能量转换与存储原理教学资料recombina

第十三课复合机制(recombination),朱嘉,2014年10月20日,南京大学现代工程与应用科学学院,测试,N(1E19),P(1E17),N(1E19),P(1E16),a),b),Wa,Wb,Vbia,Vbib,WN,WP,1),2),Banddiagram(n:Si,p:Si)(能带图)0V,positivebias,negativebias,3),Banddiagram(n:Si,p:Ge)Thermalequilibrium(热平衡),4),5),10Sun,b)1SunJscaJscbVocaVocb,6)bonus,feedback,Opencircuitvoltage(VOC),TheopencircuitvoltageissimplythevoltagewhereJ=0.AtVoc,SolvingforV,givesus,ThisisaveryimportantconceptthatmanyresearcherswhoaredevelopingnewPVtechnologiesdonotseemtoappreciate.,4,RecombinationinSemiconductors(半导体中的复合),(1)Radiative,(2)Auger俄歇复合),(3)Surface,(4)Shockley-Read-Hall(trapassisted)Recombination(间接复合),Whyisrecombinationimportant?,5,Photoexcitedelectronsinthebaseneedtodiffusetothejunctiontobecollected.Theprobabilityofthemreachingthejunctionbeforerecombiningis,wherexisthedistancetothequasineutralregion(拟中性区域)/depletionregioninterfaceand,BanddiagramofatypicalcrystallineSicell,6,Whyisrecombinationimportant?,PreviouslectureshaveshownhowJscandVocarerelatedtothedarkcurrent(J0),J0ishighlydependentontheminoritycarrierdiffusionlength,Ln.,Weneedtounderstandrecombinationtoknowhowtomaximize.,Earlyviewofthekeypoints:Thedominantmechanismdependsonthedopinglevelandthedefectdensity,7,Fig.8.7Green,p.148.,8,RadiativeRecombination,Radiativerecombinationistheoreticallyunavoidable.Itdoesnotinvolvedefects.,Therateconstantar(速率常数),9,ardependsontheoverlapoftheelectronwavefunctionandtheholewavefunction(电子波函数和空穴波函数的重叠).Iftheoverlapisweak,radiativerecombinationwillbeslow,butabsorptionwillbeweakbecauseitdependsonthesamewavefunctionoverlap.Wewouldlikestrongabsorberswithslowradiativerecombination,butwecanthavethatcombination.,DirectandIndirectBandGaps,Streetman,pg63,=2p/kRadiationandabsorptionoccurwhenkisthesamefortheinitialandfinalstatebecausethewavefunctionshavethesamewavelength.Directbandgapmaterialsabsorbandemitstrongly.Indirectbandgapmaterialsdont.,e.g.GaAs,CdTe,CIGSa-Si,organicsemiconductors,e.g.Si,Ge,11,AugerRecombination,Achargecarrier(e.g.electron)givesitsenergytoanotherchargecarrier,whichgoesdeeperintotheband.Thiscarrierthengoesbacktothebandedge,givingofftheenergytophonons.,Inann-typesemiconductor,therateofAugerrecombinationis,Fig3.10,p.53Green,12,AugerRecombinationinSilicon,Augerrecombinationforap-typesemiconductor,TotalRecombinationintheBulk,Thefactthatthattherateofrecombinationincreaseslinearlywithexcesscarrierconcentrationforallofthemechanismsisreallyimportant.Ifthiswerentthecase,thesuperpositionprinciple(叠加原理)wouldntbevalid.,Thedominantmechanismofrecombinationdependsonthedopingdensity,Fig.8.7Green,p.148.,Augerrecombinationdominatesinheavilydopedmaterialandmakesefficientcarriercollectiontherenearlyimpossible.,HowRecombinationoccursviaImpurities,EC,EV,Introduceadensityofimpurities(NT)nearthemiddleofthegapwithenergy(ET).,TheFermilevel(费米能级)determinesfractionofoccupiedtraps.,Recombinationviatrapsisatwostepprocess.,1)Electroncapturebythetrap.,(1),2)Holecapturebythetrap.,(2),NoticethepopulationoftheelectronsinthetrapsisunchangedbuttherearefewerelectronsintheCB.,WhySRHrecombinfasterthanradiativerecomb,1.)Whenenergymustbedumpedintophonons,theprocesswillslowdownasmorephononsneedtobeemitted.2.)Thedefectstatesarelocalized,whichmeansthewavefunctiondescribingthemhascomponentsinitwithmanydifferentvaluesofk.Oneofthosecomponentshasthesamevalueofkastheelectron.,TheSRHLifetime,Where:,ItcanbeshownthattheelectronlifetimeduetoSRHrecombis,DeviceElectronicsforIntegratedCircuits,MullerandKamins(2003)pp228-235,WhatyouneedtoknowaboutSRHrecombination,Therateisfastestwhenthedefectisnearthecenterofthebandgap.Theratehasthecomplicateddependenceofdopingdensityshowninthepreviousslidebecausetheratedependsonwhatfractionoftherecombinationcenters(defects)areoccupiedwithanelectronorhole.,SRHRecombinationDependsontheDopingDensity(掺杂密度),Assumingp-typedopingandrecombinationofelectrons.,Ef=Et=Ei,Tohavearecombinationeventyouneedbothholeandelectroncapture.Atlowdopinglevelsbothprocessesarefairlyslow.,Rp-capture,Rn-capture,SRHRecombinationDependsontheDopingDensity,Assumingp-typedopingandrecombinationofelectrons.,Ef,Athighdopinglevels,almostalloftherecombinationcentershaveaholeandcanthereforeacceptanelectron.Theratenolongerdependsondopingdensity.,Rp-capture,Rn-capture,Et,21,SurfaceRecombinationSchematic,Danglingbondsatthesurfaceoftencreatedefectlevelsnearthecenterofthegap.Aswejustsaw,theselevelspromoterecombination.,22,RecombinationviaSurfaceStates,Electron/holecapturecross-sectionarea,Thermalvelocityofelectrons*,*TakenfromP.Wurfel(pg76)butveisdifferentthanthermalvelocitycalculatedbyMaxwell-Boltzmanndistribution.,Thesurfacerecombinationvelocityis,Densityofsurfacestatesperareaoccupiedbyahole,Dependentonsurfacepassivation(表面钝化),latticemismatc(晶格失配),andthefermilevel.,SurfaceRecombinationVelocities,BareSihas2*105cm/sSiterminatedbyametalis106cm/sPassivatingthedanglingbondsbyreactingthemwithsomethinghelpsSiterminatedbySiO2has50-500cm/sSiterminatedbySiO2+Alwithannealhas20-50cm/s(usedinrecordSiPVdevice),Takehomemessagesregardingsurfacerecombination,24,Weshouldavoidhavingexcesscarriersnearsurfaces.Thisisbadnewsfornanostructuredcells.Surfacesshouldbepassivated.Interfacesinthehetostructuresshouldhavegoodlatticematchingifpossibletoavoiddefectformation.,25,MoreInformationAvailable,RadiativeRecombination:P.Wurfel,Thechemicalpotentialofradiation,J.Phys.C.15(1982)pp3967-3985.AugerRecombination:E.Yablonovitch,AugerRecombinationinsiliconatlowcarrie