Efficient - Hybrid - Solar - Cells - Based - on - Meso-Superstructured - Organometal - Halide - Perovskites1 - 图文

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Tofurthercorroboratethissuccessfulentan-glementcreation,wetransferredonlyonepho-tontol=T300andmeasuredtheotherphotoninthepolarizationbases.Themeasuredwitnessvaluewas1.628T0.004.Therefore,ourresultsdemonstratethatsinglephotonscancarry300?ofOAM(where?isPlanck’sconstantdividedby2p)andthatentanglementbetweentwopho-tonsdifferingby600inquantumnumbercanbeachieved.Eveninclassicaloptics,thehighestvalueofOAMthathadbeencreatedwithanSLMwasl=200(30).

Apartfromthefundamentalinterestofen-tanglinghighquantumnumbers,wealsodem-onstratetheuseofhigh-OAMentanglementforremotesensing.Forthisweusethesamemeth-odasbeforeforcreatinghigh-OAMentangledstates(foldedinterferometricschemeincludingSLM)andanalyzingthem(slitwheelmethod).WhenwetransferonephotontohighOAMval-uesandkeeptheotherinitspolarizationstate,thepaircanbeusedtoremotelymeasureanan-gularrotationwithaprecisionthatisincreasedbyafactorlrelativetothesituationwhenonlypolarization-entangledphotonpairsareused(Fig.4)(22).Thiscanleadtonotableimprove-mentsforapplicationsinthefieldofremotesensing,especiallywherelowlightintensitiesarerequired,suchasinbiologicalimagingexperi-mentswithlight-sensitivematerial.Ananalogousimprovementcanbeachievedclassicallyifdiag-onallyorcircularlypolarizedlightentersourtrans-fersetup.However,theimportantdifferenceisthatentanglementenablesthemeasurementstobedoneremotely,withthephotonsbeingspatiallyseparatedoreveninunknownlocationsatsomelatertime.

Ourapproachcouldbegeneralizedtohigher-dimensionalentanglementforspatialmodes—forexample,bystartingwithhigher-dimensional(hybrid)entanglementandamorecomplexinter-ferometricscheme.Suchadevelopmentwouldhavepotentialbenefitsinapplicationssuchasquantumcryptography,quantumcomputation,andquantummetrology.

ReferencesandNotes

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2.J.S.Bell,Physics1,195(1965).

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6.J.C.Howell,R.S.Bennink,S.J.Bentley,R.W.Boyd,Phys.Rev.Lett.92,210403(2004).

7.S.Ramelow,L.Ratschbacher,A.Fedrizzi,N.K.Langford,A.Zeilinger,Phys.Rev.Lett.103,253601(2009).8.L.Allen,M.W.Beijersbergen,R.J.C.Spreeuw,J.P.Woerdman,Phys.Rev.A45,8185(1992).9.N.K.Langfordetal.,Phys.Rev.Lett.93,053601(2004).

10.J.Leachetal.,Science329,662(2010).

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240401(2002).

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E.Andersson,Nat.Phys.7,677(2011).

13.B.-J.Pors,F.Miatto,G.W.’tHooft,E.R.Eliel,

J.P.Woerdman,J.Opt.13,064008(2011).

14.A.J.Leggett,J.Phys.Condens.Matter14,R415(2002).15.M.Aspelmeyer,P.Meystre,K.Schwab,Phys.Today65,

29(2012).

16.A.K.Jha,G.S.Agarwal,R.W.Boyd,Phys.Rev.A83,

053829(2011).

17.J.Romero,D.Giovannini,S.Franke-Arnold,S.M.Barnett,

M.J.Padgett,http://arxiv.org/abs/1205.1968(2012).18.H.DiLorenzoPires,H.C.B.Florijn,M.P.vanExter,

Phys.Rev.Lett.104,020505(2010).

19.M.?ukowski,J.Pykacz,Phys.Lett.A127,1(1988).20.E.Nagalietal.,Phys.Rev.Lett.103,013601(2009).21.E.J.Galvez,S.M.Nomoto,W.H.Schubert,M.D.Novenstern,

paperpresentedattheInternationalConferenceonQuantumInformation,Ottawa,6June2011;www.opticsinfobase.org/abstract.cfm?URI=ICQI-2011-QMI18.

22.SeesupplementarymaterialsonScienceOnline.23.S.Chávez-Cerdaetal.,J.Opt.B4,S52(2002).

24.J.B.Bentley,J.A.Davis,M.A.Bandres,J.C.Gutiérrez-Vega,

Opt.Lett.31,649(2006).

25.G.A.Siviloglou,J.Broky,A.Dogariu,D.N.Christodoulides,

Phys.Rev.Lett.99,213901(2007).

26.T.Kim,M.Fiorentino,F.N.C.Wong,Phys.Rev.A73,

012316(2006).

27.A.Fedrizzi,T.Herbst,A.Poppe,T.Jennewein,A.Zeilinger,

Opt.Express15,15377(2007).

28.G.Campbell,B.Hage,B.Buchler,P.K.Lam,Appl.Opt.

51,873(2012).

29.O.Gühne,G.Tóth,Phys.Rep.474,1(2009).30.A.Jesacher,S.Fürhapter,C.Maurer,S.Bernet,

M.Ritsch-Marte,Opt.Express14,6342(2006).Acknowledgments:SupportedbytheEuropeanResearchCouncil(advancedgrantQIT4QAD,227844)andtheAustrianScienceFund(FWF)withintheSpecialResearchPrograms(SFB)F40(FoundationsandApplicationsofQuantumScience;FoQuS)andW1210-2(ViennaDoctoralProgramonComplexQuantumSystems;CoQuS).R.F.participatedinthedesignandbuildingoftheexperimentalapparatus,collectedandanalyzedthedata,andwrotethemanuscript.R.L.,C.S.,andS.R.participatedinthedesignandbuildingoftheexperimentandassistedontheexperimentalside.W.N.P.,S.R.,andM.K.assistedonthetheoreticalside.A.Z.initiatedtheworkandsupervisedtheexperiment.Allauthorscontributedtoconceivingtheexperiment,discussingtheresults,andcontributingtothefinaltextofthemanuscript.

SupplementaryMaterials

www.sciencemag.org/cgi/content/full/338/6107/640/DC1MaterialsandMethodsSupplementaryTextTableS1Fig.S1

9July2012;accepted20September201210.1126/science.1227193

EfficientHybridSolarCellsBasedonMeso-Superstructured

OrganometalHalidePerovskites

MichaelM.Lee,1Jo?lTeuscher,1TsutomuMiyasaka,2TakurouN.Murakami,2,3HenryJ.Snaith1*Theenergycostsassociatedwithseparatingtightlyboundexcitons(photoinducedelectron-holepairs)andextractingfreechargesfromhighlydisorderedlow-mobilitynetworksrepresentfundamentallossesformanylow-costphotovoltaictechnologies.Wereportalow-cost,

solution-processablesolarcell,basedonahighlycrystallineperovskiteabsorberwithintensevisibletonear-infraredabsorptivity,thathasapowerconversionefficiencyof10.9%inasingle-junctiondeviceundersimulatedfullsunlight.This“meso-superstructuredsolarcell”

exhibitsexceptionallyfewfundamentalenergylosses;itcangenerateopen-circuitphotovoltagesofmorethan1.1volts,despitetherelativelynarrowabsorberbandgapof1.55electronvolts.Thefunctionalityarisesfromtheuseofmesoporousaluminaasaninertscaffoldthatstructurestheabsorberandforceselectronstoresideinandbetransportedthroughtheperovskite.nefficientsolarcellmustabsorboverabroadspectralrange,fromvisibletonear-infrared(near-IR)wavelengths(350to

~950nm),andconverttheincidentlighteffec-tivelyintocharges.Thechargesmustbecollected

generatedbythesolarcellundersimulatedairmass(AM)1.5solarilluminationof100mWcm?2(9).Forinstance,galliumarsenide(GaAs)solarcellsexhibitVocof1.11Vandanopticalbandgapof1.4eV,givingadifferenceof~0.29eV(2).Fordye-sensitizedandorganicsolarcells,thisdifferenceisusuallyontheorderof0.7to0.8eV(2,9).Fororganicsolarcells,suchlossesarepredominantlycausedbytheirlowdielectricconstants.Tightlyboundexcitonsform,whichrequireaheterojunctionwithanelectronaccep-torwithalargeenergyoffsettoenableion-izationandchargeseparation(10,11).Likewise,dye-sensitizedsolarcells(DSSCs)havelosses,bothfromelectrontransferfromthedye(orab-sorber)intotheTiO2,whichrequiresacertain“drivingforce,”andfromdyeregenerationfrom

ClarendonLaboratory,DepartmentofPhysics,UniversityofOxford,OxfordOX13PU,UK.2GraduateSchoolofEngi-neering,ToinUniversityofYokohama,1614Kurogane,Aoba,Yokohama225-8503,Japan.3ResearchCenterforPhotovoltaicTechnologies,NationalInstituteofAdvancedIndustrialSci-enceandTechnology,Central5,1-1-1Higashi,Tsukuba,Ibaraki305-8565,Japan.

*Towhomcorrespondenceshouldbeaddressed.E-mail:h.snaith1@physics.ox.ac.uk

1A

atahighvoltagewithsuitablecurrentinordertodousefulwork(1–8).Asimplemeasureofsolarcelleffectivenessatgeneratingvoltageisthedif-ferenceinenergybetweentheopticalbandgapoftheabsorberandtheopen-circuitvoltage(Voc)

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theelectrolyte,whichrequiresanoverpotential.EffortshavebeenmadetoreducesuchlossesinDSSCsbymovingfromamultielectroniodide–tri-iodideredoxcoupletoone-electronouter-sphereredoxcouples,suchascobaltcomplexesorasolid-stateholeconductor(1,4,12,13).

Inorganicsemiconductor–sensitizedsolarcellshaverecentlybecomeafocusofinterest(14,15).Anextremelythinabsorber(ETA)layer,2to10nminthickness,iscoatedupontheinternalsurfaceofamesoporousTiO2electrodeandthencontactedwithanelectrolyteorsolid-stateholeconductor.Thesedeviceshaveachievedpowerconversionefficienciesofupto6.3%(15).How-ever,theETAconceptsuffersfromratherlowVoc;theproblemmaylieintheelectronicallydisor-dered,low-mobilityn-typeTiO2(16).Perovskitesarerelativelyunderexploredalternatives(Fig.1A)thatprovideaframeworkforbindingorganicandinorganiccomponentsintoamolecularcompos-ite.Withcarefulconsiderationoftheinteractionbetweenorganicandinorganicelementsandsuit-ablecontrolofthesize-tunablecrystalcell(17),rudimentarywetchemistrycanbeusedtocreatenewandinterestingmaterials.Era,Mitzi,andco-workershaveshownthatlayeredperovskitesbasedonorganometalhalidesdemonstrateexcellentper-formanceaslight-emittingdiodes(18,19)andtransistorswithmobilitiescomparabletoamor-phoussilicon(20).Organometalhalideperovskiteshavebeenusedassensitizersinliquidelectrolyte–basedphotoelectrochemicalcellswithconversionefficienciesfrom3.5to6.5%(21,22).Recently,aCsSnI3perovskitewasshowntofunctioneffi-cientlyasaholeconductorinsolid-stateDSSCs,deliveringupto8.5%powerconversionefficiency(23,24).

Wereportonasolution-processablesolarcellthatovercomesthefundamentallossesoforganicabsorbersanddisorderedmetaloxides.Wefol-lowedtheETAapproachandusedaperovskiteabsorber,mesoporousTiO2asthetransparentn-typecomponent,and2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)9,9′-spirobifluorene(spiro-OMeTAD)asthetransparentp-typeholeconductor.Thesedevicesexhibitedpowercon-versionefficienciesnear8%.Remarkably,wealsofoundthatreplacementofthemesoporousn-typeTiO2withinsulatingAl2O3improvedthepowerconversionefficiency.TheAl2O3isaninsulatorwithawidebandgap(7to9eV)andpurelyactsasa“scaffold”uponwhichtheperovskiteiscoated.Weobservedthatelectrontransportthroughtheperovskitelayerwasmuchfasterthanthroughthen-typeTiO2.Inaddition,weobservedanin-creaseinVoc(movingfromtheTiO2totheinsu-latingAl2O3scaffold)ofafewhundredmillivoltsandapowerconversionefficiencyof10.9%undersimulatedAM1.5fullsolarillumination.

Thespecificperovskiteweusedisofmixed-halideform:methylammoniumleadiodidechlo-ride(CH3NH3PbI2Cl),whichwasprocessedfromaprecursorsolutioninN,N-dimethylformamideviaspin-coatinginambientconditions.X-raydif-fractionanalysisforCH3NH3PbI2Clprepared

onglass(fig.S1)(25)showeddiffractionpeaksat14.20°,28.58°,and43.27°,whichweassignedasthe(110),(220),and(330)planes,respectively,ofatetragonalperovskitestructurewithlatticeparametersa=8.825?,b=8.835?,c=11.24?,similartotheCH3NH3PbI3previouslyreported(21).Theextremelynarrowdiffractionpeakssug-gestthatthefilmshavelong-rangecrystallinedo-mains(>200nm,peakwidthlimitedbyinstrumentbroadening)andarehighlyorientedwiththeaaxis(21,26).Incontrasttothemethylammoniumtrihalogenplumbatespreviouslyreportedinsolarcells(i.e.,CH3NH3PbI3)(21,22),thisiodide-chloridemixed-halideperovskitewasremarkablystabletoprocessinginair.Theabsorptionspectra(Fig.1B)demonstratedgoodlight-harvestingcapabilitiesoverthevisibletonear-IRspectrumandwasalsostabletoprolongedlightexposure,asdemonstratedby1000hoursofconstantillu-minationundersimulatedfullsunlight.Theab-sorbanceofthefilmat500nmremainedaround1.8throughouttheentiremeasurementperiod(absorbanceof1.8correspondsto98.4?sorp-tion)(Fig.1B,inset).Notethatthescaleisopticaldensity,whereabsorbanceof~0.5at700nmcorrespondsto~70%attenuationinasinglepass;inthesolarcell,therearetwopassesoflightleadingto~91?sorptionatthiswavelength.Thesolarcellswerefabricatedonsemitrans-parentfluorine-dopedtinoxide(FTO)–coatedglasscoatedwithacompactlayerofTiO2that

actedasananode.Theporousoxidefilmswerefabricatedfromsol-gel–processedsinterednano-particles.Theperovskiteprecursorsolutionwasinfiltratedintotheporousoxidemesostructureviaspin-coatingandwasdriedat100°C,whichenabledtheperovskitetoformviaself-assemblyoftheconstituentions.Darkcolorationwasob-servedonlyafterthisdryingstep.

Withrespecttotheperovskitecoatingpro-cess,therehasbeenextensiveworkdoneonin-vestigatinghowsolution-castmaterialsinfiltrateintomesoporousoxides(27–32).Iftheconcen-trationofthesolutionislowenoughandthesolubilityofthecastmaterialhighenough,thematerialwillcompletelypenetratetheporesasthesolventevaporates.Typically,thematerialformsa“wetting”layerupontheinternalsurfaceofthemesoporousfilmthatuniformlycoatstheporewallsthroughoutthethicknessoftheelec-trode(28–31).Thedegreeof“porefilling”canbecontrolledbyvaryingthesolutionconcentra-tion(29–32).Iftheconcentrationofthecastingsolutionishigh,thenmaximumporefillingoc-curs,andany“excess”materialformsa“cappinglayer”ontopofthefilledmesoporousoxide.Fortheoptimumperovskiteprecursorcon-centrationsweused,therewasnoappearanceofacappinglayer,whichimpliesthattheperovskitewaspredominantlyformedwithinthemesoporousfilm.Weverifiedthattheperovskitewaswithinanduniformlydistributedthroughoutthemeso-

A

c

B

a

AbsorbanceAbsorbanceTime (hours)

C

Wavelength (nm)

AgSpiro-OMeTADPhotoactive LayerPerovskiteMesoporous OxideCompact TiO2FTOGlassFig.1.(A)Left:Three-dimensionalschematicrepresentationofperovskitestructureABX3(A=CH3NH3,B=Pb,andX=Cl,I).Right:Two-dimensionalschematicillustratingtheperovskiteunitcell.(B)Ultraviolettovisible(UV-Vis)absorbancespectraofthephotoactivelayerinthesolarcell(mesoporousoxide;perovskiteabsorber;spiro-OMeTAD)sealedbetweentwosheetsofglassinnitrogenandexposedtosimulatedAM1.5sunlightat100mWcm?2irradianceforupto1000hours.NoadditionalUVfiltrationwasusedforthesolarirradiance.Inset:Extractedopticaldensityat500nmasafunctionoftime.(C)Left:Schematicrepresentationoffulldevicestructure,wherethemesoporousoxideiseitherAl2O3oranataseTiO2.Right:Cross-sectionalSEMimageofafulldeviceincorporatingmesoporousAl2O3.Scalebar,500nm.

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porousoxidefilmsbyperformingcross-sectionalscanningelectronmicroscopy(SEM)withelemen-talmappingviaenergy-dispersivex-ray(EDX)analysis(fig.S2)(25).Tocompletethephoto-activelayer,theperovskite-coatedporouselectrodewasfurtherfilledwiththeholetransporter,spiro-OMeTAD,viaspin-coating;asshowninFig.1C,thespiro-OMeTADformsacappinglayerthatensuresselectivecollectionofholesatthesilverelectrode.

InFig.2A,theincidentphoton-to-electronconversionefficiency(IPCE)actionspectrumisshownforthedevicesthatusemesoporousTiO2andAl2O3,exhibitingspectralsensitivityspanningfromthevisibletothenear-IR(400to800nm)withapeakIPCEof>80%forbothoxides.Theslightdifferenceinshapearisesfromtheslightlydifferentperovskiteconcentrationsintheoptimizeddevices.InFig.2B,weshowcurrentdensity–voltage(J-V)curvesmeasuredundersimulatedAM1.5illuminationof100mWcm?2.ThesensitizedTiO2solarcellexhibitedashort-circuitphotocurrent(Jsc)=17.8mAcm?2,Voc=0.80V,andafillfactorof0.53,yieldinganoverallpowerconversionefficiency(h)of7.6%.WepresenttwodifferentJ-VcurvesfortheAl2O3-baseddevice.ThemostefficientdeviceexhibitedJsc=17.8mAcm?2,Voc=0.98V,andafillfactorof0.63,yieldingh=10.9%.Thethirdcurve(dashedtrace)showsadevicewithJsc=15.4mAcm?2andVoc=1.13Vbutalowfillfactorof0.45,yieldingh=7.8%.[See(25)forhistogramsofdeviceperformanceparametersfortheTiO2-andAl2O3-baseddevices(fig.S3)].ThegeneraltrendisthattheAl2O3cellsgen-eratedopen-circuitvoltagesthatwere>200mVhigherthanthosegeneratedbythesensitizedTiO2solarcells,withcomparableshort-circuitcurrentsandslightlylowerfillfactors.Fromthesolarcellmeasurementsonalumina-baseddevices,itwasapparentthattheperovskitelayercouldfunctionasbothabsorberandn-typecom-ponent,transportingelectronicchargeoutofthedevice.Wefurtherillustratethe“semiconducting”

natureoftheperovskitebytheconstructionofaplanar-junctiondiodewiththestructureFTO/compactTiO2/CH3NH3PbI2Cl/spiro-OMeTAD/Ag.Theperovskitefilmwas~150nmthickinthisconfiguration,andthesolarcellexhibitedJsc=7.13mAcm?2,Voc=0.64V,afillfactorof0.4,andh=1.8%.

IfwetaketheopticalbandgapofCH3NH3PbI2Cltobe1.55eVfromtheIPCEonsetat800nm(33)andtheopen-circuitvoltagetobe1.1V,thisrepresentsadifferenceinenergyofonly0.45eV,competitivewiththebestthin-filmtechnologies(2).Tounderstandwhyweobservedsuchanin-creaseinvoltageovertheTiO2cells,weneedtoconsidertheoperationalmodeofthetwoconcepts(Fig.3A).ForsensitizedTiO2devices,wewouldexpectthatafterlightabsorptionintheperovskite,electronswouldbetransferredtotheTiO2(withsubsequentelectrontransporttotheFTOelectrodethroughtheTiO2)andholeswouldbetransferredtothespiro-OMeTAD(withsubsequenttransporttothesilverelectrode).ForAl2O3-basedcells,theelectronsmustremainintheperovskitephase(34)untiltheyarecollectedattheplanarTiO2-coatedFTOelectrode,andmusthencebetransportedthroughoutthefilmthicknessintheperovskite.Holetransferfromthephotoexcitedperovskitetothespiro-OMeTADshouldoccurinmuchthesamewayasinthesensitizeddevice.Al2O3didnotactasann-typeoxideinDSSCs(fig.S4)(25).Toexaminethechargegenerationinthesedevices,weperformedphotoinducedabsorption(PIA)spectroscopyontheoxidefilmscoatedwiththeperovskite,bothwithandwithouttheaddi-tionofspiro-OMeTAD.ForthemesoporousTiO2filmcoatedwithperovskite,thePIAspectrumrevealedfeaturesinthenear-IRassignedtothefreeelectronsinthetitania(35),confirmingeffec-tivesensitizationofthetitaniabytheperovskite.Incontrast,filmsmadeofAl2O3coatedwithperovskiteexhibitednoPIAsignal,confirmingtheinsulatingroleofalumina.Afteradditionofspiro-OMeTAD,wecouldefficientlymonitortheoxidizedspeciesofspiro-OMeTADcreatedafter

photoexcitationoftheperovskite.Theyhadab-sorptionfeaturesat525and750nm,aswellasabroadbandaround1200nm,assignedtotheholelocatedonthetriarylaminemoieties(28,36),whichdominatedthespectrainboththeTiO2-andAl2O3-basedsamples.Theseresultsindicatethatholetransferishighlyeffectivefromthephotoexcitedperovskitetospiro-OMeTAD,andspecificallythataholeconductorisrequiredtoenablelong-livedchargespecieswithintheperovskitecoatedontheAl2O3.WenotethatthePIAsignaldependedbothontheconcentrationandlifetimeofthespeciesmonitored;hence,fromthismeasurementalone,quantificationoftherelativecharge-generatedyieldisnotpossible.Toprobetheeffectivenessoftheperovskitelayerattransportingelectronicchargeoutofthedevice,weperformedsmall-perturbationtran-sientphotocurrentdecaymeasurements(37).Thesolarcellswereexposedtosimulatedsunlightand“flashed”withasmallredlightpulse;insuchex-periments,thedecayrateofthetransientphoto-currentsignalisapproximatelyproportionaltotherateofchargetransportoutofthephotoactivelayer(37).AsshowninFig.3C,weobservedthatchargecollectionintheAl2O3-baseddeviceswasfasterthanintheTiO2-basedsensitizedde-vicesbyafactorof>10,indicatingfasterelec-trondiffusionthroughtheperovskitephasethanthroughthen-typeTiO2.

Becausethereisnon-typeoxideintheAl2O3-basedcells,thedevicesarenot“sensitized”solarcells,butrathertwo-componenthybridsolarcells.Asdesigned,theAl2O3issimplyactingasamesoscale“scaffold”uponwhichthedeviceisstructured;wetermthisconcepta“meso-superstructuredsolarcell”(MSSC).Theabovemeasurementsdemonstratethatlong-livedchargecarrierscanbegeneratedviaholetransferfromtheperovskitetospiro-OMeTADandthattheperovskitelayerisfasterattransportingelectronicchargethanthemesoporousTiO2.However,theydonotexplaintheincreaseinVocvalues.TheVocisgeneratedbythebuild-upofelectronsin

AB

Current density (mA cm-2)Wavelength (nm)

IPCE (%)Voltage (V)

cellexhibitinghighefficiency(redsolidtracewithcrosses)andoneexhibit-ingVOC>1.1V(reddashedlinewithcrosses);foraperovskite-sensitizedTiO2solarcell(blacktracewithcircles);andforaplanar-junctiondiodewithstructureFTO/compactTiO2/CH3NH3PbI2Cl/spiro-OMeTAD/Ag(purpletracewithsquares).VOL338

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Fig.2.(A)IPCEactionspectrumofanAl2O3-basedandperovskite-sensitizedTiO2solarcell,withdevicestructureasfollows:FTO/compactTiO2/mesoporousAl2O3(redtracewithcrosses)ormesoporousTiO2(blacktracewithcircles)/CH3NH3PbI2Cl/spiro-OMeTAD/Ag.(B)Currentdensity–voltagecharacteristicsundersimulatedAM1.5100mWcm?2illuminationforAl2O3-basedcells,one

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A

-?T/T (x10-6)B

Wavelength (nm)

Photocurrent decayTransport lifetime (s)C

Time (ms)

Current density (mA cm-2)

Fig.3.(A)Schematicillustratingthechargetransferandchargetransportinaperovskite-sensitizedTiO2solarcell(left)andanoninjectingAl2O3-basedsolarcell(right);arepresentationoftheenergylandscapeisshownbelow,withelectronsshownassolidcirclesandholesasopencircles.(B)Photoinducedabsorbance(PIA)spectraofthemesoporousTiO2films(blackcircles)andAl2O3films(redcrosses)coatedwithperovskitewith(solidlines)andwithout(dashedlines)spiro-OMeTADholetransporter,then-typematerialandholesinthep-typema-terial,resultinginsplittingofthequasiFermilev-elsforbothelectronsandholes.FormesoporousTiO2,thereexistsitesinthetailofthedensityofstatesthatextendintothebandgap(38).Thesefillwithelectronsunderillumination;theresultisthatthequasi–Fermilevelforelectrons(E*Fn)isfartherfromtheconductionband,foranygivenchargedensity,thanwouldbethecaseifthesestatesdidnotexist(i.e.,inahighlycrystallinesemiconductor).Theincreasedcharge-storingcapacityofmaterialswithahighdensityofsub–bandgapstatesistermed“chemicalca-pacitance”(38).Thereis,inessence,nochemicalcapacitanceoftheAl2O3,andfortheMSSCsalltheelectronicchargeresidesintheperov-skite,movingtheE*Fninthismaterialnearertotheconductionbandforthesamechargeden-sity.Thehighervoltageindicatesthattherearefewersurfaceandsub–bandgapstatesintheperovskitefilmsthaninthemesoporousTiO2.Hence,theincreasedvoltageiscausedbyasub-stantialreductionofthechemicalcapacitanceofthesolarcell.WeusedacompactlayerofTiO2astheelectron-selectiveanode,butthechemicalcapacitanceofthisextremelythin(50to100nm)TiO2layerwasverylowbecauseofthelowvolumeandsurfacearea(i.e.,flat).Inaddition,thecompactlayerdepositedviaspraypyrolysishasadonordensityof~1018cm?3(39),andthesub–bandgapsitesresponsibleforthechemicalcapacitancemaybefull.

AcentralquestioniswhethertheMSSCisexcitonicoradistributedp-njunction.Thepe-

under496.5nmexcitationat23Hzrepetitionrate.(C)Chargetransportlifetimedeterminedbysmall-perturbationtransientphotocurrentdecaymeasurementofperovskite-sensitizedTiO2cells(blackcircles)andAl2O3cells(redcrosses),bothwithlinestoaidtheeye.InsetshowsnormalizedphotocurrenttransientsforAl2O3cells(redtracewithcrossesevery7thpoint)andTiO2cells(blacktracewithcirclesevery7thpoint),settogenerate5mAcm?2photocurrentfromthebackgroundlightbias.

(MSSC),hasproventobeextraordinarilyeffec-tivewithann-typeperovskite,deliveringmorethan10.9%powerconversionefficiencyunderfullsolarillumination.Furtheradvancesinover-allpowerconversionefficiencyareexpectedbyextendingtheabsorptiononsettoward940nm,throughtheimplementationofnewperovskitesorbroadeningthisconcepttoothersolution-processablesemiconductors.Enhancingthelightabsorptionnearthebandedgethroughcarefullyengineeredmesostructuresorbetterphotonman-agementwouldleadtoincreasedphotocurrent.Reducedseriesresistancethroughtheuseofhigher-mobilityholetransporters,orbettercon-troloverthecappinglayerthickness,wouldim-provethefillfactor.Finally,extendingthissystemtomultijunctiondevices(withouttherequire-mentforlatticematching,asinconventionalmultijunctionsolarcells)wouldfurtherenhanceperformance.

ReferencesandNotes

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3.L.Hanetal.,EnergyEnviron.Sci.5,6057(2012).4.A.Yellaetal.,Science334,629(2011).

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rovskitestendtoformlayeredstructures,withcontinuoustwo-dimensionalmetalhalideplanesperpendiculartothezaxisandthelowerdi-electricorganiccomponents(methylamine)be-tweentheseplanes.Thepossiblequasi–two-dimensionalconfinementoftheexcitonscanresultinanincreasedexcitonbindingenergy,whichcanbeuptoafewhundredmillielectronvolts(40).Thereasonablyhighphotocurrentsfromtheplanar-junctionsolarcells(Fig.2B)couldbeexplainedbyeithermoderatelydelo-calizedandhighlymobileexcitonsbeingquenchedattheperovskite–spiro-OMeTADinterface,orthegenerationoffreechargesinthebulkoftheperovskitefilmswithreasonablygoodelectronandholemigrationoutofthedevices.

ThekeylimitationinperformanceoftheMSSCatpresentisabalancebetweenseriesandshuntresistance.Theperovskiteabsorberisreasonablyconductive,measuredtobeontheorderof10?3Scm?3;thus,short-circuitingofthedeviceoccursifcontactexistsbetweenthesilverelectrodeandtheperovskiteabsorber.Athickcappinglayerofp-typespiro-OMeTADreadilyresolvesthisissue,however;spiro-OMeTADislessconductive(~10?5Scm?1),soathickercap-pinglayerresultsinhighseriesresistance.Thus,wearepresentedwithacompromise.

Ourworkrepresentsanevolutionofthesolid-statesensitizedsolarcellwithlowfundamentallosses.Theapplicationofamesostructuredin-sulatingscaffolduponwhichextremelythinfilmsofn-typeandp-typesemiconductorsareassem-bled,termedthemeso-superstructuredsolarcell

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