光伏发电英文文献Ultra-High-Efficiency-Photovoltaic

Ultra-HighEfficiencyPhotovoltaicCellsforLargeScaleSolarPowerGenerationYoshiakiNakanoAbstractTheprimarytargetsofourprojectaretodras-ticallyimprovethephotovoltaicconversionefficiencyandtodevelopnewenergystorageanddeliverytechnologies.Ourapproachtoobtainanefficiencyover40%startsfromtheimprovementofIII–Vmulti-junctionsolarcellsbyintroducinganovelmaterialforeachcellrealizinganidealcombinationofbandgapsandlattice-matching.Furtherimprovementincorporatesquantumstructuressuchasstackedquantumwellsandquantumdots,whichallowhigherdegreeoffreedominthedesignofthebandgapandthelatticestrain.Highlycontrolledarrangementofeitherquantumdotsorquantumwellspermitsthecouplingofthewavefunctions,andthusformsintermediatebandsinthebandgapofahostmaterial,whichallowsmultiplephotonabsorptiontheoreticallyleadingtoaconversionefficiencyexceeding50%.Inadditiontosuchimprovements,microfabricationtechnologyfortheintegratedhigh-effi-ciencycellsandthedevelopmentofnovelmaterialsystemsthatrealizeshighefficiencyandlowcostatthesametimeareinvestigated.KeywordsMulti-junctionQuantumwellConcentratorPhotovoltaicINTRODUCTIONLarge-scalephotovoltaic(PV)powergenerationsystems,thatachieveanultra-highefficiencyof40%orhigherunderhighconcentration,areinthespotlightasanewtechnologytoeasedrasticallytheenergyproblems.Mul-tiplejunction(ortandem)solarcellsthatuseepitaxialcrystalsofIII–VcompoundsemiconductorstakeontheactiveroleforphotoelectricenergyconversioninsuchPVpowergenerationsystems.Becausethesesolarcellsoperateunderasunlightconcentrationof5009to10009,thecostofcellsthatusetheepitaxialcrystaldoesnotposemuchofaproblem.InconcentratorPV,theincreasedcostforacelliscompensatedbylesscostlyfocusingoptics.Thephotonsshiningdownonearthfromthesunhaveawiderangeofenergydistribution,fromthevisibleregiontotheinfraredregion,asshowninFig.1.Multi-junctionsolarcells,whicharelaminatedwithmultilayersofp–njunctionsconfiguredbyusingmaterialswithdifferentbandgaps,showpromiseinabsorbingasmuchofthesephotonsaspossible,andconvertingthephotonenergyintoelec-tricitywithminimumlosstoobtainhighvoltage.Amongthevarioustypesofmulti-junctionsolarcells,indiumgalliumphosphide(InGaP)/galliumarsenide(GaAs)/ger-manium(Ge)triple-junctioncellsthatmakefulluseoftherelationshipbetweenbandgapsanddiverselatticecon-stantsofferedbycompoundsemiconductorshavetheadvantageofhighconversionefficiencybecauseoftheirhigh-qualitysinglecrystalwithauniform-sizecrystallat-tice.Sofar,aconversionefficiencyexceeding41%underconditionswheresunlightisconcentratedtoanintensityofapproximately5009hasbeenreported.Thetunneljunctionwithafunctionequivalenttoelec-trodesisinsertedbetweendifferentmaterials.Thepositiveholesaccumulatedintheplayerandtheelectronsintheadjacentnlayerwillberecombinedandeliminatedinthetunneljunction.Therefore,threep–njunctionsconsistingofInGaP,GaAs,andGewillbecomeconnectedinseries.Theupperlimitoftheelectriccurrentissetbythemini-mumvalueofphotonfluxabsorbedbyasinglecell.Ontheotherhand,thesumofvoltagesofthreecellsmakeupthevoltage.AsshowninFig.1,photonsthatcanbecapturedintheGaAsmiddlecellhaveasmallfluxbecauseofthebandgapofeachmaterial.Asaresult,theelectriccurrentoutputRoyalSwedishAcademyofSciences2012(Supplement2):125–131DOI10.1007/s13280-012-0267-4fromtheGaAscelltheoreticallybecomessmallerthanthatoftheothersanddeterminestheelectriccurrentoutputoftheentiretandemcell.Todevelopahigherefficiencytandemcell,itisnecessarytouseamaterialwithabandgapnarrowerthanthatofGaAsforthemiddlecell.Inordertoobtainmaximumconversionefficiencyfortriple-junctionsolarcells,itisessentialtonarrowdownthemiddlecellbandgapto1.2eVandincreasetheshort-circuitcurrentdensityby2mA/cm2comparedwiththatoftheGaAsmiddlecell.Whenthematerialisreplacedwithanarrowerbandgap,theoutputvoltagewilldrop.However,theeffectofimprovingtheelectriccurrentbalanceout-performsthisdropinoutputvoltageandbooststheeffi-ciencyoftheentiremulti-junctioncell.WhenacrystalwithsuchanarrowbandgapisgrownonaGebasematerial,latticerelaxationwilloccurinthemiddleofepitaxialcrystalgrowthbecausethelatticeconstantsofnarrowerband-gapmaterialsarelargerthanthatofGe(asshowninFig.2).Asaresult,thecarriertransportpropertieswilldegradeduetodislocation.ResearchersfromtheinternationalresearchcenterSolarQuest,theUniversityofTokyo,aimtomovebeyondsuchmaterial-relatedrestrictions,andobtainmaterialsandstructuresthathaveeffectivenarrowbandgapswhilemaintaininglatticematchingwithGeorGaAs.Toachievethisgoal,wehavetakenthreeapproachesasindicatedinFig.3.Theseapproachesareexplainedindetailbelow.DILUTENITROGEN-ADDEDBULKCRYSTALIndiumgalliumnitridearsenide(InGaNAs)isabulkmaterialconsistsofInGaAs,whichcontainsseveralpercentofnitrogen.InGaNAshasahighpotentialforachievinganarrowbandgapwhilemaintaininglatticematchingwithGeorGaAs.However,InGaNAshasafatalproblem,thatis,adropincarriermobilityduetoinhomogeneousdistributionofnitrogen(N).ToachievehomogeneoussolidsolutionofNincrystal,wehaveappliedatomichydrogenirradiationinthefilmformationprocessandadditionofaverysmallamountofantimony(Sb)(Fig.3).Theatomichydrogenirradiati