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NanomaterialsforSolar CellApplications

Editedby

SABUTHOMAS

SchoolofChemicalSciences, MahatmaGandhiUniversity,Kottayam,India; InternationalandInterUniversity, CentreforNanoscienceandNanotechnology, MahatmaGandhiUniversity,Kottayam,India

ELHADJIMAMOURSAKHO

DepartmentofChemicalSciences (FormerlyAppliedChemistry), UniversityofJohannesburg

NANDAKUMARKALARIKKAL

InternationalandInterUniversity, CentreforNanoscienceandNanotechnology, MahatmaGandhiUniversity,Kottayam,India

SAMUELOLUWATOBIOLUWAFEMI

DepartmentofChemicalSciences (FormerlyAppliedChemistry), UniversityofJohannesburg

JIHUAIWU

ProfessorofMaterialsandChemistry,Vice-PresidentofHuaqiao University,DirectorofEngineeringResearchCenterof Environment-FriendlyFunctionalMaterials,MinistryofEducation, DirectorofInstituteofMaterialsPhysicalChemistry, HuaqiaoUniversity,Xiamen,Fujian,P.R.China

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ListofContributors

AntonioAbate

HelmholtzCenterforMaterialsandEnergy,Berlin,Germany

Y.Akila

DepartmentofPhysics,CoimbatoreInstituteofTechnology,Coimbatore,India

AlexanderV.Akkuratov

InstituteforProblemsofChemicalPhysicsofRussianAcademyofSciences, Chernogolovka,RussianFederation

MarcieBlack

AdvancedSiliconGroup,Lincoln,MA,UnitedStates

DanielChemisana

AppliedPhysicsSectionoftheEnvironmentalScienceDepartment,PolytechnicSchool, UniversityofLleida,Lleida,Spain

SaniatAhmedChoudhury

IndependentUniversity,Bangladesh(IUB),Dhaka,Bangladesh

MustafaHabibChowdhury

IndependentUniversity,Bangladesh(IUB),Dhaka,Bangladesh

WenqiDuan

ElectricalandComputerEngineeringDepartment,UniversityofIowa,IowaCity,IA, UnitedStates

FatemaFairooz

IndependentUniversity,Bangladesh(IUB),Dhaka,Bangladesh

LeandroA.Faustino

LaboratoryofPhotochemistryandMaterialsScience,InstituteofChemistry,Federal UniversityofUberlandia,Uberlandia,Brazil

NaokiFukata

InternationalCenterforMaterialsNanoarchitectonics,NationalInstituteforMaterials Science,Tsukuba,Japan

BingtaoGao

ElectricalandComputerEngineeringDepartment,UniversityofIowa,IowaCity,IA, UnitedStates

PengGao

CASKeyLaboratoryofDesignandAssemblyofFunctionalNanostructures,andFujian KeyLaboratoryofNanomaterials,FujianInstituteofResearchontheStructureofMatter, ChineseAcademyofSciences,Fuzhou,P.R.China;LaboratoryofAdvancedFunctional Materials,XiamenInstituteofRareEarthMaterials,HaixiInstitute,ChineseAcademyof Sciences,Xiamen,P.R.China

ChangKookHong

PolymerEnergyMaterialsLaboratory,SchoolofAppliedChemicalEngineering, ChonnamNationalUniversity,Gwangju,SouthKorea

RajanJose

NanostructuresRenewableEnergyMaterialsLaboratory,FacultyofIndustrialSciences& Technology,UniversitiMalaysiaPahang,Gambang,Malaysia

HyungjinKim

PolymerEnergyMaterialsLaboratory,SchoolofAppliedChemicalEngineering, ChonnamNationalUniversity,Gwangju,SouthKorea

P.SenthilKumar

DepartmentofChemicalEngineering,SSNCollegeofEngineering,Chennai,India

IliyaE.Kuznetsov

InstituteforProblemsofChemicalPhysicsofRussianAcademyofSciences, Chernogolovka,RussianFederation

ChrysovalantouLamnatou

AppliedPhysicsSectionoftheEnvironmentalScienceDepartment,PolytechnicSchool, UniversityofLleida,Lleida,Spain

JunmingLi

HelmholtzCenterforMaterialsandEnergy,Berlin,Germany

XiaoxiaoLin

SawantaS.Mali

PolymerEnergyMaterialsLaboratory,SchoolofAppliedChemicalEngineering, ChonnamNationalUniversity,Gwangju,SouthKorea

AndressaV.Muller

FederalUniversityofABC,SantoAndré,Brazil

N.Muthukumarasamy

DepartmentofPhysics,CoimbatoreInstituteofTechnology,Coimbatore,India

Mu.Naushad

DepartmentofChemistry,CollegeofScience,KingSaudUniversity,Riyadh,Saudi Arabia

BarbaraN.Nunes

LaboratoryofPhotochemistryandMaterialsScience,InstituteofChemistry,Federal UniversityofUberlandia,Uberlandia,Brazil

TakeoOku

DepartmentofMaterialsScience,TheUniversityofShigaPrefecture,Hikone,Japan

OluwatobiS.Oluwafemi

CentreforNanomaterialsScienceResearch,UniversityofJohannesburg,Johannesburg, SouthAfrica;DepartmentofChemicalSciences(FormerlyAppliedChemistry),University ofJohannesburg,DoornfonteinCampus,Johannesburg,SouthAfrica

GerkoOskam

DepartamentodeFísicaAplicada,CINVESTAV-IPN,Mérida,México

SundararajanParani

DepartmentofChemicalSciences(FormerlyAppliedChemistry),Universityof Johannesburg,DoornfonteinCampus,Johannesburg,SouthAfrica;Centrefor NanomaterialsScienceResearch,UniversityofJohannesburg,Johannesburg,SouthAfrica

JyotiV.Patil

PolymerEnergyMaterialsLaboratory,SchoolofAppliedChemicalEngineering, ChonnamNationalUniversity,Gwangju,SouthKorea;ThinFilmMaterialsLaboratory, DepartmentofPhysics,ShivajiUniversity,Kolhapur,India

PramodS.Patil

ThinFilmMaterialsLaboratory,DepartmentofPhysics,ShivajiUniversity,Kolhapur, India

AntonioOtavioT.Patrocinio

LaboratoryofPhotochemistryandMaterialsScience,InstituteofChemistry,Federal UniversityofUberlandia,Uberlandia,Brazil

K.GracePavithra

DepartmentofChemicalEngineering,SSNCollegeofEngineering,Chennai,India

AndreS.Polo

FederalUniversityofABC,SantoAndré,Brazil

DenaPourjafari

DepartamentodeFísicaAplicada,CINVESTAV-IPN,Mérida,México

RashidAhmedRifat

IndependentUniversity,Bangladesh(IUB),Dhaka,Bangladesh

A.Riverola

AppliedPhysicsSectionoftheEnvironmentalScienceDepartment,PolytechnicSchool, UniversityofLleida,Lleida,Spain

ElHadjiMamourSakho

SubramaniThiyagu

InternationalCenterforYoungScientists(ICYS),NationalInstituteforMaterialsScience, Tsukuba,Japan;InternationalCenterforMaterialsNanoarchitectonics,NationalInstitute forMaterialsScience,Tsukuba,Japan

SabuThomas

InternationalandInterUniversityCentreforNanoscienceandNanotechnology, MahatmaGandhiUniversity,Kottayam,India

FatimaToor

ElectricalandComputerEngineeringDepartment,UniversityofIowa,IowaCity,IA, UnitedStates

PavelA.Troshin

InstituteforProblemsofChemicalPhysicsofRussianAcademyofSciences, Chernogolovka,RussianFederation;SkolkovoInstituteofScienceandTechnology, Moscow,RussianFederation

R.JoseVarghese

InternationalandInterUniversityCentreforNanoscienceandNanotechnology, MahatmaGandhiUniversity,Kottayam,India;DepartmentofChemicalSciences (FormerlyAppliedChemistry),UniversityofJohannesburg,DoornfonteinCampus, Johannesburg,SouthAfrica;CentreforNanomaterialsScienceResearch,Universityof Johannesburg,Johannesburg,SouthAfrica

DhayalanVelauthapillai

DepartmentofChemicalSciences(FormerlyAppliedChemistry),Universityof Johannesburg,Johannesburg,SouthAfrica

A.Vossier

CNRS-PROMES,Odeillo,France

QamarWali

NUTECHSchoolofAppliedSciencesandHumanities,NationalUniversityof Technology,Islamabad,Pakistan

QiongWang

HelmholtzCenterforMaterialsandEnergy,Berlin,Germany

JihuaiWu

ProfessorofMaterialsandChemistry,Vice-PresidentofHuaqiaoUniversity,Directorof EngineeringResearchCenterofEnvironment-FriendlyFunctionalMaterials,Ministryof Education,DirectorofInstituteofMaterialsPhysicalChemistry,HuaqiaoUniversity, Xiamen,Fujian,P.R.China

Preface

Theinterestsofdevelopingrenewable,sustainable,andcleanenergy sourceshavebecomeveryhighbecauseoftheemergenceofglobal warmingandthevastuseofnonrenewableenergysources,suchasnatural gas,oil,andcoal.Severalrenewableenergysources,suchaswaveand tidalpower,windturbines,hydropower,solarcells,fuelcells,andsolar thermalarebeinginvestigatedtoevaluatetheirpotentialtoaddresslargescaledemand.Amongthesesources,solarphotovoltaic(PV)technology, whichusessolarradiationenergy,hasbeenconsideredasthemostabundant,inexhaustible,clean,andsustainableenergyresource.Solarcells directlyconverttheincidentsolarradiationintoelectricityviathePV effectandcanconvertuptoabout20%ofincomingsolarradiation.Solar cellsareclassifiedintothreegenerations,whicharebasedontheirmaterialsandmanufacturingprocess.Silicon(Si)singlecrystalwafersandbulk polycrystallineSiwafersarethefirstgenerationofsolarcells.Thesecells, accordingtothemanufacturingproceduresandwaferquality,givesolar conversionefficienciesbetween12%and16%andarelargelyleadingthe solarcellsmarket.Thethin-filmsolarcellsthataremadefromdifferent materials,suchasamorphoussilicon,a-Si,cadmiumtelluride,cadmium indiumselenide,orthinsiliconfilmsonindiumtinoxide,t-Si,arethe secondgenerationofsolarcells.Thistechnologyprovideslessexpensive solarcellswithlowersolarenergyconversionincomparingtothesilicon waferstechnology.Thethirdandemergingsolarcellsgeneration,which canproducehighefficiencydevicesatlowproductioncostsofsolarcells, arebasedonpolymersolarcells,dyesynthesizedsolarcells,quantumdots solarcells,andperovskitesolarcells.

Recently,nanomaterialshaveemergedasthenewbuildingblocksto constructsolarcellassemblies.Theuseofnanomaterialsinsolarcellapplicationisgainingtremendousinterestandbuildinggreatexpectationsin theacademiccommunity,industry,andgovernments.Amotivationto develophighefficiencyandcost-effectivenanostructuredmaterialsfor solarcellsisgrowingandaspecificcontributionofnanotechnologyto varioussolarenergyisbeingdeveloped.Nanomaterialsprovidenew methodstoapproachsolarenergyconversionwithaflexibleandpromisingmaterialplatform.Thereforenanostructuredmaterials,suchasmetal oxide,quantumdots,perovskite,graphene,carbonnanotubes,and

fullereneplayasignificantroleinsolarcellapplications.Hence,ithas beendemonstratedthatnanostructuredmaterialscanimprovetheperformanceofsolarcellsbyenhancingbothlighttrappingandphoto-carrier collection.Furthermore,thesynthesis,characterizations,andutilizationof thesenovelnanostructureslieattheinterfaceamongphysics,chemistry, engineering,andmaterialsscience.Thestructure,size,andshapeofthese nanomaterialshavesignificanteffectovertheefficiencyofthesolarenergy conversion.

Overthelasttwodecadestherearenumerousresearchpaperson nanostructuredmaterialsforsolarcellapplications.Afewresearchpapers arebasedonmetaloxide-basedsolarcells,quantumdotsensitizedsolar cells,dyesensitizedsolarcells,andpolymernanocompositessolarcells. Recently,nano-carbonbasedmaterialssuchasgraphene,graphenederivatives,carbonnanotube,andfullerenehavebeenextensivelyinvestigated onsolarcells.However,uptonow,nosystematiceffortshavebeenmade tocomeoutwithabookthatexclusivelycoversthesynthesis,characterizations,andpropertiesofnanomaterialsforsolarcellapplicationsthatare verymuchrequiredforacademeandindustry.

Thusthisbookreportsonthedevelopmentsinthesynthesisand characterizationsofnanomaterialsforsolarcells.Thebookstartswitha discussiononthefundamentalsofnanomaterialsforsolarcells,includinga discussiononthelife-cycleassessmentsandcharacterizationtechniques.It thenfollowswithareviewofthevarioustypesofsolarcells:thinfilm, metaloxide,nanowire,nanorods,andnanoporousmaterials,andconcludeswithnanocarbonmaterials.Inaddition,itincludesareviewof quantumdotsensitized,perovskite,andpolymernanocomposites-based solarcells.

CHAPTER1 Fundamentalsofsolarcells

A.Riverola1,A.Vossier2 andDanielChemisana1

1AppliedPhysicsSectionoftheEnvironmentalScienceDepartment,PolytechnicSchool,Universityof Lleida,Lleida,Spain

2CNRS-PROMES,Odeillo,France

Contents

1.1 Introduction3

1.2 Thesolarresource,solarenergy4

1.3 Principlesofphotovoltaicenergyconversion7

1.4 Semiconductors7

1.4.1 Bands,electrons,andholes8

1.4.2 Doping,nandptypes9

1.4.3 Generationandrecombinationofelectron holespairs11

1.5 Solarcellstructure,operation,andmainparameters13

1.5.1 p nJunction13

1.5.2 Structure,operation,andmainparametersofsolarcells15

1.6 Upperlimitforsolarenergyconversion20

1.7 ReducingBoltzmannlosses:opticalconcentrationandangularrestriction22

1.7.1 Opticalconcentration23

1.7.2 Angularrestriction25

1.8 Reducingthermalizationandbelow-Eg losses:advancedconceptsof photovoltaiccells26

1.8.1 Multijunction(MJ)solarcells26

1.8.2 Otherconcepts28 References32 Furtherreading33

1.1Introduction

Duringthelastdecades,photovoltaics(PVs)havebecomeoneofthe mostpromisingrenewableenergytechnologies,withinstalledcapacityof PVpanelsapproaching100GWin2018.Highconversionefficienciesat reasonablecostsundoubtedlyrepresenta sine-qua-non conditiontobefulfilledtowardpromotinganevenwiderdeploymentofsolarelectricity.

NanomaterialsforSolarCellApplications

DOI: https://doi.org/10.1016/B978-0-12-813337-8.00001-1

ThedevelopmentofstrategiesaimingatanimprovedPVefficiencyhas instigatedabroadrangeofresearchactivitiesinthemostrecentdecades. Withthisobjective,strategiesinvolvingnanomaterials,implementationof nanoobjects,ormanipulationoflightatananometerscale,hasprompted aconsiderableamountofresearch.Thesedifferentstrategieswillbecarefullyreviewedinthenextbookchapters.Inthischapter,weaimtoprovideseveralfundamentalconceptsnecessarytobettergrasptheunderlying physicalmechanismsgoverningPVcells(Adetailedexplanationofthese conceptscanbefoundinothertextbooks [1,2]).

ThePVeffect,whichwasdiscoveredbyEdmundBecquerelin1839, basicallyimpliesdirectconversionofsunlightintoelectricityusingaPV cellmadeofasemiconductormaterialtailoredtoensurebothahigh absorptionofsunlightandanefficientextractionofthephotogenerated carriers.

1.2Thesolarresource,solarenergy

Thespectraldistributionofsunlightspansabroadrangeofwavelengths rangingfromtheultraviolettothenearinfrared.Therelationbetween thephotonenergy(E)anditswavelength(λ)isgivenby:

where c isthelightspeedinvacuum(approximately3.00 3 108 ms 1) and h isthePlanck’sconstant(6.63 3 10 34 Js).

Thespectraldistributionofsunlightmayvarynoticeablydepending on(1)thepositionofthesuninthesky(whichisfunctionofthecharacteristiclatitudeofthesitewherethePVcellissupposedtooperate,the timeoftheday,andthedayintheyear)and(2)typicalatmosphericparametersvalues,whicharelikelytochangenoticeablydependingontheclimaticandatmosphericconditions.

Airmass(AM)istheatmosphericvariabletowhichthesolarspectrum isnormallymoresensitive.Itisdefinedasthedistance,relativetotheshortest(vertical)pathlength,thatsunraystraversethroughtheatmosphere beforeimpactingontheEarth’ssurface.AMcansimplybedefinedas:

where θ isthesocalled solarzenithangle,thatis,theanglebetweenthe zenithandthecenterofthesun’sdisc.

Nonetheless,amoreaccurateexpressionthatconsiderstheEarth’ scurvatureiscommonlyusedtopredictordefinethesolarspectrum [3]

Fig.1.1 showstwocommonlyusedsolarspectra:AM0(standard extraterrestrialsolarspectrummainlyusedbytheaerospacecommunity) andAM1.5Global(wherethereceivingsurfaceisdefinedasaninclined planeat37degreestilttowardtheequator,facingthesun).

ThespectraldistributioncorrespondingtoAM0solarspectrumcanbe approximated,withagoodaccuracy,tothespectrumofablackbodyat 5758K(Thespectraldistributionforblackbodyradiationbeingonly determinedbyitstemperature,asstatedbyPlanck’slaw).

TheAM1.5Globalspectrumoftenservesastheterrestrialstandard (reference),andismeasuredonasurfacethatfacesthesun,withatilt angleof37degreesoverthehorizontalplane,underspecifiedatmospheric conditions[aerosolopticaldepth(AOD)of0.084,precipitablewater

Figure1.1 Extraterrestrialsolarspectrum(AM0)andthestandardterrestrialspectrum(AM1.5Global). RetrievedfromASTM,G173-03Standardtablesforreferencesolar spectralirradiances:directnormalandhemisphericalon37°tiltedsurface,Bookof Standards,14.04.2004 [4].

(PW)of1.42cmandtotalcolumnozoneequivalentof0.34cm].An AMof1.5correspondstoasolarzenithangleofapproximately48 degrees.Passingthroughtheatmosphere,thespectrumisattenuateddifferentlyforeachwavelengthduetoabsorptionorscatteringbyatmosphericparticles.Forinstance,watervaporabsorptionbandsaremainly locatedinthenear-infraredandinfraredregionsofthespectrum (around0.94,1.10,and1.40 µm).Theamplitudeoflightscatteringin theatmosphereiscorrelatedtotheAMvalue:thehighertheAM,the higherthelightscatteringbyatmosphericmolecules(suchasnitrogen andoxygen).Consequently,theterrestrialirradiance(whichiscommonlynormalizedto1000Wm 2 )islowerthantheextraterrestrial irradiance(around1353Wm 2 ).Thepeaksolarirradiance,whichcorrespondstowavelengthstypicallycomprisedbetween0.4and0.8 µm,is associatedwith “ visible ” lightinthesensethathumanvisionevolvedto beparticularlysensitivetothisspectralrange.Oneshoulddistinguish differentdefinitionsforsolarirradiance:directnormalirradiance(DNI) referstothephotonscomingdirectlyfromthesun.Itshouldbenoted thatthedefinitionofDNIisnotunivocal.Thisambiguitystemsfrom thefactthattheangulardistancefromthecenterofthesunandthe penumbrafunctionarenotwelllimited.SeveraldefinitionsofDNIcan befoundintheliterature,explicitlyorimplicitlyreferringtodifferent limitanglesandpenumbrafunctions,whichinherentlyleadtovarying amountsofintegratedradianceinthevicinityofthesun [5].Global HorizontalIrradiancereferstothetotalirradiancereceivedfromabove byahorizontalsurface,andincludesboththecontributionsofDNIand diffuseradiation,associatedtophotonsscatteredintheatmosphere.The amountofdiffuseradiationchangesdependingontheclimate(and especiallythecloudcover)andthelatitude,andtypicallyrepresent B15%ofthetotalradiation.AM1.5Dsolarspectrumiscommonlyused asareferencespectrumforthecharacterizationofconcentratorsolar cells(becauseofthefundamentalinabilityofthesecellstoconcentrate diffuselight).

Theotheratmosphericvariablesthatsignificantlyaffectthesolar spectrumcharacteristicsareAODandPW.AODcharacterizestheradiativestrengthofaerosols(urbanhaze,s mokeparticles,desertdust,seasalt ...)intheverticaldirectionwhilePWistheamountofcondensed watercorrespondingtothetotalwatervaporcontainedinavertical atmosphericcolumnaboveanylocation.Watervaporhasstrongabsorptionbandsinthenearinfrared,whichdirectlyimpactsthespectrum.

1.3Principlesofphotovoltaicenergyconversion

Solarcellsshouldbedesignedtoensuremaximumabsorptionofphotons comingfromthesun,andtopromoteelectronstohigh-energystates wheretheyareabletomove.Thematerialshouldhaveatleasttwoenergeticallyseparatedbandstoguaranteeanefficientextractionofthecharges carriedfromthePVcell.Thebandgap(Eg)ofPVcellcorrespondstothe energygapseparatingthemaximumenergylevelinthelow-energyband [referredas “valenceband” (VB)],fromtheminimumenergylevelinthe high-energyband[knownas “conductionband” (CB)],wheretheelectronsshouldbepromoted.Thetypicaltimeduringwhichtheelectronis maintainedinahigh-energystateshouldbehighenoughtoguaranteean efficientextractionoftheexcitedcarriers[aconstraintthatmaybefulfilled ifthebandgapishigherthanthethermalenergy kBT (where kB isthe Boltzmann’sconstantand T thetemperature)].

Onlyphotonswithenergyhigherthan Eg areabletopumpelectrons fromtheVBtotheCB.Thechargeseparationmechanism,whichis requiredtoextractchargecarriersfromthePVcells,involvestheuseofa “membrane” toseparatethedifferentchargecarriers.Thisiscommonly achievedwithanelectricfieldoriginatingfromthepotentialdifference betweencontacts.

Semiconductormaterialshavehistoricallybeenseenasaveryattractive optiontowardefficientlyconvertingsunlightintoelectricityusingthePV effect.Emergingtechnologiesusingorganicor/andinorganicsubstances suchasPerovskiteorpolymersolarcellsarecurrentlyinstigatingagreat amountofresearchwork,butthesetechnologieswillnotbeaddressedin thischapter,sincetheunderlyingphysicalmechanismsaresensiblydifferent(thereadershouldrefertothefollowingchaptersfordeeperinsights intothesetechnologies).

1.4Semiconductors

Materialscanbeclassifiedintothreemaincategories,dependingontheir typicalelectronicproperties:Semiconductorsandinsulatorsbothshowan energygapbetweentheirvalenceandCBs,whereasmetalsshowanoverlap betweenenergylevelsintheVBandtheCB(and,asaconsequence,no energygap).ThedevelopmentofefficientPVcellsrequiresbothanefficient absorptionofsolarphotons,andtheestablishmentoftwodistinctchargecarrierpopulations,whichcanonlybeachievedwithsemiconductormaterials.

Inthissection,somebasicconceptsrelatedtosemiconductorphysics willbeintroduced.Electrons,holes,andelectronicbandswillfirstbe explained.Theprinciplesofsemiconductordopingwillthenbedetailed, beforeconcludingthissection,byadescriptionofgenerationandrecombinationofelectron holespairsinsemiconductors.

1.4.1Bands,electrons,andholes

Inanatom,electronsmoveinorbitalsaroundthenucleusandcanonly havecertainenergyvalues,called energylevels.Inasolidmaterialconsisting ofanimmenselyhighnumberofatoms,theoriginalorbitalsarecombined toformorbitalswithalargenumberofenergylevels.Becauseofthehuge numberofatomsinvolved,theselevelsareverycloseonefromanotherso thattheyform energybands.Thebondsbetweenatomsandtheirelectronic propertiesdeterminethebands’ energydistribution,aswellasthecrystalline structure.Forinstance,siliconatomssharefourelectronsoftheoutermost shell(valenceshell)withtheneighboringatoms,creatingstableandstrong covalentbondsthatresultinadiamondlatticetypecrystallinestructure.

Theatoms’ chemicalpropertiesaredeterminedtoagreatextentby thenumberofelectronsinthevalenceshell.Inasimilarmanner,thelast occupiedbandsdefinetheelectronicpropertiesofcrystals.Theoccupied bandwiththehighestenergy,whichcontainsthevalenceelectrons,is calledtheValenceBand(VB),whereastheunoccupiedbandwiththe lowestenergyiscalledtheConductionBand(CB).Theenergybetween bothbandsisthepreviouslymentionedbandgapenergy(Eg).

Inmetals,electronsmovewithoutdifficultyfromoneenergylevelto another,sincethevalenceandtheconductionbandsoverlapinenergy (Eg 5 0),givingrisetoahighelectricalconductivity.Insemiconductors, thevalenceandconductionbandsareseparated(0.5 , Eg , 3eV),and theVBisfilledwithbondedelectronsthatdonothavesufficientenergy toovercometheenergygapandfreelymoveinthecrystallinenetwork. Atatemperaturehigherthan0K,afractionoftheseelectronshassufficientlyhighthermalenergytobeexpelledtotheCB(thisfractionbeing afunctionofboththetemperatureandtheenergygapofthesemiconductormaterial).Insulatorshaveveryhighbandgaps,whichpractically avoidelectronsfromtheVBtobeejectedtotheCBbecauseofthehigh energyrequiredtoovercomethebandgap.Asaconsequence,theabsence offreeelectronsintheCBprecludesefficientelectricaltransport,and thesematerialsarecharacterizedbyalowconductivity. Fig.1.2 showsa schemeofinsulators,semiconductors,andconductors.

Electronswithenergyhighenoughtoovercometheelectronicgapof thematerial,becauseoftheirthermalenergyorafterabsorptionofasolar photon,maybreakfreefromtheatomsandbecomeafreeelectroninthe CB.TheremainingbrokenbondintheVBisassociatedwithavacancy referredasa “hole.” Semiconductortheorypredictsthatholesbehaveasif theywerepositivecharges.Inthepresenceofholes(orvacancies),other valenceelectronsintheVBcanmoveintothesevacancies,thusleading toanapparentmovementof “holes” intheoppositedirection.Because theconcentrationofelectronsintheVBlargelyoutnumberstheconcentrationoftheremainingvacanciesassociatedwithelectronsejectedinthe CB,itispracticallymoreconvenienttodescribethismechanismasa “holes” movement.

Semiconductorscharacterizedbyidenticalconcentrationsoffreeelectronsandholesarecalled “intrinsic.” Theconcentrationoffreecarriers (oftenreferredtoas “intrinsiccarrierconcentration”),iscorrelatedtoboth theelectronicgapofthesemiconductorandthetemperature,andtranslatestheabilityofchargecarrierstomovefromonebandtoanother underthesoleeffectoftemperature.Therefore,thehigherthetemperature,thehighernumberofelectronsintheCBandthehighertheconductivity(unlikeconductormaterialsthatshowdecreasingconductivity withincreasingtemperature).

1.4.2Doping,nandptypes

Aspreviouslyexplained,theconductivityofsemiconductorsincreasesas thetemperaturerisesandthebandgapdecreases.Forexample,theelectricalconductivityofGalliumArsenide(GaAs),whichhasabandgapof 1.42eV,istwoordersofmagnitudelowerthantheconductivityof Silicon(1.11eV).

Figure1.2 Schemeofconductor,semiconductor,andinsulatorbandgaps.

Ameantocontroltheconductivityofsemiconductors,knownas doping,consistsofintroducingimpurityatomsinthecrystallinenetwork, characterizedbydifferentelectronicstructure(and,inparticular,adifferentnumberofvalenceelectrons).Onecandistinguishtwodifferentkinds ofimpurityatoms:

• Donor:Theypossessoneextravalenceelectronthatissharedwiththe lattice,asafreeelectron.Inasiliconstructure,consistingoffour valenceelectrons,phosphorousatomsaretypical donor impurities. Theseatoms,whichcomprisefivevalenceelectrons,sharefourof themwiththeirneighboringSiatomsundertheformofcovalent bonds,theremainingonebeingfreetomoveinthecrystallinenetwork.Thephosphorousatomsbecomeionized(positivelycharged) andboththeelectrondensityandtheelectricalconductivityare increased,relativetointrinsicsilicon.

• Acceptor:Unlike donors, acceptor atomscomprisefewervalenceelectronsthanthebulkatoms,andtheirintroductioninthenetworkgives risetothegenerationofextra holes:Theimpurityatomsbecomenegativelyionizedbytakingavalenceelectronfromanotherbondand thenreleasinga hole totheband,thusleadingtoincreased hole concentrationaswellashigherconductivity.Boronatomsaretypicalacceptor atomsinsiliconlattices.

Doping isthustheprocessbywhichboththeconductivityandthe concentrationofonekindofchargecarriers(eitherelectronsorholes)are increased,throughtheintroductionofimpurityatomsshowingdifferent electronicpropertiesthanthebulkatoms.Dopingallowsincreasingthe conductivitywithoutanyexternalenergyinput(light,heat ...),and

Figure1.3 Structuresofanintrinsic, n-type and p-type semiconductors.

semiconductorswithelectronicpropertiescontrolledusingthismeansare knownas extrinsic semiconductors.

Thetypeofdopingisgovernedbythenatureoftheimpurityatoms introducedinthenetwork:ifthe donor impurityconcentrationexceeds theintrinsiccarrierconcentration,thedopingis n-type.Conversely,ifthe acceptor impurityconcentrationexceedstheintrinsiccarrierconcentration, thesemiconductorbecomes p-type. Fig.1.3 schematicallyillustratesintrinsicandextrinsicsemiconductors.

1.4.3Generationandrecombinationofelectron holespairs

TheprocessbywhichelectronsareexcitedfromtheVBtotheCB,creatinganelectron holepair,iscalled generation.Theinverseprocessiscalled recombination andinvolvestherelaxationoffreeelectronsfromtheCBto avacancy(hole)intheVB,thusleadingtotheannihilationofanelectron holepair.Underthermalequilibrium, Generation and Recombination occursatthesameratewithinthecelltomaintainthepopulationsofelectronsandholes.

Ifthe generation processrequiresaninputenergyprovidedbyphotons, phonons(vibrationalenergyofthelattice),orkineticenergyofotherparticles, recombination isarelaxationprocessinwhichenergyisreleased throughthesamemechanisms.

1.4.3.1Absorption

Photogenerationistheprocessleadingtothecreationofane hpairin thecellafterphotonabsorption.Onlyphotonswithenergieshigherthan thebandgapmaygiverisetothegenerationofe hpairs.Photonswith energylowerthanthebandgapcannotparticipatetothephotogeneration process.Inaddition,photonswithenergyexceedingthebandgapareonly partiallyused:thedifferencebetweentheincidentphotonenergyandthe electronicgapofthecelliswastedasheat.Theprocessbywhichexcited electronsquicklyreleasetheirexcessenergyuntiltheyreachtheedgeof theCBisknownas thermalization (see Fig.1.4).Thiscoolingprocessis veryfast(typicallyoccurringatapicosecondtimescale)andfundamentally explains,togetherwiththetransparencyofPVcellstolow-energy photons,thewidediscrepancybetweenthehighefficiencywithwhichit istheoreticallypossibletoconvertsunlightintoelectricity(B90%)and thebestPVefficiencyexperimentallyachievable(whichdoesnotexceed 29%forsingle-junctionsolarcells).Photogenerationischaracterizedby the absorptioncoefficient(α) thatquantifiesthesemiconductorabsorptionas

Figure1.4 Sketchofthephotogenerationprocess,depicting(left)transparencyloss mechanism,(center)photogeneration,(right)thermalizationloss.

Figure1.5 Schemeofthemainrecombinationprocesses[Shockley Read Hall (SRH),Auger,andRadiative].

afunctionofwavelength,andwhichtranslatestheabilityforaphotonof agivenwavelengthtobeefficientlyabsorbedinthePVcell.Theabsorptionprocessiseasierindirectbandgapsemiconductorsduetotheirband structures,leadingtoveryhighabsorptioncoefficientand,asaconsequence,reducedthicknesses(thematerialthicknessrequiredtoensure completeabsorptionoftheincidentlightbeingmuchsmallerthaninthe caseofindirectbandgapsemiconductors,suchassilicon).

Therearethreemainrecombinationprocesses(Fig.1.5),whoseamplitudelargelydependonthenatureandthequalityofthesemiconductor materialsinvolved,aswellasonthetypicaldensityofchargecarriersinthe cell:(1) band-to-band recombinationreferstotheannihilationofane hpair followedbytheemissionofaphotonofcorrespondingenergy.These unavoidable recombination(inthesensethat,unlikeotherrecombination

mechanisms,theymustoccurinanyPVcell)areparticularlyeffectivein directbandgapmaterials,suchasGaAs.(2) Shockley Read Hall (SRH) recombinationinvolvesimpuritiesordefectsinthecrystallinestructure,givingrisetounwantedenergylevelsactingliketrapsintheforbiddengap: annihilationofane hpairmayoccurifbothafreeelectronintheCB, andaholeintheVB,simultaneouslyfallintoanimpuritytrap.

SRHrecombinationisoftenstronginmanysemiconductormaterials, andaparticularcareshouldbebroughttowardminimizingthedefectdensityinthePVcellthroughappropriatefabricationanddopingconditions.

Trapstatesarealsolikelytoappearatthesurfaceofthecellbecauseof materialdiscontinuities.Theserecombinationmechanisms,knownassurfacerecombination,maybeminimizedwithhigh-qualitysurface passivation.

(3) Auger recombinationreferstoathree-particlemechanismwhere theenergyofanelectronintheCB(or,alternatively,theenergyofa holeintheVB)istransferredtoanotherelectron(orhole).Theexcess energyisrapidlydissipatedasheatinthecrystallinenetwork.

Carrierlifetime(τ ) isameasureofthemeanlifetimeofafreecharge carrierbeforerecombinationoccurs.Thisparameter,whichshouldbe keptlongenoughtoensureanefficientcarrierextractionfromthePV cell,islargelydependentonthesemiconductorandthedoping.

The diffusionlength(L) expressesthemeandistancethatafreecarrier cantravelinthecellbeforearecombinationeventoccurs.Thediffusion length,whichshouldbehighenoughtoguaranteethatthecarrierstravel thedistanceseparatingthemfromthep njunction,isrelatedtothelifetimeandthediffusivity(D)bythefollowingequation:

(1.4)

Thediffusivitydetermineshowcarriersrepealeachother,whereasthe mobility(μ)allowscalculatingthecarriers’ velocityunderanelectricfield. ThesequantitiesarerelatedbyEinsteinequation:

(1.5)

1.5Solarcellstructure,operation,andmainparameters

1.5.1p nJunction

EfficientphotogenerationoffreechargecarriersisafundamentalrequirementinPVcells.However,separatecollectionofholesononeelectrode,

andelectronsontheother,requiresanadditionalmechanismtoeffectivelyextractthesetwotypesofcarriers.Thischargeseparationisusually achievedusingap njunction:theelectricfieldappearingattheinterface betweenthe p-side andthe n-side ofthesolarcellactsasamembrane, repellingthedifferentchargecarriersindifferentregionsofthecellwhere theycanbeseparatelyextracted(Fig.1.6).

Thep njunctionisrealizedbybringingtogetherann-typeandaptypesemiconductorlayer.

Onthen-side,electronsmovebydiffusiontowardthep-side(where theirconcentrationisordersofmagnitudelower),leavingpositively chargedionsbehindthem.Similarly,holesonthep-sidetendtodiffuse tothen-side(wheretheirconcentrationissignificantlylower),thuscreatingnegativelychargedions.Thepresenceofnegativelyandpositively chargedionsinclosecontactgivesrisetoanelectricfieldattheinterface betweenthetworegions,repellingelectronsinthen-sideandholesin thep-side.Theregionwheretheelectricfieldarisesiscommonlyreferred as depletionregion (D)sinceitisdepletedofcarriers.Consequently,two competingmechanismsconstitutethedrivingforcesforthemovementof chargecarriersinthecell: diffusion,causedbythegradientincarrierconcentration,representsthemaindrivingforceinthepandnneutral regions,whereas drift,causedbytheinteractionbetweentheelectricfield andtheelectricalchargesholdbyelectronsandholes,principallycontrols themovementofchargecarriersinthedepletionregion.

Figure1.6 Schemeofthep njunctionshowingthedepletionregion(D),theneutralregions,andtheelectricfieldoriginated(E).

1.5.2Structure,operation,andmainparametersofsolarcells

Inpractice,solarcellsareatwo-terminaldevicethatcanprovideelectrons toanexternalcircuitwhileilluminatedwithsufficientlyhigh-energy photons.Metalfrontandbackcontactsareusedtoextractcarriers.Since thepresenceofametalgridontopofthecellmayavoidasignificant fractionoftheincidentlighttobeabsorbed,thefrontcontactshouldbe designedtominimizeshadingonthecell.However,becausethemetal gridgeometryisalsoconstrainedbyseriesresistancelosses,theoptimal gridgeometrystemsfromacompromisebetweenshadingandseries resistance.

Thefrontsurfaceiscommonlytexturedtobothincreasethelight absorptionandlowerthereflectivity.Inaddition,antireflectioncoatings withadequaterefractiveindexesaredepositedatopofthetextureto reduceFresnellosses.

Fig.1.7 summarizestheoperationofaPVcell:(1)lightisabsorbedin thecellandcreatese hpairs(2)chargecarriersmoveunderthecombinedeffectof diffusion (intheneutralregions)and drift (inthedepletion region)(3)thep njunctionattheinterfacebetweenthen-andp-side behavesasamembrane,repellingelectronsinthen-sideandholesinthe p-side(4)electronsandholesareseparatelycollectedandinjectedinthe externalcircuit.

Applyingavoltagebetweentheelectricalcontactsofthecellwill affectthecelloperation:whennovoltageisapplied(or,alternatively, whenthecellisshort-circuited),thecellissaidtooperatein short-circuit, andthecorrespondingcurrent,whichiscalled short-circuit current(ISC),

Figure1.7 Sketchofaphotovoltaic(PV)cell.

representsthemaximumelectricalcurrentonecanextractfromaPVcell. ApplyingavoltagebiasonthePVcellleadstolarger diffusion current associatedwiththeflowofelectronsfromthen-sidetothep-side,and holesfromthep-sidetothen-side.Thiscurrent,whichflowsinopposite directiontothephotogeneratedcurrent,growsexponentiallywiththe appliedvoltage,andlowersthetotalcurrentonecanextractfromthePV cell.Forasufficientlyhighvalueoftheappliedvoltage,thediffusioncurrentequalsthephotogeneratedcurrent,andthetotalcurrent extractablefromthecellisthusequaltozero.Thecorrespondingvoltage valueisknownas open-circuit voltage(VOC),andcorrespondstothemaximumvoltagethatcanbeextractedfromaPVcell.

Theshort-circuitcurrentdependsonthespectraldistributionofthe incidentsunlight:Achievinghigh ISC necessarilyrequiresanimportant fractionoftheincomingphotonstopossessanenergyexceedingtheelectronicgapofthecell.Inaddition,eachphotonwithsufficientlyhigh energyshouldideallybeconvertedintoanelectron holepair.Theability ofanyparticularcelltofulfillthisrequirementisusuallycharacterizedby quantumefficiency (QE)measurements,whichindicatetheprobabilitythat agivenphotonofacertainwavelength(λ)willprovideanelectrontothe externalcircuit.

Fig.1.8 showstheQEofacrystallinesiliconsolarcell.QEcurvesprovidekeyinformationforsolarcellmanufacturers,suchastheabilityofthe celltoefficientlycollectchargecarriers,theamplitudeoffrontsurface recombination,orreflectionlosses.

Figure1.8 Quantumefficiency(QE)ofacrystallinesiliconsolarcell.

Consideringthatthespectralincidentphotonfluxdensity F(λ)isknown, theshort-circuitcurrentcanbeobtainedusingthefollowingequation:

where e istheelectronchargeand A isthesolarcellarea.

Thespectralresponse(SR)ofasolarcellisanalogoustotheQEbut expressedinamperes-per-wattofincidentlight.Botharerelatedbythe followingequation:

1.5.2.1Darkcurrentduetovoltage

Applyingapotentialdifferencebetweentheelectricalcontactsgivesrise toareversecurrentflowinginoppositedirectiontothephotogenerated current,whichiscalled darkcurrent.Thiscurrent,whichisassociatedwith theflowofmajoritycarriers(electronsfromthen-sidetothep-side,holes fromthep-sidetothen-side),growsexponentiallywiththevoltage,thus reducingnoticeablythecurrentextractablefromthecellathighvoltage values.Thedarkcurrent(ID)canbeexpressedasafunctionofthepotentialdifference(V)bythefollowingequation:

where Io isthediodereversesaturationcurrent(associatedtothemovementofminoritychargecarriersinreversebias), m thediodeidealityfactor,and T thetemperatureinKelvin.Thediodereversesaturation currentdependslargelyonthetemperature,aswellasonthematerial quality.Theidealityfactortypicallyrangesfrom1to2.

1.5.2.2Superpositionand IV curve

Solarcellsfollowthesuperpositionprinciple,whichmeansthatthecurrent voltagecurveofaPVcellunderilluminationsimplycorrespondsto thesumofthedark IV curveandthephotogeneratedcurrent.TheequationgoverningPVcelloperationcanthusbewritten: