能量转换与存储原理教学资料designofc-sisolarcell

DesignofSiliconSolarCells,HighlyrecommendedreadingJennyNelson,ThePhysicsofSolarCells,Chapter7,MonocrystallineSolarCells,WhySilicon?,Sihasabandgapof1.1eV,whichisnotfarfromtheoptimalvalueof1.4eVforasinglejunctioncellSiisgreatforMOSFETsbecauseitssurfaceiseasilypassivatedbythermaloxidation.Massiveamountsofresearchhavebeendoneonsilicon.Thesolarcellresearchcommunityborrowedallofthemethodsformakingwafers,doping,patterningandmakingelectricalcontacts.Siisperhapsthebestunderstoodmaterialintheworld.Siisverystable.Siisthesecondmostabundantelementintheearthscrust.,Designofabasicsiliconsolarcell,Nelson,p.187,Whyaren+pSijunctionspreferredoverp+njunctions?Whyisthereathinheavilydopedlayerandathicklightlydopedlayer?Whythosethicknesses?,Howthickshouldthewaferbe?,IssuestoconsiderAbsorptionThediffusionlengthWillthewaferbreak?Thisisusuallymostimportantforwafers.,determineswherethelightisabsorbed,Nearthebandedge,notallofthelightisabsorbedandsomeminoritycarriersarelostduetobacksurfacerecombination.Ath=3.5eV,thelightisabsorbedrightatthesurface.,Nelsonp.182,transmission=e-ax,AbsorptiondepthinSi,Surfacetexturing,Nelsonp.190,Surfacetexturingscatterslightsothatittravelsmorehorizontallythoughthecell.Thishelpsabsorbphotonswithenergyjustabovethebandgap.Itreducesreflectionalso.,Lnmustbelongenoughtoallowthecarrierstoreachthejunction,Ifp1018cm3,thentwillbegreaterthan1msandprimarilydeterminesbythedensityofrecombinationcenters.ModernelectrongradeSihasalifetimeabove1ms.,Thediffusivitydependsonthedopingdensity,Impurityconcentration(cm-3),Mobility(cm2/Vs),Streetmanp.99,TheEinsteinRelationship(Streetmanp.129)tellsusthatTheelectronmobilityishigherthantheholemobilityThemobilitydropssubstantiallywhennorpexceeds1017cm-3,Designofabasicsiliconsolarcell,Nelson,p.189,Thebaseshouldbep-typesothattheminoritycarrierswillbeelectrons,whichdiffusefasterthanholes.ThebaseshouldbelightlydopedsothatrecombinationwillbeslowerandDwillbehigher.,WhydontSisolarcellsworkwellatl1100nm?,Eg=1.1eVl=1240nm-eV/E,Backsurfacefield,HeavilydopingtheSinearthebackcontactcreatesabarrierforelectronsandreducessurfacerecombination.Thisbecomesveryimportantforharvestingthelowenergyphotonsasthewafergetsthin.,Nelsonp.190,Whatshouldthedopinglevelofthesubstratebe?,TheminoritycarrierdiffusionlengthisL=(Dnn)1/2andshouldbeaslongaspossible.Thedopinglevelmustbe1018cm-3toavoidrapidAugerrecombinationandshouldbeaslowaspossible.Ontheotherhand,togetahighVoc,weneedalowIo.,AhighdopingdensityisneededtominimizeI0.(NotethatDnandLnvarywithdoping,butNAisstillthedominantfactor.)Adopinglevelof1016cm-3endsupbeingagoodcompromisebetweenthetwo.,Thetopofthecellisheavilydopedtominimizeseriesresistance,Theresistivityofthen-typelayerisThesheetresistanceis,ThepowerlossduetotheseriesresistanceisThefractionofpowerlossisForatypicalcommercialSicell,s=40/Jmpp=30mA/cm2Vmpp=450mVSmustbe4mmtokeepthepowerlossbelow4%.,Green,SolarCellsp.146,PassivatedEmitterSolarCell(PESC),MartinGreenetal.inventedtheconceptofpassivatingmostofthetopsurfacewithathinoxidetopreventsurfacerecombination.ThePESCtooktheefficiencyto20%in1985.,Nelsonp.196,Rearpointcontactcell,IntroducedbyDickSwansonatStanfordin1992withanefficiencyof22%.Frontsurfaceistexturedandpassivated.ThistechnologyisbeingcommercializedquitesuccessfullybySunPower.Sincethereisnometalontop,therearenoshadowinglosses.Thesecellslookgreatbecauseverylittlelightisreflectedfromthem.,Nelsonp.196,Passivatedemitter,rearlocallydiffused(PERL)cell,DemonstratedbyMartinGreenin1994.HoldstheworldrecordforSiwith=24%.Rearpointcontactsminimizetheareaofmetalandrearsurfacerecombination.,Nelsonp.197,SolarCellEfficiencyTable(v31),LabEfficiencies,M.Green,ProgressinPhotovoltaics(2008),MinorityCarrierLifetimesaffectDarkCurrent,TypicalSiliconPVDoping,Mobilitiesareaffectedbydoping.,Pr