Using SiOx nano-films to enhance GZO Thin films properties as front electrodes of a-Si solar cells

(1)

ContentslistsavailableatSciVerseScienceDirect

Applied

Surface

Science

j o ur na l ho m e p age :w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c

Using

SiO

x

nano-ﬁlms

to

enhance

GZO

Thin

ﬁlms

properties

as

front

electrodes

of

a-Si

solar

cells

Kow-Ming

Chang

a

_,

_Po-Ching

_Ho

b,∗

_,

_Shu-Hung

_Yu

b

_,

_Jui-Mei

_Hsu

c

_,

_Kuo-Hui

_Yang

c

_,

Chin-Jyi

Wu

c

_,

_Chia-Chiang

_Chang

c

a_Electronics_Engineering_Dept.,_National_Chiao_Tung_University;_College_of_Electrical_and_Information_Engineering,_I-Shou_University,_Republic_of_China b_Department_of_Electronics_Engineering_&_Institute_of_Electronics,_National_Chiao_Tung_University,₁₀₀₁_Ta_Hsueh_Road,_Hsinchu,_Taiwan_30010,_Republic

ofChina

c_Industrial_Technology_Research_Institute,_Mechanical_and_Systems_Research_{Laboratories,}_195,_Sec._4,_Chung_Hsing_Rd.,_Chutung,_Hsinchu,_Taiwan_31040,

RepublicofChina

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received26January2013

Receivedinrevisedform20March2013 Accepted25March2013

Available online 1 April 2013

Keywords:

Atmosphericpressureplasma Light-trappingeffect Transparentconductiveoxide

a

b

s

t

r

a

c

t

Oneoftheessentialapplicationsoftransparentconductiveoxidesisasfrontelectrodesforsuperstrate siliconthin-filmsolarcells.TexturedTCOthinfilmscanimproveabsorptionofsunlightforana-Si:H absorberduringasingleopticalpath.In thisstudy,high-hazeandlow-resistivitybilayerGZO/SiOx thinfilmspreparedusinganatmosphericpressureplasmajet(APPJ)depositiontechniqueanddc mag-netronsputtering.Thesiliconsubdioxidenano-filmplaysanimportantroleincontrollingthehazevalue ofsubsequentdepositedGZOthinfilms.ThebilayerGZO/SiOx(90sccm)samplehasthehighesthaze value(22.30%),thelowestresistivity(8.98×10−4cm),andreachesamaximumcellefficiencyof6.85% (enhancedbyapproximately19%comparedtoasampleofnon-texturedGZO).

1. Introduction

Transparentconductiveoxide(TCO)hasattractedsubstantial attentionbecauseofhightransparencyinthevisibleregionand low resistivity. Oneof the mostimportant applications of TCO is asa textured front electrode forsuperstrate pin-type silicon thin-filmsolarcells[1–3].TexturedTCOthin filmscanenhance sunlightabsorptionforthehydrogenatedamorphous silicon (a-Si:H)absorber duringoneopticalpathand furtherimprovethe cellefficiencybecauseofthelight-trappingeffect.Thereare sev-eraltypesofTCOmaterials,includingtin-dopedindiumoxide(ITO), fluorine-dopedtinoxide(FTO),andzincoxidedopedwithgroupIII elements(AZO,GZO).Foramorphoussiliconthin-filmsolarcells, bothAZOandGZOaresuitablefrontelectrodematerialsbecauseof theireasilymodifiedsurfacemorphologies.ComparedwithITOand FTO,theyalsohavehigherhydrogenandsilane(SiH4)plasma

resis-tanceandlowercost.GZOhasahighertransmittancethanAZOin thenear-infraredregion[4]andbettermoistureresistancebecause galliumexhibitslowreactivitytooxygen[5].

The morphologies of the TCO front contact strongly affect theirhazevalue.Severaldepositiontechniqueshavebeenused topreparetexturedZnOﬁlms[6–8].In thisstudy,roughsilicon

∗ Correspondingauthor.Tel.:+886921841971.

E-mailaddress:raymondsam.ee98g@nctu.edu.tw(P.-C.Ho).

subdioxide(SiOx)bufferlayers weredepositedusing the

atmo-sphericpressureplasmajet(APPJ)tomodifythemorphologiesand toinfluencetheoptoelectronicpropertiesofthesputteredGZOthin films.Afterpost-annealinginvacuum,bilayerGZO/SiOxthinfilms

obtainedbetteroptoelectronicpropertiesandimprovedthe efﬁ-ciencyofana-Sisolarcellcomparedtoanon-texturedGZOsingle layer.

2. Experiment

Rough SiOx thin ﬁlms were deposited at 75◦C on

100mm×100mm×3.2mm glass substrates (Taiwan Glass IndustryCo.)usingtheAPPJdepositiontechnique.Fig.1ashows a schematic diagram of the APPJ deposition system. The APPJ depositionsystemismainlycomposedofahigh-voltageACpower supply,a plasmajet,aprecursorbottle,ahot plate,and anx–y directionalscansystem.Theplasmapowerandfrequency were 450Wand20kHz,respectively.Thedistancebetweentheplasma jetnozzletothesubstratewas15mm.Thescanningrouteisshown inFig.1b.They-directionscanspeedwasﬁxedat250mm/s,and thepitchwasﬁxedat2mm.Hexamethyl-disiloxane[(HMDSO), AlfaAesar]wasusedastheprecursortodeposittheSiOxbuffer

layers.ToobtainvarioussurfaceroughnessesofSiOxthinﬁlms,the

argoncarriergasflowratewassettovaryat30sccm,60sccm,or 90sccm,andtheflowrateofthecompresseddryair(CDA)maingas wassetat40SLM.ThethicknessoftheSiOxnano-filmswas10nm,

(2)

Fig.1.Schematicdiagramof(a)theAPPJdepositionsystemand(b)thescanning route.

25nm,and30nmfor30sccm,60sccm,and90sccm,respectively. GZOthinﬁlmsweredepositedat100◦ConSiOxbufferlayersusing

direct-current(DC)magnetronsputtering.AceramicGZOtarget wasdoped with3.2wt.%Ga2O3.DC magnetronsputtering was

performedataworkingpressureof2mTorrinArworkinggasand ataDCpowerdensityof3W/cm2_._The_thickness_of_the_GZO_thin

ﬁlmswas1␮m.Allas-depositedbilayerGZO/SiOxthinﬁlmswere

annealedinhighvacuum(<1×10−6Torr)at500◦Cfor5minto improveadhesionbetweenGZOandSiOxthinﬁlmsandremove

chemisorbedoxygenionsonthesurfaceofGZOthinﬁlms.Finally, thebilayerGZO/SiOxthin ﬁlmswereappliedtotheamorphous

siliconthin-ﬁlmsolarcellsasafront electrodewiththe follow-ing structure: 3.2-mm-thick glass substrate/SiOx buffer layers

(10–30nm)/GZO(1000nm)/p-typea-Si:H(10nm)/intrinsica-Si:H absorber (300nm)/n-type ␮c-Si:H (15nm)/GZO (100nm)/Ag (200nm).

To investigatethe surface morphologies of SiOx and bilayer

GZO/SiOxﬁlms,ascanningelectronmicroscope(Hitachi,S-4700I)

wasused.TheX-raydiffraction(XRD;BRUKER,D2PHASER) mea-surementswereusedtocharacterizethecrystalorientationand qualityoftheﬁlms.Thebindingenergyandchemicalshiftofthe samplesweremeasuredusingX-rayphotoelectronspectroscopy (XPS).TheelectricalpropertiesofbilayerGZO/SiOxthinﬁlmsand

sputteredGZOthinﬁlmswereinvestigatedusingHalleffect mea-surements (ACCENT, HL5500PC). The optical transmittance and hazemeasurements wereperformed using a UV–vis–NIR spec-trometer(Jasco,V570).Theperformanceofthea-Sisolarcellswas measuredat100mW/cm2_using_an_AM_1.5_solar_simulator.

3. Resultsanddiscussion

Fig.2shows SEMsurfacemorphologiesof SiOxbufferlayers

andbilayerGZO/SiOxﬁlms.Thegrainsizeand roughnessofthe

SiOxbufferlayerswerefoundtoincreasewiththeargoncarrier

gasflowrate.Becauseofalargedifferencebetweenthemaingas andtheargoncarriergasflowrate,avortexwasformedinsidethe plasma-dischargingarea.Thevortexcausedradicalstostayinthe airforalongtime,andthisresultedintheformationofparticleson thesubstrates[9].Astheargoncarriergasflowrateincreased,the

(3)

Fig.3.XRDpatternsofGZOthinﬁlmsdepositedonvariousmorphologiesofSiOx

bufferlayers.TheargoncarriergasﬂowrateoftheSiOxnano-ﬁlmswassetfrom30

to90sccm.

roughsurfaceoftheSiOxbufferlayerscausedlowsurfacekinetics

ofthesputteredatomsandadecreaseinthenucleidensity[10]. Consequently,thebilayerGZO/SiOxthinﬁlmshadroughsurface

microstructureandalargegrainsize.The90sccmsamplehasthe largestrootmeansquare(RMS)roughnessof60.76nm.

Fig.3presentstheXRDpatternsofdissimilarbilayerGZO/SiOx

thinﬁlms.TheargoncarriergasﬂowrateoftheSiOxbufferlayers

wassetfrom30to90sccm.XRDpatternsofallsamplesexhibited mainlyZnO(002)and(101)diffractionpeaks.Astrongc-axis pref-erentialorientationindicatedthatthegraingrowthoftheGZOthin ﬁlmisnormaltotheplaneoftheglasssubstrate.Theintensityof the(002)diffractionpeaksharplyincreasedwiththeargoncarrier gasﬂowrate,meaningthatthecrystallinityofthebilayerGZO/SiOx

thinﬁlmsimproved.

Fig.4showsthediffractionangle,thefullwidthathalf max-imum (FWHM), and the calculated grain size along the(002) orientation. The (002) peak position of all samples is shown tohaveshiftedtoa higherBragganglecomparedtothatofthe standardpatternofZnO(JCPDS:36-1451),implyingthatgallium incorporatesintoZnO thinﬁlms[11].Thisresultwasconsistent withthecarrierconcentrationdeterminedbytheHall measure-ment,asshowninTable1.ThecrystallitesizeoftheGZOthinﬁlms

Fig.4.ThediffractionangleoftheGZO(002)peak,FWHMoftheGZO(002)peak andcalculatedcrystallitesizealongthe(002)orientationforthebilayerGZOﬁlms asafunctionoftheargoncarriergasﬂowrate.

Table1

Halleffectmeasurementsofnon-texturedGZOﬁlmandbilayerGZO/SiOxﬁlms.

Sample Resistivity (cm) Mobility (cm2_/Vs) Carrierconcentration (cm−3₎ Non-textured 1.10×10−3 _23.1 _2.448_×₁₀20 GZO/SiOx(30sccm) 1.35×10−3 27.3 1.694×1020 GZO/SiOx(60sccm) 9.72×10−4 28.7 2.238×1020 GZO/SiOx(90sccm) 8.98×10−4 29.1 2.387×1020

increasedwiththesurfaceroughnessoftheSiOxthinﬁlms.The crystallitesizealongthe(002)orientationwasestimatedusing thefollowingDebye-Scherrerformula[12]:

D=0.94/(ˇcosq)

whereisthewavelengthofCuK_␣line,_ˇistheFWHM,andis theBraggdiffractionangle.AlowerFWHMindicateshigher crys-tallinity.Therefore,the90sccmsamplehadthelargestcrystallite sizebecauseofthelowestFWHM.

Thewide-scanXPSspectraofthebilayerGZO/SiOx ﬁlmsare

shownin Fig.5a.ThephotoelectronicpeaksofZn, O,C, andGa wereobserved.Thecarbonpeak(C1s)waspositionedat285.43eV

(4)

Fig.6.Narrow-scanXPSspectraofO1softhebilayerGZO/SiOxthinﬁlmsdeposited

onSiOxbufferlayerswithanargoncarriergasﬂowrateof(a)30sccm(b)60sccm

(c)90sccm.

[13].Thecarboncontaminationmaycomefromsamplefabrication andsubsequentmeasurement.NometallicZnpeakwascentered at1021.1eV[14,15],whichconﬁrmsthatZnatomsappearonlyin theoxidizedstate.Fig.5bshowstheGa2pXPSspectraofbilayer GZO/SiOx ﬁlmsdeposited ontheSiOx bufferlayers withvaried

Fig.7. ElectricalpropertiesofthebilayerGZO/SiOxﬁlmsasafunctionoftheargon

carriergasﬂowrate.

argoncarriergasflow rates.The30sccm,60sccm,and 90sccm sampleshavebindingenergiesofGa2p3/2locatedat1119.66eV, 1119.33eV,and1118.46eV,respectively.Theresultconfirmsthat GaatomswereincorporatedintoZnOfilmsduringdeposition.

Fig.6showsthenarrow-scanXPSspectraofO1softhebilayer GZO/SiOxﬁlms.BeforerunningtheXPSmeasurements,thebilayer

GZO/SiOxthinﬁlmswerepre-sputteredwithArplasmatoremove

surfaceoxidesandcontamination.Theoxygenspectrumis asym-metricandcanbeﬁttedbytwoGaussiandistributions,locatedat 530.62±0.11eV(OI)and531.31±0.05eV(OII).Thelowbinding

energycomponent(OI)isrelatedtothelatticeoxygenoftheZnO

(wurtzite)structure,andthehighbindingenergy(OII)component

isattributedtooxygenvacancieswiththeZnOmatrix.Thespectra showedthattheareapercentofOIIincreasedwiththeargoncarrier

gasﬂowrate,implyingthatGZOthinﬁlmsdepositedonrougher SiOxbufferlayerscanleadtoanincreaseofoxygenvacancies(Vo)

insideﬁlms.Nevertheless,Vohavebeenreportedtoactasadeep

donor[16–18],andithasnotinﬂuenceontheelectricalproperties oftheGZOthinﬁlms.

ElectricalpropertiesofthebilayerGZO/SiOxﬁlmsasafunction

oftheargoncarriergasﬂowrateareshowninFig.7.Bothmobility andcarrierconcentrationincreased,whichmightbeattributedto theimprovementofthecrystallinityofbilayerGZO/SiOxthinﬁlms.

Thedefectsinsidethinﬁlms,suchasgrainboundaries,vacancies, andinterstitials,resultinthescatteringofthecarriersandreduce thecarrierconcentration.Atahighelectrondensity(>1020_cm−3_),

thedominantscatteringeffectofmobilitymeasuredbytheHall equipmentwasregardedasionizedimpurityscattering[19,20].An enhancementinthecrystallinityofthebilayerGZO/SiOxthinﬁlms

couldreduceionizedimpurityscattering,furtherincreasingboth thecarriermobilityandthecarrierconcentration.Theresistivity ofthebilayerGZO/SiOx ﬁlmsdeclined,resultinginthe

achieve-mentofthelowestresistivity(8.98×10−4cm)atthe90sccm argon carriergas ﬂow rate. TheHall measurement dataof the bilayerGZO/SiOxandnon-texturedGZOthinﬁlmsaresummarized

inTable1.Thecarrierconcentrationsofallbilayersampleswere lowerthanthatofthenon-texturedGZOthinﬁlm.Thismaybedue tounstableoxygenintheSiOxbufferlayersdiffusingintotheGZO

ﬁlmsduring500◦Cpost-annealingandactingasscatteringcenters toreducethecarrierconcentration.

Fig.8showstheopticaltransmittanceandhazeofthebilayer GZO/SiOxﬁlmsasafunctionoftheargoncarriergasﬂowrate.All

(5)

Fig.8. (a)Transmittance(b)hazeofthebilayerGZO/SiOxﬁlms.

Table2

Comparisonofperformancefora-Sisolarcellswithnon-texturedGZOandtextured GZO/SiOxfrontcontacts.

Sample Voc(V) Jsc(mA/cm2) FF(%) (%)

Non-textured 0.710 13.25 58.9 5.55

GZO/SiOx(90sccm) 0.849 13.96 57.7 6.85

inthevisibleregion.Theinsetimagepresentsthemagnifiedcurve oftheabsorptionbandedge.Theblueshiftsoftheabsorptionedge occurredastheargoncarriergasflowrateincreasedfrom30to 90sccm.ThisphenomenonisknownastheBurstein–Mosseffect, whichoccursasthecarrierconcentrationexceedstheconduction bandedgedensityofstates(DOS)[21–23].Therefore,theoptical bandgapofthebilayerGZO/SiOxfilmsincreased(datanotshown

here) dueto anincrease in thecarrierconcentration.Withthe increaseinargoncarriergasﬂowrate,thebilayerGZO/SiOxﬁlm

showedanincreaseinhazevalueinthevisibleregion.

Thecurrent-voltageparametersforthea-Sisolarcellswith non-texturedGZOandtexturedGZO/SiOx(90sccm)frontcontactsare

showninTable2.TheconductivityofthebilayerGZO/SiOxthinﬁlm

wassuperiortothatofnon-texturedGZOthinﬁlm,andthisresulted inalargerVoc[24]becauseofadecreaseinrecombinationatthe

frontcontact/p-typea-Si:Hlayerinterface.TheJscofthesolarcell

usingthebilayerGZO/SiOxfrontelectrodewashigherthanthatof

solarcellusinganon-texturedGZOfrontelectrode,implyingthat thebilayerGZO/SiOxthinﬁlmhassuperiorphotoncollection

effi-ciencytonon-texturedGZOthinfilm.Anexplanationforthisisthat thebilayerGZO/SiOxthinfilmhasahigherhazevalue(22.3%)and

abetterlight-trappingeffectcomparedtoanon-texturedGZOthin ﬁlm.Theefﬁciencyofthea-SisolarcellusingthebilayerGZO/SiOx

frontcontactwas6.85%andimprovedbyapproximately19% com-paredwiththatofthea-Sisolarcellusinganon-texturedGZOfront contact.

4. Conclusion

Inthisstudy,bilayerGZO/SiOxandnon-texturedGZOthinﬁlms

weredepositedonglasssubstratesbyusingtheAPPJtechnique andDCmagnetronsputtering.Thereafter,a-Si:HlayersandGZO/Ag backcontactweredepositedtocompletea-Sisolarcells.Roughness oftheSiOxbufferlayersdepositedusingAPPJcanbecontrolledby

modifyingtheargoncarriergasﬂowrate.BasedontheXRDresults, thecrystallinityofallbilayerGZO/SiOxthinﬁlmsimprovedasthe

surfaceroughnessoftheSiOxbufferlayersincreased.Becauseof

theenhancementincrystallinity,bothcarriermobilityandcarrier concentrationincreasedandresultedinadecreaseofresistivity. TheXPSdataconfirmedthatGaatomsincorporatedintotheZnO filmsandtheVonumberincreasedwiththeargoncarriergasflow

rate.However,Vohasbeenreportedtoactasadeepdonor,andit

isdifficulttocontributethecarrierconcentrationofGZOfilms.The transmittanceofallthebilayerGZO/SiOxfilmsismorethan80%in

thevisibleregion.RoughmorphologiesofSiOxthinﬁlmsresulted

inanincrementofhazevalueofthebilayerGZO/SiOxthinﬁlms.

ThesampleofthebilayerGZO/SiOx(90sccm)hasthehighesthaze

value(22.30%)andthelowestresistivity(8.98×10−4cm),and reachesamaximumcellefﬁciencyof6.85%(approximately19% improvementcomparedtothenon-texturedGZOsample).

References

[1]L.Prusakova,S.Flickyngerova,M.Fischer,I.Novotny,M.Tijssen,M.Netrvalova, M.Zeman,V.Tvarozek,P.Sutta,Vacuum86(2008)765.

[2]J.Krc,M.Zeman,O.Kluth,F.Smole,M.Topic,Thinsolidﬁlms426(2003)296.

[3]O.Kluth,B.Rech,L.Houben,S.Wieder,G.Schope,C.Beneking,H.Wagner,A. Lofﬂ,H.W.Schock,Thinsolidﬁlms351(1999)247.

[4]S.H.Park,J.B.Park,P.K.Song,CurrentAppliedPhysics10(2010)S488.

[5]V.Assuncao,E.Fortunato,A.Marques,H.Aguas,I.Ferreira,M.E.V.Costa,R. Martins,ThinSolidFilms427(2003)401.

[6]K.Sato,Y.,Gotoh,Y.,Hayashi,K.,Adachi,H.,Naishimura,ReportsRes.LabAsahi GlassCo.,Ltd,vol.40,1990,p.233.

[7]S.Fay,S.Dubail,U.Kroll,J.Meier,Y.Ziegler,A.Shah,Proc.16thEUPVSEC, Glasgow,2000,p.p361.

[8]D.W.Kang,S.H.Kuk,K.S.Ji,S.W.Ahn,M.K.Han,JapaneseJournalofApplied Physics49(2010)031101.

[9]S.H.Yang,C.H.Liu,C.H.Su,H.Chen,ThinSolidFilms517(2009)5284.

[10]B.D.Ahn,Y.G.Ko,S.H.Oh,J.H.Song,H.J.Kim,ThinSolidFilms517(2009)6414.

[11]K.M.Chang,S.H.Huang,C.J.Wu,W.L.Lin,W.C.Chen,C.W.Chi,J.W.Lin,C.C. Chang,ThinSolidFilms519(2011)5114.

[12]A.R.Babar,P.R.Deshamukh,R.J.Deokate,D.Haranath,C.H.Bhosale,K.Y.Rajpure, JournalofPhysicsD:AppliedPhysics41(2008)135404.

[13]W.T.Yen,Y.C.Lin,P.C.Yao,J.H.Ke,Y.L.Chen,ThinSolidFilms518(2010)3882.

[14]F.Wu,L.Fang,Y.J.Pan,K.Zhou,Q.L.Huang,C.Y.Kong,PhysicaE43(2010)228.

[15]S.S.Lin,J.L.Huang,P.Sajgalik,SurfaceandCoatingsTechnology190(2005)40.

[16]A.Janotti,ChrisG.VandeWalle,AppliedPhysicsLetters87(2005)122102.

[17]X.J.Wang,L.S.Vlasenko,S.J.Pearton3,W.M.Chen,I.A.Buyanova,Journalof PhysicsD:AppliedPhysics42(2009)175411.

[18]M.D.McCluskey,S.J.Jokela1,JournalofAppliedPhysics106(2009)071101.

[19]T.Yamada,H.Makino,N.Yamamoto,T.Yamamoto,JournalofAppliedPhysics 107(2010)123534.

[20]J.J.Robbins,J.Harvey,J.Leaf,C.Fry,C.A.Wolden,ThinSolidFilms473(2005) 35.

[21]X.Y.Gao,Q.G.Lin,H.L.Feng,Y.F.Liu,J.X.Lu,ThinSolidFilms517(2009)4684.

[22]E.Burstein,PhysicsReview93(1954)632.

[23]S.M.Park,T.Ikegami,K.Ebihara,ThinSolidFilms513(2006)90.

[24]Y.C.Lin,T.Y.Chen,L.C.Wang,S.Y.Lien,JournalofTheElectrochemicalSociety 159(2012)H599.