Solar cells

solarcells组员：张天行张帆张茂峰SolarcellsSolarcells(asthenameimplies)aredesignedtoconvert(atleastaportionof)availablelightintoelectricalenergy.Theydothiswithouttheuseofeitherchemicalreactionsormovingparts.StructureModernsolarcellsarebasedonsemiconductorphysics--theyarebasicallyjustP-Njunctionphotodiodeswithaverylargelight-sensitivearea.Thephotovoltaiceffect,whichcausesthecelltoconvertlightdirectlyintoelectricalenergy,occursinthethreeenergy-conversionlayers.DiagramcourtesyU.S.DepartmentofEnergyStructureThefirstofthesethreelayersnecessaryforenergyconversioninasolarcellisthetopjunctionlayer(madeofN-typesemiconductor).Thenextlayerinthestructureisthecoreofthedevice;thisistheabsorberlayer(theP-Njunction).Thelastoftheenergy-conversionlayersisthebackjunctionlayer(madeofP-typesemiconductor).StructureAsmaybeseenintheabovediagram,therearetwoadditionallayersthatmustbepresentinasolarcell.Thesearetheelectricalcontactlayers.Theremustobviouslybetwosuchlayerstoallowelectriccurrenttoflowoutofandintothecell.Theelectricalcontactlayeronthefaceofthecellwherelightentersisgenerallypresentinsomegridpatternandiscomposedofagoodconductorsuchasametal.StructureThegridpatterndoesnotcovertheentirefaceofthecellsincegridmaterials,thoughgoodelectricalconductors,aregenerallynottransparenttolight.Hence,thegridpatternmustbewidelyspacedtoallowlighttoenterthesolarcellbutnottotheextentthattheelectricalcontactlayerwillhavedifficultycollectingthecurrentproducedbythecell.StructureThebackelectricalcontactlayerhasnosuchdiametricallyopposedrestrictions.Itneedsimplyfunctionasanelectricalcontactandthuscoverstheentirebacksurfaceofthecellstructure.Becausethebacklayermustbeaverygoodelectricalconductor,itisalwaysmadeofmetal.OperationSolarcellsarecharacterizedbyamaximumOpenCircuitVoltage(Voc)atzerooutputcurrentandaShortCircuitCurrent(Isc)atzerooutputvoltage.Sincepowercanbecomputedviathisequation:P=I*VOperationThenwithonetermatzerotheseconditions(V=Voc/I=0,V=0/I=Isc)alsorepresentzeropower.Asyoumightthenexpect,acombinationoflessthanmaximumcurrentandvoltagecanbefoundthatmaximizesthepowerproduced(called,notsurprisingly,themaximumpowerpoint).ManyBEAMdesigns(and,inparticular,solarengines)attempttostayat(ornear)thispoint.Thetrickypartisbuildingadesignthatcanfindthemaximumpowerpointregardlessoflightingconditions.OperationForsolarcellselectionandcomparisoninformation,seethesolarcellsectionoftheBEAMReferenceLibrary'sBEAMPiecescollection.AlsoseetheStartingBlockarticleonsolarcells.Thankyou!