Modulation of Photoinduced Iodine Expulsion in Mixed Halide Perovskites with Electrochemical Bias - Dubose et al. - 2021 - Unknown

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pubs.acs.org/JPCLLetterModulationofPhotoinducedIodineExpulsioninMixedHalidePerovskiteswithElectrochemicalBias∥∥JeffreyT.DuBose,PreethiS.Mathew,JunsangCho,MasaruKuno,andPrashantV.Kamat*CiteThis:J.Phys.Chem.Lett.2021,12,2615−2621ReadOnlineACCESSMetrics&MoreArticleRecommendations*sıSupportingInformationABSTRACT:Holetrappingatiodine(I)sitesinMAPbBr1.5I1.5mixedhalideperovskites(MHP)isresponsibleforiodinemigrationanditseventualexpulsionintosolution.WehavenowmodulatedthephotoinducediodineexpulsioninMHPthroughanexternallyappliedelectrochemicalbias.Atpositivepotentials,electronextractionatTiO2/MHPinterfacesbecomesefficient,leadingtoholebuildupwithinMHPfilms.Thisimprovedchargeseparation,inturn,favorsiodinemigrationasevidentfromtheincreasedapparentrateconstantofiodineexpulsion(k=0.0030s−1).expulsionConversely,atnegativepotentials(−0.3VvsAg/AgCl)electron−holerecombinationisfacilitatedwithinMHP,slowingdowniodineexpulsionbyanorderofmagnitude(k=0.00018s−1).TheexpulsiontuningoftheEFermilevelthroughexternalbiasmodulateselectronextractionattheTiO2/MHPinterfaceandindirectlycontrolsthebuildupofholes,ultimatelyinducingiodinemigration/expulsion.Suppressingiodinemigrationinperovskitesolarcellsisimportantforattaininggreaterstabilitysincetheyoperateunderinternalelectricalbias.nmixedhalideperovskites(MHPs)suchasAPbBr3−xIxandenergybarrierforhalidesegregation/expulsion.WhilemostofIAPbBrCl(Aisacation,e.g.,Cs+ormethylammonium;3−xxthespectroscopicstudiesofhalideionmigrationarefocusedonMA),halideionmobilityleadstolight-inducedandfield-fastertimescalemeasurements,longertimescalemigration1−6inducedsegregationintodistincthalide-richphases.Suchstudiesareneededtoelucidateitsimpactontheoverallphotoinducedhalidesegregationnegativelyimpactsthestabilityphotostability.andefficiencyofdevicesasiodide-richdomainsactaschargeOfnoteisaspectroelectrochemicalapproachthathasbeenrecombinationcenters,greatlydiminishingtheperformanceofusedtodeterminetheconductionandvalencebandpositionsof7,89solarcellsandlight-emittingdiodes.Interestingly,themixedhalideperovskitesandhasrevealedtheirdependenceonentropyofmixingfavorsthemixedhalidephaseinthedark.1028,29halidecomposition.ThecontrolofdefectdensitiesinAthreshold-limitedexcitationenergyisneededtoovercomethisCsPbBr3filmsthroughexternallyappliedbiashasbeenshowntoentropyofmixingtoinducephasesegregation.11,12Upon30modulatetheemissionintensity.Ourrecentsuccessinstoppingphotoirradiation,thesegregatedphasesremixtomaintainingMHPstabilityindichloromethane(DCM)hasrestorethehomogeneousstartingcomposition.Thesecomposi-enabledelectrochemicalandspectroelectrochemicalstudiestoDownloadedviaUNIVOFCONNECTICUTonMay16,2021at06:14:49(UTC).tionalchanges,whichcanbereadilymonitoredthroughopticalproberelevantchargetransferprocessesatperovskite/liquidchanges(e.g.,absorptionandemissionspectra)inthemixedinterfaces.31,32Wehavenowutilizedspectroelectrochemicalhalidefilm,thusenabledirecttrackingofthehalideionmeasurementstosystematicallyvarythedrivingforceforchargeSeehttps://pubs.acs.org/sharingguidelinesforoptionsonhowtolegitimatelysharepublishedarticles.13movement.ThesemethodshavegiveninsightsintotheseparationbymodulatingtheFermilevelthroughexternalbiasmobilityofhalideionsinphysicallypaired3Dand2Dandmonitoringitsinfluenceonhalideionmigration.The14−16perovskites.macroscopictrackingofiodinemigrationunderappliedbiasThemobilityofiodineinmixedhalideperovskitefilmscanprovidesnewinsightsintodegradationpathways.leadtoitsexpulsionunderlong-termirradiation,thusinducingPhotoirradiationofDryversusElectrolyte-ContactingPerovskite17−20instability.ThisphotoinducediodineexpulsioncanbeFilms.Whenmixedhalideperovskites(e.g.,MAPbBr1.5I1.5)arereadilyseenuponirradiatingperovskitefilmsincontactwitha18subjectedtocontinuousphotoirradiation(abovegap),theysolvent(suchasDCMortoluene).Theselectiveremovalof1,2phasesegregateintoBr-andI-richregions.AnincreaseiniodinefromMHPsuponcontinuedirradiationtransformstheabsorbanceinthered(∼675nm)indicatesnewabsorptionperovskitefilmintoabromide-richphase.ItwasalsoshownthatsubstitutingtheMHPA-sitecationwithcesium(Cs)slowsdowniodineexpulsionduetoincreasedthermodynamicReceived:February1,2021stabilizationofMHPlattices.18,21,22SimilarlatticestabilizationAccepted:March2,2021hasbeenshowntoreducephasesegregationinCl-alloyedPublished:March9,202115,23,24perovskites.Surfacetreatmentsand2Dinterfaceshave14,25−27suppressedhalideionmigrationtosomeextent.Suchstrategiesprovideacompositionalmeansofmodifyingthe©2021AmericanChemicalSocietyhttps://dx.doi.org/10.1021/acs.jpclett.1c003672615J.Phys.Chem.Lett.2021,12,2615−2621

1TheJournalofPhysicalChemistryLetterspubs.acs.org/JPCLLetterFigure1.SpectroelectrochemcialdataforFTO/TiO2/MAPbBr1.5I1.5films(0.5mV/ssweeprate)duringthe(A)reductionand(B)oxidationhalfcyclestogetherwiththeabsorbancechangeattheexcitonicpeak(615nm).TheshadedregionineachtracerepresentthestablewindowinwhichnoFaradaicprocessesoccur.Measurementswereperformedin0.01MBu4NPF6inDCMunderdegassedN2conditions(prebubbledthroughDCM).Figure2.AbsorbancespectraofFTO/TiO/MAPbBrIfilmsduring405nmCWphotoirradiation(50mW/cm2)whileheldatappliedbiasesof21.51.5(A)0.5,(B)0,and(C)−0.3VvsAg/AgCl.(D−F)CorrespondingΔAbsorbancegraphsforeachappliedbias,whichhighlightsthelossofabsorbance(bleach)near625nm.DataforallappliedbiasesareshownintheSupportingInformation.Measurementsweremadein0.01MBu4NPF6inDCMunderdegassedN2conditions(prebubbledthroughDCM).arisingfromtheformationofiodine-richdomains,withaabsorbanceduringsteady-statephotoirradiationisobserveddueconcomitantdecreaseinabsorbanceat∼625nmduetototheselectiveexpulsionofiodinespeciesintothesolventdepletionofthemixedphase(FigureS1A;reactions1and2).(reaction5;notethatiodide,iodine,andotherI-speciesplaya18,32roleintheoverallexpulsionprocess).Reactions3−5are−+MAPbBrI1.51.5+→hνMAPbBrI(e1.51.5+→h)facilitatedbyTiO2asitcaptureselectrons,thusaccumulatingMAPbBrI1.51.5+′hν(1)holesinthefilm.Theaccumulatedholescontributetoboth4,18photoinducedphasesegregationandiodineexpulsion.−+11MAPbBrI(e1.51.5+→h)MAPbBr3+MAPbI3MAPbBrI(e,h)−++TiO22(2)1.51.52Uponstoppingphotoirradiation,segregatedregionsremixto+−→+MAPbBrI(h)1.51.5TiO(e)2(3)12attainentropicstabilization(FigureS1B).Interestingly,whenperovskitefilmsareincontactwitha−−solventorelectrolyteacontinuousblueshiftofthebandedgeTiO(e)22+→+FTOTiOFTO(e)(4)2616https://dx.doi.org/10.1021/acs.jpclett.1c00367J.Phys.Chem.Lett.2021,12,2615−2621

2TheJournalofPhysicalChemistryLetterspubs.acs.org/JPCLLetterFigure3.(A)Monoexponentialfitstothedifferentialabsorbancebleachfeature(625nm)forFTO/TiO2/MAPbBr1.5I1.5filmsphotoirradiatedandheldatpotentialsrangingfrom0.5to−0.3VvsAg/AgCl.DataforallappliedbiasesareshownintheSupportingInformation.(B)Dependenceoftheapparentrateconstantofiodineexpulsion(kexpulsion)determinedfrommonoexponentialfitsin(A)ontheappliedpotential.Thedashedlineismeanttoguidetheeye.++1PhotoinducedIodineExpulsionunderAppliedBias.Thesetup2MAPbBrI1.51.5+→hMAPbBr3+++PbI2MAI2employedforspectroelectrochemicalmeasurementsisshownin2(5)SchemeS1(SI).TheabsorbancespectraofMHPfilmsatthreedifferentbiaspotentials(Figure2A−C)withinthestableFigureS2AshowstheabsorbanceofaMAPbBr1.5I1.5filminelectrochemicalwindowwererecordedduringirradiationwithacontactwith0.01Mtetrabutylammoniumhexafluorophosphate405nmCWdiodelaser(50mW/cm2).Changesinabsorbance(Bu4NPF6)indichloromethane(DCM).Whereasadryduringiodineexpulsionarebettervisualizedthroughdifferenceperovskitefilm(nosolventorelectrolyte)segregatesintoBr-absorbancespectra(Figure2D−F).Ineachcase,alossofandI-richdomains,theperovskitefilmincontactwiththeabsorbanceisseenthroughouttheMAPbBr1.5I1.5absorbanceelectrolyteexpelsiodineintothesolution(FigureS2B).Aswindow,withadistinctbleach(negative)featurenear625nmdiscussedinourpreviousstudy,CsalloyingofMAPbBr1.5I1.5(excitonicpeakofMAPbBr1.5I1.5).Theabsorbancelossandfilmsslowsdowniodinemigrationandrevealstheformationofaconcurrentblueshiftinthespectrumareassociatedwithchargephase-segregatedstatepriortoiodineexpulsion.18Thisstrategy32separationfollowedbyiodineexpulsion(reactions1−5).ofA-sitecationmodificationgreatlysuppressesthethermody-Whentheappliedpotentialwasmaintainedat0.5V,namicsofiodineexpulsion.photoinducediodineexpulsionwasrapidandcompletewithinStableElectrochemicalWindowforaMixedHalidePerovskite.15min(Figure2A,D).At0VvsAg/AgCl,weobserveaToinvestigatetheeffectofanexternallyappliedbias(andthusrelativelysloweriodineexpulsionprocess(Figure2B,E).EvenEFermi)onthephotoinducediodineexpulsionprocess,wefirstafter60minofirradiation,filmsretainasmallabsorptionfeatureestablishedastableelectrochemicalworkingwindowinwhichataround615nm,indicatingtheincompleteexpulsionofiodinenoFaradaicprocessesoccur(i.e.,noelectrochemicallyinducedfromfilms.Forafilmheldatanappliedbiasof−0.3VvsAg/redoxevents)inthedark.FilmsofMAPbBr1.5I1.5weredepositedAgCl,onlyasmallchangeinabsorptionisseen(Figure2C,F).ontoTiO2-coatedFTO(fluorine-dopedtinoxide)electrodesThisslowerexpulsionatthenegativepotentialindicatesthatthe4usingpreviouslyreportedmethods.TiO2isusedasanelectroniodinemigrationbecomeslessefficientandonlyasmallamounttransportlayer(ETL)inperovskitesolarcellsandwasthusofiodineescapesfromthefilmintothesolution.(Absorptionselectedforthisstudy.TheresultantFTO/TiO2/MAPbBr1.5I1.5spectraoftheelectrolytesolutionattheseappliedbiasesarefilmswereimmersedin0.01MBu4NPF6inDCM,andshowninFigureS5.)WehaveestimatedthequantumefficiencyelectrochemicalexperimentswereperformedunderdegassedofiodineexpulsionbasedonthesolutionabsorbanceofI−and3conditions(N2wasprebubbledthroughDCMtoreducesolventtheincidentlaserpower.Thequantumefficiencyforiodine31,32loss).expulsionwas0.042%at0.5Vand0.0044%at−0.3VandDuringthisreductionscan(Figure1A),wecanclearlyseeaagreedwiththetrendobservedinFigure2.(SeetheSupportinglargedecreaseintheabsorbanceoftheperovskiteduetotheInformationforquantumefficiencymeasurements.)reductionofPbtoPb0(−0.77V).Similarly,whenwescannedTheabsorbanceandcorrespondingdifferenceabsorbancethepotentialinthepositivedirection(Figure1B)wecouldseeaspectraofphotoirradiatedMHPfilmsatotherappliedpotentialsdistinctfeaturecorrespondingtotheoxidationofthehalideions(between+0.5and−0.3VvsAg/AgCl)areshowninFiguresataround1V.TheabsenceofaFaradaiccurrentandnolossinS6−S9.Theabsorbancelossat625nm,whichrepresentsthetheabsorbancewithintheelectrochemicalwindow,−0.3VanddisappearanceofMHP,wasrecordedatdifferenttimestoobtain+0.5V(shadedregionsinFigure1A,B),ensuredthatwecouldiodineexpulsionkinetics(Figures3AandS10).EachtracewascontroltheEFermioftheelectrodewithoutinducinganfittoamonoexponentialdecaytodeterminetheapparentrateelectrochemicalreactionattheperovskite−electrolyteinterface.constantsofiodineexpulsion.At0.5VvsAg/AgCl,theapparentToconfirmthestabilityofourperovskitefilmsinthispotentialrateconstant,k,wasdeterminedtobe0.003s−1andisanexpulsionwindowoverlongertimes,werecordedabsorbancespectraoforderofmagnitudegreaterthantheoneobservedatnegativethefilmsheldatselectbiaspotentialsfor60mininthedarkpotentials(k=0.00018s−1at−0.3V).Thedependenceexpulsion(FiguresS3andS4).oftheapparentrateconstantofphotoinducediodineexpulsion2617https://dx.doi.org/10.1021/acs.jpclett.1c00367J.Phys.Chem.Lett.2021,12,2615−2621

3TheJournalofPhysicalChemistryLetterspubs.acs.org/JPCLLetterFigure4.(A)J−VcurvesforMAPbBrIfilmsonanFTO/TiOelectrodein(a)thedarkand(b)under405nmCWirradiation(50mW/cm2).1.51.52Measurementswereperformedin0.01MBu4NPF6inDCMunderdegassed(N2saturated)conditions.(B,C)BanddiagramschemedepictingthepathwaysforchargecarrierswhenFTO/TiO2/MAPbBr1.5I1.5filmsarephotoirradiatedandheldat(B)positivebias(+0.5V)and(C)negativebias(−0.3V)(vsAg/AgCl).TheshiftingoftheFermilevel(EFermi)oftheelectrodeanditseffectonelectronpathwaysisexplicitlyshown.Conductionandvalencebandenergylevelsofsemiconductorsareobtainedfrompreviouswork(refs1and10).Figure5.Top-viewSEMmicrographsofFTO/TiO2/MAPbBr1.5I1.5films.(A)Beforeapplyingbiasorphotoirradiation(pristine).(B−D)Filmsirradiatedfor15minwitha405nmCWlaserwhileheldat(B)−0.3,(C)0,and(D)0.5VvsAg/AgCl.Thevoidspaceswhichstarttoappearat0Vincreaseinsizeat0.5Vappliedbias.Thesemorphologicalchangesseenin(C)and(D)aretheresultofiodineexpulsion.ontheappliedpotentialisshowninFigure3B.Atpotentialsredoxcouplepresentinthissetup,theobservedphotocurrentismorenegativethan0VvsAg/AgCl,kexpulsionremainsunaffected.relativelylow.Followingband-gapexcitation,electronsfromHowever,withtheincreaseintheappliedpotential(greaterthanMAPbBr1.5I1.5arequicklytransferredtoTiO2(reaction3)due4,37,380V),weobserveanincreaseinkexpulsion.Itisinterestingthatthetoitsfavorablyalignedconductionband.TheseelectronselectrochemicalbiasappliedtotheTiO2/MHPfilminfluencesareextractedintotheFTOelectrode(reaction4)togenerateathephotoinducedmigrationofiodineandtheultimatephotocurrent.TheJ−VcharacteristicsinFigure4oftheFTO/expulsionofiodine.ThisfurtherconfirmsourabilitytoTiO2/MAPbBr1.5I1.5filmsunder(a)darkand(b)405nmCWmodulatethephotoinducedmigrationofiodineintheMHPirradiation(50mW/cm2)exhibitn-typesemiconductorfilmthroughanexternallyappliedelectrochemicalbias.behavior.Thus,theappliedbiasdirectlydictatestheFermiPreviously,weshowedthatanexcitationintensitythresholdleveloftheperovskitefilmand,inturn,chargeseparationwithin11,12,15,33existsforlight-inducedhalidesegregation.Athermody-thespacechargelayer.namicbalancebetweenthedrivingforceforhalidesegregationTheapplicationofexternalbiasinthepotentialrangeof−0.3(whichisproportionaltolightintensity)andtheentropyofto+0.5Vaffectsthechargeseparationintheperovskitefilm,mixingdictatestheinteraction.WefoundthatiodineexpulsionshiftingtheapparentFermilevel(EFermi)oftheelectrodeeitherfromfilmsheldatdifferentpotentialsexhibitsasimilarexcitationclosertoorawayfromtheTiO2conductionband.Figure4B,Cintensitythreshold.Detailsofthethresholdintensitydetermi-illustratesthephotoinducedchargeseparationandthenationcanbefoundintheSupportingInformation,andinterfacialchargetransferundertheinfluenceofanexternalindividualabsorbancespectraareshowninFigureS11.At0.5Vbias.Atpositivebias(e.g.,+0.5VvsAg/AgCl),EFermifavorsvsAg/AgCl,theIvaluewasthelowest(∼65μW/cm2).electronflowfromMHPtoTiOandfromTiOtoFTOThreshold22AsEFermiisincreasedbyapplyingamorenegativepotential,the(pathways2and3,Figure4B).TheaccumulatedholesinthresholdintensityrequiredtoobserveiodineexpulsionMAPbBr1.5I1.5gettrappedattheiodidesite,whichintroduces39increases4-foldfornegativepotentialsof0and−0.3VvsAg/instabilitywithinthelattice.BecauseofthelargerfractionofAgCl,respectively(∼290μW/cm2;FigureS12).holeaccumulationatpositivepotentials,iodinemigrationPhotoelectrochemicalResponseofMAPbBr1.5I1.5Filmsonanfollowedbyitsexpulsionoccursatafasterrate.However,FTO/TiO2Electrode.Anotherwaytoconfirmtheappliedwhenweapplyanegativebias(0to−0.3VvsAg/AgCl),EFermipotentialdependenceofchargetransferprocessesistoprobeshiftstowardtheconductionbandofTiO2.Thislowersthe34−36photocurrentgenerationatperovskite/liquidinterfaces.Athermodynamicdrivingforceforelectroninjection,givingrisetophotocurrentdensityof∼5μA/cm2isgeneratedwhentheagreaterdegreeofchargerecombination(pathways4and5,FTO/TiO2/MAPbBr1.5I1.5filmsareirradiatedinaphoto-Figure4C).Thischargerecombinationminimizesholeelectrochemicalcell(Figure4A).BecauseofthelackofaaccumulationinMAPbBr1.5I1.5,andthusweseesloweriodine2618https://dx.doi.org/10.1021/acs.jpclett.1c00367J.Phys.Chem.Lett.2021,12,2615−2621

4TheJournalofPhysicalChemistryLetterspubs.acs.org/JPCLLetterexpulsion.Thehole-inducedoxidationoranodiccorrosionofpresentedinthisstudyfurtherconfirmsthenecessityofholeshortbandgapsemiconductorssuchasmetalchalcogenidesinlocalizationattheiodinesiteastheprimarystepresponsiblefor40−42photoelectrochemicalcellshasbeenwellstudied.Insuchthemigrationifiodineisinthefilm.instances,thesurfaceoxidationcreatesapassivelayeronthesurfaceofthesemiconductorasthesurfacegetsoxidized.In■ASSOCIATEDCONTENTMHPfilms,however,theinteractionwithholesmakesiodine*sıSupportingInformationmigratetotheinterfaceandgetexpelledintothesolution.TheTheSupportingInformationisavailablefreeofchargeatprocesscontinuesuntilalloftheiodineisexpelled,leavinghttps://pubs.acs.org/doi/10.1021/acs.jpclett.1c00367.behindaMAPbBr3perovskitefilm.Materials;preparationmethodsforFTO/TiO/perov-2MorphologicalChangesAssociatedwithIodineExpulsion.Theskiteelectrodes;electrochemicalmeasurements;iodineimpactonthemorphologyoftheperovskitefilmsduetoexpulsionmeasurements;XPSdataandanalysis;detailsofphotoinducediodineexpulsionunderappliedbiaswasprobedtheSEMmeasurement;quantumefficiencycalculation;usingscanningelectronmicroscopy(SEM).PristineMHPfilmsthresholdintensitycalculation;filmstabilitytests;and(beforeirradiation/appliedbias)showacleansurfacewithnoindividualabsorbancespectra(PDF)noticeablepinholesorvoids(Figure5A).However,uponexposureto405nmirradiationfor15minthefilmsexhibitvoids,■thedegreetowhichisdependentontheappliedbias.At−0.3VAUTHORINFORMATIONvsAg/AgCl,weobserveonlyasmallnumberofpinholes(FigureCorrespondingAuthor5B).At0V,largerpinholesareobserved,andat0.5V,thesePrashantV.Kamat−NotreDameRadiationLaboratory,pinholesgrowintolargervoids,indicatingagreaterdegreeofDepartmentofChemistryandBiochemistry,andDepartmenttransformationofthematerial(Figure5C,D).AsseeninourofChemicalandBiomolecularEngineering,UniversityofNotrepreviouselectrochemical51andphotochemicalstudies,18theDame,NotreDame,Indiana46556,UnitedStates;perovskitestructureisreconfiguredfollowingiodineexpulsion,orcid.org/0000-0002-2465-6819;Email:pkamat@leadingtopoorfilmmorphology.nd.eduWealsoperformedXPSmeasurementsontheperovskitefilmsAuthorstoassessthechangeincompositionthatoccurredduringiodineJeffreyT.DuBose−NotreDameRadiationLaboratoryandexpulsion.TheXPSspectraareshowninFigureS13,andtheDepartmentofChemistryandBiochemistry,UniversityofresultsaresummarizedinTableS3forfilmsheldat0.5,0,andNotreDame,NotreDame,Indiana46556,UnitedStates;−0.3VvsAg/AgCl.Thefilmheldat0.5Vappliedbiasshowedaorcid.org/0000-0002-7708-4703largeenhancementinbromineduetothegreaterdegreeofPreethiS.Mathew−NotreDameRadiationLaboratoryandiodineremovalfromthefilm.WhenEFermiisshiftedhigher(0VDepartmentofChemistryandBiochemistry,UniversityofvsAg/AgCl),thefilmsstillexpeliodineandbecomebromine-NotreDame,NotreDame,Indiana46556,UnitedStatesrich,buttoamuchsmallerdegree.WhenEFermiisshiftedtotheJunsangCho−NotreDameRadiationLaboratory,Universityhighestvalue(−0.3VvsAg/AgCl),theBr/IratioremainsofNotreDame,NotreDame,Indiana46556,UnitedStates;mostlythesame,inlinewiththesmallamountofiodineorcid.org/0000-0003-4211-4113expulsion.MasaruKuno−DepartmentofChemistryandBiochemistry,VariousmodelshavebeenproposedtoexplainphotoinducedUniversityofNotreDame,NotreDame,Indiana46556,ionicconductivityandhalidemigration.Theseincludethe43UnitedStates;orcid.org/0000-0003-4210-8514miscibilitygapbetweendarkandilluminatedconditions,polaronformationandassociatedlatticestrain,13,44−46trappedCompletecontactinformationisavailableat:carriergradientasaresultofinhomogeneousexcitationandhttps://pubs.acs.org/10.1021/acs.jpclett.1c0036747defect-assistedmigration,andpositivefreeenergiesofmixingunderillumination.12,33InaphotosegregatedMHPfilm,AuthorContributions∥isoenergeticconductionbandpositionsforMAPb(BrxI1−x)3,J.T.D.andP.S.M.contributedequally.MAPbI3,andMAPbBr3andvalencebandoffsetsofupto0.26−Notes0.67eVfavorholelocalizationortrappinginI-richregions.48−50Theauthorsdeclarenocompetingfinancialinterest.Thetimescalewithwhichvariousprocessescontributetohalidemigrationvary.WhereasthefunnelingofchargecarrierstotheI-■ACKNOWLEDGMENTSrichregionhasbeenconfirmedspectroscopicallytooccurintheTheauthorsthankDr.IanLightcapoftheNotreDame51,52timeframeof10−100ps,halidesegregationoccursoveraMaterialsCharacterizationFacility(MCF)forhelpwithandperiodofseveralsecondstominutes.TheultimateexpulsionofdiscussionsaboutXPS,alongwiththeNotreDameIntegratediodineoccursonamuchlongertimescale(minutestohours).ImagingFacility(NDIIF)foruseoftheSEM.J.T.D.andP.S.M.SuchslowerexpulsionofiodinehasalsobeenobservedinfilmsthankGergelySamuforhelpfuldiscussionsregardingelectro-17incontactwithtoluene.Themacroscaleiodineexpulsionchemicalexperiments.J.T.D.acknowledgessupportfromtheprobedinthepresentstudyfurtherservesasarapidtesttoPatrickandJanaEilersGraduateStudentFellowship.J.T.D.andevaluatetheperovskiteinstabilitycausedbyiodinemigration.P.V.K.acknowledgesupportbytheDivisionofChemicalThetrappingofholesatiodinesitesduringphotoirradiationSciences,Geosciences,andBiosciences,OfficeofBasicEnergycausesiodineionstomigratetowardgrainboundaries,thusSciencesoftheU.S.DepartmentofEnergy(awardDE-FC02-inducingtheformationofiodine-richphasesandbromide-rich04ER15533)forconductingspectroelectrochemicalmeasure-44,53−56phasesindryfilms.Whenmixedhalideperovskitefilmsmentsanddiscussions.P.S.M.andM.K.acknowledgetheareincontactwithsolution,however,themigrationofiodineDivisionofMaterialsSciencesandEngineeringOfficeofBasicextendsbeyondphasesegregationasitgetsexpelledintoEnergySciencesoftheU.S.DepartmentofEnergythroughsolution.TheelectrochemicalmodulationofiodineexpulsionAwardDE-SC0014334forcarryingoutiodineexpulsion2619https://dx.doi.org/10.1021/acs.jpclett.1c00367J.Phys.Chem.Lett.2021,12,2615−2621

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