitized into Dye SolarCells(DSSC) soaked a dye are in Ru-based andsoabsorption betuned dyes, can bythechosen dye typicallysolution, 156
  • 9. Adsorption kinetics TiOzwere undertaken, determination dye concentration for the ofwas done with a UV-Visspectrometer measuring by absorbance 1,a"(535nm). at Thetime profileof dye adsorption a singleand continues is curve leadingto saturation,suggesting the possiblemonolayercoverageof dye on the surfaceof TiOz. Theadsorptionexperiments were conductedat varioustemperatures C18oC,20oC and37.50C)and at various concentrations (0.8m9/ml, 0.2mg/mtand 0.02mg/ml). Theamount of adsorptionvariationis shown. lt is seen that temperature significanflyinfluencesthe adsorptioncapacity.High temperature results in high adsorption,suggesting endothermic an property.The adsorption function timecan be fittedwitha pseudo in of second-order rate: 1 t 1. i=ne q.qr (andqe)is the amount dye adsorbed TiO2at timet (andat equilibrium).is the of on kpseudosecond-order constant dye adsorption, is expressed a function rate of it as oftemperature the Arrhenius- by typerelationship [17]. lnK=lnA-3 RTIPCEIncident Photon current to (IPCE)is a technique measure conversion Efficiency to the ofincidentphotonsand the percentage those photonsthat effectively of generated anelectron. halogenlamp in combination A with a monochromator used to produce ismonochromatic light.This light is passedthrougha beam splitter that the incident solighton the samplesolarcellis known.IPCEis determined [18]: by 4pce = 1Lue Dcr lcce qrne:Lightharvesting (amount absorbed efficiency of photons) qcrE: Chargeinjection efficiency rtccE: Chargecollection efficiency t57
  • 10. + 37.5oC0.2m9/mlRu535 + o.2mglml20qc Ftltes + Zo"Co.2mg/nrlRu535 + 0.02mg/m120rc Ru535 ---- laoo 0.zmg/rnlnu535 + 20C R!535 o.8mgyml -18C0 2mg/mlRu535 2o6Co 2s€/ml Fu535 37.5"CO-2mSln Ru535 0.8 mg/ml20"C Ru535 c2 mg/ml2CfC8u535 o.o2 mg,ml2o"c Ru535 3ff S?lil?;^*;ff ;:?:*"#:nU""tJlHH*1.l"::is"JT.:i3 ;,,ff Belerence Gall Test Cdl Monoafi€mator (lPcE)testsetup Photon currentEfficiency Figure8r Incident to be absorbtionthere should on the rLHEby saturating dye theSince we focusedmainly ""nu showsthat other parameter Frg::..n without"n"ng;ng ;nvan increasein lPcE will increase dye the efficiency DSSCs ofby changing the amountof adsorDed 158
  • 11. |lJ Wav€lengdx{r:m} Figure g: lpCE for DSSCas functionof soakinqtimeConclusionsAn effort has been made to increasethe Incidentphoton to Current Efficiencyof DyeSensitized Solar Cells by focusing on the sinter process and on the soaking timeprocess Positiveresurtswere obtainedby tuningthe sinteringprofirein such way that aconductive network of TiO2 was obtained without destroying the pore size of themesoporousnetwork. These pores promote the active surface where dye moleculescouldanchorand providea directchargeinjectionin the TiO2layer_substrates were syslematicallyimmersed in solutionswith differentconcentrationatvariabre temperaturesresurting a moderto describethe dye uptake.This resurted in in apositivechange of lpCE by a factor of 3. The proposed approachthereforeresultedtobe efficienttowards a better understanding and improvementof dye sensitizedsolarce s- 159
  • 12. AcknowledgementsTheauthor likesto acknowledge universiteit the Hasselt xios Hogeschool financial / forsupportFurthermore poryspec to the FWo thanksis due to the rwr-sBo project an(Phd.Koen Vandewal)Refurences[l] E.Becquerel, Acad. C.R. i4S - Sci.9, (1839). .[2] A. Goetzberger, Knobtoch, B.Voss,Ctystaltine J. and Siticon So/arCels (Wtey,199g).[3] M.A.Green,SiticonSolarCe s: Advancedprinciples practice & (Bridgeprintery, 199S).[4] H. Hoppe, N.S.Sariciftci, and Joumat materials of research1S,1S24 (2004).[5] A. coetzberger, C. Hebting, and SotarEnergyMaterials Sotar and Celts I (2000). 62,[6] B. ORegan, M. Gr.itzet, and Nature 353,737(1991).f/l M. Gratzel,CoordinationChemistry Reviews1tl, j67 (j991).[8] J-M.Kroon, Bakker, H.J.p.Smit,progress photovoltaics (2OO7l. N.J. and in 15,.1[9] J. Nelson, Physical Review phys.Rev.B pRB59, 15374 B (j999).[10] M. cratzel,Journal photochemistry photobiotogyA: of and Chemistry 164,3(2004).[1ll K.D. Benkstein, Kopidakis, van de Lagemaat, A.J. Frank,Joumatof phvsical N. J. and Chemistry 107,7759(2003). B[12J B. C.J,andA. Journat F, oftheAmerican Ceramjc Society80,3157 (1997).[13] H. Tributsch, Coordination Chemistry Reviews 248, 1511(2004.[14] H. Tang,K. Prasad, R. Sanjines, and Joumal Applied of physics 2042 75, (1gg4r.[15] R. OHayre, Nanu, Schoonman, A. Goossens, M. J. and Journal physical of Chemistry C 111,4809 (20071.[16] J. vande Lagemaat, park,andA.J.Frank, N.G. Journat physical of Chemistry 1O4,2044 B (2000).[17] S.Wang, H. Li,Journat Hazardous and of Materiats.t26,71 (2OOS.[18] J. Halme,G. Boschtoo, Hagfetdt, p. Lund,Journat physical A_ and of Chemistry 1.12, C 5623(2008). 160