一种新型的背面钝化技术PECVD-ONO

Volume2008,ArticleID485467,10pagesdoi:10.1155/2008/485467

ResearchArticle

PECVD-ONO:ANewDepositedFiringStableRearSurfacePassivationLayerSystemforCrystallineSiliconSolarCells

M.Hofmann,1S.Kambor,1C.Schmidt,1D.Grambole,2J.Rentsch,1S.W.Glunz,1andR.Preu1

1Fraunhofer2Forschungszentrum

InstituteforSolarEnergySystems,Heidenhofstrasse2,79110Freiburg,Germany

Dresden-Rossendorf,BautznerLandstrasse128,01328Dresden,Germany

CorrespondenceshouldbeaddressedtoM.Hofmann,marc.hofmann@ise.fraunhofer.deReceived17January2008;Accepted6March2008RecommendedbyArminAberle

Anovelplasma-enhancedchemicalvapourdeposited(PECVD)stacklayersystemconsistingofa-SiOx:H,a-SiNx:H,anda-SiOx:Hispresentedforsiliconsolarcellrearsidepassivation.Surfacerecombinationvelocitiesbelow60cm/s(afterfiring)andbelow30cm/s(afterforminggasanneal)wereachieved.Solarcellprecursorswithoutfrontandrearmetallisationshowedimpliedopen-circuitvoltagesVocvaluesextractedfromquasi-steady-statephotoconductance(QSSPC)measurementsabove680mV.Fullyfinishedsolarcellswithupto20.0%energyconversionefficiencyarepresented.Afitofthecell’sinternalquantumefficiencyusingsoftwaretoolPC1Dandacomparisontoafull-areaaluminium-backsurfacefield(Al-BSF)andthermalSiO2isshown.PECVD-ONOwasfoundtobeclearlysuperiortoAl-BSF.Aseparationofrecombinationatthemetallisedandthepassivatedareaatthesolarcell’srearispresentedusingtheequationsofFischerandKray.Nuclearreactionanalysis(NRA)hasbeenusedtoevaluatethehydrogendepthprofileofthepassivationlayersystematdifferentstages.

Copyright©2008M.Hofmannetal.ThisisanopenaccessarticledistributedundertheCreativeCommonsAttributionLicense,whichpermitsunrestricteduse,distribution,andreproductioninanymedium,providedtheoriginalworkisproperlycited.

1.Introduction

Electricalrearsurfacepassivationisbecomingincreasinglyimportantincrystallinesiliconsolarcelltechnology.Themainreasonforthatistheneedforacostdecreaseinthephotovoltaicsector.Thisleadstowafersthatarebecomingthinnerandthinnerandasteadilyincreasingneedforhigherenergyconversionefficiencies.Theresultofbothtrendsisanincreasedimportanceofaneffectivelypassivatedrearsurface.InFigures1to3,simulationsonrearsurfacepassivationandcellthicknessarepresentedusingPC1D[1].Open-circuitvoltageVocandenergyconversionefficiencyηdependstronglyontherearsurfacerecombinationvelocity(SRV)Seffwhenhigh-qualitysiliconsubstratesaretakenintoaccount(τbulk=750μs,excellentoptics,frontemitter,andpassivation,Figure1).ForSeffvaluesintherangeof5∗101cm/sto5∗103cm/s,thestrongestinterdependencewasfound.Thethinnerthesolarcellsthestrongerthiseffectappears.Notethatverythinsolarcells(50μm)showahigherVoclevelatverylowSeffvalueswhilethe50μmcellssufferfromadecreasedlighttrappinginoursimulationand

convertthelightalittlelessefficientlytoelectricityatthemaximumpowerpoint.Whenlookingatmedium-efficientsolarcells(τbulk=100μs,excellentoptics,frontemitter,andpassivation,Figure2)theinfluenceoftherearpassivationincreaseswhenthecellsaregettingthinner.Wefoundthatthethinnestcellsshowevenbetterefficienciesthanthickercellswhenanexcellentrearpassivationisapplied.Low-efficiencycells(τbulk=10μs,excellentoptics,standardfrontemitter50Ω/sq,andlow-levelfrontpassivation,Figure3)showonlyaslightdependenceontherearpassivationduetoalowdiffusionlength(τbulk=10μs→approximatelyLeff=170μm).Hence,mostminoritycarriersdonotprofitfromalowrearrecombinationvelocitybutrecombineinthebulksilicon.Thethinnerthelow-efficientcellsbecome,themorepronouncedwillbetheinfluenceofagoodrearpassivation.

Agostinellietal.[2]haveshownthatsiliconsolarcellsonverythinsubstrates(downto105μm)canreachhigherefficienciescomparedtoastandardAl-BSFrearstructurewhenanadaptedrearsurfacepassivationisapplied.Kray[3]haspresentedenergyconversionefficienciesabove20%for

2

0.72Highefficiency230.7Simulatedopen-circuitvoltageVoc(V)22Simulatedefficiencyη(%)0.6824AdvancesinOptoElectronics

Highefficiency210.6620190.64180.62170.6100101102103104105106RearsurfacerecombinationvelocitySeff(cm/s)

300μm250μm200μm

(a)

150μm100μm50μm

16100101102103104105106RearsurfacerecombinationvelocitySeff(cm/s)

300μm250μm200μm

(b)

150μm100μm50μm

Figure1:Simulatedopen-circuitvoltageVoc(left)andsimulatedenergyconversionefficiencyηversustheeffectiverearsurfacerecombinationvelocitySeffforhigh-efficiencysolarcells.CalculatedusingPC1D.Parameters:NA=3∗1016cm−3,τbulk=750μs(approx.Augerlimit),texturedfront,frontemitter:120Ω/sq,Sfront=1000cm/s.

substrateswithlessthan50μmthickness.Bothpublicationsmakeuseofthepassivatedemitterandrearcell(PERC)concept[4].

FraunhoferISEhasdevelopedaneconomicallyfeasibletechnologyformassproductionofthelocalrearcontactsofPERCs[4]byimplementingalasertechnology.Thispatentedlaser-firedcontacts(LFCs)approach[5]allowsthelocalaluminiumcontactfiringthroughapassivating—andinmostcasesisolating—layersysteminapproximately1to3secondspercell.Withinthesamestep,aluminiumisalloyedintothesiliconformingap+regimeunderneaththelocalcontactcreatingaverygoodohmiccontactatthecell’srear[6,7].

RearsidesurfacepassivationschemesneedtobeadaptedtotheLFCprocessandtotheindustrialproductionenvi-ronment.Themostcommonlayersarethermallygrownsilicondioxide(SiO2),plasma-enhancedchemicalvapourdeposited(PECVD)hydrogenatedamorphoussiliconnitride(a-SiNx:H,inshort:SiN),andPECVDhydrogenatedamor-phoussilicon(a-Si:H,inshort:a-Si).Asthermallygrownsilicondioxide(SiO2)layersaremanufacturedbyatime-andenergy-intensivehightemperatureprocess,theyarenotthefirstchoiceformassproduction,althoughtheypossiblyprovideaverygoodandthermallystablepassivation[8].Unfortunately,thetemperaturelevelisnotsuitableforeverymaterialquality.Especially,low-costwafersmightdegradeduringanoxidationprocess.

SiNfilmsarefabricatedbyalowtemperatureprocessingstep(approximately300–400◦C).Theyalsoprovideaverygoodpassivationquality[9–15]butarenotasthermallystableasSiO2layers[8].ThethermalstabilityofoptimisedSiNlayerscanbeimprovedtoadegreethatissufficientforthetypicalthermalbudgetofthesolarcellproductionlineexcludingemitterdiffusion.Additionally,thecellspassivatedwithSiNfilmstypicallysufferfromaninversionlayerwhichisshuntedbythelocalrearcontacts[16,17].TheinversionlayerisduetoahighdensityoffixedpositivechargeswhichtypicallycanbefoundinSiNfilms.

Gooda-Sifilmsprovideaveryefficientsurfacepassiva-tionwithoutformingsuchaninversionlayerasa-SifilmsdonotmainlypassivateviaafieldeffectbutthroughtheloweringofthedensityofstatesattheinterfaceDit.Differentgroupshavebeenusinga-Siaspassivation,eitherassinglelayera-Sipassivation[18],a-Sistacks[19,20]ora-SistackedwithSiN[21,22],orPECVDsiliconoxide(SiOx)[23].Notonlyintrinsicbutalsodopeda-Silayershavebeenused.Themajordrawbackwhenusinga-Sipassivationlayersincrystallinesiliconsolarcelltechnologyistherelativelylow

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0.7Mediumefficiency0.68Simulatedopen-circuitvoltageVoc(V)2122Mediumefficiency3

200.66Simulatedefficiencyη(%)150μm100μm50μm

(a)

190.64180.62170.6160.58150.56100101102103104105106RearsurfacerecombinationvelocitySeff(cm/s)

300μm250μm200μm

14100101102103104105106RearsurfacerecombinationvelocitySeff(cm/s)

300μm250μm200μm

(b)

150μm100μm50μm

Figure2:Simulatedopen-circuitvoltageVoc(left)andsimulatedenergyconversionefficiencyηversustheeffectiverearsurfacerecombinationvelocitySeffformedium-efficiencysolarcells.CalculatedusingPC1D.Parameters:NA=7∗1015cm−3,τbulk=100μs,texturedfront,frontemitter:120Ω/sq,Sfront=1000cm/s.

thermalstabilityofthepassivationproperties.Afteratypicalscreenprintedcontactfiringstepwithtemperaturesofupto∼850◦Cthepassivationpropertiesarestronglyreduced[24].Therefore,analternativepassivationlayersystemthat(i)canbedepositedquicklyandeasilyatlowtempera-tures,preferablyusingPECVD,

(ii)providesgoodpassivationwithoutasignificantshun-ting,

(iii)isthermallystableinacontactfiringprocessat

∼850◦C,

(iv)providesacompatibleinternalrearsurfacereflec-tancewouldbeaverygoodwaytoincorporateaneffectivesurfacepassivationlayersystemintoday’sindustrialsolarcells.

2.LifetimeInvestigation2.1.SymmetricSampleStructure

Asdescribedabove,thermallygrownsilicondioxidelayersprovideallmajorrequirementstobesuitablefortherearpassivationforsiliconsolarcells:goodsurfacepassivation,

goodrearreflectance,andthermalstabilityinsolarcellproductionprocesses.Onlytheprocessthroughputandcostandthepossibledegradationofthesolarcellprecursorsseemtobethemajorandcrucialproblem.

PECVDsiliconoxidelayers(SiOx)looklikeapossiblealternativetocombinethegoodlayercharacteristicsofanSiO2filmwiththebenefitsofPECVD.Regrettably,noPECVDSiOxlayerscouldbefoundbytheauthorsthatshowagoodsurfacepassivationandthermalstability,nomatterifthelayerhasbeenannealedindifferentgasambient(e.g.,forminggas)ornot.Hence,differentstacklayershavebeenunderinvestigationwiththePECVDSiOxlayerbeingthefirstlayerdeposited(sittingdirectlyonthewafer’ssurface).ItshouldbenotedthataninvestigationofdepositedoxidelayersstackedwithPECVDSiNxhasbeenpresentedbyAgostinellietal.[25].

Hoexetal.havepresentedwell-passivatingSiOxfilmsdepositedbytheexpandingthermalplasma(ETP)technique[26].

Thelifetimeinvestigationhasbeenperformedusingsiliconwafersassubstrateswiththefollowingproperties:floatzone,p-type,borondoped,1Ωcm,250μmthick,andshinyetchedsurfaceswithacrystalorientationof(100)[27].ThewaferswerecleanedwithawetchemicalRCAclean

4

0.62Lowefficiency16AdvancesinOptoElectronics

Lowefficiency15Simulatedopen-circuitvoltageVoc(V)0.6Simulatedefficiencyη(%)150μm100μm50μm

(a)

140.5813120.56100101102103104105106RearsurfacerecombinationvelocitySeff(cm/s)

300μm250μm200μm

11100101102103104105106RearsurfacerecombinationvelocitySeff(cm/s)

300μm250μm200μm

(b)

150μm100μm50μm

Figure3:Simulatedopen-circuitvoltageVoc(left)andsimulatedenergyconversionefficiencyηversustheeffectiverearsurfacerecombinationvelocitySeffforlow-efficiencysolarcells.CalculatedusingPC1D.Parameters:NA=7∗1015cm−3,τbulk=10μs,texturedfront,frontemitter:50Ω/sq,Sfront=106cm/s.

WetchemicalRCAclean

PECVDofSiOx+SiNx

PECVDofSiOx+SiNx+SiOx

Carrierlifetimemeasurement

Annealing425◦C,15minFiring850◦C,∼3s

Carrierlifetimemeasurement

Figure4:Workflowofthelifetimeexperimentonsymmetricsamplestructures.

platePECVDreactorusingaplasmaexcitationfrequencyof13.56MHzandagasmixtureofmonosilane(SiH4)andnitrousoxide(N2O)fortheSiOxlayerandmonosilane,hydrogen(H2)andnitrogen(N2)fortheSiNxlayer.Thedepositiontemperatureforalllayerswas350◦C.Theoxidelayerscanbedepositedveryquicklyatadepositionrateof∼120nm/min.Thenitridelayertakeslonger,atarateof∼8nm/min.Thicknessesof120nmfortheoxideand70nmforthenitridelayershavebeenapplied.SeeFigure4fortheprocesssequenceandFigure5forthesamplestructures.Minoritycarrierlifetimesweremeasuredusingthequasi-steady-statephotoconductance(QSSPC)method[29].ThesurfacerecombinationvelocityhasbeencalculatedfromthemeasuredlifetimesassumingthatonlyAugerrecombinationtookplaceinthesample’sbulk(i.e.,“perfectbulk”).TheAugermodelbyGlunzetal.[30]hasbeenappliedforthecalculationusingthefollowingequation[31]:

1W1W1

=++τeffτb2SeffDnπ󰀄

󰀂

󰀃2󰀅−1

bathsequence[28]includingafinaldipinhydrofluoric

acid(HF)andsubsequently,thedepositiononbothsurfacesofthewafershasbeenperformed.TwodifferentPECVDstacklayersystemshavebeeninvestigated:(i)SiOxandSiNxand(ii)SiOx,SiNx,andSiOx.SiOxalwayshasbeenthefirstlayerdeposited.Thedepositiontookplaceinaparallel

,(1)

withthesiliconwafer’sbulklifetimeτb,thewaferthicknessW,theeffectivesurfacerecombinationvelocity(SRV)Seff,andthediffusionconstantforelectronsDn.

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SiOxSiNxSiOxFZ-c-SiSiOxSiNxSiOx

5

SiNxSiOxFZ-c-SiSiOxSiNx

Figure5:Symmetricallifetimesamplestructures.Left:doublelayersamplestructureSiOx+SiNx.Right:triplelayersamplestructureSiOx,SiNxandSiOx(PECVD-ONO).

3−1000

mnco41it0a1ni∗bm5o=100

cnreΔce@af)rsu/smcve(10

itffceSffeyEticolve1

As-depositedAfterannealingAfterfiring425◦C850◦CSiO.SiNSiO.SiN.SiOFigure6:EffectivesurfacerecombinationvelocitiesofPECVDstacksofSiOx+SiNx(leftredcolumns)andSiOx,SiNxandSiOx(rightgreenstripedcolumns,PECVD-ONO)as-deposited,afterannealing(425◦C,15min,forminggas)orfiring(850◦C,∼3s).

ThestacksystemofPECVDSiOxandSiNxledtoas-depositedSeffvaluesbelow700cm/s.Subsequently,anannealinginforminggasat425◦Cfor15minuteshasbeenperformed.MinoritycarrierlifetimemeasurementsatthisstageshowedSeffvaluesbelow50cm/s.Next,athermaltreatmentcomparabletofiringofscreenprintedfrontcontactswithapeakwafertemperatureofapproximately850◦Cforabout3secondswasdonewithSeffincreasingto<70cm/s.

Forthetriplelayersystem(SiOx,SiNx,andSiOx,inshort:PECVD-ONO),thesamethermalprocessingasforthetwolayersamplestructurehasbeenapplied.Thisledtothefollowingsurfacerecombinationvelocities:as-deposited:below240cm/s,afterannealinginforminggasat425◦Cfor15minutes:below30cm/s,afterthermalprocessingcomparabletofiringofscreenprintedfrontcontactswithapeakwafertemperatureofapproximately850◦Cforabout3seconds:below60cm/s.

TheSeffresultsaresummarisedinFigure6.

Bothstacksystemsaresuitableforthesurfacepassivationofp-typeSiwafersbeforeandafterathermaltreatment

SiO2n-Sip-Si

SiOSiNxSiOxx

Figure7:Samplestructureofthesolarcellprecursors.

likethecontactfiring.Afterdepositionandafterannealing,astrongerpassivationeffectwasfoundforthetriplestackstructureSiOx+SiNx+SiOx(PECVD-ONO,Patentpend-ing).Thiseffectwillbeinvestigatedfurtherinthefuture.However,onebeneficialeffectistheincreasedoveralllayerthicknessprovidingahigherprobabilityforhydrogentodiffusetothesilicon/siliconoxideinterfaceandloweringtheamountofrecombinativelyactiveinterfacetraps.

2.2.SolarCellPrecursors

AninvestigationofthesuitabilityofthenewlydevelopedPECVD-ONOstacksystemforpassivationofsolarcellrearsidesispresentedhere.Ashortsummaryhasbeenshownin[27].

Floatzonewaferswiththefollowingcharacteristicswereused:p-type,borondoped,0.5Ωcm,250μmthick,andshinyetchedsurfacewithacrystalorientationof(100).Thewaferswereoxidised,frontsidewindowswereopenedbyphotolithography.Then,anemitterwasdiffusedintothefront(120Ω/sq.)followedbyawetchemicaletchofthephosphoroussilicateglass(PSG).Inasecondoxidationstep,anantireflectingSiO2layerwasgrownonthefront(andtheoxideonthebackwasthickened).Afterremovingtheoxideontherearandcleaningthesurfaceinawetchemicalbathsequenceof(i)HNO3and(ii)HF,thenovelpassivationstackPECVD-ONOhasbeendepositedontherearandthelifetimewasmeasured.Subsequently,thesampleswereannealedat425◦Cfor15minutesinforminggasand/orathermaltreatmentcomparabletothefiringofscreenprintedfrontcontactswasperformed.Aftereachthermalprocess,thelifetimewasmeasuredusingtheQSSPCtechnique.ThesamplestructurecanbefoundinFigure7andtheprocesssequenceinFigure8.

6OxidationOpeningfrontwindowsEmitterdiffusion120Ω/sq

PSGetchOxidationARCRemovalrearoxideDepositionrearpassivationCarrierlifetimemeasurementAnnealing“Contactfiring”425◦C,15min∼850◦C,∼3sCarriermeasurement

lifetimeCarriermeasurement

lifetimeFigure8:Processsequenceofthesolarcellprecursorexperiment.ThecolourscorrespondtothecoloursinFigure9.

Itcanbesummarisedthatthesamplestructureequalsafinishedsolarcellexceptthemetalcontactsonfrontandrear.CuevasandSintonhaveshownthatimpliedVocvaluescanbeextractedfromthemeasuredeffectiveminoritycarrierlifetimes[32,33]usinganapproximationequationforVoc:

󰀂

󰀃

Voc=kT

qlnnpn2,(2)

i

withBoltzmann’sconstantk,temperatureT,elementary

chargeq,free-electrondensityn,free-holedensityp,andintrinsicfree-electrondensityni.

Forp-typewafersandlongdiffusionlengths,(2)canbewrittenas[32,33]

󰀂

󰀃

Voc=kT

qln

Δn(Δp+NA)n2,(3)

i

withNAbeingthedensityofacceptors.

nicanbecalculatedfrom

ni=NCNVe−Eg/kT,

(4)

withsilicon’sdensityofstatesintheconductionbandNC

andinthevalencebandNVandthesiliconbandgapenergyEg.Hence,onecancalculateImpliedVocvaluesfromQSSPCmeasurements.TheimpliedVocwasextractedat1sun.Initially,Vocvaluesofapproximately631mVhavebeenobserved.Afterannealingorfiring,theimpliedVocvaluesincreasedstronglytovaluesabove680mV.Here,theresultsafterfiringwereverysimilarregardlessofwhetheranannealingwasdonepriortothefiringstepornot.Theeffectissuspectedtobeduetoanenhancementofthefrontandrearsurfacepassivation.SeeFigure9.

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)690nus1@680CPS670SQ()V660m(CO650Vdeil640pmI630

Reardeposition

Annealing425◦C

Firing850◦C

Figure9:Solarcellprecursors.ImpliedVocvaluesextractedfromQSSPCmeasurementsat1sunshowastrongincreaseafterthermaltreatment.ColourscorrespondtoFigure8.

3.HydrogenDepthProfiling

Forthepassivationeffect,thediffusionofhydrogenfromthestronglyhydrogenatedsiliconnitridelayertotheSiOx/bulkSiinterfacemightberesponsible.Typically,ahighdensityofrecombinativelyactivestatesatthisinterfacecanbefound.Thesestates,mostlySidanglingbonds,canbesaturatedbyhydrogenandimplicitlymaderecombinativelyinactive.Therefore,thehydrogenconcentrationandthechangeinhydrogenconcentrationindifferentlayerswithinthepassivationstacksystemisofgreatinterest.

Nuclearreactionanalysis(NRA)measurementshavebeenperformedatForschungszentrumDresden-Rossendorf,Germany[34].NRAisbasedonareactionbetweenanitrogenisotope15NwhichisacceleratedtoseveralMeVandahydrogenatom1Hinthesample.Thereactiononlytakesplacewhenthenitrogenatomhasaspecifickineticresonanceenergy(6.385MeV).Atlowerenergy,nonuclearreactionwillhappen.Athigherenergy,thenitrogenatomloosesspeed(energy)whilepassingthroughthefirstatomiclayersofthefilm.Whenthenitrogenisdeceleratedtotheresonanceenergy,thereactionwillhappenifthereisahydrogenatomtoreactwith(Figure10).Thereactionofhydrogenandnitrogenwillfinallyleadtoa12Catomandanalphaparticleandagammaquantumthatareradiated.Thereactionequationsareasfollows:

15N+1H

−→16O∗−→12C∗+α,12C∗

−→12C+γ.

(5)

Countingthemeasuredgammaquantaforacertainnitrogenkineticleadstothehydrogenconcentrationandthedepthprofile[34].

SiliconwafersamplescomparabletothesymmetricallifetimesampleswereusedfortheNRAmeasurement[27].Theprocessstateofthesamplesunderinvestigationwasas-deposited,◦afterannealing(425◦C,15min)andafterfiring(850C,∼3s).Thehydrogendepthprofilingoftheas-depositedsampleshowedahydrogenconcentration

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HHKineticenergy∼6.385MeVHHHHKineticenergy>6.385MeV15NHHHH15NγHHHSampleγHHH(a)

Sample(b)

xFigure10:Measurementprincipleofthenuclearreactionanalysis(NRA)technique(after[35]).Theintensityofgammaradiationforaknownkineticenergyof15Nabovethethresholdenergyof6.385MeViscorrelatedwiththedepthofhydrogen.

1816Hconcentration(at%)141210864200SiOxSiNxSiOxSisubstrateEvaporatedTiPdAgfrontcontacts,Agelectroplated

Randompyramids

SiO2

n+emitterp-Si

LocalAl-BSFSiOxSiNxSiOxAlLaser-firedcontacts(LFC)

100200Depth(nm)

300400As-deposited

Annealed425◦C15minFired850◦C3s

Figure12:StructureofthefabricatedsolarcellswiththenewlydevelopedpassivationstacksystemPECVD-ONOontherearandLFCcontacts.

Figure11:ResultsofthehydrogendepthprofilingusingtheNRAmethod.SampleswithPECVD-ONOafteravariationofthermaltreatmentareshown:as-deposited,afterannealing(425◦C,15min),afterfiring(850◦C,∼3s,samplenotannealedpriortofiring).

nitridefilmtothesurroundingoxidelayerstookplace.NosignificanteffectcanbeobservedattheSiOx/c-Siinterface(seeFigure11).

4.SolarCellFabrication

ofapproximately8at%inthePECVDSiOxfilmsandapproximately16at%intheSiNxfilm.Almostnohydrogencouldbefoundwithinthecrystallinesiliconbulk.Afterannealingthehydrogendepthprofiledidnotchangemuch.Hence,nostronghydrogendiffusioncouldbeobserved.Thesamplethatwassubjectedtothe“contactfiring”showedasignificantlymodifiedhydrogenprofile.Asexpectedatthistemperaturerange,hydrogenwasverymobile.Theoverallhydrogenconcentrationwaslowered,withintheSiOxlayersapproximately5at%hydrogencouldbefoundandapeakconcentrationofapproximately8at%wasobservedwithintheSiNxlayer.Thisshowsthatastrongout-diffusion(i)fromthehydrogenatedlayerstotheambientand(ii)fromthe

Finally,solarcellswiththenewlydevelopedpassivationstacksystemPECVD-ONOwerefabricatedtoshowthequalityofthepassivationonafinalcell.

Startingthecellfabricationusingfloat-zoneSisubstrateswiththesamecharacteristicsasintheaboveexperiments,thefinalcellsexhibitevaporatedTiPdAgfrontcontacts,athermallyoxidisedantireflectioncoatingthatalsoservesasfrontpassivationlayer,a120Ω/sqn-typeemitter,fabricatedbydiffusionusingaPOCl3ambient,a1Ωcmp-typebulk,thenewlydevelopedPECVD-ONOstackpassivationsystem,andanevaporatedAllayeratthebackandlaser-firedcontactsthatledtoalocalAl-BSFunderneath(aboveiflookingatFigure12)thepointcontacts.Therearsurfacewas

8

10.9ycn0.8eicffi0.7em0.6utn0.5auql0.4anr0.3tenI0.20.10300400500600700800900100011001200Wavelength(nm)

ThermalSiO2Al-BSF

PECVD-ONOFigure13:InternalquantumefficiencyforcellsofthisbatchwiththermalSiO2andPECVD-ONOrearpassivationandhigh-qualityAl-BSFrearpassivationofanotherbatch.

Table1:Bestsolarcellsofthisinvestigation.

RearpassivationAreaVocJscFFηPECVD-ONO4.0cm2664mV38.2mA/cm278.7%20.0%ThermalSiO24.0cm2676mV38.3mA/cm280.4%20.8%

cleanedinawetchemicalbathsequenceof(i)HNO3and(ii)HF.

ThecellschemecanbefoundinFigure12.

Thefinalcellwasannealedat425◦Cinforminggasfor15minutes.Athermalprocesscomparabletothefiringofscreenprintedfrontcontacts(asusedinthelifetimeexperimentsdescribedabove)wasnotapplied.Thisprocesswouldharmthesolarcellstructureusedinthisbatch.

ThebestsolarcellwiththenovelPECVD-ONOstacksystemasrearpassivationledtoacellefficiencyof20.0%(seeTable1).

Forcomparisonalsothermallygrownsilicondioxidewasusedforrearpassivationwithinthesamesolarcellbatchthatisknownforitsexcellentrearpassivationquality.20.8%peakefficiencycouldbeobtainedforthesereferencecells.

Internalquantumefficiency(IQE)measurements(Figure13)allowforacomparisonofthequalityofthenovelrearpassivationPECVD-ONOstackwiththermallygrownSiO2aswellaswithfullareahigh-qualityAl-BSF(theAl-BSFcellsoriginatedfromadifferentbutsimilarsolarcellbatch).ItcouldbeproventhatPECVD-ONOissuperiortoAl-BSFrearsidesbutshowslowerqualitythanthermallygrownSiO2.

Toquantifytherearpassivationqualityinthefinishedsolarcell,anumericalfittotheexperimentalexternalquantumefficiency(EQE),IQE,andreflectionpropertieswaspreparedusingPC1D[1].ThegraphicalresultscanbefoundinFigure14.ThetotalfrontandrearsurfacerecombinationvelocitiescanbederivedfromthePC1Dfitas3200cm/s±500cm/s(front)and550cm/s±50cm/s

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1,EQn0.9Ioiyt8cc0.neefli0.7creffi,eE0.6mQEu.t0.5nE.auQql0.4alannr0.3rtetexne0.2I0.10300400500600700800900100011001200Wavelength(nm)

IQEPECVD-ONO

EQEPECVD-ONOPC1DfitIQEPECVD-ONOPC1DfitReflection

EQEPECVD-ONO

Figure14:Externalquantumefficiency,IQEandreflectionvaluesforthesolarcellusingPECVD-ONOrearpassivation.Experimentaldata(symbols)andcalculatedvalues(lines)aredisplayed.ThecalculationwasdoneusingPC1D[1].

(rear),respectively.Thisincludestherecombinationatthepassivatedareaaswellasthecontactarea.FischerpresentedanequationdescribingtheSRVoflocallycontactedsolarcellrearsides[36]

󰀄

󰀄

󰀇SLeff=

Dπ󰀅󰀅−1

W2W󰀆pπfarctan2WLp

f−e−W/Lp+DfWSmet

+Spass

1−f

,(6)

withtheminoritycarrierdiffusionconstantD,thewaferthicknessW,thecontactpitchLp,themetallisationfractionf,theSRVofthemetallisedsurfaceSmet,andtheSRVofthepassivatedsurfaceSpassoftherearside.Forthecellspresentedhere,thefollowingvaluesarevalid:DnL=27.1cm2/s,W=240μm,pexperimentally=1investigatedmm,andfand=modelled1%.KraytheandrecombinationGlunzhavevelocityatthelaser-firedcontactsleadingtotheexpression[37]

Smet󰀈

NA󰀁

=S0+αeβ(NA+N0),

(7)

withS0andN=−900cm/s,α=22.1cm/s,β=1.29×10−16cm3,

0=3the.40substrate×1016cm−3andNAbeingthedensityofacceptorsin(inourcase1.5×1016cm−3).Hence,byusingtheaboveequationsandtheSeffvalueextractedfromPC1D,Spasscanbecalculatedto500cm/s±50cm/s.

5.Conclusion

AnovelsurfacepassivationstackcomprisingofthreePECVDlayerswaspresented.Thestackconsistsofa-SiOx:H,a-SiNx:H,anda-SiOx:H.Lifetimemeasurementswiththe

AdvancesinOptoElectronics

novelstacksystemonbothsurfacesofp-typesiliconwafersshowedsurfacerecombinationvelocitiesof<240cm/s,afterannealingat425◦C:<30cm/s,afterfiring:<60cm/s.SolarcellprecursorswithoutmetallisationbutwithPECVD-ONOrearpassivationexhibitedimpliedVocvaluesextractedfromtheQSSPCmeasurementsof>680mV.HydrogendepthprofilingusingtheNRAmethodwasconductedresultinginhydrogenconcentrationsofapproximately8at%inthePECVDSiOxfilmsandapproximately16at%intheSiNxfilmintheas-depositedandannealedstate.Afterfiring,thehydrogenconcentrationswerestronglyloweredto5at%inthePECVDSiOxfilmsandapproximately8at%intheSiNxfilm.

Solarcellswiththenovelpassivationstackwereprocessedandshowedefficienciesofupto20.0%.IQEcharacterisationandmodellingusingPC1DledtoatotalSRVattherearsurfaceof550cm/s±50cm/s.Additionally,themodelfortherecombinationatlocallycontactedrearsidesbyFischerandKrayledtoanSRVof500cm/s±50cm/satthepassivatedarea.

ApatentonthenovelPECVD-ONOpassivationstacksystemispending.

Acknowledgments

TheauthorswouldliketothankA.Leimenstoll,A.Her-bolzheimer,andS.Seitzforcleanroomprocessing.ThelaserprocessingwassupportedbyJ.F.Nekarda,B.Fleischhauer,C.Harmel,andA.Grohe.Additionally,theauthorsthankK.Kr¨ugerforAgplating,T.Rothforlifetimemeasurements,andE.Sch¨afferandK.KordelosforI-Vmeasurements.ThisworkwaspartlysupportedbytheEUfundedproject“CrystalClear”undertheprojectnumberSES6-CT2003-502583.

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