8/11/2015 MultijunctionsolarcellWikipedia,thefreeencyclopediahttps://en.wikipedia.org/wiki/Multijunction_solar_cell#Comparison_with_other_technologies 1/20BlacklighttestofDawn' striplejunctiongalliumarsenidesolarcells[1]MultijunctionsolarcellFromWikipedia,thefreeencyclopediaMultijunction(MJ)solarcellsaresolarcellswithmultiplepnjunctionsmadeofdifferentsemiconductormaterials.Eachmaterial'spnjunctionwillproduceelectriccurrentinresponsetodifferentwavelengthsoflight.Theuseofmultiplesemiconductingmaterialsallowstheabsorbanceofabroaderrangeofwavelengths,improvingthecell'ssunlighttoelectricalenergyconversionefficiency.Traditionalsinglejunctioncellshaveamaximumtheoreticalefficiencyof34%.Theoretically,aninfinitenumberofjunctionswouldhavealimitingefficiencyof86.8%underhighlyconcentratedsunlight.[2]Currently,thebestlabexamplesoftraditionalcrystallinesiliconsolarcellshaveefficienciesbetween20%and25%,[3]whilelabexamplesofmultijunctioncellshavedemonstratedperformanceover43%.[4][5]Commercialexamplesoftandem,twolayer,cellsarewidelyavailableat30%underonesunillumination,[6]andimprovetoaround40%underconcentratedsunlight.However,thisefficiencyisgainedatthecostofincreasedcomplexityandmanufacturingprice.Todate,theirhigherpriceandhigherpricetoperformanceratiohavelimitedtheirusetospecialroles,notablyinaerospacewheretheirhighpowertoweightratioisdesirable.Interrestrialapplications,thesesolarcellsareemerginginconcentratorphotovoltaics(CPV),withagrowingnumberofinstallationsaroundtheworld.[7]Tandemfabricationtechniqueshavebeenusedtoimprovetheperformanceofexistingdesigns.Inparticular,thetechniquecanbeappliedtolowercostthinfilmsolarcellsusingamorphoussilicon,asopposedtoconventionalcrystallinesilicon,toproduceacellwithabout10%efficiencythatislightweightandflexible.Thisapproachhasbeenusedbyseveralcommercialvendors,[8]buttheseproductsarecurrentlylimitedtocertainnicheroles,likeroofingmaterials.Contents1Description1.1Basicsofsolarcells1.2Lossmechanisms1.3Multijunctioncells1.3.1MaterialChoice1.4Structuralelements1.4.1Metalliccontacts1.4.2Antireflectivecoating1.4.3Tunneljunctions1.4.4Windowlayerandbacksurfacefield1.5JVcharacteristic8/11/2015 MultijunctionsolarcellWikipedia,thefreeencyclopediahttps://en.wikipedia.org/wiki/Multijunction_solar_cell#Comparison_with_other_technologies 2/201.5JVcharacteristic1.6TheoreticalLimitingEfficiency2Materials2.1Galliumarsenidesubstrate2.2Germaniumsubstrate2.3Indiumphosphidesubstrate2.4Indiumgalliumnitridesubstrate3Performanceimprovements3.1Structure3.2Spectralvariations3.3Useoflightconcentrators4Fabrication5Comparisonwithothertechnologies6Applications7References8Furtherreading9SeealsoDescriptionBasicsofsolarcellsTraditionalphotovoltaiccellsarecommonlycomposedofdopedsiliconwithmetalliccontactsdepositedonthetopandbottom.Thedopingisnormallyappliedtoathinlayeronthetopofthecell,producingapnjunctionwithaparticularbandgapenergy,Eg.Photonsthathitthetopofthesolarcellareeitherreflectedortransmittedintothecell.TransmittedphotonshavethepotentialtogivetheirenergyhtoanelectronifhEg,generatinganelectronholepair.[10]Inthedepletionregion,thedriftelectricfieldEdriftacceleratesbothelectronsandholestowardstheirrespectivendopedandpdopedregions(upanddown,respectively).TheresultingcurrentIgiscalledthegeneratedphotocurrent.Inthequasineutralregion,thescatteringelectricfieldEscattacceleratesholes(electrons)towardsthepdoped(ndoped)region,whichgivesascatteringphotocurrentIpscatt(Inscatt).Consequently,duetotheaccumulationofcharges,apotentialVandaphotocurrentIphappear.Theexpressionforthisphotocurrentisobtainedbyaddinggenerationandscatteringphotocurrents:Iph=Ig+Inscatt+Ipscatt.TheJVcharacteristics(Jiscurrentdensity,i.e.currentperunitarea)ofasolarcellunderilluminationareobtainedbyshiftingtheJVcharacteristicsofadiodeinthedarkdownwardbyIph.Sincesolarcellsaredesignedtosupplypowerandnotabsorbit,thepowerP=VIphmustbenegative.Hence,theoperatingpoint(Vm,Jm)islocatedintheregionwhereV>0andIphEgEmitterandEgBSF>EgEmitter.Furthermore,thelatticeconstantmustbeclosetotheoneofInGaPandthelayermustbehighlydoped(n1018cm3).[20]JVcharacteristicFormaximumefficiency,eachsubcellshouldbeoperatedatitsoptimalJVparameters,whicharenotnecessarilyequalforeachsubcell.Iftheyaredifferent,thetotalcurrentthroughthesolarcellisthelowestofthethree.Byapproximation,[21]itresultsinthesamerelationshipfortheshortcircuitcurrentoftheMJsolarcell:JSC=min(JSC1,JSC2,JSC3)whereJSCi()istheshortcircuitcurrentdensityatagivenwavelengthforthesubcelli.BecauseoftheimpossibilitytoobtainJSC1,JSC2,JSC3directlyfromthetotalJVcharacteristic,thequantumefficiencyQE()isutilized.Itmeasurestheratiobetweentheamountofelectronholepairscreatedandtheincidentphotonsatagivenwavelength.Leti()bethephotonfluxofcorrespondingincidentlightinsubcelliandQEi()bethequantumefficiencyofthesubcelli.Bydefinition,thisequatesto:[22]Thevalueof isobtainedbylinkingitwiththeabsorptioncoefficient ,i.e.thenumberofphotonsabsorbedperunitoflengthbyamaterial.Ifitisassumedthateachphotonabsorbedbyasubcellcreatesanelectron/holepair(whichisagoodapproximation),thisleadsto:[20]wherediisthethicknessofthesubcelliand isthepercentageofincidentlightwhichisnotabsorbedbythesubcelli.Similarly,because,thefollowingapproximationcanbeused: .Thevaluesof arethengivenbytheJVdiodeequation:TheoreticalLimitingEfficiencyWecanestimatethelimitingefficiencyofidealinfinitemultijunctionsolarcellsusingthegraphicalquantumefficiency(QE)analysisinventedbyC.H.Henry.[23]TofullytakeadvantageofHenrysmethod,theunitoftheAM1.5spectralirradianceshouldbeconvertedtothatofphotonflux(i.e.,numberofphotons/m2/s).Todothat,itisnecessarytocarryoutanintermediateunitconversionfromthepowerof8/11/2015 MultijunctionsolarcellWikipedia,thefreeencyclopediahttps://en.wikipedia.org/wiki/Multijunction_solar_cell#Comparison_with_other_technologies 9/20electromagneticradiationincidentperunitareaperphotonenergytothephotonfluxperphotonenergy(i.e.,from[W/m2/eV]to[numberofphotons/m2/s/eV]).Forthisintermediateunitconversion,thefollowingpointshavetobeconsidered:Aphotonhasadistinctenergywhichisdefinedasfollows.(1):Eph=hf=h(c/)whereEphisphotonenergy,hisPlancksconstant(h=6.626*1034[Js]),cisspeedoflight(c=2.998*108[m/s]),fisfrequency[1/s],andiswavelength[nm].Thenthephotonfluxperphotonenergy,dnph/dh,withrespecttocertainirradianceE[W/m2/eV]canbecalculatedasfollows.(2): =E/{h(c/)}=E[W/(m2eV)](109[m])/(1.9981025[Jsm/s])=E5.031015[(#ofphotons)/(m2seV)]Asaresultofthisintermediateunitconversion,theAM1.5spectralirradianceisgiveninunitofthephotonfluxperphotonenergy,[numberofphotons/m2/s/eV],asshowninFigure1.Figure1.Photonfluxperphotonenergyfromstandardsolarenergyspectrum(AMof1.5).Basedontheaboveresultfromtheintermediateunitconversion,wecanderivethephotonfluxbynumericallyintegratingthephotonfluxperphotonenergywithrespecttophotonenergy.ThenumericallyintegratedphotonfluxiscalculatedusingtheTrapezoidalrule,asfollows.(3):Asaresultofthisnumericalintegration,theAM1.5spectralirradianceisgiveninunitofthephotonflux,[numberofphotons/m2/s],asshowninFigure2.8/11/2015 MultijunctionsolarcellWikipedia,thefreeencyclopediahttps://en.wikipedia.org/wiki/Multijunction_solar_cell#Comparison_with_other_technologies 10/20Figure2.Photonfluxfromstandardsolarenergyspectrum(AMof1.5).Itisshouldbenotedthattherearenophotonfluxdatainthesmallphotonenergyrangefrom0eVto0.3096eVbecausethestandard(AM1.5)solarenergyspectrumforh