Characterization of silicon heterojunctions for solar cells

Characterization of silicon heterojunctions for solar cells

Kleideretal.NanoscaleResearchLetters2011,6:152http://doc.guandang.net/content/6/1/152

Characterizationofsiliconheterojunctionsforsolarcells

Jean-PaulKleider1*,JoseAlvarez1,AlexanderVitalievitchAnkudinov2,AlexanderSergeevitchGudovskikh3,EkaterinaVladimirovnaGushchina2,MartinLabrune4,5,OlgaAlexandrovnaMaslova1,3,WilfriedFavre1,Marie-EstelleGueunier-Farret1,PereRocaiCabarrocas4,EugeneIvanovitchTerukov2

Introduction

Inthefieldofsiliconsolarcells,recentprogresshasbeenachievedintwodirections:siliconheterojunctionsandsiliconnanowires.Thesetwotopicsarebrieflyaddressedhereandweshowsomenewcharacterizationresultsthatuseconductive-probeatomicforcemicro-scopy(CP-AFM)measurements.

Siliconheterojunctionsareformedbetweencrystallinesilicon(c-Si)andhydrogenatedamorphoussilicon(a-Si:H).Solarcellefficienciesofupto23%havebeendemon-stratedonhighqualityn-typec-Siwaferswithlayersofp-typea-Si:Hdepositedatthefront(astheemitter)andn-typea-Si:Hdepositedattheback(asthebacksurfacefield),respectively[1].Sincetransportpropertiesarequitepoorina-Si:Hduetothelargeamountofdefectsandbandgapstatesandlowcarriermobilities,thedopeda-Si:Hlayersareusedtoformthejunctions,buttheirthicknesshastobekeptverylow.Thefronta-Si:Hlayerhastobeverythininordertominimizeabsorptionofincomingphotonsandtoprivilegeabsorptioninc-Si.Onekeyfea-tureoftheSiheterojunctionsistheverygoodpassivationpropertyofthec-Sisurfacebya-Si:H.Thisisevenimprovedbyinsertingathinundopeda-Si:Hlayer

*Correspondence:jean-paul.kleider@lgep.supelec.fr1

LaboratoiredeGénieElectriquedeParis,CNRSUMR8507,SUPELEC,UnivP-Sud,UPMCUnivParis6,11rueJoliot-Curie,PlateaudeMoulon,91192Gif-sur-YvetteCedex,France.

Fulllistofauthorinformationisavailableattheendofthearticle

(so-called“intrinsic”layer,whichleadstothe“HIT”-heterojunctionwithintrinsicthinlayerdenomination[2]).Thislimitsinterfacerecombinationandleadstoveryhighopencircuitvoltages[3].Bandoffsetsbetweena-Si:Handc-Sialsoplayacrucialrolebecausetheydeterminethebandbending,whichinfluencesthecarriercollection.Weheredemonstratetheexistenceofaconductionchannelalongboththe(n)a-Si:H/(p)c-Siandthe(p)a-Si:H/(n)c-SiinterfacesfromdirectCP-AFMmeasurementsper-formedoncleavedsectionsofsolarcells.Weshowfromadditionalplanarconductancemeasurementsandsimula-tionsthatthesearerelatedtostronginversionregionsattheinterfaces.Fromthetemperaturedependence,wedeterminethevaluesofbandoffsets.

Experimentaldetails

Solarcellstructure

Atypicalsolarcellstructurebasedona-Si:H/c-Sihet-erojunctionsformedwithn-typec-SiispresentedinFigure1.Asimilarstructurestandsforp-typec-Si,replacingthen-typea-Si:Hbyp-typea-Si:Handviceversa.Forn-typec-Si,weusedFloatZone,n-typec-Siwafers, 100 oriented,withresistivity:r=1-5Ωcm,andthickness:W=300μm.Forthep-typec-Si,weusedCzochralski(CZ)c-Siwafers, 100 oriented,withresistivity:r=14-22Ωcm,andthickness:W=300μm.Weusedindiumtinoxide(ITO)asthefront

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Characterization of silicon heterojunctions for solar cells

Kleideretal.NanoscaleResearchLetters2011,6:152http://doc.guandang.net/content/6/1/152

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transparentconductiveoxide(TCO),andaluminumasthebackmetalcontact.Thea-Si:Hlayersweredepos-itedatEcolePolytechniqueinaradiofrequency(13.56MHz)plasma-enhancedchemicalvapordeposition(PECVD)reactoratasubstratetemperatureof200°C.Spectroscopicellipsometrymeasurementsandmodelingwereusedtocheckthatthedepositedsiliconthinlayersweretrulyamorphous,andthatnoepitaxialgrowthoccurredonthec-Sisubstrate.

CP-AFM

CP-AFMmeasurementswerecarriedoutusingtwodif-ferentsetups(i)inIoffePhysical-technicalInstitute(NT-MDTNtegraAura)and(ii)inLaboratoiredeGénieÉlectriquedeParis(DigitalInstrumentsNano-scopeIIIaMultimodeAFMwiththeRESISCOPEexten-sion[4]).ThesesetupsallowonetoapplyastableDCbiasvoltagetothedeviceandtomeasuretheresultingcurrentflowingthroughthetipasthesamplesurfaceisscannedincontactmode.SchematicAFMsetupisshowninFigure2.Inbothmeasurementsdiamond-coatedconductiveprobesmadeofsiliconwereused,thecontactinteractionforcebeingintherange100-500nN.

Withthehelpofthistechniqueonecansimultaneouslyexamineonthesamplecleavagesthesurfacetopographyandconductivepropertiesofthelayersconstitutingthesolarcells.Notethat,duetodifferentsoftwares,thefirstsetupprovidesimageswithcurrentvalues(currentflow-ingthroughthetip),whilethesecondoneprovidesresistancevalues,theresistancebeingdefinedastheratiooftheappliedvoltagetothemeasuredcurrent.FortheseCP-AFMmeasurements,thenormalsolarcellstructurewasreplacedbyasimplersymmetriccon-figuration,seeFigure3a,wherethesamea-Si:Hlayerwasdepositedonbothsidesofthec-Siwafer.ThenITOelectrodesweredepositedontopofbothsidesofthewafer,beforethesamplewascleaved.SometestswerealsoperformedwithaluminuminsteadofITOaselectrodes.TheobtainedCP-AFMresultsweregloballythesame.However,aluminumelectrodesformedhighridgesatthecleavededgeandtheircross-sectionwerepoorlyconductiveduetostrongoxidationofaluminum,whatinducedsomeproblemsinAFMimaging.There-fore,herewefocusonsampleswithITOonbothsides.Thus,cleavedsectionsofITO/(n)a-Si:H/(p)c-Si/ITOandITO/(p)a-Si:H/(n)c-Si/ITOsampleswith

different

Characterization of silicon heterojunctions for solar cells

Kleideretal.NanoscaleResearchLetters2011,6:152http://doc.guandang.net/content/6/1/152

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thicknessesofthea-Si:Hlayer(20,100,300nm)wereinvestigated.

Planarconductance

ThesamplestructureforthesemeasurementsisshowninFigure3bforp-dopeda-Si:H.Thea-Si:Hlayerwasdepositedinthesamerunonbothn-typec-Siandglass(Corning1737).Topcoplanaraluminumelectrodeswerethendepositedonthetopofa-Si:H.WemeasuredtheD …… 此处隐藏：17374字，全部文档内容请下载后查看。喜欢就下载吧 ……