综述.docx

综述.docx

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综述

RecentprogressonCdS/CdTethinfilmsolarcells

Abstract

TheCdS/CdTethinfilmsolarcellisthemostsuitabletobefabricatedontheformofthinfilms.Theprocessesusedtomakeallthefilms,whichcomposethecell,arequitesimpleandfast.Anefficiencyof16.5%hasbeenreachedonlaboratoryscale.TheresearchsonpreparationmethodsofCdSandCdTelayer、CdCl2heattreatmentandthebackcontacthavebeenrepresentedinthispaper.

Keywords:

CdS;CdTe;thinfilmsolarcell;CdCl2heattreatment;backcontact

1Introduction

Solarenergyisoneoftheabundant,non-pollutingrenewableenergyofourplanet.Duringthelastthreedecadesconsiderableprogresshasbeenmadeindevelopingtechnologiestoharnesselectricityfromsolarradiation.PolycrystallinethinfilmCdTecontinuestobealeadingmaterialforthedevelopmentofcosteffectiveandreliablephotovoltaicsduetoitsbandgap（1.5eV）,closetotheidealforphotovoltaiccon-versionefficiency（1.45eV）,anditshighopticalabsorptioncoefficient（90%）.ThinfilmCdTesolarcellsaretypicallyhetero-junctionswithCdSbeingthen-typepartner,orwindowlayer[1].BonnetandRabenhorstpublishedaninterestingpaperonCdS/CdTethinfilmsolarcellsin1972reportinganefficiencyof6%[2].The10%efficiencyvaluewasovercomebyTyanandPerez-Albuerne[3]andfinallyanefficiencyof15.8%hasbeenreachedbyFerek-ides[4]etal.VeryrecentlythegroupofNRELreportedarecordefficiencyof16.5%[5].Theidealphotoelectricconversionefficiencyis30%.TheefficiencycanbeimprovedbytheimprovementofCdS/CdTeheterojunction、CdCl2heattreatmentandthebackcontact.WebrieflyreviewedthepreparationofCdSandCdTethinfilm、CdCl2heattreatmentandthebackcontactinthispaper.

2StateandMechanismofCdS/CdTesolarcell

Fig.1.SchematicsofatypicalsuperstrateCdS/CdTesolarcellstructure[6]

AsseeninFig.1,thetypicalsuperstratestructureofaCdS/CdTesolarcelliscomposedof4layers:

1.atransparentandconductingoxide（FTO）whichactasafrontcontact;

2.aCdSfilmwhichisthesocalledwindowlayer;

3.aCdTefilmwhichistheabsorberlayermadeontopofCdS;

4.thebackcontactontopoftheCdTelayer.

Fig.2.Mechanismofasolarcell[7]

AsshowninFig.2,theworkingmechanismofthesolarcellcanbebrieflyexplantedasfollowing:

theabsorberuptakesthesun-lightandgenerateselectron-holepairs,withtheconductionbandproducingelectronsbandandthevalancebandproducingholes.Thentheelectrontransportmaterialstransferelectrons,theholetransportmaterialstransferholesatthesametime.Finallytheexternalconnectedformacurrent.

3TheCdSlayer

CdSispreparedbyseveralmethodsthatareevaporationfromasinglecrucible,sputtering,chemicalbathdeposition（CBD）andclosedspacesublimation（CSS）.

AllthesemethodsaresuitabletoprepareaCdSlayerthat,whenusedintheCdS/CdTesolarcell,cangiveefficiencylargerthan10%.R.Tedeschi,V.Canevarideposited,viamagnetronRFsputtering,withoutremovingthesamplefromtheapparatus,aCdSlayer[8].WuetalobtainedthehighestefficiencybyusingaCdSpreparedbyCBD[9].AcheaperandsafermethodfortheactivationstephasbeenintroducedbyN.Romeoetal.onclosedspacesublimation（CSS）-CdS[10].

ThechoiceoftheCBDmethodwasprobablyduetothefactthatCBDmakesaverycompactfilmthatcoversperfectlytheTCOlayer.HoweveritmustbeconsideredthatCBDisnotsuitableforlarge-scaleproductionsinceitisnotfastandgivesawastethatneedstoberecycled.RFsputteringisthemostsuitableforindustrialproductionforF-dopedCdShasrevealedtobemorestablewithrespecttotheCdS-CdTemixingwhentheCdTe/CdSstructureistreatedat400℃inthepresenceofchlorine[11].Fromthepointofviewofthecostofequipment,depositionrateandfinalcellperformance,theCSStechniqueseemstobebestsuited.

4TheCdTelayer

CdTeisamaterialthatexhibitsaforbiddengapof1.45eVveryclosetothemaximumofsolarenergyconversion.SeveralmethodshavebeenusedtodepositCdTe,namelyclosedspacesublimation（CSS）、electrodeposition（ED）、screensrinting、physicalvapordeposition（PVD）、chemicalvapordeposition（CVD）、metalorganicchemicalvapordeposition（MOCVD）、magnetronsputtering（RF）、vacuumthermal-evapor、molecularbeamepitaxy（MBE）elat.

N.Romeoetalhavemanufactureann-CdS/p-CdTewithaareaof1.05cm-2andaefficiencyof15.8%byclosedspacesublimation（CSS）[12].A.AshourelatdiscoveredthatthethicknessofCdTepolycrystallinethinfilmhadmucheffectonitsresistivitywithelectricalparameterssuitingforFuchs-SondheimerfuctionbyvacuumthermalevaportoproduceCdTethinfilm[13].Johnsonfirstlypreparedann-CdS/p-CdTewithaareaof0.02cm-2andaefficiencyof14.2%byelectrodepositionsecondlyonlytoclosedspacesublimation[14].

closedspacesublimation（CSS）technologyisdevelopedfromⅢ-Ⅴcompoundsemiconductorbodyepitaxyinthe1960s.Itisahighdepositionrateofthenewdepositiontechnique.CSSmethodcancombinewiththeconventionglassrawproductionprocess,greatlyimprovingproductionefficiencyandreducingproductioncosts.Vacuumthermalevaporationprocessissimple、easytocontrolandlowcost.Henceitisoneofthemorepracticalpreparationmethod.ElectrochemicalmethodofdepositiontheCdTethinfilmhasitsuniqueadvantages,itslowcost、easyprocesscontrol、securityandcanbedepositedinalargeactivityarea.

5CdCl2heattreatment

OneofthecriticalstagesindevicefabricationistheCdTefilm,withCdCl2,aprocessthatisessentialintheproductionofhigh-efficiencycells.Firstly,CdCl2treatmenthasbeencarriedoutassolutiontreatment[15].Inthis,CdCl2isfirstdissolvedinmethanolandthenappliedonas-depositedmethod,thinlayerofCdCl2isdepositedonCdTefilm.Sincethen,therearealternativestechniques,calledtreatmentsinchloridevapors.Inoneofthesetechniques,CdTefilmsareheatedinthepresenceofCdCl2vapors.Inavariantofthistechnique,CdTefilmsarealsoheatedinthepresenceofchlorinevaporgeneratedfromHCl[16].TheuseofCdCl2vaporhasbeeninvestigatedasanalternativeviabletotheuseofsolutionCdCl2[17].VaportreatmentreducesprocessingtimesincecombiningtheexposuretoCdCl2andannealingintoonestep.Itimprovestheexperimentalreproducibilityandtheprocesscontrolandalsoeliminatestheremainingliquidassociatedwiththesolutionmethod[18].Recently,anothertreatmentwithagashasbeenmade;amixofAranddifluoro-chloromethane（HCF2Cl）wasused,andalso,inthiscaseitisnotnecessarytoheatthesampleinair[19].

ItiswellknownthattheCdCl2treatmentoffersseveralsubstantialbenefitssuchasincreasedgrainsize,grainboundariespassivationandreducedlatticemismatch[20].ItimprovestheCdS/CdTejunctionbehaviorbyenhancingtheformationofanalloyedCdTexS1-x/CdSyTe1-yinterface,wherexandyarelessthanorequaltothesolubilitylimitsatabout400℃（x～0.03andy～0.06）[21].Duetothisfact,itimprovesthestructural,morphologicalandelectricalpropertiesoftheCdTesolarcells,whicheventuallyresultinasignificantincreaseinsolarcellconversionefficiency（g）andinallthreesolarcellparameters:

open-circuitvoltage（Voc）,shortcircuitcurrentdensity（Jsc）andfillfactor（FF）[22].

6Thebackcontact

OneofthemajorproblemsinCdTesolarcellsisthehighcontactresistanceandpoorcontactstabilitybetweenp-typeCdTeandbackcontactclectrode[23].CdTeisap-typesemiconductorwithahighelectronaffinityandbandgap.TypicalmetalsusedforbackcontactstoCdTedonothaveworkfunctionslargeenoughtomakegoodohmiccontactstoCdTe,andtendtoformSchottky,orblockingbarriers.ItisbelievedthatCuisnecessarytomakeanohmiccontactonCdTesinceCdS/CdTesolarcellsmadewithcontactsnotcontainingCuhaveahighseriesresistance[26].ButithasbeenreportedthatCuwassegregatedneartheCdS/CdTejunction.

MostresearchersmakeabackcontactonCdTewithamaterialcontainingCu-Aualloy,Cu2Te,ZnTe：

Cu[24]andCu2S[25].JaeHoYun,KiHwanKim,DooYoulLeeusedCu2TeasaCu-dopingsourceandasanelectrodeinCdTesolarcellsToavoidtheCusegregationattheCdS/CdTeinterface[23].Metal/Sb2Te3thinfilmdouble-layersystemsareusedbyAnkeE.AbkenasamodelforaninnovativebackcontactforCdTe/CdSthinfilmsolarcellsofferinganimprovedlong-termstability[27].S.H.Demtsu,D.S.Albin,J.W.PankowstudiedtheAgandNiback-contactsindicatingthatdeviceswithoutthegraphitelayershowedlargerreductionsinefficiencyrangingfrom25%to45%forAgandNicontacts,respectively[28].AnovelMoOxbackcontactbufferhasbeensuccessfullydevelopedbyHaoLin,Irfan,WeiXiaforthin-filmCdS/CdTesolarcells.WithMoOxasthehighworkfunctionbuffer,variousmetalscanbeusedastheelectrodetorealizeanohmicbackcontacttop–CdTe[29].

7Conclusion

Inrecentyears,themaingoalofthesolarcellresearchisahighconversionefficiency、lowcostandhighstability.CdTeisrecognizedasefficientandlowcostthinfilmbatterymaterials.Alsoitistherapiddevelopmentofacompoundsemiconductorthinfilmsolarcells.InthispaperwedescribedtheformationandtechnologyoftheCdS/CdTesolarcell.WehavepointedthatRFsputteringisthemostsuitableforindustrialproductionandelectrochemicalmethodofdepositiontheCdTethinfilmhasitsuniqueadvantageindepositinginalargeactivityarea.AmixofAranddifluoro-chloromethane（HCF2Cl）wasusedtoimprovetheCdS/CdTejunctionbehavior.AnovelMoOxbackcontactbufferhasbeensuccessfullydeveloped.Thenewlyrecordefficiencyis16.5%