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NANOSTRUCTUREDCARBONNANOTUBESCHOTTKYJUNCTIONSOLAR CELLS By MAUREENK.PETTERSON ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOL OFTHEUNIVERSITYOFFLORIDAINPARTIALFULFILLMENT OFTHEREQUIREMENTSFORTHEDEGREEOF DOCTOROFPHILOSOPHY UNIVERSITYOFFLORIDA 2013 1
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c r 2013MaureenK.Petterson 2
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Tomyfamily 3
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ACKNOWLEDGMENTS Iwouldliketothankmyadviser,Dr.AndrewRinzler,forhisunwaverin gsupport andguidancethroughoutthepastfewyears.Hisknowledgeofbot hexperimentaland theoreticalaspectsofphysicsresearchiscomprehensiveandine xhaustible;hewasalways presenttoanswertheoreticalquestionsorhelpdesignandbuildex perimentalapparatus. Hisabilitytoplacecontemporaryresearchinhistoricalperspective dissuadesamyopic viewofgraduateresearch,insteadfosteringanappreciationfor thewideapplicabilityof experimentalresults.Dr.Rinzlergavemefreedomtospenddays, weeks,andmonths tediouslytroubleshootingexperimentsandhisencouragementand availabilityexpedited thesuccessfulresults,whilehispatienceandcommiserationalleviat edfrustrationoverthe failures. I'dliketothankmycommitteemembersfortheirongoingsupport.Dr .Stantonand Dr.TannerwereamongtherstprofessorsImetwithinthephysic sdepartment,and I'mgladtohavehadtheircounseloverthepastseveralyears.Co llaborationswithDr. Hebardandhisgrouphavebeenenlighteningandfruitful,culminating insomeexcellent publishedwork.Evenbeforehewasonmycommittee,Dr.Biswaslent alivelyatmosphere tothedepartmentwithoutunderminingtheethosofgraduateres earchanddiscussions withhimregardingprofessionalpursuitshavebeenbenecial.I'mth ankfultoDr.Sofor hisexcellentobjectiveinput;hisownresearchgavehimparticularlyg oodinsightintomy researchprojects. Ialsoowealotofgratitudetomylabmates:Dr.MitchellMcCarthy,D r.BoLiu, Dr.RajibDas,Dr.SvetlanaVasilyeva,Dr.MaxLemaitre,Dr.Pooja Wadhwa,Dr.Evan Donoghue,Dr.Po-HsiangWang,YuShen,XiaoChen,NanZhao,JieH ou,MattGilbert, andKyleDorsey.Informationgleanedfromdiscussionswiththemgr eatlyfacilitatedmy understandingofphysicsandchemistry.Theywereeagertohelpw ithanyproblemsI encounteredandoeredtheirexpertiseandguidanceonmanyasp ectsofmyexperiments. 4
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Notrelegatedtojustprofessionalcohorts,theyalsolivenedupt heatmosphereandmade comingintolabanenjoyableexperience. I'dliketothankDarleneLatimerandPamMarlinforwadingthroughthe academic bureaucracyonmybehalfandalwaysensuringIwasontracktogra duate.Darlene's genuineconcernforallofthegraduatestudentsisheartwarming andhercontributionsto thephysicsdepartmentarebeyondmeasure.I'dliketothankJayH orton,TimNoland, andthemachineshopforprovidingtheirtechnicalexpertisetoour labandforfabricating andxingmost(ifnotall)ofourexperimentalapparatus.I'dalsolike tothankPete Axsonandtherestoftheelectronicsshopforkeepingoursolarsim ulatorinworking order. I'dliketothankmyfriendsforoeringunlimitedandunconditionalsup port, encouragement,andadvice;withoutwhomIneverwouldhaveappr eciatedthecamaraderie inducedbysportingeventsortoleratedsummersinFlorida.I'despe ciallyliketothank EvanDonoghueforbestowinguponmethelessonshelearnedinhistim eduringgraduate schoolandforhelpingmendtheperfectbalancebetweenhardwo rkandpersonal development.Physicistsarenotgenerallyknownfortheirsocialap titudeorenthusiastic inclusionofnewcomers,butthefriendsIhavegainedthroughthed epartmentdefythe stereotypesandhavemademygraduateschoolyearsonesofbo thacademicandpersonal growth. Finally,I'dliketothankmyfamily.Myparents,JohnPettersonandLo rettaKelley, forsupportingmeemotionally,intellectually,andnanciallyforthepa stthreedecades. Bothhavingexperiencedthetrialsandtribulationsofearninganadv anceddegree,their empathyandunderstandingwasgreatlyappreciated.Iowemucht omysiblings,Alyssa andCareyPetterson,forshapingmypersonalityandultimatelyput tingmeonthepathto beingasuccessfulphysicist. I'dliketoacknowledgetheNationalScienceFoundationforfundings upportunder awardECCS1232018. 5
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TABLEOFCONTENTS page ACKNOWLEDGMENTS ................................. 4 LISTOFTABLES ..................................... 9 LISTOFFIGURES .................................... 10 LISTOFABBREVIATIONS ............................... 13 ABSTRACT ........................................ 15 CHAPTER 1INTRODUCTION .................................. 17 2INTRODUCTIONTOCARBONNANOTUBES ................. 21 2.1HistoryandStructure ............................. 21 2.2Synthesis ..................................... 26 3INTRODUCTIONTOSOLARCELLS ....................... 30 3.1Fundamentals .................................. 30 3.1.1GenerationandSolarSpectrum .................... 30 3.1.2Recombination .............................. 32 3.1.2.1Radiative ........................... 33 3.1.2.2Auger ............................. 33 3.1.2.3ShockleyReedHall ...................... 34 3.1.3Characterization ............................. 35 3.1.4SeriesandShuntResistance ...................... 37 3.2TheoreticalLimitations ............................. 39 3.3TypesofSolarcells ............................... 41 3.3.1P-NJunction .............................. 41 3.3.2Organic ................................. 44 3.3.3PhotoelectrochemicalDevices ..................... 45 3.3.4Multi-junction .............................. 47 3.3.5Schottkyjunction ............................ 47 3.3.6InversionLayerCells .......................... 48 4INTRODUCTIONTOSCHOTTKYBARRIERS ................. 50 4.1Fundamentals .................................. 50 4.1.1BasicSchottkyModel .......................... 50 4.1.2CurrentTransport ............................ 51 4.1.2.1ThermionicEmission ..................... 52 4.1.2.2ThermionicFieldEmissionandFieldEmission ....... 52 4.1.2.3MinorityCarrierInjection .................. 53 6
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4.1.3BeyondSchottky-Mott ......................... 53 4.1.3.1FermiLevelPinning:BardeenModelandMetalInduced GapStates .......................... 55 4.1.3.2BondPolarization ...................... 56 4.2SchottkyJunctionSolarCells ......................... 57 4.2.1HistoricalBackground .......................... 57 4.2.2CNTonSiliconSchottkyJunctionSolarCells ............ 58 4.2.2.1ExperimentalDetailsandEquipment ............ 58 4.2.2.2ElectronicGating ....................... 60 4.2.2.3InversionLayerModeling .................. 60 5NANOSTRUCTURINGFORENHANCEDLIGHTABSORPTION ....... 66 5.1Overview .................................... 66 5.2PotassiumHydroxideEtching ......................... 66 5.3SiliconNanowires ................................ 68 5.3.1ProcedureandCharacterization .................... 69 5.3.2Integrationinsolarcellsandinitialperformance ........... 71 5.3.2.1RemoteGating ........................ 73 5.3.2.2PassivationofNanowireSidewalls .............. 74 5.3.2.3SWNTlmtransferonSiNW ................ 76 5.3.3DiscussionofinversionlayerinSiNWs ................. 78 5.4EectofOxygenandWateronDevicePerformance ............. 79 5.4.1Eectofambientoxidation ....................... 79 5.4.2Reversibledopinginambientenvironment .............. 81 5.4.3Watervaporandoxygencontamination ................ 82 5.4.3.1CVmeasurementsshowingILcontamination ........ 83 5.4.3.2Exclusiononplanardevice .................. 84 5.5ConcludingRemarks .............................. 86 6PASSIVATIONOFSILICON ............................ 89 6.1AtomicLayerDepositionofAl 2 O 3 andHfO ................. 90 6.1.1Al 2 O 3 andHfOresults ......................... 93 6.2Hydroquinone .................................. 96 6.3Sulfur ...................................... 98 6.4DiscussionandSummary ............................ 100 7ADDITIONALPROJECTS ............................. 104 7.1TFSADopingofGraphene-SiandCarbonNanotube-SiDevices ...... 104 7.1.1Graphene-SiSolarCells ......................... 104 7.1.2TFSAwithcarbonnanotubes ..................... 107 7.2BacksideDoping ................................ 108 7.3ConcludingRemarksandPathForward .................... 110 APPENDIX 7
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AFULLSIMULATIONSFORTHEINVERSIONLAYERCELL ......... 113 BSOLARCELLPARAMETERSWITHINCREASINGOXIDATIONTIME ... 114 REFERENCES ....................................... 115 BIOGRAPHICALSKETCH ................................ 121 8
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LISTOFTABLES Table page 3-1Currentmaximumecienciesforvariousphotovoltaicdevices[1 ] ......... 49 4-1TheoreticalvsExperimentalSchottkyBarrierHeights:Barr ierheightsmeasured at300K,theoreticalvaluesdeterminedfromSchottky-Mottrela tion ....... 65 5-1Performanceforvariouslmdepositiontechniquesandthickne sses ........ 88 6-1PerformanceforALDdevicesforV G =-1.0V ................... 103 7-1PerformancesummaryforTFSAdopedgrapheneandSWNTsola rcells ..... 112 7-2Performanceforbacksidedopedsubstrates ..................... 112 9
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LISTOFFIGURES Figure page 2-1Graphenelatticeandgeometricclassicationsforcarbonnano tubes ....... 22 2-2Bandstructureforcarbonnanotubes ........................ 23 2-3Densityofstatesforsemiconductingandmetalliccarbonnanot ubes ....... 24 2-4ElectronicgatingofSWNTlm ........................... 25 2-5DedopingofSWNTlmduringhightemperaturebake .............. 29 3-1ThesolarspectrumreceivedbothoutsidetheEarth'satmosph ereandatthe surfaceoftheEarth ................................. 31 3-2Threerecombinationrouteswithinsemiconductors. ................ 34 3-3Maximumgeneratedpowerdensity(bluebox),denedby P = V M J M ...... 36 3-4Circuitequivalentshowingseriesandshuntresistance. .............. 38 3-5TheeectsofseriesandshuntresistanceontheJ-Vcurve ............ 39 3-6TheShockley-Queisserlimitshowingthemaximumtheoreticalec iencyasa functionofbandgapforasinglep-njunctionsolarcell. ............. 41 3-7Schematicforap-njunction ............................. 42 3-8BanddiagramofP-Njunctionwithnobias,reversebias,andfor wardbias ... 43 3-9Schematicofthebilayerandbulkheterojunctionsolarcells. ........... 45 3-10Schematicofsimplesemiconductor/liquidjunctionsolarcellsho wingredoxreactions occurringbothatthesemiconductorsurfaceandatametalcoun terelectrode. .. 46 4-1Schottkybarrierbanddiagrams ........................... 51 4-2InterfacestatesshownintherealisticmodelofaSchottkyju nction. ....... 55 4-3Carbonnanotube-siliconSchottkyjunctioncell .................. 59 4-4SchematicandresultsforelectronicallygatedSWNT-Sicell ........... 61 4-5Schematicandperformanceforgridcell ...................... 62 4-6Simulationsshowinginversionlayerinsiliconextendingacrossentir esurfacein betweencarbonnanotubestrips ........................... 64 5-1KOHschematicandperformance. .......................... 67 5-2Themechanismforsiliconnanowiregrowth .................... 69 10
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5-3SiliconnanowiresgrowninanHF/AgNO 3 solution ................ 70 5-4Orientationofsiliconnanowires ........................... 71 5-5Rerectanceofthesiliconnanowiresubstratesascomparedto untexturedsilicon. 72 5-6InitialperformanceoftheSWNT-SiNWdevice .................. 73 5-7SchematicforremotegatingandSEMofSWNT-SiNWactivearea ....... 74 5-8J-VofaSWNT-SiNWdeviceshowingtheeectofsidewallpassivat ionviaoxidation ontheperformanceofthedevice. .......................... 75 5-9J-Vcurvesfor V G =-1.0V,0V,+1.0VontheSiNWdevice. .......... 77 5-10EvolutionofJ-Vcurvewithoxidationinambientatmosphere. .......... 80 5-11ReversibilityoftheJ-Vcurveuponalternatingexposuretoar gonandambient atmospheres ...................................... 82 5-12CyclicvoltammogramsoftheglassycarbonelectrodeinEMI-BT Iionicliquid at50 mV s ........................................ 84 5-13Stabilityofplanardevicewithoxygenandwaterexcludedbygat ingininert atmospherewithV G =-1.0V. ............................ 86 5-14DegradationoftheplanarSWNT-SiNWdeviceuponexposureto atmosphere withV G =-1.0V. ................................... 87 6-1ALDgrowthprocess ................................. 90 6-2SEMimageofALDdeposition ............................ 92 6-3J-VcurvesfortheALDAl 2 O 3 coatedSWNT/SiNWcell ............. 93 6-4J-VcurvesforALDSWNT-SiNWdevicevsdevicewithoutALD ........ 94 6-5J-VfortheALDHfOdeviceshowingaloweringJ SC duetothehighrerectance ofthedevice. ..................................... 95 6-6Siliconsubstrateandhydroquinonemolecule .................... 97 6-7J-VcurveoftheHQtreatedplanarcellbefore,during,andaf terelectronicgating withEMI-BTI. .................................... 99 6-8J-Vcurvesforsulfurandhydroquinonepassivateddevices ............ 101 7-1JVcurveforthemonolayergraphenedevice. .................... 106 7-2SchematicandperformanceforgraphenePVcell. ................. 107 11
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7-3J-VcurvesshowingeectofTFSAdopingandsubsequentgatin gonSWNT-Si device. ........................................ 108 7-4Blisteringonthesurfaceofthesiliconfollowingahightemperatur ebaketodope thebackside. ..................................... 110 A-1Modelingoftheinversionlayeratthesiliconsurfaceinthecarbon nanotube gridsolarcell ..................................... 113 B-1FF,J SC ,V OC ,andPCEforaSWNT-SiNWdeviceforvariousoxidationtimes inthelabatmosphere. ................................ 114 12
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LISTOFABBREVIATIONS A RichardsonConstant ALD AtomicLayerDeposition AM 1 : 5 G AirMass1.5Global CNT CarbonNanotube CVD ChemicalVaporDeposition D n=p Electronorholediusionconstant DOS DensityofStates E F Fermienergy EMI BTI 1-Ethyl-3-methylimidazoliumbis(triruoromethylsulfonyl)imide FF FillFactor J M Currentdensityatmaximumpowerpoint J photo Photocurrentdensity J O Saturationcurrentdensity/darkcurrent J SC Shortcircuitcurrentdensity MIGS Metalinducedgapstates MIS IL Metal-insulator-semiconductorinversionlayercell P M Maximumpowerdensity PCE Powerconversioneciency m Workfunctionofmetal Bn 0 Schottkybarrierheightton-typesemiconductor 0 Neutrallevel(aboveE V )ofinterfacestates Potentialacrossinterfaciallayer Electronanityofsemiconductor bi Built-inpotential Thicknessofinterfaciallayer q Electroncharge 13
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Q sc Space-chargedensityinsemiconductor Q ss Interface-trapcharge Q M Surface-chargedensityonmetal D it Interface-trapdensity i Permittivityofinterfaciallayer(vacuum) s Permittivityofsemiconductor PLV PulsedLaserVaporization R S SeriesResistance R SH ShuntResistance SBH SchottkyBarrierHeight SRH Shockley-ReedHallRecombination SiNW SiliconNanowire SWNT Single-WallNanotube e=p Electronorholemobility V M Voltageatmaximumpowerpoint V OC OpenCircuitVoltage V G GateVoltage 14
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AbstractofDissertationPresentedtotheGraduateSchool oftheUniversityofFloridainPartialFulllmentofthe RequirementsfortheDegreeofDoctorofPhilosophy NANOSTRUCTUREDCARBONNANOTUBESCHOTTKYJUNCTIONSOLAR CELLS By MaureenK.Petterson August2013 Chair:AndrewG.RinzlerMajor:Physics Thisdissertationexploresandexploitsthephysicalprocessesunc overedduring experimentsaimedatimprovingsolarcelleciencyinanovelelectron icallygatedsolar cellthroughsurfacetexturing.Besidestheincreaseddeviceec iency,thendingsshed lightonthepreviouslimitationsinsimilardevicesandmayhaveimplications forother semiconductorbaseddevices. Siliconnanowireshavelongbeenknownfortheirexcellentantirerect ionproperties, buthavesueredsubstantiallyfromrecombinationatthesurface .Here,wedeposita dispersecarbonnanotubenetworkonthetipsofaforestofvert icalsiliconnanowiresand exploitelectronicgatinginanovelSchottkyjunctionsolarcell.Prev iousexperimentson carbonnanotube-siliconsolarcellsmadeuseofanionicliquidtomodulat ethenanotube Fermilevelviaelectronicgating.ThismodulationchangedtheSchot tkybarrierheightof thedeviceanddecreasedthecarbonnanotubelmresistance,lea dingtopowerconversion ecienciesofupto12%foragatevoltageof-0.75V.Furtherexper imentsuncovered anadditionalmechanisminwhichtheionicliquidinducedaninversionlayer withinthe silicon,greatlyfacilitatingholeextractionbyrepellingelectronsfrom thesiliconsurface (andconsequentlyreducingrecombination).Weexploitthisinduced inversionlayerwithin oursiliconnanowiresolarcellsandshowagreatlyincreasedpowercon versioneciency exceeding15%,thehighestreportedeciencyforsiliconnanowireb aseddevicestodate. 15
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Wealsoinvestigatethephysicalandchemicalprocessesresponsib lefordegradation inthesedevices.Weshowthatcontaminationoftheionicliquidwithoxy genor waterleadstoredoxreactionsforgatevoltagespreviouslythoug httobewellwithin theelectrochemicalwindow.Wesubsequentlydemonstratethatb yexcludingthese contaminants,stableperformanceoftheelectronicallygatednan otube/siliconsolarcell ispossible.Advancedpassivationtechniquesareusedtoalleviatesu chdegradation. Specically,depositionofaluminumoxideviaatomiclayerdepositionwas usedtocreate ahighquality,conformal,dielectriclayerthatinhibitselectrochemica lreactionsbetween theionicliquidandthesilicon,leadingtominimalreductioninperformanc easthegate voltageisapplied. 16
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CHAPTER1 INTRODUCTION Photovoltaicshavebeenanextremelyactiveareaofresearchsinc etheearly1970s,yet onlyafewdevicestructureshavepassedthetestofaordability, longevity,andeciency. Volatilemarkets,decreasingresources,andapenchantforinnov ationfuelthecontinued researchintodierentnovelarchitecturesutilizingavarietyofor ganicandinorganic materials.Concomitantwiththeincreaseinsolarcelleciencyisadee perunderstanding oftheunderlyingphysicalprocessespresentinsuchdevices,som ethingwhichalsohas moregeneralscienticvalue.Thehigheciencyofthesolarcellspre sentedinthisthesis wasachievedbyeortstounderstandtheunderlyingphysicsofth edevicesandusingthat knowledgetoimprovelightabsorptionwhileminimizinglossesanddegrad ation.Thehigh eciencieswererealizedthroughmultiplemethods,asdiscussedinde tailinChapters4-7. Abriefsummaryofthedissertationispresentedbelow. Chapter2startsobydiscussingthetheoreticalbackgroundof singlewallcarbon nanotubes(SWNT),withparticularemphasisontheabilitytomodulat etheFermilevel oftheSWNTsduetotheirlowdensityofstates.Thismodulationcanb eexperimentally veriedbyobservingthechangeintransmittanceofthelmduringe lectronicgating,as demonstratedintheworkofDr.ZhihongChenandDr.Zhuangchun Wu,wholedthat eortintheRinzlergroup.[ 8 ] Chapter3givesabriefintroductiontosolarcells.Testing,charact erization,and dierenttypesofsolarcellsarepresented,alongwithsomeofthec hallengesconfronting researchersintheirpursuittodevelophigheciencydevices.Histo ricalinformationon solarcellsismentionedwiththepurposeofshowinghowworkdescribe dlaterinthis thesiscanaddressandsolveproblemsencounteredinthephotovo ltaicdevicesdevelopedin the1970s. Chapter4presetsamorethoroughdescriptionofSchottkyjunc tionsolarcells, withemphasisonthephysicsofSchottkybarriers,includinghowsur facepreparation 17
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aectsdeviceperformance.Thecarbonnanotube-siliconSchott kyjunctionsolarcell isintroduced,alongwiththeworkdonebyDr.PoojaWadhwainwhichs uchadevice waselectronicallygated.Modulatingthegatevoltagebetweenthea ctivearealmand agatelmmodulatestheSWNTFermilevel,changingtheSchottkybu ilt-inpotential andmodulatingdeviceperformance.Lastly,theinversionlayergrid cellisdescribed.This device,inwhichImademyrstcontributionstothisclassofdevices, demonstrateda newmechanismbywhichtheionicliquidusedduringelectronicgatingoft heSWNTlm simultaneouslyformsaninversionlayerwithinthesilicon.[ 10 ]Thisallowedforecient collectionofphotogeneratedcarriersfarfromtheSWNTgridlines, boostingtheeciency ofthedevicefrom10.9%to12%. Chapter5detailsmyworkonnanostructuringthesiliconsurfaceto improve lightabsorption.Thisworkwasmotivatedbytheresultsoftheinver sionlayerdevice discussedinChapter4.Theabilitytoinduceaninversionlayerwithinth esiliconno longerconstrainsustohavethecarbonnanotubelmtouchingthe entiresiliconsurface, allowingexplorationofalternativearchitectures.Siliconnanowires( SiNW),knownfor theirexcellentanti-rerectionproperties,wereintegratedintoaS WNT-SiNWdeviceand tookfulladvantageoftheionicliquid-inducedinversionlayeralongth enanowiresidewalls. Thegreatlyincreasedsurfaceareaofthenanowiresrequiredmod icationstothesolar cells,specicallyanincreasedsurfaceareagatelmtocompensate fortheionsneeded toinducetheinversionlayer,andaSWNTspraydepositedlmtoimpro veconnection betweenthesiliconnanowiresandthecarbonnanotubes.Integra tionofthesetwoledtoa greatlyimprovedpowerconversioneciencyofover15%,thehighe stPCEforanysilicon nanowiredevicetodate. ThelatterpartofChapter5addressesthestabilityofthesedevic esduringelectronic gating.Experimentsonthecarbonnanotube-siliconSchottkyjun ctionssolarcells showedareductionofthePCEasthedevicewaselectronicallygated .Characteristics ofdegradationsuggestedthatredoxreactionswerefacilitatingo xidationofthesilicon 18
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surface,formingabarriertocarrierextractionanddecreasingp erformance.Thegreatly increasedsurfaceareaofthesiliconnanowires,versustheplanar devices,exhibiteda greaterdegradation(andhencemoreredoxreactions),asevide ncedbyanincreased parasiticgatecurrent.Experimentstotesttheelectrochemical windowofourionicliquid showedasubstantialreductionofthewindowduetocontamination ofwaterandoxygen. Awarethattestinginambientatmospherewouldleadtoimmediatecon tamination, Itestedaplanardeviceinagloveboxusingdriedionicliquidandobserve dstable performanceovernumeroushours.Adramaticreductioningatec urrentsindicating negligibleredoxreactionsconclusivelydemonstratedthatthedegr adationwasdueto contaminationofwaterandoxygenatthesiliconsurface. Chapter6describesmyworktoreducedegradationoftheSWNT-S iNWdevices duringelectronicgating.Thoughsimpleencapsulationcaneliminatede gradation,I exploredbothatomiclayerdepositionandchemicalpassivationasam eanstoelicitstable performance.Hydroquinoneandsulfurpassivationledtoimproved deviceperformance priortoelectronicgating,butultimatelyprovedtobeincompatiblewit htheionic liquid.Atomiclayerdepositionofaluminumoxideonthefullyfabricatedd evicelimited contactbetweentheionicliquidandthesiliconsurfaceduringelectro nicgatingwithout sacricingtheinversionlayer.Thisreducedcontactlimitedredoxre actionsandlowered thegatecurrentbyafactorof60.Thoughthedevicewastested intheatmospherewith "contaminated"ionicliquid,theALDlayerimprovedthestabilityofthe devicesandstill producedahighpowerconversioneciencyof14.8%. Finally,Chapter7discussestwosideprojects:graphene-siliconSc hottkyjunction solarcellsandbacksidedopingofthesiliconsubstrates.Theformer demonstrateda greatlyenhancedpowerconversioneciencyuponintroductionof theorganicdopant, bis(triruoromethylsulfonyl)amide(TFSA).Theimprovementinecie ncyisattributedto anincreaseintheSchottkybarrierheight,decreaseinseriesresis tance,andtheabilityof theTFSAtoactasananti-rerectionlayer.Lastly,backsidedoping wasfoundtoimprove 19
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thepowerconversioneciencybylimitingrecombinationatthebackc ontact.Aspin ondopantdepositedontothebacksideofthesiliconsubstratespr oducedathink,highly dopedregiononthebacksideofthesilicon.Thepowerconversione ciencyoftheplanar deviceswasimprovedfrom9.7%to13.4%. 20
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CHAPTER2 INTRODUCTIONTOCARBONNANOTUBES 2.1HistoryandStructure Firststructurallyinterpretedin1991bySumioIijimaattheNipponEle ctric Company(NEC),carbonnanotubeshavebeenthefocusofintens eresearchand developmentforthepasttwodecades.Fundamentalproperties andnovelapplications havebeenexploredinthephysicalsciencessincetheirdiscovery,w hilewithinthemedical andbiologicalsciencesmuchworkhasbeendonetoincorporatecar bonnanotubesinto variousdevicesrangingfromprostheticstomoleculartransporte rs.[ 2 { 5 ]Thewiderangeof potentialapplicationsisderivedfromtheuniqueelectrical,physical, andopticalproperties possessedbythesefullerenes.Aquasi-onedimensionalstructur e,carbonnanotubescanbe thoughtofasasheetofgraphenerolledintoaseamlesstubewithdia metersapproximately 1-10nmandaspectratiosupto10 5 Theelectronicandopticalpropertiesofcarbonnanotubescanbe calculatedfromthe bandstructureofgrapheneduetothelocalstructuralsimilarity ofthetwo.Grapheneisa simpletwodimensionalhexagonallatticecomposedof sp 2 bondedcarbonatoms.Along, narrowrectangularstripcutfromthislatticeandrolledupalongthe narrowdimension (withbondsreformedacrosstheseam)generatesthestructur eofasinglewallnanotube (SWNT).Dependingupontheorientationofthestripdirectionrelat ivetothegraphene latticeSWNTsofthreedistinctstructuralclassicationscanform :armchair,zigzag,or chiral.Asidefromtheirgeometricclassications,SWNTscanbesubd ividedintoeither metallicorsemiconductingtypesbasedonthenanotuben,mindex(de nedbelow).[ 6 7 ] Figure 2-1A showsthegraphenelatticeandcorrespondingunitcellwithprimitive vectors, a 1 and a 2 alongwiththechiralvector C .Denedas C = n a 1 + m a 2 forintegern,m,( m
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AGraphenelattice BCarbonnanotubegeometries Figure2-1.Graphenelatticeandgeometricclassicationsforcarb onnanotubes.FigureA: Graphenelatticeshowingprimitivevectors a 1 and a 2 thatcomprisetheunit cell.Alsoshownisthechiralvectorthatdeterminesnanotubetype .FigureB: Thethreegeometrictypesofcarbonnanotubes.Reprintedwithp ermission fromR.Saito,G.Dresselhaus,andM.S.Dresselhaus,Physicalpr opertiesof carbonnanotubes(ImperialCollegePress,1998) thechiralvectorisalinearcombinationofprimitivevectorsgovernin gthedirectionsin whichagraphenesheetcanberolledup,subsequentlydeterminingt hespecicproperties ofthenanotube;n=mresultsinanarmchairnanotube,m=0corres pondstoazigzag nanotube,andallothercombinationsofn,mresultinachiralnanotu be.Theconditions forametallicnanotuberequirethat( n m )=3 j ,wherejisaninteger.Consequently, allarmchairnanotubesaremetallicwhilezigzagandchiralnanotubes canbeeithertype; theseselectionrulesleadtoanoverall2:1ratioofsemiconductingto metallicnanotubes acrossallallowedn,m. Theenergydispersionrelationsanddensityofstates(DOS)forna notubesarederived byplacingtheappropriatecircumferentialboundaryconditionson theenergydispersion relationforgrapheneandsolvingfortheallowedkvaluesandassocia tedenergystates. Forasinglesheetofgraphene, 22
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E g 2 D ( k x ;k y )= t ( 1+4cos p 3 k x a 2 cos k y a 2 +4cos 2 k y a ) 1 2 ; (2{2) wheret=-3.033eVand k x and k y correspondtothexandyaxesinreciprocalspace.[ 7 ] Theboundaryconditionsrequirekatobemultiplesof ,withtheexactdispersion relationforagivennanotubedependingonthechiralvector.Figur e 2-2 showstheband structureforbothametallicandsemiconductingnanotube.Shown inFigure 2-3 isthe densityofstatesforbothasemiconductingandmetallicnanotube, withthegraphene DOSoverlayedasadottedline. Figure2-2.Bandstructureforcarbonnanotubes.Shownisamet allica)(5,5)andb)(9,0) nanotubeandc)asemiconducting(10,0)nanotube.Reprintedwith permission fromSaito, etal. Physicalpropertiesofcarbonnanotubes(ImperialCollege Press,1998) TheDOSofgraphenegoestoexactlyzeroatthesixDiracpointsinth eBrillioun zone,makinggrapheneazerogapsemiconductor.Thecyclicalsym metryofcarbon nanotubesrestrictstheallowedwavevectorsalongthecircumfer ence.Divergencesofthe densityofstates,calledVanHovesingularities,areduetotheoned imensionalityofthe carbonnanotubesandarethevisiblespikesinFigure 2-3 23
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Figure2-3.Thedensityofstatesforasemiconductingnanotube( left),andametallic nanotube(right).Thedottedlineoverlayedinbothimagesistheden sityof statesforgraphene.NotethelowDOSforthemetallicnanotubesn earthe Fermienergy. r 0 is3.13eVandcorrespondstotheC-Ctightbindingoverlap energy.ReprintedwithpermissionfromSaito, etal. Physicalpropertiesof carbonnanotubes(ImperialCollegePress,1998) Carbonnanotubesarenotablefortheirrelativelylowdensityofsta tesandeasily manipulatedFermilevel.Bothelectronicgatingandchemicalcharge transferdopingcan beutilizedtoshiftthecarrierconcentrationofthenanotubes,allo wingintegrationinto devicesthatdemandcontroloverconductance.[ 8 { 10 ]Thischangeinelectronicpopulations concomitantwithaFermilevelmodulationcanbedemonstratedbya changeinthe lmtransmittance.Figure 2-4B showstheexperimentalsetupfordemonstratingsuch modulationviaelectronicgating.Twocarbonnanotubelmsaretran sferredtoaquartz substrateandthenbakedat600Ctodedopethelms,afterwhich theentiresubstrate issubmergedinaliquidelectrolyteandagatevoltageisappliedbetween thetwolms, drivingtheionsintheelectrolytetoeitherlminaresponsetoColuomb icforces. Inclusionofionicliquidbooststhecapacitanceofthenanotubelm,a kintoinsertinga dielectricbetweenaparallelplatecapacitor.Figure 2-4A showsthetransmittancedata takenwithaUV-Visspectrophotometerasagatevoltagebetween thetestlmandgate lmwasheldatincrementalvoltagesfrom-1.8Vto+1.8V.TheS1,S2, andM1peaks correspondingtoelectronictransitionsbetweenVanHovesingular ities.Asanegativegate 24
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voltageisapplied,thetestlmbecomesmorep-dopedastheFermile velispushedfarther fromvacuum,resultinginanincreasedtransmittanceaselectrons aredepletedfromthe VanHovesingularitiesthatcomprisethegroundstateforthecorr espondingtransition. Asapositivegatevoltageisapplied,thetrendreversesandthetra nsmittancedecreases. Thiscontradictstheintuitiveideathatanyappliedgatevoltage(pos itiveornegative)will resultinanincreasedtransmittanceduetoamid-gapFermilevelat zeroappliedvoltage. Intheidealizedcase,anyappliedgatevoltagewilldecreasethenumb erofavailablestates forelectronictransitions,loweringthetransmittanceandleadingt oambipolarbehavior. Theexperimentalresultsseemtobeliethetheory.Thoughinitiallyat tributedtopossible contaminationeectivelyp-dopingthecarbonnanotubes,wenowb elievethisdiscrepancy isattributedtoelectronsfromthecarbonnanotubesbeingdonat edtoanoxygen/water redoxcouple,aswasstudiedindepthbyMartelin2009.[ 11 ]Thehydrophilicquartz substrateandwatercontaminatedionicliquidprovideampleamounts ofwaterandoxygen tofacilitateelectrochemicalreactions,leadingtop-typebehavior AElectronicGating BExperimentalSetup Figure2-4.ElectronicgatingofSWNTlm.FigureA:Changeintransm ittanceofa45nm thickSWNTlmwithvariousappliedgatevoltage.FigureB:Experiment al set-upfortransmittancemeasurements. 25
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2.2Synthesis Carbonnanotubescanbesynthesizedthroughfourdierentmet hods:arcdischarge, pulsedlaservaporization(PLV),plasmatorch,andchemicalvapor deposition(CVD). Theinitialprocedureforcarbonnanotubefabrication,arcdischa rgegrowthisachievedby applyingapotentialbetweentwographite/carbonelectrodeswith theresultantdischarge heatingthecarbontargets,catalyzingtheformationofcarbonn anotubesandother fullerenes.[ 12 ]PLValsousesahighenergybeamtoformcarbonnanotubes,bute mploysa greenandinfraredlasertoablateacarbontarget.[ 13 14 ]Environmentalconditionswithin thecarbonaceousvaporaectthespecicpropertiesofthecar bonnanotubes;temperature relatestothediameterwhilethepresenceofmetalcatalystspart iclesfacilitateformation ofsinglewallnanotubesinsteadofmulti-wallednanotubes.[ 15 { 17 ]Duringthegrowth, severalindividualcarbonnanotubesadheretogetherviaVander Wallsforcestoform nanotubebundlestensofnanometersindiameter.Plasmatorchgr owthisanother permutationofgrowthviathermaldecompositionofcatalystprec ursorsinacarbonaceous gas.Inthismethod,acarbonsourceandametalcatalystsource aresimultaneouslyfed throughaplasmatorch,producingcarbonnanotubesintheheate dvapor.[ 18 ] CVDgrowthcomprisesafewdierentmethods:conventionalCVD, plasmaenhanced CVD(PECVD),HiPCo(high-pressureconversionofcarbonmonoxid e),Ferrocene injection,androatingcatalystmethod,tonameafew.Therstge nerationofCVD growthdevelopedbyNikolaev, etal. usedrowingcarbonmonoxideinconjunctionwitha metalcatalystcontaininggasinacontinuousrowreactor.Growth ofcarbonnanotubes occurredthoughthermaldecompositionofthemetalcatalystwit hintheheatedcarbon monoxiderow,afterwhichtheywouldadheretothesidesofthequa rtztube.[ 19 ]. Alternatively,metalcatalystparticlescanbepre-depositedonto acarriersubstrateand thenplacedintothegrowthchamberwhereuponthecarbonconta iningreactantgaswould befed.Decompositioninthehightemperaturegrowthchamberwou ldbefollowedby carbonnanotubegrowthfacilitatedbythemetalparticlesonthec arriersubstrate.[ 20 { 22 ] 26
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Thecarbonnanotubesusedfortheexperimentsdescribedinthef ollowingchapters weresynthesizedbyPLVandsubsequentlyunderwentanextensiv epuricationprocess toeliminateresidualamorphouscarbon,catalystparticles,andot hernon-nanotube contaminants.A2.4Mnitricacidreruxremovedamorphouscarbon( andotherforms ofcarbonexhibitingrelativelyweak sp 3 bonding)andmetalimpuritiesinadditionto p-dopingthecarbonnanotubes.Multiplecentrifugationsat6000R PMfollowedby decantationoftheacidicsupernatantneutralizedthecarbonnan otube/acidsolutionand allowedtheirdispersionina1%Triton-Xsurfactantsolution.Next,c rossrowltration eliminatedreactionproductsandneparticulatesbyrepeatedlypa ssingthesolution throughlong,hollowberswithsmallporesalongthesidewalls.These smallporesallow thepassageofsmallparticulateswhilepreventingthepermeationo fcarbonnanotubes. Thisltrationcontinueduntilthepermeateisclear,indicatingamajo rityofsmall particleshavebeenremovedfromthesolution.Anadditionalcentr ifugation(6000-10000 RPM),thistimeretainingthesupernatant,separatedoutcontam inantswithadensity greaterthanthatofthesurfactantbuoyedcarbonnanotubes ..Lastly,altrationthough a650 m membraneremovedlargeparticulatesnotbrokendownbythenitric acidrerux oreliminatedbyprevioussteps,ultimatelyproducingapuried,surf actantbasedcarbon nanotubesuspension.[ 8 23 { 25 ] InthestudiesdiscussedinthisworktheSWNTsweretypicallyusedint heform ofthintransparentlms.Suchlmformationproceededasfollows. Startingwiththe puriedsolution,thecarbonnanotubesarevacuumlteredontoa mixedcellulose (MCE)membraneandcopiouslyrinsedwithdeionizedwatertoremove residualTriton-X surfactant.Afterdryingunderanincandescentlamp,thecarbo nnanotubelmisready tobetransferredtoasubstrateorstoredinaninertatmospher euntilneeded.Totransfer toasubstrate(suchasglass,ITO,PET,silicon,etc),thenanotu belmisrstplaced againstthesubstrate(MCEsideup),ontopofwhichisplacedaporo usTeronmembrane followedbyahydratedsheetofporousplastic.Thisassemblyissand wichedbetween 27
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twoaluminumplatesthatareclampedtogethertosupplypressureb etweenthecarbon nanotubelmandthesubstrate.Uponbeingplacedina100Coven,t hewaterinthe porousplasticevaporatesandwetsthenanotubelm.Asthestea mslowlydiusesout oftheassemblywithcontinuedheating,thenanotubesarebrough tintointimatecontact withthesubstrateandaretherebyretainedviaVanderWallsforce s.Afterseveralhours, theassemblyisremovedandthesubstrate/lmisplacedinanaceton evaporbathto dissolvetheMCEmembrane,leavingbehindthecarbonnanotubelm. Subsequentliquid acetonebathsensurecompleteremovalofthecellulose,afterwh ichthesubstrateisplaced inanisopropolbath(agenerallycleanersolventthanacetone,inwh ichthelatteris miscible),removed,andthoroughlydriedinanitrogenstream. Asmentionedabove,thecarbonnanotubesarechargetransfer p-dopedduringthe nitricacidrerux.Nitrogenbasedcations, NO x ,intercalatethenanotubebundlesand sequesterelectrons,eectivelyshiftingtheFermileveltowards thevalencebandedge andholedopingthenanotubes.[ 26 ]Thisresultsinachangeintransmittanceinboththe opticalandIRregimesthatcanbeobservedwithaUV/VIS/NIRspe ctrophotometer. Bakingat600Cprovidesenoughthermalenergytode-adsorbthe dopantspecies,resulting inalowerconductivityandreducedtransmittanceatwavelengthsc orrespondingtoVan Hovesingularities,asshowninFigure 2-5 ControlleddopingcombinedwiththeabilitytomodulatetheFermilevel via electronicgatingmakeitpossibletotailorcarbonnanotube'selectric alandoptical propertiesonanas-neededbasis.Thephotovoltaicdevicesprese ntedinthisdissertation takefulladvantageofthismalleabilityandexhibitsuperiorperforma ncecomparedto similarphotovoltaicdevices.Additionally,informationregardinginter actionsbetween thecarbonnanotubesandothermaterials(crystallinesemiconduc torsandelectrolytes) hasbeengleanedthroughexperimentsaimedatimprovingPVecienc y,elucidating fundamentalphysicalinteractionswithinthedevice. 28
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Figure2-5.DedopingofSWNTlmduringhightemperaturebake.The shiftin transmittancefora45nmsinglewallednanotubelmbakedat600C,indicatingdedoping.Theinsetshowsthedensityofstatesofasemic onducting andmetallicnanotube,withtheshadedregionsindicatingelectronicpopulations(inthiscase,p-typedoping). 29
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CHAPTER3 INTRODUCTIONTOSOLARCELLS 3.1Fundamentals Ariseingaspricescoupledwithheightenedenvironmentalawarenes shasprompted increasingamountsofresearchintoalternativeenergysources. Thoughmuchresearch hasbeenputintotheadvancementofvarioustypesofsolarcells,s iliconbaseddevices continuetodominatethemarket.Siliconisreadilyavailable,wellunder stood,andastaple oftheelectronicsmanufacturingindustry.Combinedwithnuclear, wind,andhydro,solar powerpromisestobecomeoneofthefrontcontendersintheener gymarket.It'smodest andlocalizedinstallationrequirementsmakeitattractiveforindividua lswhodesire morecontrolovertheirenergyuseorforurbanlocationswithminu teamountsofspare realestate.Otherformsofalternativeenergyrequirevasttra ctsoflandorarehighly geographicdependent,whilesolarpowercanbeecientlyharveste dmostanywheresouth ofthemid-latitudes.Asingletypeofsolarcellwillnotbesucienttom eettheneedsof everyapplication,andassuchcontinuingresearchinalltypesofso larcellsisvitaltothe futureofalternativeenergy. Unlikeabattery,whichsuppliesaconstantvoltage,asolarcellismor eakintoa currentgeneratorthatislimitedbythephotonruxincidentonthea ctiveareaofthe device.Excludingthosedevicesexhibitingmultipleexcitongeneration ,everyphoton withenergyabovethebandgapthatisabsorbedwithintheactivema terialgeneratesan electron-holepair.TheprimaryaimsofR&Dinthisareaaretobothab sorballphotons hittingthesolarcellandtocollectallthephotogeneratedcarriers .Table 3-1 attheendof thechaptershowsthecurrentprogressofvarioustypesofpho tovoltaicdevices. 3.1.1GenerationandSolarSpectrum Notallsemiconductingmaterialsaresuitableforbroadbandabsorp tionofthesolar spectrum;alargebandgapisidealforcollectinghighenergyphoton s,whileasmaller bandgapwastesmuchoftheenergyofhigherenergyphotonsast hephotogenerated 30
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electronsdecaytothebottomoftheconductionbandbygenerat ingheatwithinthecell. Assuch,smallbandgapmaterialsaregenerallyrelegatedtoanIRab sorptionlayerwithin tandemcells.Thesun'sspectrumisthatofablackbodywithanavera getemperatureof 5800K,howevertheradiationreceivedatthetopoftheEarth'sat mospherediersfrom thatinspaceduetoenhancedabsorptionandscatteringfromgas andwatervaporandthe obliqueangleatwhichtheradiationhitstheterrestrialsurface.Sh owninFigure 3-1 are therelevantsolarspectraatthetopandbottomofearth'satmos pherecomparedtoa5250 Cblackbodyspectrum..[ 27 ] Figure3-1.ThesolarspectrumreceivedbothoutsidetheEarth'sa tmosphereandatthe surfaceoftheEarth.ImagecreatedbyRobertA.RohdeforGlob alWarming Art. Comparisonsofsolarcelleciencyareonlyvalidifallthedeviceshave beentested underthesameconditions.Inordertocreateconsistencyamong reportedperformance, thecommunityhasagreeduponasolarspectrumcorrespondingto theaverageradiation receivedatthemid-latitudes,designatedAirMass1.5Global(AM1.5G ).Equivalentto roughly100 mW cm 2 ,thisspectrumapproximatesthesolarirradianceattheEarth'ssu rface whenthesunis48.2 ozenithandaccountsforscatteringbytheatmosphere.AM0 31
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isthesolarirradianceatthetopoftheatmosphere(roughlyequiva lenttoablackbody spectrumat5800K),ahigherruencethatoverestimatesphoton ruxforsolarinstallations intheUnitedStates. Aphotonincidentonasolarcellthatisabsorbedwithinthesemicondu cting materialwillexciteanelectrontotheconductionband,leavingbehin daholeinthe valenceband.Extractionofthiselectrononlyoccursifthereisadr ivingforcemoving theelectronorholetowardstheelectrodes.Extractionofthech argesispredicatedon transportofthechargetotheelectrodes,necessitatinganinte rnalmechanismtomovethe photogeneratedcarriers.Whilebothdrift(duetoaninternalelec triceld)anddiusion (duetoconcentrationgradient)cancontributetothecurrent, theformerismoreecient atquicklyseparatingelectronholepairsandferryingthemtotheirir respectiveelectrodes. Thisdrivingforceisfoundwithindiodes,andthetotalcurrentinaso larcellcanbe representedbytheShockleydiodeequationwithanadditionalterm correspondingtothe photocurrent.[ 27 ] J total = J photo + J diode (3{1) J diode = J o exp qV kT 1 ; where J o isreferredtoasthedarkcurrentandyieldsinformationregarding recombination atthejunction(amoredetailedanalysisoftheShockleyequationisg iveninChapter3). Thebindingenergyoftheelectronholepairvariesdependingonthea bsorber material:insiliconitisonly14.7meV,lowenoughforthepairtobedissocia tedatroom temperature.Incontrast,thebindingenergyinorganicmaterials isontheorderof0.51eV,necessitatingamechanismfordissociationinordertoseparat eandthenextractthe photogeneratedcarriers.[ 28 29 ] 3.1.2Recombination Recombinationisthecombiningofaphotogeneratedelectronandho le.Itremainsthe primarylossmechanismwithinsolarcells;carriersthatrecombinecan notbeextractedas 32
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usableelectricalenergyandoverallcelleciencydecreases.The rearethreerecombination typespertinenttosolarcells:Radiative,Auger,andShockley-Ree d-Hall(SRH).The dominantsourceofrecombinationisdependentonthetypeofmate rialused:electron holepairsindirectbandgapsemiconductorsarepredisposedtocom bineradiatively, whilecarriersinindirectbandgapsemiconductorsaremorelikelytoun dergoSRH recombination.Augerrecombinationispossibleinbothtypesofsemic onductors,though typicallynegligibleatmostdopingdensities.[ 30 ]Thetypeofbandgapisn'ttheonly contributingfactortorecombinationrates;crystallinityplaysalar geroleaswell. Amorphoussilicon,whichcanpossesshugelyvaryingdensitiesofdef ectsanddangling bonds,hascarrierlifetimesontheorderof10 9 s,farshorterthanthevalueforit's crystallinecounterpart,whichhasaminoritycarrierlifetimeof2.5x1 0 3 s.[ 30 31 ] 3.1.2.1Radiative Radiativerecombinationisatwobodyinteractioninwhichanelectronc ombines withaholeandproducesaphoton.Conservationofcrystalmomen tumdictatesthat thisprocessonlyoccurinadirectbandgapsemiconductorwheread irectbandto bandtransitionmayoccurwithoutanadditionalinteraction.Hence ,directbandgap materialsarepronetohavinghighlevelsofrecombinationinthebulk,n ecessitating thinnersemiconductorlayersinordertoextractphotogenerate dcarriersbeforethey recombine.Thisresultsinarelativelyshortdiusionlengthandsmaller carrierlifetime. Oneofthemostpopulardirectbandgapsemiconductors,GaAs,ha saminoritycarrier lifetimeofapproximately10 8 s,severalordersofmagnitudeshorterthanthatofsilicon. Consequently,directbandgapmaterialsareoftenrelegatedtomu lti-junctionsolarcells. ShowninFigure 3-2 aretheschematicsforthethreetypesofrecombination. 3.1.2.2Auger Unlikeradiativerecombination,Augerrecombinationisathreecarrie rinteraction thatcanoccurinbothdirectandindirectbandgapsemiconductors .Anelectronand holerecombinewithakineticenergytransfertoathirdcarrierwhich issubsequently 33
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Figure3-2.Threerecombinationrouteswithinsemiconductors.Ad aptedfromPrinciples ofSemiconductorPhysics,VanZeghbroeck,B.unpublished. excitedtoahigherenergylevel.Thethirdcarrierthenrelaxesdown viaphononemission. Althoughthisprocessispermittedinallsemiconductors,itrequires highcarrierdensities tocontributesignicantlytorecombination,especiallyindirectband gapsemiconductors. Thoughthemajorityofrecombinationinindirectbandgapsemicondu ctorsisdueto defects(mostlyatthesurface),amajorityofrecombinationinth ebulkcanbeattributed toAugerifthedefectdensityislow.3.1.2.3ShockleyReedHall ShockleyReedHall(SRH)recombinationisrecombinationofanelectr onandhole thatiscatalyzedbyadefectortrapstate.Thisremainsthedomina ntrecombination mechanisminindirectbandgapsemiconductors,aspurelyradiativer ecombinationis impossibleandAugerisonlylikelywithhighcarrierdensities.Defectsint hecrystal createnewenergystates,allowinganelectronandholetorecombin eradiativelyor non-radiativelywhilestillconservingcrystalmomentum.Trapstat escan"trap"carriers foraniteamountoftime,duringwhichtheycanrecombinewithanot hercarrieror bethermallyexcitedoutofthetrap.Thisrecombinationmechanisme xplainsthelong diusionlengthinindirectbandgapmonocrystallinesemiconductors, asphotogenerated 34
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carriersarelosttorecombinationonlyatthesurface.Thisallowsre lativelythicksolar cellsmodulescapableofabsorbinglowerenergyphotons.Comparat ively,amorphousand polycrystallinesiliconpossessahighdensityofgrainboundariesthat actasdefectsand facilitaterecombination,limitingtheoptimalabsorberthickness. Therecombinationrateatthesurfacecanbequantiedbytherec ombinationvelocity parameter,S: S n = v th s N n (3{2) wherev th isthethermalvelocity(typically 10 7 cm s ), s isrecombinationcrosssection (typically 10 15 cm 2 ),andN n isthedensityoftrapstatesatthesemiconductorsurface. Ahighersurfacerecombinationvelocityindicatesincreasedcarrier recombination.[ 30 ] Highlyqualitypassivatedsiliconcanachievesurfacerecombinationve locitiesaslowas 0.25 cm 2 vs forundopedsilicon.[ 32 ],ascomparedto2590 cm 2 vs forpolycrystallinesilicon.[ 33 ] 3.1.3Characterization Inlieuofattachingavariableresistiveloadtothesolarcelltocalculat eeciency,the deviceisconnectedtoapowersourceandthevoltageisrampedfro mnegativetopositive voltages.Thecurrentateachvoltageisrecordedandplottedasa J-Vcurve.Eachpoint inthefourthquadrantrepresentsaparticularloadresistancean danidenticalJ-Vcurve canbegeneratedinthatquadrantbyconnectingthesolarcelltoa variableresistorand recordingthecurrentandvoltageateachresistancevalue.Thou ghthedevicegenerates poweronlyinthefourthquadrant,biasingthedeviceoverafullran geofvoltagesyields importantinformationaboutthesolarcellinregardstobothphoto generationanddiode behavior. Thefourparametersusedtoevaluateasolarcell'sperformancear epowerconversion eciency(PCE),shortcircuitcurrent(J SC ),opencircuitvoltage(V OC ),andllfactor (FF).Thebiasvoltagewherethephotocurrentisequalandoppos itetothediodecurrent (ie,J total =0)iscalledtheopencircuitvoltage,V OC .Theoutputcurrentwhen V bias =0V isdenedastheshortcircuitcurrentdensity,J SC .Anoverallmeasurefortheeciencyof 35
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Figure3-3.Maximumgeneratedpowerdensity(bluebox),denedb y P = V M J M acell,PCEisdenedasaratioofthemaximumpowergeneratedbythe celltothepower oftheincidentradiationontheactiveareaofthedevice. TheshortcircuitcurrentdensityistakendirectlyfromtheJ-Vcur veandis approximatelyequaltothephotocurrent(thedarkdiodecurren tisgenerallyordersof magnitudesmaller).The V OC isdenedasthevoltageatwhichthephotocurrentisequal andoppositetothediodecurrent, J photo = J diode : J photo = J O exp qV OC kT 1 (3{3) Solvingfor V OC yields: V OC = kT q ln J photo J O +1 (3{4) TheV OC isextracteddirectlyfromtheJ-Vandcanbeusedtocalculateothe r parameters,suchasthedarkcurrent(andsubsequentlySchot tkybarrierheight).This equationelucidatesthefactorscontributingtoahighorlow V OC ,namelyandincreasein 36
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shortcircuitcurrentand/oradecreaseindarkcurrent.Increa sing V OC byincreasingthe lightintensityisawellknownphenomenaexploitedthroughsolarconc entrators,though thesemiconductingmaterialmusthavehighenoughmobilitiestoavoid carriersaturation (andinversion).Theinverselogarithmicdependenceondarkcurre ntpointstoareduction injunctionrecombinationasanotherwaytoincreasethe V OC Thellfactorisameasureofhowmuchthesolarcellfunctionsasan idealdiode, withaFFof1correspondingtoacompletelysquareshapedJ-Vcurv einthefourth quadrant.Table 3-1 inthebeginningofthechaptershowshighperformingsolarcells possessingaFFofapproximately0.8-0.9. FF = V M J M V OC J SC (3{5) V M and J M arethevoltageandcurrentdensitycorrespondingtothemaximum power point:thepointontheJ-Vcurvewherethemaximumpowerisgenera tedbythesolar cell.Thisisfoundbygraphingthepowerdensityvsvoltageandsolving for dP dV =0. Thepowergeneratedatthemaximumpowerpointisrepresentedby thebluesquarein Figure 3-3 Withthoseparametersdened,wecannowsolveforthePCE: = J M V M P incid = J SC V OC FF P incid (3{6) Notethatthisistheeciencyatthemaximumpowerpoint;theecien cywillbelower atotherequivalentloadsinthefourthquadrant.Consequently,m aximizingthepower extractedfromasolarcellinvolvesmatchingtheloadtotheresista nceatthemaximum powerpoint.3.1.4SeriesandShuntResistance Bothseriesandshuntresistancehaveadetrimentaleectonsola rcellperformance, withthebestperformanceextractedbyminimizingtheformerandm aximizingthelatter. 37
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Figure3-4.Circuitequivalentshowingseriesandshuntresistance. Theexpressionforthetotalcurrentcanberewrittentoincludeb othseriesandshunt resistance: J total = J photo J O exp q ( V + IR S ) kT V + IR S R SH (3{7) Seriesresistanceistheresistanceencounteredbycarriersasth eyareextractedfrom thedevice.Seriesresistanceshouldideallybeaslowaspossible;highs eriesresistances leadtoalowerFFandultimatelylowerPCE.Highseriesresistancescan bedueto employingpoorlyconductivecontactsorhavinginsucientelectrica lcontactsuchthat thephotogeneratedcarrierscannotbeecientlyextracted.Sh untresistanceisideallyas highaspossible,asitrepresentsallcurrentpathsthatcarrythe chargesthroughacircuit inparallelwiththeload,i.e.,thephotogeneratedcarriersdonousef ulworkanddonot contributetotheoveralleciency.Onepossiblecontributortolow shuntresistancein bothp-njunctionsandSchottkyjunctionsislosingphotogenerat edcarriersouttheedgeof adevice.Acircuitschematicdepictingseriesandshuntresistances isshowninFigure 3-4 whileFigure 3-5 showsthedegradationoftheJ-Vcurvewithlargeseriesandsmalls hunt resistances. 38
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Figure3-5.TheeectsofseriesandshuntresistanceontheJ-Vc urve.Optimalvaluesare R series =0ohmcm 2 and R shunt = 1 ohmcm 2 3.2TheoreticalLimitations Nophysical,biological,orchemicalprocessis100%ecientduetothe rmodynamic limitations,andsolarcellsarenoexception.In1961ShockleyandQue isserpublished theirexhaustivecalculationonthemaximumtheoreticaleciencyof singlep-njunction solarcells,oftencalledthedetailedbalancelimit.Thismaximumeciency isattributed tothreeprimarymechanisms:emissionofblackbodyradiation,spec trumlosses,and recombination.[ 34 35 ]Allobjectsemitblackbodyradiationasafunctionoftheir temperature.Solarcellsoperatingatroomtemperatureemitradia tioncorresponding to300K,andthisemissionaccountsforlosesof7%. Spectrallossesarethelossesofphotonenergyexceedingtheba ndgapoftheabsorber material.Creationofasingleelectron-holepaironlyrequiresenergy equaltotheband gapofthesemiconductor,anythinginexcessiscarriedawaybythe chargesaskinetic energy,subsequentlylosttophonons(heat)astheelectronsre laxtothebottomofthe conductionbandandholestothetopofthevalenceband.Heatingo fthedevicecanbe especiallydegradingtoperformanceasthedarkcurrentisexpone ntiallydependenton temperature,leadingtoaloweredV OC andloweredPCEwithanincreaseintemperature. Mitigationofspectrallossesisachievedbyintroducingmulti-junctio ncellscomposedof individuallayers,eachspecicallytailoredtocollectionphotonsofap articularfrequency. 39
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Lastly,upordownconverters,luminescentlayersthatabsorbhig h(low)frequenciesand re-emitlow(high)frequencyphotons,canbeutilizedtobettermat chthephotonfrequency tothebandgapandminimizeenergylossandavoidheating.Upconver ters,typically placedatthebacksideofthecell,aretheorizedtoincreasethemax imumtheoretical eciencyto47.6%fornon-concentratedsolarlight,anoticeableimp rovementfromthe Shockley-Quiesserlimit.Downconverters,whichmustbeplacedont hetopofthecell, reapbenetduetoavoidanceofthermalizationofthephotogener atedcarriers,leading tolessheatingofthedevice.Theoreticaleciencyforthesereac h38.6%,onlyamodest improvementfromtheShockley-Quiesserlimit.[ 36 ] Recombinationisinevitableinsolarcellsandmucheortisaimedatminimizin g losses.Photogeneratedelectronsandholesmusttraveltothee lectrodestobeextractedas usableelectricalenergy,andpoorconstructionofPVcellsorusing incompatiblematerials leadtohighratesofrecombination.Evenwithcarefulcontrolove rsurfaceproperties anduseofcompatiblematerials,recombinationwillstilloccurduetoin herentproperties ofsolarcells.Dieringeectivemassesofelectronsandholesleadto dierentdiusion lengths.Afastmovingelectroncancollidewithaslowmovingholeleftov erfroma previousphotonabsorption,recombiningthroughoneofthemech anismsdiscussedabove. Anincreaseinphotonruxcorrelateswithanincreaseinthedensityo fphotogenerated carriers,increasingtheprobabilityofrecombination.Materialswhic hpossessahigher minoritycarrierdiusionlengtharelesslikelytosuerfromincreased recombinationat higherphotonruxes. Combiningthesethreelossmechanismstogetherleadstothemaximu mtheoretical eciencyforasinglep-njunctionsolarcellasafunctionofbandgap (Figure 3-6 ).The maximumpossibleeciencyis33.7%forabandgapof1.34eV.Withaband gapof1.12 eV,siliconcanattainapeakeciencyofonly29%.Utilizingmorecomplex architectures suchastandemsolarcellswithconcentratedlight,increasestheth eoreticaleciency 40
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Figure3-6.TheShockley-Queisserlimitshowingthemaximumtheoret icaleciencyasa functionofbandgapforasinglep-njunctionsolarcell. eciencydramatically;aninnitelylayeredsolarcellunderconcentr atedlightcan theoreticallyreach86%.[ 37 ] 3.3TypesofSolarcells 3.3.1P-NJunction Themostprevalentsolarcellsarecomposedofap-typeandn-type semiconductor broughtintocontacttoformap-njunction.P-Njunctionscanbe bothheterojunctionsor homojunctions,thoughinthecaseoftheformer,caremuchbeta kentomatchthelattice constantstominimizestrainanddefectsatthejunction.Whenaptypesemiconductor isbroughtintocontactwithann-typesemiconductor,mobilecharg esrearrangeinorder toestablishequilibriumofFermilevels.Theosetinworkfunctionbet weenthetwo materialsdriveselectronsinthen-typesemiconductortowardsth ep-typesemiconductor, wheretheycombinewiththeholesandleavetheregionwithanetnega tivecharge. Then-sideisleftwithanetpositivecharge.Thiscontinuesuntilther esultingbuilt-in potential, V bi ,preventsanyfurthermigrationofchargeduetoColoumbicrepuls ion.The builtinpotentialandalteredcarrierdensitynearthejunctionarem anifestedasabending ofthesemiconductorconductionandvalencebands,withtheregio noverwhichtheband bendingoccurscalledthedepletionregion(Figure 3-7 ).[ 30 38 ] 41
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Figure3-7.Schematicforap-njunction.Shownisthespacecharg e(depletion)regionin bothpandnside.ElectrontransferdrivenbyFermilevelosetes tablishesa builtinpotential(yellowarrow)opposingfurthertransferofchar ge. SolvingPoisson'sequationyieldsthewidthofthisregioninboththepan dn-type semiconductor; w p = 1 N a vuut 2 s V bi q 1 N a + 1 N d ; (3{8) w n = 1 N d vuut 2 s V bi q 1 N a + 1 N d ; (3{9) wherethetotalwidthisthesumofthetwo: w total = s 2 s q 1 N a + 1 N a V bi (3{10) where V bi isthebuiltinpotential,and N d and Na arethedonorandacceptordensities, respectively.Notethatasthecarrierdensityincreases,thedep letionwidthdecreases;in thecasewhereonesideisheavilydoped,thedepletionlayerexistsalm ostentirelyinthe otherside.Thisbuiltinpotentialactsasabarriertomajoritycarrie rswhilefacilitating transportofminoritycarriers,makingsolarcelleciencyofp-nba seddevicesmore sensitivetominoritycarrierdiusionlengths. 42
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Figure3-8.BanddiagramofP-Njunctionwithnobias,reversebias, andforwardbias. V bi isthebuiltinpotentialand V bias isthebiasvoltage.Inreversebias,the barrierincreasesandthedepletionlayerexpands.Underforward bias,the barrierdecreasesandthedepletionlayershrinks,leadingtoahighe rcurrent. Underillumination,electronholepairsarecreatedwithinthebulk;UVp hotonstend tobeabsorbednearthesurfacewhileIRphotonsareabsorbedde eperinthesubstrate. Minoritycarriersgeneratedclosetothedepletionlayerarecarried bythebuiltinpotential acrossthejunction,wheretheybecomemajoritycarriersandmu stdiusetoanelectrode tobeextracted.Carriersgeneratedfartherfromthedepletion layereitherdiusetothe junctionandaresweptbythebuiltinpotentialtotheelectrodes,o rtheyrecombine. Inp-nsolarcells,thedopedsemiconductormaterialdoesnotposs essahighenough conductivitytofunctionasanelectrode,necessitatingmetalgrid linesevenlyspaced acrossthetopofthedevice.Thesegridlinespreventincomingphot onsfromreachingthe bulk,lowingtheshortcircuitcurrentandPCEcommensuratewithth epercentageof 43
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surfaceareacoveredbythemetal.Increasesincarrierlifetimeca nmitigatetheselossesby allowinggridlinestobespacedfurtherapart.3.3.2Organic Organicbasedphotovoltaicshavebecomeincreasinglyinvestigated duetolow temperatureprocessingandthepotentiallowercostofmaterials ,aneasywaytoreduce thecostofmanufacturingthatwouldallowsolarenergytobemorec ompetitiveonthe openmarket.Organicmoleculesarerexibleandcanbetailoredtopos sessthedesired electroniccharacteristics(e.g.tailoredbandgaps),twoattribut esdistinctlylackingin silicon.However,excitonsinorganicmaterialsgenerallyhavebindinge nergiesonthe orderof0.5-1eV 1 .[ 39 ]Separationcannotoccurwithinthebulk,butmustoccuratthe junctionofthedonorandacceptormaterials.Thestronglybounde lectronholepairshave ahighprobabilityofrecombining,leadingtodiusionlengthsoftensof nanometerswithin mostorganicphotovoltaicmaterials,leadingtorecombinationlosses .Abalancemustbe struckbetweenhavingsucientabsorberthicknesstocapturet hemajorityofthelight incidentonthedeviceandminimizingthepathtothejunction.[ 29 40 41 ] Twomainarchitecturesfororganicsolarcellsarethinbilayerdevices andbulk heterojunctiondevices.Theformerreliesonusingthinlayersofhig hconductivitypand n-typeconjugatedpolymers(mobilitiescomparabletoamorphouss ilicon)toformap-n junction,whilethelatterusesasinglesolutionmixtureofthesamepo lymerstominimize thedistancebetweenthebulkandthejunction. Thebilayerdevice,showninFigure 3-9A ,hasthetwolayereddonorandacceptor materialssandwichedbetweenelectrodes.Inmostorganicdevices ,theanodeisconstructed ofindiumtinoxide(ITO)andthecathodeofthinaluminum.Poly(ethyle ne-dioxythiophene) (PEDOT:PSS)isoftendepositedontopoftheITOtominimizebandos et.Duetothe 1 Measurementsofsomeorganics,MEH-PPVandPPVhaveyieldedbind ingenergiesfromzeroto 1 eV.Discrepanciesarepossiblyattributedtoinaccuratetreatmen tofelectron-phononcouplingorother electroniceects. 44
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ABilayerOrganicSolarCell BBulkHeterojunctionSolarCell Figure3-9.Schematicofthebilayerandbulkheterojunctionsolarc ells. shortdiusionlengthofexcitons,onlythosegeneratedwithin20nm ofthejunction willbedissociatedandcollected,leadingtorelativelypooreciencies. Thebulk heterojunctioncell,showninFigure 3-9B addressthisissuebyhavingtheacceptor anddonormaterialmixedtogetherpriortodeposition,improvinge cienciesbycombinga thickerabsorberlayerwithadonor-acceptorjunctiondistribute dthroughoutthebulk. Morerecentlytherehasbeeninterestinorganic-inorganichybridd evices.Inaddition tolowtemperaturefabrication,thesetakeadvantageofthelong diusionlengthswithin siliconandtheexcellentconductivityandantirerectionpropertieso fthepolymerto produceasolarcellwithaPCEof13%.[ 42 ] Themajorbarriertolargescaleimplementationoforganicsolarcellsis theirlackof longtermstability.Inorganicdevicescontinuetoworkdecadesaft erinstallation,whereas organicpolymerssuerfrombothlightinduceddegradationandoxy gen/watervapor degradation.Simpleencapsulationsolvesthelatterproblem,butav oidingthedegradation associatedwithlightinvolvesusinglterstoblockoutselectivewavele ngths,whichalso lowersthefractionofthesolarspectrumbeingabsorbedandturn edintoavailableenergy. [ 29 ] 3.3.3PhotoelectrochemicalDevices Intheelectrolyteofaphotoelectrochemicalcell,theelectrochem ical(Nernst) potentialoftheincorporatedredoxcouplesetstheequilibriumdist ributionofthecouple 45
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Figure3-10.Schematicofsimplesemiconductor/liquidjunctionsolar cellshowingredox reactionsoccurringbothatthesemiconductorsurfaceandatam etal counterelectrode. betweenitsreducedandoxidizedstates.Whentheelectrolytecom esintocontactwith thesemiconductor,thetwoexchangecharge,simultaneouslyshif tingtheelectrochemical potentialoftheredoxcoupleandtheFermilevelofthesemicondu ctoruntiltheyare inequilibrium(thusestablishingthedepletionlayerinthesemiconducto r).Under illuminationphotocarriersarecreatedinthesemiconductorandcha rgeofonesignis repelledfromthejunctionwhilechargeoftheothersignisdriventot hejunctionwhereit reactswithonememberoftheredoxcouple.Thatchargeisthende liveredbyiondiusion toacounterelectrodethatcomprisesthesecondterminalofthe cell.Thoughinitially promising,siliconbasedliquidjunction(socalled)solarcellssuerfrom electrochemical reactionsatthesiliconsurface,creatingahighsurfacedefectde nsityinadditionto creatingnewspecieswhichwouldcontaminatetheelectrolyte. Anotherpopulartypeofphotoelectrochemicalcellisthedyesens itizedsolarcell, orGratzelcell.Theseuseasthesemiconductortitaniumdioxidewhic hhastoolargea bandgaptoitselfbeusefulasanabsorberbutthisis"sensitized"b ytheincorporation ofsmallerbandgapdyemoleculesthataredepositedontothesurfa ceoftheTiO 2 46
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Otherwisetheyoperateasdiscussedabovewitharedoxcouplefer ryingchargetoa counterelectrode.[ 43 ]Similartoorganiccells,Gratzelcellsdependinhighsurfaceareasfo r chargeseparationandmustemployabulkheterojunctionstructu retoachieveappreciable powerconversioneciencies.3.3.4Multi-junction Multi-junctioncellsaddresstheneedtocapturetheincidentradiat ionwithout wastingabovebandgapenergytonon-radiativerelaxationbyhavin gmultiplelayers.Tandem devicesaretwo,three,ormoredistinctlayers,withthetoplayers absorbingshorter wavelengthsandthebottomcellsabsorbingatlongerwavelengths. Duetotheirmultiple layers,thesedevicesarenotheldtotheShockley-Quiesserlimitimpo sedonsinglejunction Schottkyandp-njunctionsolarcells,alreadyachievingover40%e ciency.Thebiggest drawbackiscostduetothecomplicated,multi-stepprocessingand useofexpensive semiconductingmaterials,relegatingthesedevicestospecializedhig hvalueapplications (satellitesandmilitary).3.3.5Schottkyjunction Schottkyjunctionsolarcellsaresimilartop-njunctionsolarcellswith oneof thesemiconductorsreplacedwithametal.Theequilibrationoftheme talwiththe semiconductorcreatesthedepletionlayerwithinthelattermustge nerallybemadeso thickthatitisopaque.Togetlighttothesemiconductorthemetalis patternedasa gridofnarrowlinesallowingthelighttogetintothesemiconductorbet weentheopaque lines.Schottkyjunctionsalsobenetfrombeingmajoritycarrierd evicesfabricated withlowtemperatureprocesses,butupuntilthemid-1970ssuer edfromaloweropen circuitvoltagethanp-njunctionsolarcells.[ 44 ]Thislow V OC ispartlyduetothehigher darkcurrentinherentindevicesthatrelyonthermionicemissionofm ajoritycarriers fortransportacrossthejunctions(asinSchottkyjunctions). Additionally,surface statesthatpintheFermilevelarealsoresponsibleforincreasedr ecombinationanda correspondingincreaseindarkcurrent.A V OC andcorrespondinglowPCEkeptSchottky 47
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junctiondevicesfromcompetingwithp-nsolarcellsuntilGodfreyan dGreendevelopeda 17.6%metal-insulator-semiconductor(MIS)PVcellwithanopencirc uitvoltageofover 0.65V.[ 45 46 ]Thisexcellentperformancewasrealizedthoughathin( < 2nm)insulating layerbetweenthemetalandsemiconductinglayer,passivatingthe siliconsurfaceand reducingrecombinationwhilenegligiblyaectingcurrenttransport. Thecarbonnanotube -siliconsolarcellspresentedinthisdissertationareSchottkyjunct iondevices,andamuch morecompletedescriptionoftheunderlyingphysicsispresentedint hefollowingchapter. 3.3.6InversionLayerCells Inversionlayercellsinduceaninversionlayerwithinthedevicetoenha nceperformance. TherstofthesedeviceswasdevelopedbyRLCallintheearly1970s andwascomposed ofagridbasedSchottkyjunctioncellpossessinglargerspacingbe tweenthemetalgrid linesandathickinsulatinglayersandwichedbetweenatransparente lectrodeandthe semiconductor.Avoltagebetweenthetransparentelectrodean dthesemiconductor formedaninversionlayeratthesurface.Thissocalledinducedjunc tioncellbeneted fromecientchargecollectionduetothesurfacedepletionlayer,w hichrepelledthe majoritycarrierswhileattractingtheminoritycarriersavoidingthe irrecombinationas theminoritycarriersdiusedtothewidelyspacedelectrodes.[ 47 ]Callabandonedsuch electronicallyinducedjunctionsbecauseofthechallengesofgettin gpinholefreeinsulators overthelargeareasneededbutdiscoveredthatthedepositionof certaininsulators simultaneouslytrappedchargeofasignthatcreatedtheinversion layerwithoutneeding todosoactively.SuchcellswerefurtherrenedbyGodfreyandGr eeninthelate'70s. Thesedevicessueredfromimpermanenceofthetrappedcharge andovertimethe inversionlayerwoulddisappear.[ 44 ]Thedevicespresentedinthisdissertationconstitutein somesensearediscoveryofthephenomenaexploitedbyCall,Godfr eyandGreen. 48
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Table3-1.Currentmaximumecienciesforvariousphotovoltaicdev ices[ 1 ] TypeV oc (V)J sc mA cm 2 FF(%)Eciency(%) MonocrystallineSilicon(PERL)0.70642.782.825.0PolycrystallineSilicon0.66438.080.920.4CommercialSilicon---13MonocrystallineGaAs1.10728.386.728.3TripleJunctionGaInP/GaInAs/Ge2.69114.786.034.1DyeSensitised0.71421.9370.311.0OrganicTandem0.89916.7566.112.1 49
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CHAPTER4 INTRODUCTIONTOSCHOTTKYBARRIERS 4.1Fundamentals Schottkyjunctionsolarcellsarecomposedofametalincontactwit hasemiconductor anddisplayrectifyingbehavior.Analogoustoap-njunctioninwhicho nesideis degeneratelydoped,thedepletionlayerexistssolelyononesideoft hejunction. Semiconductordoping,Schottkybarrierheight,andinterfacedy namicsaectthe transportofcarrierswithinthedevice,aectingthefunctionality andPCE.The highlyconductivemetalcontactalsofunctionsastheelectrode,e liminatingtheneed foradditionalgridlinestoextractphotogeneratedcarriers.4.1.1BasicSchottkyModel Torstorder,theSchottkybarrierheightformedattheinterfa cebetweenametal andsemiconductorcanbeapproximatedbytheSchottky-Mottmo del.Assumingametal ofworkfunction m andasemiconductorofworkfunction n andelectronanity ,upon placingthetwointoelectricalcontacttheenergydierencebetwe entheworkfunction ofthemetalandtheFermilevelofthesemiconductordriveselect ronstomovefromone materialtotheother.Chargerearrangementcontinuesuntilele ctrostaticequilibrium isestablished,formingapotentialgradientwithinthesemiconductin gmaterialthat opposesanyfurthertransferofelectrons.Thisinducesbandbe ndingoftheconduction andvalencebandsinthesemiconductor,withthedepletionlayerwidt hdeterminedby thespatialregionoverwhichthebandbendingoccurs.Ahigherdop ingdensityyields steeperbandbendingandashorterdepletionwidth.ShowninFigure 4-1 belowarethe bandprolesforann-typesemiconductorandametalforwhich m > ,andforap-type semiconductorandmetalforwhich m < .Forann-typesemiconductor,electrons approachingthemetal-semiconductorjunctionfromthesemicond uctorsideseeapotential of qV bi .Electronsapproachingthejunctionfromthemetalseeapotent ialbarrierequalto theSchottkybarrierheight.[ 30 38 48 ] 50
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AN-TypeSemiconductor BP-TypeSemiconductor Figure4-1.Schottkybarrierbanddiagrams.AdaptedfromAyalew ,T,"SiC SemiconductorDevicesTechnology,Modeling,andSimulation",2004 Torstorder,thebarrierheightisgivenbytheSchottky-Mottre lation: q SBH = q ( m )forn-typesemiconductor(4{1) q SBH = E g q ( m )forp-typesemiconductor(4{2) 4.1.2CurrentTransport Schottkybarriersfunctionasdiodes,allowingnegligiblecurrentund erreversebias andexhibitinganexponentialcurrentwithforwardbias.Underrev ersebias,thebuilt inpotentialincreasesandverylittlechargerowsuntilbreakdown. Underforwardbias, thebandbendingdecreasesasthemetalisraisedtoahigherpoten tialrelativetothe semiconductor,resultinginanexponentialincreaseofcurrent,p rincipallyascarriersspill overthebarrier.InbothcasestheidealizedSchottkybarrierrem ainsunchanged.Carrier 51
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transportinSchottkydiodesoccursthroughseveralmechanism s:thermionicemission, thermioniceldemission,eldemission(tunneling),andminoritycarrie rinjection.[ 49 ] 4.1.2.1ThermionicEmission Thermionicemission,theprimarytransportmechanismformoderat elydoped semiconductorsandtheonemostrelevanttotheworkpresented inthefollowingchapters, isthetransportofenergeticcarriersoverthepotentialbarrier atthejunction.Thoroughly investigatedbyHansBetheinthe1940s,thetotalcurrentisasimp lesumofthecurrent rowingfromthesemiconductorintothemetalplusthecurrentrow ingintheopposite direction.[ 50 ]Theformerisgivenby J s m = A T 2 exp q SBH kT exp qV kT (4{3) A = 4 qm k 2 h 3 whereVisthebiasvoltageandAistheRichardsonconstantwhichisde pendentonthe eectivemass(m )ofthecarriers. A =120 A cm 2 K 2 and A A n Si =2 : 1forn-typesilicon. Currentrowingfromthemetalintothesemiconductorisindepende ntofthebiasvoltage andisgivenby J m s = A T 2 exp q SBH kT (4{4) Summingthetwoequationsyields J total = J o exp qV kT 1 (4{5) with J o = A T 2 exp q SBH kT asthesaturationcurrentdensity(oftencalleddarkcurrent). 4.1.2.2ThermionicFieldEmissionandFieldEmission Forheavilydoped( > 10 17 )semiconductorsatlowtemperature,tunnelingbeginsto contributesignicantlytocarriertransport.Fieldemissionisthetu nnelingofcarriers throughthepotentialbarrieratthesemiconductor/metaljunc tion.Thesecarriers 52
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generallypossesslittlekineticenergyandlieneartheFermilevel.The rmioniceld emissionconsidersthetunnelingofmoderatelythermallyexcitedcar riersacrossthe junction.Carriesthatalreadypossesssomethermalenergy(th oughlessthanthat requiredtosurmountthebarrier)"see"asmallerpotentialwidtht hancarriersthatare notthermallyexcited.Aparameterforevaluatingthetransportr egimeisgivenas: E 00 q ~ 2 r N m s (4{6) With kT E 00 ,thermalemissionisthedominanttransportmechanism.If kT E 00 eldemission(tunneling)istheprimarymodeoftransportacrossth ebarrier.Finally,if kT E 00 ,thenthermaleldemissiondominates.[ 49 ] 4.1.2.3MinorityCarrierInjection Schottkyjunctiondiodesareprimarilythoughtofasmajoritycarr ierdevicesdue totheextremelysmallcontributionfromminoritycarrierdiusion.A tlargeforward bias,however,driftofminoritycarriersbecomescomparabletoth ethermalemissionof majoritycarriersoverthebarrier.Thecurrentcontributiondue tominoritycarriersisthe sumofthedriftanddiusionprocesses: J = qnE| {z } Drift qD dn dx | {z } Diusion (4{7) Where isthemobility,nisthenumberconcentration,Disthediusioncoecie nt,and dn dx isconcentrationgradient,andEistheelectriceld.Thetotalcurr entislimitedby minoritycarrierrecombinationwithinthedepletionlayer.4.1.3BeyondSchottky-Mott DecadesofexperimentalresultshaveshownthattheSchottkyMotttheoryisnot anaccuratemeasureofSchottkybarrierheightsformostmetalsemiconductorinterfaces, motivatingalternatetheoriestoexplainthephysicsandchemistryo fthesejunctions. 53
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ShowninTable 4-1 attheendofthechapterarethetheoreticalandmeasuredScho ttky barrierheightsforvarioussemiconductor-metalinterfaces.[ 51 52 ] Clearlytheexperimentalresultsdeviatefromthetheoreticalvalu espredictedbythe Schottky-Motttheory.Forsimplicity,anequationtoquantitative lypredictbarrierheights onn-typesiliconhasbeenadoptedbasedonexperimentalresults: q SBH =0 : 27 q m 0 : 52. Amorecompletepictureofthe"real"junctionisshowninFigure 4-2 .Included areinterfacestatesoccurringinthebandgapofsilicon,denoted Q ss ,theoriginof whichisexploredinmoredetailbelow.It'sclearfromthisgraphicthat theunderlying physicsofmetal-semiconductorjunctionsisfarmorecomplicatedt hancapturedbythe Schottky-Mottmodel. Thegraphicrepresentsthejunctionwithoutanyexternalapplied bias.Upon introductionofanelectriceld,thecombinationoftheappliedeldan dtheinduced imagechargesresultinasmallSchottkybarrierlowering.Conseque ntly,realjunctionare somewhatbiasdependent,withtheamountthebarrierisloweredgiv enby = r q m 4 s (4{8) m = s 2 qN j s j s and s isthesurfacepotential.InadditiontoSchottkybarrierlowering,d evicescan suerfromleakageofcarriersoutthesidesoftheactivearea.Th isedgeleakageoften occursatthesharpcornersofmetalelectrodeswherehighlycon centratedelectriceld linesfacilitatetunneling.Mitigationofthiseectisachievedinbothp-n junctionsand Schottkyjunctionsbyusingguardrings. 54
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Figure4-2.InterfacestatesshownintherealisticmodelofaScho ttkyjunction. 4.1.3.1FermiLevelPinning:BardeenModelandMetalInduce dGapStates In1947JohnBardeenpublishedhispaperoninterfacestatesatth esemiconductor-metal junctionandtheirroleincreatingabarrierindependentoftheScho ttky-Mottmodel.A propertyofthesemiconductorsurface,interfacestatescana ecttheSchottkybarrier heightdramatically,producingabarrierheightcompletelyindepende ntofthemetalwork function.Thesemiconductorsurfacecanpossessaveryhighden sityofsurfacestates -farhigherthaninthebulk.ThisleadstoFermilevelpinning,aproce ssinwhichthe 55
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extremelyhighdensityofsurfacestatespreventthesemiconduc torFermilevelfrom shiftinginresponsetothemetalcontact,evenafterreachingeq uilibrium.Notonlydo thesesurfacestatespintheFermilevelandcreateaSchottkyb arrierindependentof m ,buttheyalsoactastrapsites,facilitatingrecombinationofelectr onsandholes.[ 53 ] Nearly30yearsafterBardeenpublishedhisworkonFermilevelpinn ingatsemiconductor surfaces,Tersopublishedhistheoryexploringalternatesource sofFermilevelpinning inordertoexplainthediscrepancyintheBardeenmodelforpredict ingtheSBHwithin ionicsemiconductors.Atafreesemiconductorsurface(notanMS interface),theFermi levelcanbepinnedbyarelativelysmallamountofsurfacedefects. Thescreening lengthofthesedefectstateswithinthesemiconductorisrelatively large,leadingtoa correspondinglylargesurfacedipoleandshiftinE F .AtanMSinterface,thechargesin themetalscreenthesurfacedefects,leadingtoasmallerlocaldip oleandsmallershiftin E F .Histheory,basedonmetalinducedgapstates(MIGS),positsth atthecontinuumof statesexistinginthemetalattheMSinterface"leak"intothesemic onductor,leadingto gapstatesthatdecayintothebulkandconsequentlypintheFermi level.Thebarrier heightisthesumofboththedipoleduetometallicscreeningoftheMIG Sanda surfacedipole,eitherofwhichmaydominatedependingonthebulkpr opertiesofthe semiconductor.[ 53 ] 4.1.3.2BondPolarization AmorecontemporarytheorypioneeredbyRaymondTunglookstoc hemicalbonds formedacrossthemetal-semiconductorjunctionasthesourceo finterfacestatesthatcause thebarrierheighttodeviatefromtheSchottky-Mottapproximat ion.Allpropertiesof theSchottkyjunction,includingbarrierheightandinterfacedipole ,areaconsequenceof thechemicalbondsformedbetweenthesemiconductorandmetal. Theinterfacedipole issimplyduetothepolarizationofthesebonds.Thismodelassumesa nextremelysmall interfaceregionandpredictsthebarrierheightusingthefollowinge quation: 56
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SBH;n = r B ( m s )+(1 r B ) E g 2 (4{9) r B =1 e 2 N B d MS it j j and isthebondintegral, N B isthedensityofchemicalbonds, d MS isthedistance betweentheatomsatthemetalsurfacefromtheatomsatthese miconductorsurface, it is thepermittivityoftheinterface,and accountsforcharges"hopping"fromoneatomto another(metal-metal,metal-semiconductor,semiconductor-se miconductor).[ 54 { 56 ] Thoughbondpolarizationisanexcellentmodelforchemicallyactiveme tals,carbon nanotubesarenotchemicallyreactive.Duetothislackofchemicalr eactivityofthe carbonnanotubesandthereluctancetoformchemicalbondswith substrates,theBardeen modelprovidesmoreinsightintothebehaviorsrelevanttooursolar cells. 4.2SchottkyJunctionSolarCells 4.2.1HistoricalBackground BuiltbyCharlesFrittsin1894,therstsolarcellwasaSchottkyjun ctiondevice constructedfromseleniumsandwichedbetweengoldandanotherm etal.Solarcellresearch anddevelopmentactivityremainedlowuntilthe1950s,whenhighqua litysiliconbecame available,spurringadvancementsinhomojunctionstructures.In itiallyengineeredfor spaceapplications,itwasn'tuntiltheenergycrisisinthe1970sthat solarcelldevelopment begantofocusonproductionforcommercialuse.Bothp-njunct ionsandSchottky junctionsolarcellswereextensivelyresearched,withmetal-insulat or-semiconductor inversionlayer(MIS-IL)devicesachieving17.6%.[ 45 46 ]InthedecadessinceGodfrey andGreen'shighperformingMISstructure,solarcelldesignshave expandeddramatically byexploitingadvancesinpolymersciencefororganicdevices,usingn ewtechnologiesto fabricatehighqualitythinlmstructures,developingnovellighttra ppingtechniques,and creatinghybridstructuresexploitingthestrengthsofvariedmat erials.Concomitantwith thosedevelopmentswasagreaterunderstandingofthephysical andchemicalprocesses 57
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pertinenttophotovoltaics.AdiscussionofallPVtypesisoutsideth escopeofthisthesis, butmuchoftheworkpioneeredinthepreviousdecadesgreatlyinru encedthedesignof thedevicespresentedbelow.4.2.2CNTonSiliconSchottkyJunctionSolarCells Carbonnanotubeswererstincorporatedintodevicesasconduc tiveelectrodesfor eithersupercapacitorsororganicphotovoltaics.[ 57 58 ]In2002,acarbonnanotube/polymer solarcellwithaneciencyof0.04%wasdeveloped,catalyzingtheuse ofcarbon nanotubesinphotovoltaics.[ 59 ]LowperformanceinCNT/organicPVcellsencouraged developmentusinginorganicmaterials,andin2007adoublewalledcarb onnanotubesilicondevicewithaPCEof1.38%waspublished.[ 60 ] SWNT-Sisolarcellshaveachievedhighecienciesinrecentyears,du einpartto themalleabilityoftheelectronicandopticalpropertiesofcarbonna notubes.In2010,it wasdemonstratedthatanionicliquidcouldbeusedtoelectronicallymo dulatetheFermi levelofthecarbonnanotubelm,changingthebuiltinpotential,jun ctiondynamics,and PCE.[ 9 ]Thedevicespresentedbelowhavetakentheoriginalsolarcells,sh ownbelowin Figure 4-3 andfurtheroptimizedthedeviceperformancebyexploitingtheee ctofthe ionicliquid.4.2.2.1ExperimentalDetailsandEquipment Substrateweredicedfroma500 mthick, < 100 > / < 111 > ,n-Typesiliconwafer possessingeithera200nmor1000nmthickthermaloxide.The < 100 > wafershada resistivityof0.5-0.7Ohm-cm,whilethe < 111 > waferswereslightlylessdopedwitha resistivityof4-10Ohm-cm.Ontothesurfaceoftheoxidewasden edasquare12x12mm 2 Au/Cr(60/10nm)pad,possessinga2x4mm 2 rectangularwindowatitscenter.This Au/Crpadservedseveralfunctions:itprovidedanetchmaskfor aBOEetchoftheoxide inthewindowandservedastheelectricalcontacttothenanotube sthatweredrapedas athinlmfromtheAu/Crlayerdownacrossthesilicon;andnally,itpr ovidedaliteral shadowmask,limitingthecollimated,simulatedsolarradiationtothewin dowareain 58
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Figure4-3.Carbonnanotube-siliconSchottkyjunctioncell.Includ edareboththeactive arealmandthegatelm whichtheactiveareawasdened.AfteraBOEetchremovedtheth ermaloxidewithin thegoldframedwindow,acarbonnanotubelmwastransferredfo llowingtheprocedure inWu, etal. .[ 8 ]Anotherlmtransferredtoasecondarygoldelectrodefunction edasthe gatelmandwasinsulatedfromthesiliconbythethermaloxide.Ohmic contactbetween theSiwaferbacksideandastainlesssteelsheetwasmadebyagalliu m-indium(Ga/In) eutecticspreadbetweenthetwo. Illuminationwasprovidedbya150Wxenonlamp(Oriel6255)inanOriel6 136 housingpoweredbyamodel8500powersupply.AnOriel81094AM1.5 Glterapproximated thesolarspectraldistribution.Lightfromtheinhomogeneoussou rcewasfocusedintothe apertureofa150mmlong,fusedsilicaHomogenizingRod(EdmundOpt icsP65-837)by a50mmdiameterfusedsilicalenswitha65mmfocallength.Theoutput faceofthe HomogenizingRodwasimagedinthehorizontalfocalplaneofthesam plebya50mm diameter,100mmfocallengthfusedsilicalensafterrotationby90d egreeswithabroad bandmirror(Newport66225).Theintensityatthesampleplanewas adjustedto100 mW cm 2 bytranslationofthe65mmFLlens,cuttingdownonthefractionoft helightenteringthe 59
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HomogenizingRod.Homogeneityofthelightintensityoverthe1cm 2 centralregionof thehomogenizedbeamatthesampleplanewasmeasuredtobewithin5 %. 4.2.2.2ElectronicGating ThesedevicesdieredfromconventionalSchottkyjunctionsolar cellsbyinclusionof anionicliquid1-Ethyl-3-methylimidazoliumbis(triruoromethylsulfonyl) imide(EMI-BTI) toaltertheelectronicpropertiesofthenanotubelm,whichinturn modiesthe characteristicsoftheSchottkybarrierandbuiltinpotential.Theio nicliquidsaturates boththeactiveareaSWNTlmandasecondarySWNTlm.Agatevolta geisapplied betweenthetwolms,polarizingtheionsintheionicliquidandincreasing ordecreasing thecarrierdensityoftheSWNTlmintheactivewindow.Thiscapacitiv edopingalters theFermilevelofthelmandsubsequentlychangesthebuiltinpote ntialattheSWNT-Si junction.Unlikephotoelectrochemicalcellswheretheionicliquidactu allytransfers chargebetweenelectrodes,thereisnonetcurrentrowthrough theionicliquid.EMI-BTI hasalargeelectrochemicalwindowspanningnearly5V,allowingtheap plicationof appreciablevoltagesbetweenthetwoSWNTlmswithoutelectroche mistryoccurringat theelectrodes.Thisprocess,akintocharginganelectrolyticcapa citor,drawsnocurrent onceequilibriumisestablishedateachgatevoltage.Tocharacterize thecell,axedgate voltageisappliedtotheSWNTgatelmandthevoltageacrosstheelec trodesisswept from-1Vto1VandtheresultingJ-Vcurveisanalyzedtoextractde viceperformance. ThenanothergatevoltageisappliedandanewJ-Vcurveisgenerate d.Previousresults demonstratedanoptimizedperformanceatagatevoltageof-0.75 V;positivegatevoltages resultinasmallerPCE,llfactor,andV OC .[ 9 ]TheresultingJ-Vcurvesforgatevoltages between+/-0.75VareshowninFigure 4-4B .ThenativedevicePCEwithoutgatingwas 8.5%.GatingmodulatedthePCEbetween3.6and10.94.2.2.3InversionLayerModeling ThepreviousworkbyDr.Wadhwaelucidatedthemechanismbywhicht heionic liquidimprovedtheeciencyofthesolarcells.Theintendedfunctiono ftheelectrolyte 60
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ASchematicoftheduringelectronicgating. -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 -40 -30 -20 -10 0 10 20 30 40 50 60 Current Density (mA/cm2)Voltage (V) -0.75V -0.6V -0.45V -0.3V -0.15V 0V 0.15V 0.3V 0.45V 0.6V 0.75V BResultantJ-Vcurvesatvariousgatevoltages. Figure4-4.SchematicandresultsforelectronicallygatedSWNT-Si cell.Gatevoltageis thepotentialdierencebetweentheactivearealmandthegate lm wastochangetheFermileveloftheCNTlm,increasingordecreas ingthecharge transferredbetweenthesiliconandCNTinordertoestablishequilibr iumofFermi levelsinadditiontoincreasingtheconductivityofthelm.However,a nexperimental architectureinwhichthecarbonnanotubelmwaspatteredintoth instripsvia photolithographydemonstratedanothermechanismatplay.Inth isdevice,thecarbon nanotubestripsmeasured100 m acrossandwereevenlyspacedevery300 m ,asshown inFigure 4-5A below.Theactiveareaofthedevicehaslessthan30%lmcoverage. Asexpected,thephotocurrentpriortoadditionofionicliquidwasre ducedduetothe decreasedlmcoverage.Thoughreducedbyafactorof2,theph otocurrentdidnot decreasedirectlyinproportiontothereducedSWNTlmcoverage; thelongdiusion lengthsinsinglecrystalsiliconallowscollectionofphotocarriersgene ratedoutsidethe junction.Uponadditionofionicliquidthephotocurrentincreaseddr amatically,indicating analterationofthesiliconsurfaceinawaytofacilitatechargecollect ion.Applyingagate voltagefurtherimproveddeviceperformance,consistentwithpr eviousplanardevices.[ 10 ] 61
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ATheactiveareaofthelmcom-prisingof2x4mmsiliconwindowwithpatternedcarbonnanotubelm. BFigureB:J-Vcurveforthesamedevice.Notereducedpho-tocurrentpriortoadditionofEMI-BTI. Figure4-5.Schematicandperformanceforgridcell. Weinferred,andsimulationsbyJ.GuoandJ.SeolintheElectricalan dComputer EngineeringdepartmentattheUniversityofFloridaconrmed,tha ttheionicliquid spontaneouslyformsalayerofchargeatthesiliconsurfacewhere nonanotubesare present,inducinganinversionlayerinthesiliconoutsidetheSWNT-silic onjunction.The positivecarriersthatdiusedintothedepletionregiontraveledalon gthesurfaceofthe siliconuntiltheyencounteredananotubeandwereextracted.Th isismanifestedasan increasedphotocurrentandcorrespondingincreaseineciency. Thisbehaviorcanbe qualitativelyunderstoodasfollows.WhenthenanotubesandthenSiarerstplaced inintimatecontact,thefreeenergyofelectronsinthen-Si(work function: Si =4.3eV) isreducedbytheirtransfertothecarbonnanotubes(workfunc tion: SWNT =4.9eV). SuchtransferstopswhenCoulombicrestoringforcesduetothec hargeimbalanceraise thelocalpotential(thebuiltinpotential)topreventfurtherchar geexchange,establishing equilibrium.Inthepresenceofelectrolyteions,theionsmigratetoc ompensatethe transferredchargeandthuspermittheexchangeofsubstantia llymorechargebefore 62
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theequilibriumisreached.Additionalelectronsaretransferredto thenanotubesfrom theSiregionsbetweenthenanotubegridlinescompensatedbypos itiveelectrolyteions surroundingthenanotubes,whilethepositivechargeleftbehindint hen-Siinversionlayer iscompensatedbynegativeelectrolyteionsaccumulatedattheSis urface.Theelectrolyte hereservesmuchasitdoesinanelectrolyticcapacitortoraisethec apacitanceofthe systemwithaself-potentialprovidedinternallybytheoriginalFerm ilevelosetbetween thenanotubesandthen-Si,orexternallybythegateeld.Shownin Figure 4-6 below arethesimulatedresultsatabiasvoltageof0VforV G =-0.75VandV G =+0.75V.Full simulationresultsareincludedinAppendixA.[ 61 ] Exploitationofthisdiscoveryopenedthedoorsforalternativearc hitecturesdesigned tooptimizedeviceperformance.GuoandSeolshowedthattheionic -inducedextension ofthedepletionregionoccurredoverhundredsofmicrons,allowing areductionin thefractionofnanotubelmcoveringthesiliconactiveareawithout sacricinghole extraction.Thepositivecarriersthatdiusedintothedepletionre giontraveledalongthe surfaceofthesiliconuntiltheyencounteredananotubeandwere collected.Thereduction inthenanotubelmareaenhancedthenumberofphotonsbeingabs orbedinthesilicon surfacetoincreaseboth J sc andthePCE. 63
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AModelParameters BSimulationsatV G =-0.75VandV G =+0.75V Figure4-6.Simulationsshowinginversionlayerinsiliconextendingacro ssentiresurfacein betweencarbonnanotubestrips.Notethereductionintheinvers ionlayerat +0.75V;atmorepositivebiasvoltages,thisresultsinareducedcollec tionof carriers,asshowninpreviousJ-Vcurvesforgateddevices(Figur e 4-4B ) 64
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Table4-1.TheoreticalvsExperimentalSchottkyBarrierHeights :Barrier heightsmeasuredat300K,theoreticalvaluesdeterminedfromSchottky-Mottrelation SemiconductorMetalTheoreticalSBH(eV) 1 ExperimentalSBH(eV) 2 n-SiAl0.01-0.20.81n-SiAu1.05-1.40.83n-SiPt1.1-1.90.9n-GaAsAl0.03-0.20.93n-GaAsAu1-1.381.05n-GaAsPt1-1.80.98 1 Thetheoreticalvalueisgivenasarangeastheworkfunctionofmet alsdiers dependingoncrystallographicorientation. 2 Highestmeasuredvalues.[ 62 63 ] 65
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CHAPTER5 NANOSTRUCTURINGFORENHANCEDLIGHTABSORPTION 5.1Overview Texturingofphotovoltaicstoimprovephotonabsorptionhasbeen aggressively investigatedforseveraldecadesasameanstoimprovesolarcelle ciency.[ 28 64 { 72 ]In 2000,Li etal. demonstratedthefeasibilityofusingasimplemetal-assistedchemica letch tocreateporoussilicon,openingthedoorstofacileproductionofo pticallyabsorbent substrateswithahighsurfacearea.[ 73 ]Whilefabricationofporousstructuresusing chemicaletchingisasimpleandlowcostprocess,thereislittlecontro loverthesizeand uniformityoftheresultingfeatures(thoughspatialperiodicityca nbedetrimentaldueto diractionlosses).CVDgrowthisoftenutilizedforapplicationswher ecompletecontrol overdiameteranddepthisneeded.In2005,Peng etal. demonstrateda9.31%ecient siliconnanowire,p-nhomojunctionsolarcell.[ 74 ]Continuedinnovationinnanostructured deviceshasculminatedinan18.2%ecientporoussiliconp-nhomojunc tioncell.[ 75 ] Consideringthererectivityofthedevicespresentedinthepreviou schapter(silicon +SWNT+ionicliquid)isroughly20%overwavelengthsgreaterthanthe bandgapof silicon,optimizinglightabsorptioncanresultinsignicantincreasesof thePCE.Methods fortexturingthesilicontoenhanceabsorptionarepresentedinth ischapter,alongwith complicationsinherenttoalterationofthesemiconductorsubstra te. 5.2PotassiumHydroxideEtching Anaqueoussolutionofpotassiumhydroxide(KOH)isabasicsolutionf requently usedtoetchpyramidalstructuresinsilicon.Atroomtemperature ,theratioofetch ratesbetween < 100 > and < 111 > planesisroughly100:1fora30%KOHsolution.This anisotropyleadstoregularpyramidalstructureswiththeexpose dfacescorresponding tothe < 111 > plane.[ 76 ]Thoughthepyramidsareexcellentatreducingrerectionfrom thesiliconsurface,thepointedtopswouldprovidelittlesurfacefor contacttotheSWNT lmandpromptedfabricationofpyramidalgroovesinstead.Fabric ationconsistedofrst 66
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deningtheareastobeetchedthroughphotolithographyona < 100 > substratewith a200nmthermaloxide.SiO 2 islargelyunaectedbyKOH,permittingitsuseasan etchmask.ABOEetchremovesthethermaloxideintheregionsnot protectedbythe photoresist,afterwhichthephotoresistisremovedandthesubs trateisthenplacedinthe KOHforthedesiredlengthoftime.FollowingaDIrinseandN 2 dry,theremainingoxide isstrippedwithBOEandaSWNTlmistransferredtothesiliconfollowing standard proceduresandsubsequentlytested.Theresultinggrooveswer e10 m wide,14 m deep, andextendedacrosstheentireactivearea. AResultantgroovesfromKOHetch -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 -40 -30 -20 -10 0 10 20 30 40 50 60 Current Density (mA/cm2)Voltage (V) Trenches No IL Trenches -0.75V No Trenches No IL No Trenches -0.75V BJ-VcurvesforKOHdevice Figure5-1.KOHschematicandperformance. Similartothegridlm,thereisareducedphotocurrentwithouttheio nicliquidand anincreasedphotocurrentwithadditionoftheEMI-BTI,asshown inFigure 5-1 .As withplanardevices,amarkedimprovementineciencyoccursforne gativegatevoltages. ThemaximumPCEobtainedwas11.4%,animprovementof12%overapla nar,fulllm < 100 > device(10.6%PCE).Thellfactorremainedroughlythesame:0.77f ortheKOH etcheddeviceversus0.76forthefulllmplanardevice.Theincreas edperformancecan 67
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thereforebeattributedtotheincreased J SC resultingfromtheenhancedabsorptionofthe groovedstructure.Note,however,thattheoverallPCEisstilllo werthanthatachieved bythegridlm.Thedimensionsofthegroovesarefarlargerthanth ewavelengthof visiblelight,leadingtoarelativelypoorincreaseinabsorptioncompare dtoothertextured devices.Additionally,theperiodicspacingofthegroovesleadstodi ractionatoblique angles,makingthesestructuresinferiorforlightcollection.Nonet heless,thisresult reinforcestheimprovedperformanceattainedwithareductioninS WNT-Sicontactand encouragedcontinuedexperimentsinsurfacetexturing. 5.3SiliconNanowires Duetotheirexcellentabsorptionandeasilytailoredgeometries,silico nnanowires arrayedinverticalforestsonasubstratehavebeenextensively studiedforintegration intophotovoltaicdevices.Asincommerciallyavailablesolarcells,p-nho mojunctions ofvariousdesignsarefoundwithinsiliconnanowirebaseddevices.Fa bricationinvolves takingadopedsemiconductingnanowireforestsupportedonabulk substrateandusing ahightemperaturediusionprocesstodopethesurfaceofthena nowires.Thisdoping processcanbeoptimizedtodopejustthenanowire,leavingthesup portingsubstrate withtheoriginaldopingtype,orthedopingcanbedonecoaxially,cre atingaradial p-njunction.[ 72 77 ]Therstgenerationp-nnanowiresolarcellssueredfromhigh seriesresistance,reducedlightabsorptionduetometalngerele ctrodes,anddiraction atobliqueanglesduetotheregularspacingofnanowires.[ 72 78 79 ]Asidefromp-n homojunctions,siliconnanowireshavebeenintegratedintophotoe lectrochemicalcells andevendevicesutilizingacarbonnanotubetoplayer.[ 80 81 ]Thelatter,however,were poorlyconceivedanddidnotlikelyoperateasphotoelectrochemical cellsasclaimed, butratherlikelyalongthelinesofthemechanismsdiscussedherewith aPCEofonly 1.3%.Despiteimpressiveprogressintheeciencyofsiliconnanowirec ellsingeneral,the maximumeciencyforanysiliconnanowirebaseddeviceshasnotyetp assed10%.[ 78 82 83 ]Thegeneralthoughtinthecommunityforthereasonbehindthislim itisthechallenge 68
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ofcontrollingthetrapstates(andresultingrecombination)atthe largesurfacearea createdinthenanowiresurfaces.GiventhegridSWNTlmresultswe reasonedthatwe couldavoidrecombinationviatheelectrolyteinversionlayerandthus resolvedtopursue suchSinanowiredevices.5.3.1ProcedureandCharacterization Fabricationofthesiliconnanowireswasachievedviaasimplechemicale tchadapted fromKQPeng, etal. .[ 80 84 ]Substrateswerepreparedbyevaporatingagoldelectrode framesurroundinganopenwindowtobecometheactivecellarea,a ndthenpainting photoresisteverywheresavefortheactiveareawindow.Thepho toresistprotectsthe substratefromtheaggressiveetchwhilethegoldelectrodealsofu nctionsasanetchmask tokeepthenanowire"growth"containedwithintheactivearea.Th esubstrateswere placedin6:1BOEtoremovethethermaloxidelayer,rinsedwithDIwat er,driedinanN 2 stream,andsubmergedintoa4.6M/0.02MHF/AgNO 3 solutionforavaryingamountof time.Atroomtemperature,a4minuteyielded1 m longnanowires. Figure5-2.Themechanismforsiliconnanowiregrowth.A:Silverionsa dsorbonto surface.B:Oxidationofsilicon.C:EtchingofSiO 2 andsinkingofsilver particle.AdaptedfromKQPeng etal. Theetchingsolutionproducessiliconnanowiresviaathreestepproc ess,asshownin Figure 5-2 silverions(dissociatedfromtheAgNO 3 )adsorbontothesiliconsurfaceand oxidizethesilicondirectlyunderneaththesilverparticle.Thislocallyox idizedregionof siliconisetchedawaybytheHF,creatingaspatialvacancythatthe silverparticlesinks into.Thereducedsilverundergoesaredoxreactionandreturnst oitsoxidizedstate, uponwhichtheprocessrepeatsuntilthereactionisquenched.Th eresultingstructureis 69
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Figure5-3.SiliconnanowiresgrowninanHF/AgNO 3 solution dependentonboththemolarconcentrationofthetwoactivechem icals(HFandAgNO 3 ) andthelengthofetch,withtheconcentrationslistedaboveprodu cingthehighaspect rationanowires.Alternativewetchemicaletchesalsoexploredinvo lveelectrolessmetal depositionorsputtering/evaporationofmolecularsilver/gold,res pectively,ontothe substrateandusinganoxidantsolution(typicallyH 2 2O 2 andHF)tocatalyzeredox reactionsatthemetalconglomerates.Theholeproducedbytheo xidationofthemetalis injectedintothevalencebandofthesilicon,oxidizingtheregiondirec tlybelowthemetal. Thisalternativehastheadvantageofcontrollingmetaldepositionu singphotolithography tocontroltheetchinggeometry.Althoughthesestructureswe reinitiallyconsidered forintegrationintoourSiNWdevices,SEMimagesshowedlessuniform growthof nanowireswiththismethod.Additionally,acomprehensivereviewofs iliconnanowires inphotovoltaiccellsconcludedthatnanowireforestsfabricatedth roughinHF/AgNO 3 solutionhadahigherV OC thanthosefabricatedthroughotherwetchemicaletches.[ 80 ]. ThesetwofactorsledustousesiliconnanowiresfabricatedinaHF/A gNO 3 solution. Immediatelyuponremovalfromtheetchantsolution,residualsilve rresidingat thebottomofthenanowiresorextendingacrossthenanowiresur faceareremovedin an8Mnitricacidbath,followedbytwoDIbathsandagentleN 2 dry.Thenanowires 70
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ASiliconNanowireson < 100 > crystallographic orientation. BSiliconNanowireson < 111 > crystallographic orientation. Figure5-4.Orientationofsiliconnanowires. agglomerateuponremovalfromthelastDIbath,asshowninFigure 5-3 .Whilegrowth wasinitiallyperformedonboth < 100 > and < 111 > siliconsubstrates,SEMimagesshowed adistinctdierencebetweenthewiresformedondierentcrystallo graphicorientation substrates; < 100 > gaveanisotropicdistributionofverticalnanowires,while < 111 > formedwelldenedregionsofnanowiresatotheranglescorrespo ndingtotheeasily etched < 100 > and < 110 > crystallographicplanes,asshowninFigure 5-4 .Ithasbeen demonstratedthattheselectiveetchorientationisrelatedtooxid antconcentration,with lowconcentrationsleadingtoetchingalong < 100 > planesandhighconcentrationsleading toverticaletchingindependentofcrystallineorientation.[ 85 ] Rerectancefromthenanowiresnearnormalincidencetothesubs trateonboth < 100 > and < 111 > weremeasuredusingaUV/VIS/NIRspectrophotometerfrom 400-1200nm.Thoughitwasfarlowerthanthatforuntexturedsilic on,the < 111 > still exhibitedhigherrerectancethan < 100 > ,mostlikelyduetotheanglednanowires. 5.3.2Integrationinsolarcellsandinitialperformance Thesiliconnanowiresexhibitaverylowrerectanceoverphotonener giesgreater thanthebandgapofsilicon,butsingle-wallcarbonnanotubeonsilico nnanowire (SWNT-SiNW)devicessuerfromareducedcontactbetweenthes iliconandSWNT 71
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400 500 600 700 800 900 1000 1100 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 ReflectivityWavelength (nm) Bare silicon w\IL 20nm SWNT on Si w/IL 20nm SWNT on <100> SiNW w/IL 20nm SWNT on <111> SiNW w/IL Figure5-5.Rerectanceofthesiliconnanowiresubstratesascomp aredtountextured silicon. lm.Initialdevicesperformedrelativelypoor.Infact,thereispra cticallynophotocurrent intheabsenceoftheionicliquidandthePCEisonly0.02%.Additionofthe ionicliquid yieldsamaximumgatedPCEof4.6%,relativelylowcomparedtootherde vices,but theincreaseemphasizedhowenhancedabsorptioncoupledwiththe inversionlayerwere abletopartiallycompensatefortheminimalSWNT-SiNWjunctionsatt hetipsofthe nanowires. Thoughthiseciencyispoorincomparisontotheplanargateddevice s,itwasgood enoughtoencouragefurtherinvestigationandoptimization.Aninc reaseinthegate electrodelmareacommensuratewiththeincreaseinsiliconsurface areawasimperative forinducingtheinversionlayerinthenanowiresidewalls.Employingaga telmofasize usedintheplanardevicesevidentlylimitedthecapacitanceandpartia llyexplainedthe poorperformance.Additionally,employingbettertransfermetho dstoenhancecontact betweenthecarbonnanotubelmandthesiliconnanowireswouldfur therimprovecarrier collection.Finally,theincreasedsurfaceareaprovidedmorerecom binationcenters, 72
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Figure5-6.InitialperformanceoftheSWNT-SiNWdevice.Notetha tthebluecurveis theilluminatedJ-Vpriortoadditionofionicliquid.Theinsetshowsminutepowergenerationpriortotheadditionoftheionicliquid. makingtheperformancehighlysensitivetothesurfaceproperties ofthesiliconand encouragedexperimentsaimedatpassivatingthesiliconsurface.5.3.2.1RemoteGating Theneedforalargercapacitancegateelectrodeanddesiretoavo idtakingup (potentiallyuseful)areaonthefaceofthesolarcellnecessitated thedevelopmentof aremotegateelectrodethatstillretainedahighsurfaceareawhile occupyingasmall volume.ThisgateelectrodeconsistedofacoiledPtwireontowhichat hicklayerof SWNTshadbeendeposited,placedwithina2mminnerdiameterpolyeth ylenetube.The smalltubewaslledwiththeviscousEMI-BTIelectrolyte,rendered immobilethough capillaryforces.TheSWNTsonthePtwireamountedtoa1cmx1cm,1 m thick lm,over2ordersofmagnitudelargerthanthepreviousgatelms. Theendofthis gateelectrodewastouchedtotheEMI-BTIelectrolytedrop(ove rtheAupadtoavoid shadowinglightfromtheactivearea)connectingtheelectrolytere servoirs.Notethatthis remotegateelectrodeimprovesonthepreviousdesignwheretheg ateelectrodeoccupied frontsurfacerealestateoftheSi(thus,inprinciple,precluding thatareasavailabilityfor 73
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lightcapture).Aschematiccross-sectionofthedevicearchitect ure(nottoscale)andthe wiringdiagramfortestingisshowninFigure 5-7A AExperimentalset-upforelectronicgatingwiththeremotegate BSEMimageofSWNT-SiNWactivearea,showingthenanotubelmlayingacrosstheSiNW. Figure5-7.SchematicforremotegatingandSEMofSWNT-SiNWactiv earea 5.3.2.2PassivationofNanowireSidewalls ThebenetsofathinnativeoxidepassivationlayerinplanarSWNT-Si solarcells wasnotedinthesupportinginformationofWadhwa etal. andstudiedinsomedetailfor doublewalledcarbonnanotube/planar-SicellsbyJia etal. [ 9 86 ]Suchpassivationisalso criticalforthesiliconnanowiredevices.Iftheoxidelayerbecomest oothick,however, itpresentsatunnelingbarrierthatdegradesthecellperformanc e.Aninitiallypoor performanceofSWNT-SiNWcellstestedimmediatelyafterdeposition ofthenanotube layersuggestedthatthenativeoxidehadgrowntoothickduringth edevicefabrication steps.Accordingly,abriefBOEetchoftheSiNWsthroughtheporo usSWNTnetwork wasimplementedtostripawaytheoxide,followedbyanoxideregrowt hintheambient air.Figure 5-8 showsJ-VcurvesforaSWNT-SiNWcell(noEMI-BTI)asafunctiono f timeinambientairfollowingtheBOEetch.Theinitialmeasurementsexh ibitedverypoor 74
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performance;however,theshortcircuitcurrentdensity, J SC ,opencircuitvoltage, V OC andllfactor,FF,wereallseentoimprovewithtimeupto96hr,aft erwhichthetrends reversed.Theseriesresistance,R S ,obtainedfromtheslopeatthehighestforwardbias, wasfoundtogrowmonotonicallywhilestillbeinglowenoughat96hrtha tthenative deviceperformancewasmaximizedatthattime.Plotsofthesolarce llparameterswith increasingoxidationtimeareshowninAppendix B .Electrolytegatingwasalsofoundto beoptimizedfollowingsuch96hroxidation.Ithasbeenshownthatwa terplaysanactive roleinsiliconoxidation,soitwasreasonedthatitsexclusionbythehyd rophobicionic liquidusedduringgatingwouldavoidfurthersiliconoxideformationonc etheelectrolyte wasadded. -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 -40 -30 -20 -10 0 10 20 30 40 50 60 Current Density (mA/cm2)Voltage (V) post BOE etch 5 hrs 24 hrs 30 hrs Figure5-8.J-VofaSWNT-SiNWdeviceshowingtheeectofsidewallp assivationvia oxidationontheperformanceofthedevice. TheeectofgatingwiththeEMI-BTIelectrolyteatgatevoltageso f+1.0,0,and -1.0Vunder100 mW cm 2 ,AM1.5GilluminationareshowninFigure 5-9 .Thegatevoltage 75
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inducedmodulationoftheSWNTFermilevelrelativetothatofthenSi,modulating thebuilt-inpotential(V bi )atthejunctionisindicatedbythedramaticshiftoftheopen circuitvoltage( V OC )from0.15V(at V G =+1.0V)to0.58V(at V G =-1.0V).Atthe gatevoltageof V G =+1.0V,positiveionicchargeintheelectrolyteatthesiliconsurface attractsmajoritycarriers(electrons,inthen-Si)tothesurfac eandintothenanowires. Screenedbytheseexcessmajoritycarriersfromthepositiveionic charge,photogenerated holescanalsoapproachthesiliconsurface,resultinginanenhanced surfacerecombination. Combinedwiththesimultaneousdecreaseinthebuilt-inpotentialinth eSiNWsat theirjunctionswiththenanotubes,therecombinationlossesleadt oallfactorthatis essentiallyzero.Atthegatevoltageof V G =-1.0V,negativeionsintheelectrolyteat thesiliconsurfacerepelsthemajoritycarriers,creatinganinvers ionlayeratthesurface whichlimitssurfacerecombinationinamajorfractionoftheSiNWs.Co mbinedwiththe enhanced V bi intheSiNWattheSiNW/SWNTjunctions,thellfactorincreasesto0 .76 andmaximizesthecellperformance.The35 mA cm 2 shortcircuitcurrentdensityhereismuch greaterthanthatintheplanar,gatedSWNT/Sicells( mA cm 2 ),consistentwiththeadditional lightabsorptionduetotheverticalSiNWarray.5.3.2.3SWNTlmtransferonSiNW TwodistinctmethodsfordepositingtheSWNTlayerwereexplored:u ltrasonic sprayingfromanethanolsuspensionandtransferofapre-form edSWNTlmmade bytheltrationroute.[ 8 ]PurelysprayedSWNTlayershadtobemadesubstantially thickerthanwhatisseeninFigure 5-7B toattainlowresistancecontinuitytotheAu/Cr electrode.Inourexperience,however,photonsabsorbedinthe nanotubescontribute little,ifatall,tothepowergeneration,sothatthickernanotubelay ersdegradedcell performance.[ 10 ]Agoodcompromisewastosprayathinlayerofnanotubesfollowedb y thetransferofa10nmthickltrationfabricatedlm.Theroughlyo ptimizedquantity ofnanotubesdepositedbythecombinedmethodhadasurfacenan otubeconcentrationof 1.3 g cm 2 ,approximatelyequivalenttothatina20nmthick,entirelyltrationf ormedand 76
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-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 -40 -30 -20 -10 0 10 20 30 40 50 60 Current Density (mA/cm2)Voltage (V) VG = -1V VG = 0V VG = +1V Figure5-9.J-Vcurvesfor V G =-1.0V,0V,+1.0VontheSiNWdevice. transferredlm;however,thetransferofa20nmthicklmwithou tthesprayedlayerdid notyielddevicesthatperformedaswellasthepartlysprayed,par tlytransferredlayers. Table 5-1 attheendofthechaptercomparestheperformanceofseveral SWNT/SiNW cellsat V G =-1.0Vforwhichtheprincipledierenceswerethedepositionmethod and thicknessoftheSWNTlayer.DeviceD(J-VcurveshowninFigure 5-9 )wasthebest,for whichthepowerconversioneciencywas15.1%.Deviceswithathicke r(andhencemore lightabsorbing)netSWNTlayer,(withcorrespondinglyreducedlight transmissioninto theSi)exhibitedpoorerperformance. Toexplainthedierencesbetweenthewhollytransferredversust hepartially spraydepositedlmswenotethatthickerltrationfabricatedSWN Tlmspossesses agreatermechanicalstiness.Whensuchalmistransferredacr ossverticalnanowires withvaryingheights,thestinesslimitsthelmsabilitytoconformove rshortlength scales,preventingcontacttotheshorternanowires.Thismotiva tedtheuseofmixed 77
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sprayed/transferredlmsandisconsistentwiththedatainTable 5-1 .Indeeditisthis abilityofthenanotubestotouchandextractphotocurrentfrom virtuallyeverynanowire tip,whileprovidingadirect(non-tortuous),lowimpedancepathway tothegoldelectrode (alongwiththegateinducedinversionthatavoidssurfacerecombin ation)thatexplains thedramaticallyimprovedperformanceinthesecellsoverothersilico nnanowirebased cellsreportedtodate.5.3.3DiscussionofinversionlayerinSiNWs Theimprovementgarneredfromtheinversionlayerinplanarcellsisimp ressive, thoughpredictable;thelongdiusionlengthinsinglecrystalsiliconen suresadequate collectionofcarrierswithaninversionlayerstretchinghundredsof micronslong.The passivatingnativeoxidelayercoupledwiththeinversionlayerpreven trecombinationat thesurface.Thesiliconnanowires,however,areknowntohaveex tremelyhighratesof recombination.Indeed,thishashistoricallybeenoneofthelimitingfa ctorsineciency asthehighsurfaceareaandlongpathtotheelectroderesultinsign icantlossofcarriers. Whileoxidizingthesubstratefornumeroushourspartiallypassivate sthenanowire sidewalls,performanceisstillextremelypoorduetorecombination. Theinductionofionic liquidimmediatelyimprovesperformanceandallowscarrierstothendi usethoughtthe singlecrystalsiliconnanowiresuntiltheyreachacarbonnanotube .Thegeometriesofthe siliconnanowiresaresuchthatit'spossibletheentirenanowireisinver ted.Simulations presentedinthepreviouschaptershowedtheinversionlayerreac hingadepthofover 1 m ,whilethesiliconnanowiresareonlytensofnanometerswide.Wereth eentire nanowiretobeinvertedandthereexistednopotentialgradient,t hereisnodrivingforce torepeltheelectronfromthenanowire,eventuallyleadingtoreco mbination.Itisthus possiblethatfurtherimprovementscouldbeobtainedwiththickern anowires.Eortsto experimentallytestthisarepresentlypartofacollaborationwithth eCNMSatOakRidge NationalLabs. 78
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5.4EectofOxygenandWateronDevicePerformance Encapsulationofphotovoltaicsisuniversal;oxygen,water,andot heratmospheric contaminantsshortenthelifetimeofsuchdevices.Especiallyvulner ableareorganic solarcells,whichalsosuerfromUVlightinduceddegradation.[ 29 ]Thoughsiliconsolar cellstypicallyfunctionfordecades,deleteriousinteractionswithth eenvironmentrequire encapsulationtoensurestability.Thedevicespresentedabovesh owedsomedegradation overtimeduringcontinuedtestingwiththelabatmosphere.Experim entstopinpointthe sourceofdegradationwerecarriedout.5.4.1Eectofambientoxidation Ourinitialforayintopassivationstartedwithambientgrownoxide.T hepassivating eectofsiliconoxidehasbeenutilizedontherstSWNT-Sidevicesfa bricatedbyDr. Wadhwaandwereexploredmoreindepthduetotheresultsinthepre cedingchapter.[ 9 10 ]InvestigationsintothekinkthatisseeninbothSWNT-SicellsandMI Ssolarcells showedacomplexconnectionbetweencellperformanceandoxidet hicknessatthe SWNT-Sijunction.Immediatelyuponadditionoftheionicliquid,ahump showsup intheJ-Vcurveforallsolarcellstested.Thiswasinitiallyattributedt oamodulation oftheinterfacedipolebetweenthecarbonnanotubelmandthesilic on,butfurther experimentswiththeinversionlayercellindicatedanotherpossiblem echanism:ions fromtheionicliquidsituatedatthesiliconsurfacewithintheinterstitia lregionsbetween carbonnanotubebundlescreateaeldthateitherenhancesorco unteractsthebuiltin potential.Asthedepletionlayerdecreaseswithanincreaseinbiasvo ltage,theeect oftheseionsincreases.Furthermore,thegatevoltageaectst herelativepopulationsof cationsversusanionswithintheseinterstitialregions.Thoughthe kinkcanbecausedby themechanismsexplainedabove,thissamefeaturealsoarisesfrom asub-optimaloxide thicknessatthesilicon-oxidejunction.[ 10 87 ]Planar,fulllmsolarcellswereconstructed usingboth < 100 > and < 111 > siliconandtestedcontinuouslyforupto49hourstotest thedegradationofthecellovertime.Nogatingwasperformed;on lytheatmospherewas 79
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exposedtotheactivearea.Asshownintheplotbelow,akinkisforme dbetween7and22 hours. -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 -30 -20 -10 0 10 20 30 40 Current Density (mA/cm2)Voltage 5 min 15 min 30 min 1 hr 2 hr 7hr 22 hr 49 hr Figure5-10.EvolutionofJ-Vcurvewithoxidationinambientatmosph ere. Thekinkfeatureoccursforbothlittletonooxidation,andfortoom uchoxidation. Wehypothesizethatwhenthesiliconsurfaceisfreshlyetched,itpo ssessshallowtrap statesthatinhibitcarrierextractionandleadtoasaturationincur rentdensitywhenthe biasvoltageisclosetothebuiltinpotential.Asthebiasvoltageisincrea sed,thecarriers haveenoughenergytobeexcitedoutofthetrapsandtheJ-Vcur veexhibitsexponential behaviorfollowingtheShockleydiodeequation.Conversely,toomuc hoxideresultsin abarrier.Atroomtemperature,theoxidationrateslowsdramatic allyinanambient environmentaftertherstfewnanometersassubsequentoxida tionrequirestheoxygen todiusionthroughthealreadyformedsiliconoxidetoreachthesilico nasstatedinthe Deal-Grovemodel.Thisleadstotheformationofakinkthatprogres sivelybecomesmore severeastheoxidelayergrows. 80
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5.4.2Reversibledopinginambientenvironment In2009,Martel etal. exploredtheinabilityton-dopecarbonnanotube-silicon devices.Itwasdemonstratedthatreduction-oxidationreaction swereoccurringbetween thecarbonnanotubesandwatervaporpresentonthesubstrat esurface.Thoughthe substrate(siliconoxideinsulatinglayerontopofsilicon)itselfwasnot participatinginthe redoxcouple,itshydrophilicsurfacefacilitatedsuchreactionsthr oughadsorptionofwater. Theseredoxreactionstransferredelectronsfromthecarbonn anotubesintoH 2 O/O 2 redoxcouple,eectivelyp-dopingthecarbonnanotubes.[ 11 ]Eortston-dopethecarbon nanotubesresultedinimmediatetransferofelectronstotheredo xcouple.Replacingthe siliconoxidewithahydrophobicdielectric,suchasparylene,diminished thiseectand allowedelectronconductancewithinthecarbonnanotubes.Webelie vethissameeectis occurringinoursolarcellsduringtestinginambientconditions.Areve rsiblereductionin V OC hasbeenobservedduringcharacterizationinaninert(argon)atm osphererelativeto testingintheambientlabenvironment. AfteraBOEetchtoremovenativeoxide,thedeviceswereplacedina smallvacuum chamberthatwasevacuatedandsubsequentlylledwithargongas .J-Vcurveswere takenwhilethesubstratesatinthisenvironmentfor2hoursunder illuminationbyahigh intensityberopticlamp.TheresultingJ-Vcurveischaracteristico fadevicewithout residualwatervaporintheactiveareaandwithoutapassivatingox idelayer,i.e.thereis stillakinkduetosurfacetrapstates.TheinitialV OC islowerthanwhatisnormallyseen intheSWNT-Siplanarsolarcells,consistentwithbothMartel'sconclu sionsandalsoa lackofapassivatinglayer.Thechamberdoorwasopenedandthesu bstrateexposedto ambientatmospherefornumeroushours.TheV OC steadilyincreased,albeitataslower ratethanusualduetotheinitialexclusionofwatervapor(thusslo wingtherateofoxide growth)andsaturatedat16hours.Thechamberwasthenclosed ,pumpedout,and backlledwithargongas.AnotabledecreaseintheV OC resulted,indicatingashiftin 81
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nanotubeFermilevel.Whenthechamberwasopenedbacktoambien tatmosphere,the V OC increasedtoitspreviousvalue. -0.2 0.0 0.2 0.4 0.6 -5 -4 -3 -2 -1 0 1 2 3 4 5 Current Density (mA/cm2)Voltage (V) post BOE Argon 2 hrs Atmosphere 16 hrs Argon 3 hrs Atmosphere 2 hrs Argon overnight Figure5-11.ReversibilityoftheJ-Vcurveuponalternatingexposu retoargonand ambientatmospheres.Thelegendisorderedverticallybasedonthe orderof measurements. Thisreversiblemodulationimplicateschemicalreactionsatthesilicon/ nanotube surfaceinthepresenceofwateroroxygen.ShowninFigure 5-11 aretheJ-Vcurves forthedeviceinbothargonandambientatmospheres.Concurren twiththechangein V OC wasadevelopmentofakinkwhenwaterandoxygenwereexcluded.T hiscanbe explainedasthewaterhavingamoderateelectrolyticaectonthed evicesimilar(but moresubtle)thanwhatisseenwiththeEMI-BTI.Thoughthisredox initiallyseemstobe benecialtodeviceperformancebyincreasingtheopencircuitvolt age,itbecameapparent insubsequentexperimentsthatthemoistureadsorbedontothes iliconsurfaceoxidizesthe siliconduringelectronicgatingwiththeEMI-BTI,causingadegradat ioninperformance. 5.4.3Watervaporandoxygencontamination Electrolytegatedcellsdosueraseriousproblemanalogoustoonet hatplagued initiallyverypromisingliquidjunctionsolarcells:chemicalreactionsatt hesiliconsurface 82
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degradecellperformance.[ 88 ]Inthegatedcellssuchdegradationwasacceleratedbythe appliedgatevoltagesothatwhenheld,evenforafewminutesat V G =1.0V,theJ-V curvesbegantoexhibitanincreasingseriesresistanceanddecrea singllfactor.Such characteristicsforthedegradationsuggestedacontinuedgrow thoftheoxidelayerbetween theSWNTsandthesilicon,implyingthatwater/oxygenhadaccessto thejunctiondespite thehydrophobicityoftheelectrolyte.Thisdeleteriouseectiseve nmorepronouncedin theSiNWdevices,presumablyattributabletothehighsurfacearea availableforsuch chemicalreactions.5.4.3.1CVmeasurementsshowingILcontamination Tofullyexcludewaterfromthedevicesduringelectronicgatingande nsurethat minimalelectrochemicalreactionswereoccurringatthesiliconsurf ace,measurements conrmingthewideelectrochemicalwindowoftheEMI-BTIwerecar riedout.Although theas-received,EMI-BTIelectrolytewasalwaysstoredandsamp ledfromaninert atmosphereglovebox(argon,H 2 O,O 2 each < 0.1ppm),cyclicvoltammetrymeasurements ontheelectrolyteperformedwithintheglovebox(glassycarbonwo rking-electrode,Ag wirepseudoreference,Ptcounter-electrode)revealedanelect rochemicalwindowofonly 2.7V,greatlyreducedfromitsliteraturereportedwindowof4.4V,b utconsistentwith beingcontaminatedwithwater. "Drying"theionicliquidinvolvedusingactivatedmolecularsieves(1:1mix tureof 3Aand4A)totrapwatermolecules.Thesievesweresubmergedinth eionicliquidfor4 hours,afterwhichcyclicvoltammogramswerecarriedouttocheck theelectrochemical window.Asexpected,theCVmeasurementsshowedamarkedimpro vementafterdrying, withtheelectrochemicalwindowincreasingbyroughly600mV.Remov ingthedried EMI-BTIfromthegloveboxandrepeatingtheCVmeasurementinam bientatmosphere showedagradualshrinkingoftheelectrochemicalwindowoverthe courseofafewhours astheionicliquidbecamere-contaminatedwithwater. 83
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Figure5-12.Cyclicvoltammogramsoftheglassycarbonelectrodein EMI-BTIionicliquid at50 mV s :blacklinebeforetreatment;redlineafterdryingoverthe molecularsieves.Blackandreddottedlinesspecifytheelectrochem ical windowsoftheEMI-BTIbeforeandafterdrying,respectively. ThepeaksintheCVmeasurementindicatereductionandoxidationre actions.It's unclearexactlywhatspeciesarebeingcreatedintheionicliquid,anda pplyingvarying potentialsoutsidetheelectrochemicalwindowproducedierentco mpoundswiththeir ownassociatedredoxpotentials,leadingtochemicalreactionsinsid eoftheEMI-BTI electrochemicalwindowanddegradingdeviceperformance.Thisisa pparentintheblack curveinFigure 5-12 .Thespikeat-0.25Visanoxidationreactioncorrespondingtothe reductionthatoccurredasthevoltagewassweptbeyond-1.25V.5.4.3.2Exclusiononplanardevice Knowingthatwatercontaminationoftheionicliquidcouldleadtoelectr ochemical reactionsinsidethetheoreticalelectrochemicalwindow,weproce ededtofullygatea deviceinadry,inertatmosphere.AftertransferringaSWNTlm,a nalBOEetchwas 84
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followedbyoptimaloxidationinambientatmosphere.Toterminatefu rtheroxidation andtoevaporateresidualsurfacewater,thedevicewasplacedin totheargonglovebox whereitwasstoredfor4days.Concurrently,asampleoftheelect rolytewasthoroughly driedinanactivatedmolecularsieve.Attheendofthistimetheactive cellareawas saturatedwiththethoroughlydriedelectrolyteandJ-Vmeasurem entswereperiodically recordedunderillumination,intheglovebox,withthegatevoltageinit iallymaintained ataconstant V G =-0.75V.NodegradationinanyoftheJ-Vcharacteristicswasobse rved evenafter5hoursatthisgatevoltage.Thegatevoltagewassubs equentlyraisedto V G =-1.0Vforanadditional5hourswithstillnodegradationobserved. Thegatevoltage wasturnedoandthedeviceleftinthegloveboxovernight.Thenex tdayagatevoltage of-1.0Vwasagainapplied,withnochangeintheJ-Vcurve,asshownin Figure 5-13 Thedevicewassubsequentlymovedintothelaboratoryambientatm osphereandretested. Degradationbecamenoticeablewithinonehourofexposuretothea mbientatmosphere (at V G =1.0V),becomingprogressivelyworsewithfurtherexposure,ass hownin Figure 5-14 Theseexperimentsstronglyimplicatewaterasthesourceofthede gradation inambientatmosphereandindicatethatbyavoidingitsuchrapiddegr adationcanbe overcomeinthegatedcells. Interestingly,theV OC oftheplanardevicewashigheroutsideoftheglovebox,both beforeandafterexposuretotheinertatmosphere.Thisrevers iblebehavioristhoughtto becausedbyoxygenandwaterredoxreactionswiththecarbonna notube/siliconjunction, asdiscussedinSection 5.4.2 .Exposuretotheatmosphere,evenafterionicliquidis applied,resultsinanincreaseinV OC ,indicatingcontaminationofwaterand/oroxygenat thejunction.Thistheoryiscorroboratedbythedegradationinth eJ-Vcurveconsistent withwater/oxygencontamination. 85
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-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 -30 -20 -10 0 10 20 30 40 Current Density (mA/cm2)Voltage (V) -1V 1hr 2hr 3hr 4hr 5hr next day Figure5-13.Stabilityofplanardevicewithoxygenandwaterexclude dbygatingininert atmospherewithV G =-1.0V. 5.5ConcludingRemarks Whilemetalassistedchemicaletchingofnanowiresisasimple,lowcost procedure, theresultantnanowiresareratherfragile.Theirsmalldiameterno tonlymakesthem susceptibletobreakageoragglomeration,butpossiblyallowscomple teinversionunder electronicgating.CurrentcollaborationswithOakRidgeNationalLa boratoryaimto producesiliconnanowiredeviceswithcontrolledpitchesanddiameter soptimizedfor opticalabsorption.Thiscontrolledgrowthshouldproducerobust ,uniformnanowiresthat resistagglomeration.Theabilitytorandomlyspacethenanowiresinh ibitsdiractionand furtherincreasestheopticalabsorptionanduniformheightsmigh teliminatetheneedfora sprayedSWNTlm. Producingarecordhigh15.1%siliconnanowiredeviceisamajoradvanc eover theprevious10%,butonlypracticaliftheperformanceisstable.T hepropensityfor recombinationatthenanowiresidewallsinhibitcarriercollectionwhileth ehighsurface areaandseveretopologyexacerbateanychemicalreactionswith theionicliquid.Locally 86
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-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 -30 -20 -10 0 10 20 30 40 Current Density (mA/cm2)Voltage (V) Post glovebox 1hr 3hrs 5hrs Next day Figure5-14.DegradationoftheplanarSWNT-SiNWdeviceuponexpo suretoatmosphere withV G =-1.0V. strongelectriceldscancatalyzelocalredoxreactions,producin gvariedchemicals deleterioustothedevice.Proofofstabilitywithwaterexclusionwas acriticalrststepin provingtheviabilityofthesedevices,butimprovedpassivationisalso imperativetoavoid longtermdegradation. 87
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Table5-1.Performanceforvariouslmdepositiontechniquesandt hicknesses DeviceV OC (V)J SC mA cm 2 FFEciency(%)Notes A0.5832.50.7413.920nmtransferredB0.5832.00.7313.55nmsprayed/25nmtransferredC0.5832.50.7113.245nmtransferredD0.5835.00.7615.110nmsprayed/10nmtransferred 88
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CHAPTER6 PASSIVATIONOFSILICON Theneedforexcellentpassivationofsemiconductorshasbeenext ensivelyresearched inregardstobothMISandP-Njunctiondevices.Inherenttop-nh omojunctionsis anearperfectboundarywhenthejunctionisformedbyhightempe raturediusion onacrystallinesubstrate.HeterojunctionsandSchottkyjunct ionssuerfromabrupt boundariesthatfacilitaterecombinationthroughdefectsandmidg aptrapstates.As such,mucheorthasbeenputintovariouschemicalandmechanica lmethodsaimedat creatingdefectfreesurfacesandjunctions.[ 32 89 { 91 ]Passivatingsiliconwithaninsulating layer,suchassiliconnitride,isafacilemethodtoreducerecombinatio n,butoftenrequires thicknessesinhibitingchargetransportacrossthejunction.[ 92 ]Thechallengewiththe siliconnanowiredevicesistopassivatethetipsinsuchawaythatholes canstillbe transferredtothecarbonnanotubelm,whilestillprovidingrobus tpassivationalong thenanowiresidewallstolimitrecombination.Limitationsonthicknessc onstrainthe passivationlayerbetweentheSWNTsandSiNWtobelessthanacouple nanometers, whilethepassivationofthesidewallsmustnotinterferewiththeionicliq uidinduced inversionlayer. Ourexperimentshaveshownincreaseddegradationwithinthenano wiredevices ascomparedtotheplanardevicesuponelectronicgating.Theplana rdevices,after beingheldatagatevoltageof-0.75Vfor20minutes,showedasmalld ecreaseinpower eciency:10.12%to10.06%,a0.6%decrease.Thenanowirebasedde vicedegradedfrom 15.1%to14.4%duringthesameperiodatagatevoltageof-1.0V,anov eralldecrease of4.6%.Theincreaseinsurfaceareasimultaneouslycreatesmores itesforrecombination ofphotogeneratedcarriersandadverseelectrochemicalreact ionsbetweenthesiliconand ionicliquid.Excludingwaterfromthejunctionmitigatesthelatter,bu tprovidingahigh qualitypassivationlayercouldhelpalleviatebothproblems.Presente dbelowareseveral methodsofpassivationachievedthroughchemicaltreatmentsan datomiclayerdeposition. 89
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Figure6-1.ALDgrowthprocess.1.Hydroxideterminatedsubstr ateisexposedto reactantTrimethylaluminum,whichchemiadsorbsontothesubstra te, producingmethaneasabyproduct.2.Systemispurged,removing unreacted precursorsandchemicalbyproducts.3.Oxidant(watervapor) isfedintothe system,whereitreactswiththemethylligand,formingOHgroups. 4.System isagainpurgedofexcessspecies,andtheprocessrepeats. 6.1AtomicLayerDepositionofAl 2 O 3 andHfO Onemeanstoavoidambientwateradsorptionistoencapsulatethec ellsinaninert atmosphereasmustpresentlybedoneforotherwater/oxygens ensitivesystems(e.g. organicsolarcells).Alternatively,athindielectricbarrierlayercoat ingthenanotubes andSiNWsatthejunctionsmaybesucienttopreventoxidationdue tothewater entrainedintheelectrolyte.Inanattempttocreatesuchabarrie rweturnedto atomiclayerdeposition(ALD)ofAl 2 O 3 .ALDdepositedaluminumoxidehasreceived increasinginterestasasiliconsurfacepassivationlayersincetheav ailabilityofcommercial ALDsystems.[ 93 ]Thelayerbylayerdepositionofvaporphasereactants(sequent ially trimethylaluminumandwater)impliesaconformalcoatingeventhrou ghthepre-deposited nanotubelayer. ALDisaselflimitingprocessthatdepositsamonolayerbymonolayerco nformal coatingofbyalternatingreactantgasses.Inbetweeneachreac tantgas,thechamberis purgedtovacateanyreactantsnotchemisorbedontothesubst rate.Boththermaland 90
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plasma/ozoneALDformconformallayers,thoughtheuseofunst ableandreactivespecies inthelatteroftencreatesmalldefectsinthecoatingandisonlyutiliz edforsubstrates thataresensitivetohightemperatures.[ 94 ]Exposuretoozoneforevenacoupleminutes causesextremesidewalldamagetothenanotubes,resultinginasu bstantiallossof conductivity.Consequently,eortsweremadetoavoidplasmawhile stillusingarelatively lowdepositiontemperatures.Theacidpuriednanotubesweusear ep-dopedbyitwhich increasesthebuilt-inpotentialagainstn-typesiliconsolowtempera tureswerepreferredto preventnanotubededoping. Figure 6-1 showsthefourmainphasesofALDAluminumOxidegrowth.Onceloade d intotheALDsystemandcontainedwithinaninertenvironment,the rstreactantgas, trimethylaluminum(TMA),isfedintothechamber.Reactionswiththe (OH) onthe substrateresultinchemiadsorbtionofmethlyterminatedaluminum( bondedtooxygen), creatingmethanegasasabyproduct.Apurgeremovesanyunrea ctedTMAalong withthemethane.Watervaporisthenfedintothechamberwhereit reactswiththe methylterminatedaluminum,releasingmethaneandresultinginahyd roxideterminated aluminum.Anotherpurgeremovestheexcesswaterandmethane, leavingthesubstrate inasimilarchemicalstateasitwasintherststep.Theprocessthen repeats,eachtime growingasinglelayerofaluminumoxide. PriortogrowthoftheAl 2 O 3 theSWNT/SiNWdeviceunderwentanalBOE etch,followedbyoxidationinambientforthetimethatoptimizeddevic eperformance (96hr).Al 2 O 3 wasgrownfor110reactantcyclesatasubstratetemperatureo f80 C.[ 95 ]Ellipsometryperformedonsuchalmdepositedonaratsiliconchipun derthese conditionsgavealmthicknessof8.8nm(coverageintherstsever alcyclesisincomplete sotheinitialgrowthdoesnotgivecompletelayers).Figure 6-2 showsanSEMimageof theAl 2 O 3 coateddevice.Brightspotsintheimageareenhancedsecondarye missionfrom wheretheSiNWtipsunderliethedielectriccoatednanotubes. 91
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Figure6-2.SEMimageofALDdeposition.ALDAl 2 O 3 onSWNT-SiNWdevice.Bright spotsareenhancedsecondaryemissionfromthetipsoftheunder lyingSiNW tips. Measurementofparasiticgatecurrentscanquantifythereactio nsoccurringat theSisurface,someportionofwhichshouldcorrespondtodelete riousredoxreactions (otherelectrolyteorimpurityreactionsthatdon'tdegradetheSW NT/Siinterfacecan alsooccur).ForSWNT-SiNWdeviceswithouttheALDdielectriccoatin g,thesteady stategatecurrentatV G =-1.0Vinthenon-driedelectrolytewastypically2.7 A .For thedielectriccoateddevicethiswasreducedbyafactorof60to45 nA.Unfortunately, thiswasstillafactorof110greaterthatobservedforthedevice measuredintheglove boxusingthedriedelectrolyteforwhichthegatecurrentwas0.4nA ,andwhilethe rateofdegradationofthecoateddevicewasgreatlyreducedove rtheuncoateddevice, degradationwasevidentoverthecourseofseveralhours(meas uredintheambient labatmosphereinthenon-driedEMI-BTIelectrolyte).Thisimpliesth attheALD layerremainspermeabletowateratthethicknessused.Athickerla yermayprevent this,althoughahydrophobiccoating(e.g.Parylene)maybeprefer redtothenaturally hydrophilicoxideinsuchanapplication. 92
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6.1.1Al 2 O 3 andHfOresults Figure 6-3 showstheJ-Vcurvesforthedevicebeforeandafterelectrolyte addition atgatevoltagesforthelatterofzeroand-1.0V.AtV G =-1.0Vtheopencircuitvoltage, shortcircuitcurrentdensityandllfactorwere,V OC =0.62V,J SC =33.4 mA cm 2 ,FF= 0.73resultinginaPCEof14.8%.TheslightlylowerJ SC andPCEovertheuncoated deviceislikelyduetoanincreasedlightscatteringduetotheAl 2 O 3 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 -40 -30 -20 -10 0 10 20 30 40 Current Density (mA/cm2) pre EMI-BTI V G = 0 V V G = -1 V Figure6-3.J-VcurvesfortheALDAl 2 O 3 coatedSWNT/SiNWcell.Withoutelectrolyte (red)andwiththeelectrolyteattheindicatedgatevoltages(black ,blue).The enhancedphotocurrentonadditionoftheelectrolyteisattribute dtorefractive indexmatchingreducingthescattering. TheALDresultsdemonstratedgoodpassivationoftheSiNWs.Theq uantitative reductioninthegatecurrentindicatingreducedredoxoccurringa tthesilicon/nanotube junctioniscorroboratedbythedecreaseddegradationoftheJVcurve,shownin Figure 6-4 .Thedecreaseinperformanceforanon-ALDSiNWdeviceisshownfo r comparison,albeitthemeasurementsaretakenfordierenttimes (48hoursvs72hours), 93
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-0.5 0.0 0.5 -40 -30 -20 -10 0 10 20 30 40 No ALD -0.75V initial No ALD -0.75V 72hrs No ALD -0.75V 72hrs hold for 20min ALD -0.75V initial ALD -0.75V 48hrs ALD -0.75V 48hrs hold for 60minJsc (mA/cm^2)Voltage (V) Figure6-4.J-VcurvesforALDSWNT-SiNWdevicevsdevicewithoutA LD.Includedis thedegradationoccurringbothwithappliedgatevoltageandtimesp entin ambientwithoutthegatevoltageapplied.The"NoALD-0.75V72hour s"and "ALD-0.75V48hours"graphsindicatethedevicewassittinginambien t withoutthegatevoltageon(butionicliquidstillinplace)forthespeci ed amountoftime. soanexactcomparisonisnotprovided.provided.TheAl 2 2O 3 3coatingclearlyhelpsbut itisclearlyalsonotsucienttoovercometheelectrochemicaldegra dation.Whileitis possiblethatmorewaterhaddiusedintothedeviceat72hours,an dhencecouldexhibit ahigherdegradationduetoenhancedredox,wedonotthinkthatis theprimaryfactorfor theincreaseindegradationrelativetotheALDdevice. NotingthatHafniumOxide(HfO)possessesadielectricconstanttw icethatofAl 2 O 3 wealsodepositeda25nmthickADHfOlayerontoSWNT-SiNWsubstrat es(already oxidizedfor96hoursinthelabatmosphere).Itwasreasonedthat thethickerlayer wouldfurtherreduceelectrolyteaccesstothesiliconsurfacewhile stillprovidingthe samecapacitanceastheAl 2 O 3 .Devicestability,specicallygatecurrents,weremarkedly 94
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-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 -40 -30 -20 -10 0 10 20 30 40 50 60 Current Density (mA/cm 2 )Voltage (V) 25 nm HfO ALD 8.8 nm Al2O3 Figure6-5.J-VfortheALDHfOdeviceshowingaloweringJ SC duetothehigh rerectanceofthedevice. smallerthanforthe8.8nmAl 2 O 3 .ThegatecurrentatV G =-1.0Vwas13nA,as opposedtothe60nAforthethinneraluminumoxideALDlayer,indicat ingthatathicker ALDinhibitsredoxreactionsbylimitingcontactbetweentheionicliquida ndthesilicon surface.ThethickerALDlayerwasn'twithoutdrawbacks;J SC wasonly30 mA cm 2 duetothe higherscatteringinducedbythethickerHfOlayer,whichhadanass ociatedhazevisibleto thenakedeye.ReduceddegradationintheJ-VcurveoftheHfOde vicewasquantiable -After30minutesofcontinuouslyheldgatevoltageat-1V,thered uctioninPCEwas measuredtobe0.98%,comparedtoadegradationof2.1%forthe8.8 nmAl 2 O 3 ,which washeldatalowergatevoltageof-0.75V. 1 1 Someofthemeasurementsmadeondierentdeviceswerenotident ical,leadingtodiscrepancieswhen tryingtocomparedegradation.Carewastakentotryandcompar eresultsundersimilargatingconditions.Earlydevices,unfortunately,werenotasextensivelytest edaslaterdevices,resultinginlessdatafor comparison. 95
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ToassesstheabilityoftheALDlayertoprotectthesiliconfromredo xreactions withtheionicliquidandtofullyevaluatetheeectofthedepositeddiele ctric,wealso depositedAl 2 O 3 ontoaplanarSWNT-Sidevice.Asforthepreviousdevices,thesola r cellwasallowedtooxidizeintheambientatmospherefor2hoursprior toALDgrowth, whichwasfoundtobetheoptimumtimeforplanardevices.Allsolarce llparameters improvedinthegatedALDdeviceascomparedtonon-ALDdevices.M ostnotableisthe increaseinJ SC bothwithandwithoutionicliquid,indicatingtheALDlayeractsasan indexmatchinglayerthatreducesrerection.Witharefractiveinde xof1.55[ 96 ],theAl 2 O 3 iswellbelowtherefractiveindexofsilicon,n=3.96,andsimilartorefra ctiveindexofthe ionicliquid,n=1.42. 2 DataareshowninTable 6.4 attheendofthechapter. TheALDdevicesclearlyexhibitsuperiorperformancerelativetothe nonALD devices,thoughthereisstillsomedegradation.Currenteortsa reaimedattestingthe ALDdevicesinthegloveboxwiththedriedEMI-BTI.Completeexclusio nofwater inconjunctionwithhighqualitypassivationshouldaordexcellentmea nstoavoidall sourcesofdegradation.Despitethepromisingresultsofincorpor atingALDintothe device,wealsoexploredothermethodsofproducingthesamestab ility.Photovoltaics mustbestableinadditiontoinexpensiveinordertocompeteontheop enmarket, motivatingasearchtondafacile,scale-able,andinexpensivemean stoprotectthe modulesfromdegradation. 6.2Hydroquinone Hydroquinone,anorganiccompoundcommonlyusedasableachingag entand photographicdeveloper,hasrecentlybeenshowntoproduceidea lSchottkyjunctions whenusedtotreattoasiliconsubstrate.[ 97 ]Moleculesattachedtothedanglingsilicon bondscreateasurfacedipole,changingtheeectiveelectronan ityofthesemiconductor. Throughuseofdierentadditives,thelengthofthedipolecanbealt ered,leadingtoa 2 TakenfromTCIChemicalswebsite:http://www.tcichemicals.com/esh op/en/us/commodity/E0599/ 96
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ASiliconsurfaceshowingdangingbondsafterremovalofoxidelayer. BHydroquinonemoleculewithattachedmethoxy(CH 3 ) group,formingasurfacedipoleandsimultaneouslypassivatingthesurface. Figure6-6.Siliconsubstrateandhydroquinonemolecule.Reprinted withpermissionfrom R.Har-Lavan, etal. ,AIPAdvances2,012164(2012). tunableeectiveelectronanityandsubsequentchangeinSchott kybarrierheight.Inthe paperbyHar-Lavan, etal. ,itwasshownthatmethanolyieldsthatlargestnegativedipole, yieldinganeectiveelectronanityof3.05eV.Thiscreatedabarrier heightofnearly1 eV,remarkableinthatitiscomparabletothebandgapofsilicon.Duet othedipole,the siliconatthejunctionbecamestronglyinverted,leadingtocarriert ransportdominated byminoritycarriergenerationandrecombination.Mostimportantf orourpurposes,the surfaceofthesiliconwaspassivated,withstabilityshownoverseve raldays(asopposedto hydrogenterminationwhichisstableuptoafewhoursinambient). ThepreparationwasmodiedslightlyfromtheproceduredonebyHa r-Lavan, etal. ASWNT-Sidevicewaspreparedfollowingthestandardprocedure, nishingwithanal BOEetchtoremoveanynativeoxidegrowthandleavingthesiliconhyd rogenterminated atthesurface.Thesubstratewasplacedina0.01MHydroquinonein methanolsolution for3hoursinadark,ambientenvironment.Theoriginalpapercalled forsonication inethylacetate,buttheviolenceofsuchaprocesswouldhavedam agedthecarbon nanotubelm.Treatingthesubstratepriortotransferringthec arbonnanotubelm produceddeviceswithahigherseriesresistanceandpoorllfacto r,indicatingadverse reactionsbetweenthehydroquinoneandsolventsusedinthetran sferprocess.Assuch, 97
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HQtreatmentsweredoneafterthelmhadbeentransferred,af terwhichthesubstrate wasdippedintoboilingdichloromethanefor30secondstoremoveany excessmolecules notrmlyadsorbedontothesurface.Thiswasfollowedbydryingina nN 2 streamand constructionofthebacksidecontact. ShowninFigure 6-7 aretheJ-Vcurvesforthehydroquinonedevice.Initial performanceiscomparabletothatachievedwithoutHQtreatment ,withaV OC of 0.56V,FF=0.74,J SC =23.6 mA cm 2 ,andPCEof9.73%.AdditionofEMI-BTItotheactive arearesultsinimmediatedegradation,withsevere,irreversibledeg radationafterelectronic gating,indicatingthatHQpassivationisnotcompatiblewithinclusionof ionicliquid. GatedresultsforV G =-0.75VareV OC of0.52V,FF=0.21,J SC =8.6 mA cm 2 ,andPCE of0.93%.Thoughthepreviousexperimentsonwaterandoxygenex clusionindicatethat theproblemcouldbeattributedtoadditionalchemicalinteractions betweentheionic liquidandwaterorthehydroquinoneandwater,ultimatelyformingse veralcompounds deleterioustodevicestability.Stabilitymeasurementsofthehydro quinonepassivated deviceswithoutelectronicgatingarepresentlyunderinvestigation 6.3Sulfur SulfurhasbeenusedtosuccessfullypassivateGaAs < 100 > andInPsubstratesandin 2007itwasshownthatsulfurprovidesanexcellentpassivationfor < 100 > silicon,resulting innearidealSchottkybarrierswithbothlowandhighworkfunctionm etals.[ 98 ]Though theadherenceofaforeignmoleculetodanglingsiliconbondsformsan interfacedipole, experimentsshowtheeectofthisinterfacedipoleontheSBHtobe negligiblerelative tothereducedFermilevelpinningachievedthroughareductionof surfacestates.With anelectronegativityof2.58,sulfurattractselectronswhencova lentlybondedtosilicon (electronegativityof1.9),[ 99 ]resultinginasurfacedipoleinoppositiontothebuiltin potentialinthesiliconattheSWNT-Sijunction.Nonetheless,thep assivationaordedby thischemicaltreatmentreducesthesurfacerecombinationveloc ityandsubsequentlythe 98
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-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 -40 -30 -20 -10 0 10 20 30 40 50 60 Current Density (mA/cm 2 )Voltage (V) pre EMI-BTI 0V -0.75V 0V post gating Figure6-7.J-VcurveoftheHQtreatedplanarcellbefore,during ,andafterelectronic gatingwithEMI-BTI. darkcurrent,negatinganydecreaseintheV bi andleavingV OC largelyunaected,shown inFigure 6-8A EarlyexperimentswithsulfurpassivationfollowingtheprocedureinA li etal. resultedinpoorperformanceduetolowFFandhighR S .Inlightofresearchconducted byAibin, etal. ,weconcludedthatthetimespentinthesulfursolutionallowedgrowt h oflargecrystallites,therebycontaminatingthesiliconsurfacewith largeparticulatesand impedingchargecollection.Shorteningthetimeinsolutionbyafactor of4produceda uniformmonolayerofsulfur,passivatingthesurfacewithoutcrea tingaphysicalbarrierto holeextraction.[ 100 ] SimilarlytotheHQexperiments,sulfurpassivationwasdonebothprio rtoandafter SWNTlmtransfer,withoptimalresultsfortreatmentafterlmtr ansfer.Afterthe prepareddevicesunderwentanalBOEetchtoremovenativeoxid e,theywereplacedin 99
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a0.33M/2.4MNH 4 ) 2 S/NH 4 OHsolutionat60Cfor5minutes.Thiswasfollowedbytwo DIbathsof5minuteseach,andanN 2 dry.J-Vcurvesweretakenimmediatelyafterthe backcontacthadbeenassembled.Initialresultsshowedexcellent performancerelative tounpassivateddevices.AdditionofEMI-BTIandsubsequentgat ingindicatednegative interactionsofthesulfurwiththeionicliquid.Initialgatingcurveswe reexcellent,but continuedapplicationofthegatevoltageresultedinirreversibledeg radation,asshownin Figure 6-8A Alikelyculpritinthepoorperformanceisthepresenceofwater,whic hwasshownin Chapter4todegradedeviceperformance.Furtherexperiments performedinaglovebox usingdriedionicliquidwillbeusedtocheckifthisiscorrect. 6.4DiscussionandSummary Giventhatencapsulationorabetteroptimizedpassivationlayersho uldpreventthe electrochemicaldegradation,weconsiderotheraspectsofourp resentdevicesthatcould limittheirperformance(suggestingmeanstoincreasetheirPCEsbe yondthepresent 15%).Onelimitationconcernsexcessrecombinationatthebackcon tact.Ithaslongbeen knownthatabacksurfaceeldinducedbyappropriatedopingofth eSiatthedeviceback contactcanreducerecombinationthere,withcorrespondingimpr ovementsinthedevice performance.[ 101 ]AnotherfactoralsolimitingthePCEinourpresentdesignistheir geometry.InthepresentconstructiontheSiwaferthickness( 550 m )islargerelative totheactiveareawidth(2mm)meaningthatphotocarrierscreate dneartheedgeof theactivewindowregionhaveanappreciablecross-sectionforesc apeoutthesidesof thatregion,thuscontributingtothelosses.Capturingthosecar rierscouldsignicantly boostthedevicePCE.QuiterecentlyPCEscomparabletowhatwede monstratehere wereobtainedfromplanar,chemicallychargetransferdopednano tube/Sisolarcells exploitingaTiO 2 antirerectioncoating.[ 102 ]Thebroad-bandrerectanceduetothat coatingwasnotaslowasourverticalNWarrays,exhibitingaminimumo f5%at600 nm,butrisingsmoothlyoneithersideoftheminimumtoover10%at500 nmand800 100
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-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 -40 -30 -20 -10 0 10 20 30 40 50 60 Current Density (mA/cm 2 )Voltage (V) Post BOE etch to backside 4 hours in ambient EMI-BTI -0.75V EMI-BTI -0.75V 20 minutes APerformanceofthesulfurpassivateddevice(postSWNTlmtran sfer) -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 -40 -30 -20 -10 0 10 20 30 40 50 60 Current Density (mA/cm2)Voltage (V) HQ pre EMI-BTI HQ at -0.75V S pre EMI-BTI S at -0.75V BHydroquinoneversusSulfurpassivationuponelectronicgating. Figure6-8.J-Vcurvesforsulfurandhydroquinonepassivatedde vices. 101
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nm,respectivelyandtoover20%attheextremesoftherelevants olarspectrum(400-1100 nm).Thecomparableperformance,despiteourreducedrerecta nce,suggeststhattheir devicesexhibitedlowerlosseswhichcouldbeduetotheirmoreoptimize dgeometry. Theyusedthinner(400 m )wafersandlargeractivecellarea(15mm 2 vs.our8mm 2 ) consistentwithareducedcarrierleakageoutthesidesoftheiract iveregion.Combingthe resultspresentedherewithfuturestrategiesforfurtherimpro vementbodewellforrapid advancementofnanotube-silicondevicessincetheirinitialdevelopm entin2007.[ 60 ] 102
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Table6-1.PerformanceforALDdevicesforV G =-1.0V DeviceALDtypeandthicknessV oc (V)J sc mA cm 2 FFEciency(%) SiNW8.8nmAl 2 O 3 0.6233.20.7314.84 SiNW25nmHfO0.5829.70.729.74Planar8.8nmAl 2 O 3 0.6280.813.45 103
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CHAPTER7 ADDITIONALPROJECTS 7.1TFSADopingofGraphene-SiandCarbonNanotube-SiDevic es Graphenehaslongbeenusedasanelectrodefororganicphotovolt aics,[ 103 104 ] butonlyinthepastcoupleyearshasitbeendemonstratedinSchott kyjunctionsolar cells.[ 105 106 ]TherstgenerationofdevicesachievedaPCEofapproximately1.7 %, astheaspreparedgrapheneintroducedhighlevelsofresistancein tothedevicewhich reducedboththeFFandJ SC whileincreaseR S .Nonetheless,wedecidedtointegrate grapheneintoourarchitectureinanattempttomitigatethedelete riouseectsofexcessive oxidegrowthatthejunctionandtolimitredoxreactionsbetweenth esiliconandionic liquid.Withit'shexagonallyclosepackedcarbonatomsandrelativelylow permeability ofwatervaporandoxygen,thegraphenelatticeissuitableforhind eringsiliconoxidation atthesurfaceoftheactivearea.Thisshouldstabilizetheseriesre sistanceandhinderthe formationofthekinkthataccompaniesanoxideinducedbarrier.Un lesshighlydoped,a singlemonolayerofgrapheneistooresistiveactasanelectrode,re sultinginthelowPCE reportedabove.Additionally,agraphene-CNThybriddevicecouldo erthebestofboth worlds:retentionoftheoptimaloxidethicknessandalowseriesres istance.Additionally, graphenewouldbeabletoscreenthesiliconsurfacefromtheionsinio nicliquid,avoiding redoxreactionsandleadingtomorestableperformancewithelectr onicgating. 7.1.1Graphene-SiSolarCells Thefabricationprocessforthegraphene-Sidevicewasmodieds lightlydueto theimpermeabilityofthegraphene;oxygenandwatervaporcanno teasilypenetrate thelatticetoreadilyoxidizethesiliconsurface.TheSWNTdevicesund erwentanal BOEetchtoremovenativeoxide,followedbyoxidationinambientatmo spherefor2 hoursuntiltheJVcurveexhibitedstable,maximumperformance.T hisprocessisnot practicalforgraphenedevices,astheBOEcannotpermeatethe graphenetoetchthe underlyingoxide(thoughatearinthegraphenecouldallowetchingof theunderlying 104
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oxide,reactionproductscouldnotdiuseout).Ourmodiedfabric ationprocessinvolved BOEetchingthegoldframedactiveareawindowandexposingthesub stratetothe ambientatmospherefor2hourstofacilitateoxidegrowthbeforet ransferofthegraphene. Adropofisopropanol(IPA)wasplacedonthewindowandaPMMAsup portedgraphene sheet,synthesizedandprovidedbyDr.MaxLemaitreandDr.ArtH ebard,wasplaced ontopoftheIPA(graphenesideagainstthesilicon).Pressuretoa dherethegraphene tothesiliconwassuppliedbyaluminumblocks.After4hoursatroomte mperature,the IPAhadevaporatedandthegraphenewasheldtightlytothesubst rateviaVanderWalls forces.Anacetonevaporbathfollowedbyseveralacetonesoak sremovedthePMMA membrane.Fabricationofthebackcontactandcharacterization proceededfollowing standardprocedures. Asexpected,thedeviceexhibitedahigherseriesresistancerelativ etothecarbon nanotubedevices.Thoughgraphenehashighmobilityitscarrierden sityislowresulting inaresistivitythatmakesforahighseriesresistance.Priortoaddit ionoftheEMI-BTI, PCEwas1.9%.AdditionofEMI-BTIandsubsequentgatingat-0.75Vin creasedthe eciencyto4.6%,anoverallincreaseofover140%.Whilethishugeimp rovement indicatesfacilemanipulationofgraphene'selectronicproperties,th eperformancewas severalfactorsbelowthatofthecarbonnanotube-silicondevice s.Wehypothesized thatahybrid-graphene-SWNTdevicewouldexhibitbetterperform ancethougha conruencestabilityfromthegraphenelayerandimprovedconduct ivityfromthecarbon nanotubelayer.Unfortunatelytheresultsforthehybriddevicew ereworsethanforthe graphene-onlydevice,presumablyduetosiliconsurfacestatesfo rmedduringtheexcessive amountofprocessinginvolvedintransferringgrapheneandSWNTs .TheJVcurvesare showninFigure 7-1 andcellcharacteristicsshowninlineoneofTable 7-1 IncollaborationwithDr.XiaochangMiaoandDr.SeattinTongay,ag raphene-Si solarcellwasfabricatedandthendopedwithTriruoromethanesulf onyl-amide(TFSA, (CF 3 SO 2 ) 2 NH).Extensiveresearchondopantsforgraphenehasbeendone thepastseveral 105
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-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 -40 -30 -20 -10 0 10 20 30 40 50 60 Current Density (mA/cm 2 )Voltage (V) pre EMI-BTI VG = 0 V VG = -0.75V Figure7-1.JVcurveforthemonolayergraphenedevice. years,withTFSA(alternatively,TFSI)provingtobehighlyp-doping andstabledue toitshydrophobicnature.[ 107 ]Graphenedeviceswerefabricatedfollowingthemethod outlinedabove,withPCEsof1.5-3%.A20mMsolutionofTFSAinnitrome thanewas spuncoatedontothegrapheneat1000-1500RPMfor1minute,af terwhichthedevices werecharacterized.Immediateimprovementoccurredforallsola rcellparameters,as summarizedinTable 7-1 attheendofthechapterandshowninFigure 7-2B .ThePCE jumpedto8.6%duetoadramaticincreaseinJ SC ,V OC ,andFF.Thestrongcharge transferdopingachievedwiththeTFSAprovedtobefairlystablefo rthegraphenedevice, resultinginnegligibleeciencydecreasesoverseveraldays.[ 106 108 ] TheTFSAdopingincreasedtheSchottkybarrierheightbyapproxim ately0.11eV, from0.79eVto0.89eV,resultinginanincreaseofthebuiltinpotential andareduction ofdarkcurrentduetoreducedrecombinationatthejunction.Ad ditionally,theexternal quantumeciencyincreasedbyroughly30%overvisiblewavelengths ,indicatingamore 106
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AArchitectureofthegraphenecell.IdenticaltotheSWNT-Siplanardeviceswithouttheonchipgatelm. BJ-VcurvesshowingeectofTFSAdoping Figure7-2.SchematicandperformanceforgraphenePVcell. ecientcollectionofcarriersmostlikelyattributedtotheincreased builtinpotential.The thinlayerofTFSApotentiallyactsasanantirerectionlayer,asdark eningoftheactive areawindowpostTFSAtreatmentwasnoticeable.7.1.2TFSAwithcarbonnanotubes GiventheexcellentresultswiththeTFSAdopedgraphenedevice,we setoutto incorporatethesametreatmentforthecarbonnanotubelms.T houghouras-puried SWNTsarealreadyp-doped,theTFSAprovidesahigherlevelofdop ingasevidenced bytheincreasedV OC andsmallerR S .Followingthestandardprocedureforfabricating SWNT-Sidevices,thedevicewasoxidizedintheambientatmosphere for2hoursuntil theJVcurverevealedstableperformance,afterwhichtheTFSAw asspuncoatedonto thedevice.Immediatetestingrevealedenhancedperformancere lativetotheundoped andungatedperformance.Unlikethegraphenedevice,theTFSAd opingoftheSWNTs appearedtobetransitoryandaslowdegradationinallparameters wasnoticedover time.Whilewedonotknowtheexactcauseofthediscrepancybetwe enthestability ofthegraphenedevicesvs.thecarbonnanotubedevices,webelie veitisattributedto thereduced,butstillnite,accessofatmosphericwatertothelin econtactbetweenthe 107
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nanotubesandthesilicon,permittingoxidationthere.TheTFSAdop antwasshownto behighlyincompatiblewiththeionicliquid(asexpectedduetotheirchem icalreactivity), andassuchthismethodofdopingwouldonlybeviableforungatedenc apsulateddevices. ShowninFigure 7-3 aretheJVcurvesfortheTFSAdopedcarbonnanotubedevice. -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 -40 -30 -20 -10 0 10 20 30 40 50 60 Current Density (mA/cm 2 )Voltage (V) Post BOE etch and oxidation Post TFSA TFSA at 50 min EMI-BTI V G = -0.75V Figure7-3.J-VcurvesshowingeectofTFSAdopingandsubseque ntgatingonSWNT-Si device. 7.2BacksideDoping Severalexperimentswereperformedattemptingbacksidedoping ofthesilicon substratestocreateabacksurfaceeldthere.Thisstronglydo pedregionhasbeenused extensivelyinp-njunctionsolarcellstoreducerecombination.[ 109 ]Ann + -njunction exhibitssimilarpropertiesofap-njunction:inbothcasescarriersd iusefromoneregion toanotheruntilelectrostaticequilibriumisreached.Theresultinge lectriceldserves tosweepcarriersofoppositechargesinoppositedirections.When employedonthe backsideofphotovoltaiccells,thissecondarybuiltinpotentialserv estoreducecarrier 108
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recombinationatthebackcontact.Thedopingprocedureisfairlys traightforward:a phosphorusspinondopant(FilmtronixP509)isspuncoatedat2500 RPMontothe backsideofthesiliconsubstrateandbakedat1150Cinambientatmo spherefor2 hours(rampingat1degreeCperminute).Thehightemperatureallo wsthedopantto diuseintothesilicon,creatingashallow,gradatedregionofn + doping.Preliminary testingyieldedasheetresistanceof1.5-3ohm/squareontheback sideofthewafers (versus20-60ohm/squarefortheundopedwafers)andamodes tincreaseinPCEwas obtained.Thewafersusedwerealsothinnedto250 mbeforedopingthebackside,a modicationthatshouldminimizerecombinationbyreducingthecross -sectionforcarrier leakageoutthesidesoftheactivearea.Additionally,thediusionlen gthofelectronsin lightlydopedn-typesiliconisontheorderofhundredsofmicrons(de pendingonsilicon grade,growthparameters,anddopingdensity)soreducingthep athoftravelshould resultinanincreasedphotocurrent.Thisimprovementincomesata price:slightlyless photoconversionfromphotonsnearthebandgapofsilicon.Theab sorptionlengthof siliconforlightwithenergy1.1eVisapproximately6.6mm,withhigherene rgyphotons havingashorterabsorptionlength.[ 110 ]However,theirradianceofinfraredphotonsinthe solarspectrumisrelativelylow,sothenetchangeinphotonsabsorb edwithinthesiliconis onlymarginallyaected. ThehigherV OC isattributedtoareductioninrecombination,whichsubsequently lowersthedarkcurrentandincreasestheopencircuitvoltage.Ad ditionally,less recombinationofelectronholepairsgeneratednearthebacksidein creasedJ SC ,asshown inTable 7-2 attheendofthechapter.Thoughthismethodwashighlysuccessf ulat increasingthePCE,theprocesswasonlyviableonsiliconwithathick( > 1 m)oxide layer.Thehightemperaturebakeformedblistersonthesurfaceo fthesubstrate,creating shortswithinthedevice,showninFigure 7-4 .Alteringthetemperatureandramprate wereunsuccessfulatpreventingthedefects.Futureproject sshouldlooktoalternative methodsofimplantingahighlydopedbacksideregiontoreliablyboostp erformance. 109
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Figure7-4.Blisteringonthesurfaceofthesiliconfollowingahightemp eraturebaketo dopethebackside. 7.3ConcludingRemarksandPathForward Theimprovementsdiscussedinthepreviouschaptersindicatethep otentialfor furtherimprovementoftheSWNT-SiNWdevicesbyincreasingbothe ciencyand longevity.TheALDprovedtobeanexcellentbarrierbetweenthesilic onandtheionic liquidwithoutsacricingtheinversionlayerneededtoreducerecomb ination.Though degradationdideventuallyoccurintheALDdevices,parasiticgatec urrentsindicatethat theelectrochemicalreactionsweregreatlyreducedcomparedto thenon-ALDSiNWbased devices.Additionally,itwasdemonstratedthatwatervaporandox ygencontaminationat thesurfaceofthedevicewereresponsibleforthedecreaseinper formanceduringelectronic gating,necessitatingtestingwithinaninertatmospheretoelicitsta bleperformance. Lastly,ahighlydopedregionatthebackcontactwaseectiveatre ducingrecombination andboostingPCE.Incorporatingthesethreemechanismsintopho tovoltaicdesigns promisestodeliverdevicesthatachieveaPCEabove15%,butalsost able. Futureprojectsarecurrentlyunderway,includingacollaboration withOakRidge NationalLaboratorytodevelopmoreorderedsiliconnanopillardevic es.Thesenew architectureswillbemorerobustthanthenanowire.Theabilitytos electivelycontrol diameterwillallowustoexplorethechargecollectioneciencywhenth eentirenanopillar iseithercompletelyorpartiallyinverted.Thoughbeyondthescopeo fthisthesis,the 110
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abilityofthecarbonnanotubelmtoconformtothesubstratesur facewouldmakeitan ideacandidateforformingSchottkyjunctionswithamorphoussilico n.Ashortdiusion lengthduetomultiplegrainboundariesnecessitateexcellentconnec tionbetweenthe electrodeandthesilicon.TheSWNTlmwithinourSchottkyjunctiond evicesalso actsasatransparentelectrode,makingitanidealcandidatefora -Sibaseddevices.The rexibilityofthecarbonnanotubelmcomplimentsthemalleabilityofamo rphoussilicon, raisingthepossibilityofarexible,thinlmsolarcell. Thevastmajorityofexperimentalbreakthroughsrealizedatthe academiclevel neverreachtheconsumermarket.Asdauntinganddiscouraginga sitmayseemtothe buddingscientist,noexperimentiscompletelydisconnectedfromeit hercontemporary innovationsortheresearchconductedinthepast.Theprimarygo alofscienceisto increasetheknowledgeofhowtheworldworks,disseminatethatkn owledgetothosewho areinterested,andultimatelyusethatknowledgetoadvancesocie ty.Theadvancesmade duringtheseexperimentsinphotovoltaiccellswillhopefullybenetot herscientistsand contributetotechnologicalinnovation. 111
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Table7-1.PerformancesummaryforTFSAdopedgrapheneandSW NTsolarcells DeviceV oc (V)J sc mA cm 2 FFEciency(%)Notes Graphene0.4214.20.321.9Graphenew/TFSA0.5425.30.638.6Graphenew/EMI-BTI0.5226.840.344.6V G =-0.75V SWNT0.5124.00.728.97SWNTw/TFSA0.5426.00.7510.43SWNTw/TFSAandEMI-BTI0.4525.650.212.45V G =-0.75V Table7-2.Performanceforbacksidedopedsubstrates DeviceV oc (V)J sc mA cm 2 FFEciency(%)Notes < 111 > 500 m0.54250.749.98V G =-0.75V < 111 > 250 m0.5226.70.659.74V G =-0.75V < 111 > 250 mdoped0.5829.60.7913.42V G =-0.75V 112
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APPENDIXA FULLSIMULATIONSFORTHEINVERSIONLAYERCELL FigureA-1.Modelingoftheinversionlayeratthesiliconsurfaceinthe carbonnanotube gridsolarcell 113
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APPENDIXB SOLARCELLPARAMETERSWITHINCREASINGOXIDATIONTIME FigureB-1.FF,J SC ,V OC ,andPCEforaSWNT-SiNWdeviceforvariousoxidationtimes inthelabatmosphere. 114
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BIOGRAPHICALSKETCH MaureenPettersonwasborninSanFrancisco,Californiain1983.Th eyoungestof threechildren,shefoundsupportandencouragementtoexplore theuniversethrough herparents,whobothbelievedininquirybasedlearningandalwaysfo rcedherto thinkinsteadofgivingeasyanswers.ShereceivedaBachelorsofSc ienceinPhysics (Astrophysics)fromtheUniversityofCalifornia,SantaCruzin200 6.Thoughherdegree focusedonAstrophysics,asenioryearthesisinMedicalPhysicswo rkingunderDr. HartmutSadrozinskiledtoaninterestinmorehandsonresearch. Aftergraduation sheworkedattheSantaCruzInstituteforParticlePhysicsfortw oyears,anexperience thatwasinvaluableforhowitshapedherintoananalyticandpatientr esearcherand ultimatelycementedherdecisiontopursueanadvanceddegreeinph ysics.Aftersending inhergraduateschoolapplications,shelefttheUnitedStatesand spentthenext6 monthsbackpackingaroundSoutheastAsia,theMiddleEast,andE asternEuropewith Griths'QuantumMechanicsintow.SheenrolledattheUniversityof Floridainthe fallof2008.Activewithinthephysicscommunity,shewastheGradu ateStudentCouncil representativeforthephysicsdepartment,servedonthephys icsGraduateStudent AdvisoryCommittee,andparticipatedinnumerousoutreachevent s.Inhersecondyear shestartedworkingwithDr.AndrewRinzleroncarbonnanotubeba sedphotovoltaics,a eldinwhichshecouldcombinebothgratifyingresearchandexcitingp hysicalprinciples withadesiretoimpactandcontributetosocietyinameaningfulandp ositiveway. 121
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