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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%