Highly textured ZnO:B films grown by low pressure chemical vapor deposition for efficiency enhancement of heterojunction silicon-based solar cells

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Highly

textured

ZnO:B

ﬁlms

grown

by

low

pressure

chemical

vapor

deposition

for

efﬁciency

enhancement

of

heterojunction

silicon-based

solar

cells

Jui-Chung

Hsiao

a,b

,

Chien-Hsun

Chen

b,

*

,

Hung-Jen

Yang

c

,

Chien-Liang

Wu

b

,

Chia-Ming

Fan

b

,

Chien-Fu

Huang

b

_,

_Chao-Cheng

_Lin

b

_,

_Peichen

_Yu

a

_,

_Jenn-Chang

_Hwang

c

a

DepartmentofPhotonicsandInstituteofElectro-OpticalEngineering,NationalChiaoTungUniversity,HsinChu30010,Taiwan

b

GreenEnergyandEnvironmentResearchLaboratories,IndustrialTechnologyResearchInstitute,HsinChu30010,Taiwan

c

DepartmentofMaterialsScienceandEngineering,NationalTsingHuaUniversity,Hsin-ChuCity30013,Taiwan

It is very important to have both a high energy conversion

efﬁciencyandalowproductionprocessingcostfor

commercial-izedsolarcells.Itisnodoubtthattheheterojunctionsilicon-based (HJS)solarcellisoneofthemostattractivesolarcellsamongthem forthefollowingreasons[1–3].First,thewholestructureofaHJS solarcellisverysimple.Theemitterlayerandthe

back-surface-ﬁled (BSF) layer are deposited on a crystalline silicon (c-Si)

substrate by a plasma enhanced chemical vapor deposition

(PECVD)system.Nosophisticatedstep,suchasphotolithography

orion-implantation,noranycomplicatedstructuresuchasametal wrapthroughoraninter-digitalbackcontactisinvolved[4–6]. Second,theprocessingtemperatureofaHJSsolarcellisverylow. AlloftherequiredprocessestoaHJSsolarcellcanbeaccomplished

below2508CusingaPECVDsystem,whichisespeciallysuitable

forthinnersiliconwafers[7].Thereductionofthethermalbudget, comparedtothetraditionalcrystallinesiliconsolarcell,isbeneﬁt

to the mass production. Third, there is a very high energy

conversionefﬁciencyduetoahighopenvoltageforaHJSsolarcell

[8].Anintrinsichydrogenatedamorphoussilicon(a-Si:H)layercan beinsertedbetweenanemitterlayerandac-SisubstrateoraBSF layerandac-Sisubstrate,whichcanprovideanexcellentsurface passivationtoac-Sisubstrate[9–11].Itisthereforeattractedmany

peopledevotedonthistopic.

MostoftheresearchaboutHJSsolarcellsarefocusedonthe

intrinsic a-Si:H layer [9,12,10]. Only few papers discuss the

transparentconductiveoxide (TCO)layeron theHJSsolarcells

[13–16].AnIndiumTinOxide(ITO)layerisatypicalTCOmaterial fortheHJSsolarcells.However,theshortageoftheindiumandfree carrieradsorptionatthelongwavelengthregionaretwoserious

problemstoanITOmaterial[17].AZnO-basedTCOcouldbean

alternative for its low cost and non-toxicity, attracting much

attention for its widely applications to light-emitter diode,

transparenthighpower electronicstransducers,gas-sensingand

solarcell[18,19].ItisanappropriateTCOlayerforaHJSsolarcell becauseithasthepropertiesofahightransmittance(>80%)anda lowsheetresistance(10

V

/&).

Inthiswork,aZnO:Blayersweredepositedbythelowpressure

chemical-vapor-deposition (LPCVD) system. The electrical and

opticalpropertiesofZnOﬁlms,whichwerecontrolledbytheﬂow

rateoftheB2H6andthesubstratetemperature,wereoptimized

ﬁrst.Second,thePVcharacteristicsofHJSsolarcellsvaryingwith thethicknessoftheoptimizedZnO:Bﬁlmfrom0.35

m

mto1.6

m

m wereexamined.Finally,theperformanceofHJSsolarcellswiththe

ZnO:BﬁlmandtheITOﬁlm,respectively,arediscussed.

1. Experimental

The HJS solar cells were fabricatedon textured n-type c-Si

substratescuttinginto2cm2cm.Theresistivityandthickness ofthen-typeCZc-Si(100)substrateswere1–5?cmand180

m

m, JournaloftheTaiwanInstituteofChemicalEngineers44(2013)758–761

ARTICLE INFO

Articlehistory:

Received5October2012

Receivedinrevisedform23January2013 Accepted27January2013

Availableonline30March2013

Keywords: ZnO:B

Heterojunctionsolarcell

Low-pressurechemical-vapor-deposition

ABSTRACT

Thispaperdemonstratesthegrowthofhighly-texturedboron-dopedZnO(ZnO:B)filmbyusing low-pressure chemical-vapor-deposition (LPCVD)forefficient lightharvestingand carrier collectionin heterojunctionsilicon-based (HJS) solar cells. Theoptical and electrical characteristicshave been optimizedversusthesubstratetemperatureandB2H6flowratefortradeoffsamongthesheetresistance,

free-carrierabsorption,andopticaltransmissionofblue/greenwavelengths.AHJSsolarcellwitha

1.6-mm-thickZnO:Bfilmachievesahighpowerconversionefficiencyof16.30%andfillfactorof78.05%, comparedto15.64%and72.17%,respectively,fromacounterpartwithaconventional80-nm-thick indiumtinoxidelayer.

ß2013TaiwanInstituteofChemicalEngineers.PublishedbyElsevierB.V.Allrightsreserved.

*Correspondingauthor.Tel.:+88635915170. E-mailaddress:[email protected](C.-H.Chen).

ContentslistsavailableatSciVerseScienceDirect

Journal

of

the

Taiwan

Institute

of

Chemical

Engineers

j o urn a lhom e pa g e :ww w . e l se v i e r. c om / l oca t e / j t i ce

1876-1070/$–seefrontmatterß2013TaiwanInstituteofChemicalEngineers.PublishedbyElsevierB.V.Allrightsreserved.

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respectively.Samplesweredippedin5%HFsolutiontoremovethe nativeoxidelayerandthenwererinsedinthede-ionizedwaterin 3min.Ap-typea-Si:HﬁlmusingB2H6gasasaprecursorandan

n-typea-Si:HﬁlmusingPH3gasasaprecursorweredepositedona

texturedc-Si substrate asan emitterlayerand a BSF layer.All

ZnO:BﬁlmsweredepositedbyLPCVD.Diethylzinc(DEZ)andwater

(H2O)vaporscarriedbyargongaswereusedasprecursors,and

theirﬂowsweresetto500and550sccm,respectively.Diborane

(B2H6),1%dilutedinhydrogen,wasusedasthedopinggaswiththe

ﬂowrateof8sccm.AllthedepositionparametersforZnO:Bﬁlms

aresummarizedinTable1.Asilvergridlayer,deﬁnedbyashadow

metalmask,wassputteredontheZnO:Bﬁlmwithathicknessof

200nm.TheAg/ITOlayersweresputteredonthen-typea-Si:Hﬁlm asabackcontact.

The electrical and optical properties of a ZnO:B ﬁlm were

characterized by four points probes and UV–vis spectrometer

(PerkinElmer,Lambda750s).Scanningelectronmicroscope(JEOL,

JSM5400)wasutilizedtoinvestigatethesurfacemorphologyand

thicknessofaZnO:Bﬁlm.Thephotovoltaiccharacteristicsofthe

HJSsolarcellsweredeterminedunderstandardilluminationtest

condition(258C,1000W/m2_,_AM_1.5_G).

2. Resultsanddiscussion

Fig.1showsthesheetresistance(R&)oftheZnO:Bﬁlmsasa

functionofthediborone(B2H6)dilution.Thereisadrasticdecrease

oftheR&from227

V

/&to11.2

V

/&withtheincreaseoftheB2H6

from0sccmto3sccm.TheR&canbefurtherdecreasedto7.8

V

/&

attheB2H6ﬂowrateof5sccmandseemstobesaturatedto6.3

V

/

& attheB2H6 ﬂow rateof10sccm.Bothelectrical andoptical

propertiesareimportanttothetransparentconductivelayer.The transmissionoftheZnO:BﬁlmswithdifferentB2H6ﬂowrateare

measured from thewavelengthof 300nmto 1200nm (Fig.2).

Thereisablue-shiftofacut-offwavelengthfortheZnO:Bﬁlmsin theultra-violetregionincreasingwiththeB2H6ﬂowrate.Thisis

mainlyduetotheBursteinMossshift[20].Thetransmissionsare alsoincreasedwiththeB2H6ﬂowrateatthesewavelengthranges.

On the contrary, the transmissions of these ZnO:B ﬁlms are

decreasedwiththeincreaseoftheB2H6ﬂowrateintheIRregion.

Thiscanbeattributedtothefreeelectronsadsorptionatthelong

wavelengthregion[17].Meanwhile,thetransmissionsarealmost

the same(80%) at the wavelengthof 550–850nmfor the all

ZnO:BsamplesregardlessoftheB2H6ﬂowrate.

Itisnotedthatthedepositionrateandtheorientationofthe

ZnO:Bﬁlms,determinedbytheSEMandXRD,respectively,arenot

affectedby theB2H6 ﬂow ratefrom0sccm to10sccm.On the

contrary,theincreaseofthesubstratetemperaturehasremarkable

change in the surface morphology as shown in Fig. 3. The

structuresoftheZnO:Bﬁlmschangefromgranular-likestructure

at1308Ctopyramid-likestructureat1608Candfurtherchangeto stripe-likestructure at3008Cdeposited onthetextured silicon substrate.Moreimportantly,thelowestR&canbeobtainedtobe

6.3

V

/&at1608CasshowninTable1.

Itisimportanttotaketheopticalandelectricalpropertiesofthe

ZnO:B ﬁlms as well as the deposition temperature into the

considerationforthefabricationoftheHJSsolarcells.TheB2H6

ﬂowrateandthedepositiontemperaturearechosentobe10sccm

and 1608C, respectively. Generally speaking, the optimized

thicknessofanITOﬁlmforaHJSsolarcellis80nm.Itisnoted thattheresistivityofaZnO:Bﬁlm(4103

_V

_cm)_is_higher_than

thatofanITOﬁlm(3104

_V

_cm)._The_optimized_thickness_of_a

ZnO:BﬁlmforaHJScellshouldbelargerthan80nm.Thethickness

of the ZnO:B ﬁlms are varied from 0.35

m

m to 1.6

m

m to

investigatehowtheyaffectthePVcharacteristics,includingopen circuitvoltage(Voc),shortcircuitcurrent density(Jsc),ﬁll factor

(FF),andenergyconversionefﬁciency(Eff.)oftheHJSsolarcellsas showninFig.4.TheJscvaluesofHJSsolarcellswithZnO:Bﬁlmsare

almost the same at the ZnO:B ﬁlms thicknessof 0.35

m

m and

0.8

m

m,butdecreaserapidlyto35.2mA/cm2asthethicknessof

theZnO:Bﬁlmrises to1.6

m

m. Thereisa relationbetweenthe

incidentlight(I0)andthetransmissionlight(I):

I¼I0eðatÞ (1)

where

a

andtaretheabsorptioncoefﬁcientandthicknessofthe

ﬁlm, respectively. The intensity of the transmission light is

inverselyexponentialtothethicknessoftheﬁlm,suggestingthat

the Jsc decreases with the increase of the ﬁlm thickness. The

Table1

ThedepositionparametersfortheZnO:Bﬁlmsusedinthiswork. ZnO:B sample B2H6 (sccm) Temperature(8C) Pressure (Torr) Sheetresistance (V/&) #1 0 160 0.6 227 #2 3 160 0.6 11.2 #3 5 160 0.6 7.8 #4 10 160 0.6 6.3 #5 10 130 0.6 38 #6 10 200 0.6 125 #7 10 250 0.6 390 #8 10 300 0.6 353

Fig.1.ThemeasuredsheetresistancevalueoftheZnO:Bﬁlmsasafunctionofthe diboronedilution.

Fig.2.ThetransmissionoftheZnOﬁlmswithdifferentB2H6dilutionfromthe

spectrometer.

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quantum efficiencies (QEs) of HJS solar cells with different thicknessoftheZnO:BflmsasshowninFig.5canfurtherexplain thisphenomenon.ThemajordifferenceinQEsfortheseHJScellsis atthewavelengthof600–1200nm,inwhichtheQEoftheHJScell withtheZnO:Bfilmof1.6

m

mthickissmallerthanthatwiththe

ZnO:Bﬁlm of 0.8

m

m. TheHJS cellhashighenergy conversion

efﬁciencyatthelongwavelengthregionbecausethebandgapof

thesiliconis1.1eV.Asaconsequence,thethickertheZnO:Bﬁlm, thelowertheJsc.

TheFFsvaluesofHJSsolarcellswithZnO:Blayersincreasesbya

factorof1.14withtheincreaseofthethicknessoftheBZOﬁlm

from 0.35

m

m to 1.6

m

m. It can be originated from the ﬁlm

thicknesseffectbecausetheR&isinverselyproportionaltotheﬁlm

thickness,whichresultsinthelargerFF.Furthermore,asmallerR&

canresultinalowerseriesresistance,leadingtoahigherVocvalue.

ThisexplainstheFFandVocincreasingwiththethicknessofthe

ZnO:Bﬁlm.

Theefﬁciencyresultscombinewithtotalfactorsincludingthe FF,Voc,andJsc.ThetendencyofefﬁciencyofHJSsolarcellsisthe

sameastheFFresults.Theoptimizedefﬁciencyof16.2%canbe

obtainedattheZnO:Blayerthicknessof1.6

m

m.

In most HJS solar cell research, an ITO ﬁlm is chosen as a

transparentconductiveoxidelayerbecauseofitsexcellentoptical

and electrical properties. It is of interest to compare the PV

characteristicsbetweenHJSsolarcellswithanITOﬁlmof80nm

thickandaZnO:Bﬁlmof1.6

m

mthick,respectively,asshownin

Table2.TheFFoftheHJSsolarcellwithaZnO:Bﬁlmishigherthan thatwithanITOﬁlm,becausetheR&of6.3

V

/&foraZnO:Bﬁlmis

lower than theR& of 50

V

/& for an ITOﬁlm. Meanwhile, the

thickerﬁlmresultsinalowerJscasmentionedearlier.Itisworth

noting that the most signiﬁcant feature of the low pressure

chemical vapordeposited ZnO:B ﬁlm is the textured structure

(Fig.3),whichisbeneﬁttothelightabsorptionandcontributesto theJscofthesolarcell.Otherwise,theJscwouldbelowerthanwe

measured. There are two main disadvantages for an ITO ﬁlm

thickerthan 80nm.Oneisthat theJscwilldecreaseduetothe

absorptionofthethickerﬁlm.Theotheristhattheabsorptionat

thelongwavelengthregionincreasesrapidlybecauseofthefree

carrierabsorption[17,21].

3. Conclusions

WehavefabricatedtheHJSsolarcellsusingZnO:Bﬁlmsasthe

TCO material deposited by an LPCVD system. Due to its high

transmittance, low sheet resistance as well as the textured

structure,thehigherenergyconversionefﬁciencyoftheHJSsolar cellwasobtainedthanthatwithanITOﬁlmastheTCOlayer.Other

works such as the improvements of the optical and electrical

propertiesoftheZnO:BﬁlmsontheHJSsolarcellsareinprocess.

Fig.3.TheSEMcrosssectionsofZnO:Bﬁlmsdepositedonc-Siin(a)1308C,(b)1608Cand(c)3008C.

Fig.4.Photovoltaiccharacteristicsof(a)ﬁllfactor,FF(b)shortcircuitcurrent density,Jsc(c)opencircuitvoltage,Vocand(d)conversionefﬁciency,Eff.forHJS

solarcellsasafunctionofthethicknessoftheZnO:BﬁlmandforaHJSsolarcellwith theITOﬁlm(bluespot).

Fig.5.ThequantumefﬁciencyofHJSsolarcellsfortwoZnO:Bﬁlmswiththickness of0.8mmand1.6mm.

Table2

Photovoltaic characteristicsof opencircuitvoltage (Voc),shortcircuit current

density(Jsc),ﬁllfactor(FF),andconversionefﬁciency(Eff.)forHJSsolarcellswith

theITOﬁlmandtheZnO:Bﬁlm.

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

HJScellwiththeITOﬁlm 0.593 36.51 72.17 15.64 HJScellwiththeZnO:Bﬁlm 0.593 35.26 78.05 16.3 J.-C.Hsiaoetal./JournaloftheTaiwanInstituteofChemicalEngineers44(2013)758–761

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Acknowledgements

This work was supported by Bureau of Energy under the

contractNo.B455DR1120.

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