AnalysisofOpticalAbsorptioninNanowireArraysforPhotovoltaicApplications2013-11-7WangWeiping1.Finite-differencetime-domain(FDTD)simulations

Transfermatrixmethod(TMM)

AnalysisofOpticalAbsorptionin

SiliconNanowireArraysforPhotovoltaic

ApplicationsApsorption=1-Refectance-TransmittanceGeometricparameters:Diameter,Length,FillingratioFrequencyrange：1.1~4eVFixeddiameter:50nmInhigh-energyphotonregime,theabsorptionreachesaplateauregionsharedby

alllengthsInlow-energyphotonregime,theabsorptioninthefilmismoreefficient.thereflectanceofnanowiresissignificantlylowerthanthatofthethinfilmintheentirespectralrangeForlowfrequencies,theextinctioncoefficientof

siliconissmallandinterferenceeffectsexist,resultingintheoscillationofreflectanceandtransmittanceFixedperiod:100nm,wirelength:2.33umTheelectromagneticinteractionbetweennanowirescannotbeneglected!Ultimate

efficiencyORTheabsorptionefficienciesare5.80%,

9.47%,12.50%,and15.50%forthe50,65,and80nm

nanowiresandthethinfilm,respectively.Puttingaperfect

reflecting

mirror(100%reflectivity)onthebacksidetheoverallabsorptanceefficiencyisenhancedfrom12.50%to16.09%a=100nm,diameter=80nm,length=2.33umthenanowirestructureisalmostisotropicinthex-yplaneTEpolarizationhaslowerabsorptionthanTMpolarizationbecausethelatterhasanelectricfieldcomponentalongthewireaxiswhichfacilitatestheabsorptionCONCLUSION:smallreflectioninawidespectrumrangeinthelow-frequencyregime,applylight-trappingtechniquesoruselongerwiresAnalysisofopticalabsorptioninGaAsnanowire

arraysAperfectrefractiveindexmatchingatthetopinterface

betweentheairandNWAs,henceleadstogoodcouplingoftheincidentlightintotheNWAsFixedD/P:0.5,diameters:60~240nm1.ThelightabsorptionissignificantlyenhancedbytheincreaseofNWdiametersfrom60to180nminthelongwavelengthregion(photonicresonancemode).2.Diameterupto240nmleadstoadecreaseinabsorptionefficiency(enhancedlightreflectionatthetopsurfaceoftheNWAs).3.nanowireactsasananoscalecylindricalresonatorwhichcantraplightbymultipletotalinternalreflections.Theopticalgenerationrate:D/P=0.5,Length=2um,diameter=180nm,wavelength=800nm.1.Atanglesofincidenceupto60°,thetotalabsorptionismaintainedabove80%forbothTEandTMpolarizations.2.AhigherabsorptionisobservedinTMpolarizationthaninTEpolarizationConclusion:Theoptimalresultsforthenormalincidenceareevaluatedasdiameter=180nm,length=2μm,andD/P=0.5.2.Mieresonancetheory

length=2.5um,diameter=425nmshort-circuitcurrent:256pA,opencircuitvoltage:0.43,fillingfactor:0.52photogeneratedcurrentdensityof180mA/cm2maximumpower:57pw;efficiency:40%Methods-nanowiregrowthNanowiresweregrownonoxidizedSi(111)with100nmaperturesusingaself-catalysedmethodThep-dopingofthecorewasachievedbyaddingafluxofberylliumduringaxialgrowthThen-typedopingwasobtainedbyaddingsilicontothegrowthstepDevicefabricationSU-8wasspunontothesubstrateat4,000r.p.m.for45sandcuredwith1minultravioletlightand3minonahotplateat185oCAnetch-backwitha1–3minoxygenplasmaetchwasthenperformedtofreethenanowiretipThetopcontactwasdefinedbyelectron-beamlithographyfollowedbyevaporationofindiumtinoxide(ITO)Thebottomcontactwasobtainedbysilvergluingtotheback-sideofthewaferOpticalsimulationsofasinglenanowiresolarcellTheabsorptioncross-sectionisdefinedasTheabsorptioncross-sectioninnanoscalematerialsislargerthantheirphysicalsizeThelargestabsorptionisforananowirediameterof380nmExternalquantumefficiency(EQE)andScanningphotocurrentmeasurementsTheabsorptionboostinthedeviceisduetoanunexpectedlylargeabsorptioncross-sectionoftheverticalnanowiregeometryConclusionAremarkableboostinabsorptionisobservedinsingle-nanowiresolar

cells

thatisrelatedtotheverticalconfigurationofthenanowiresandtoaresonantincreaseinthe

absorptioncross-section.Theseresultsopenanewroutetothird-generationsolarcells,localenergyharvestersonnanoscaledevices

andphotondetectors.ScatteringefficienciesofMie

calculationstransversalmagnetic(TM)polarizationtransversalelectric(TE)polarizationnon-polarizedAbsorptionandScatteringofLightbySmallParticlesBohrenCFandHuffmanDR1998Opticalscattering

properties

of

GaAs

NWsCalculated

scattering

efficienciesQsca(red

lines)and

scaled

measuredspectra(blacklines)Leaky-moderesonances(LMRs)Thisworkshowsthatlight

absorptioninnanowiredevicesisnotjustafunctionofthe

intrinsicopticalmateri