Reactive Mechanism of Cu2ZnSnSe4 Thin Films Prepared by Reactive Annealing of the Cu/Zn

14 Pages·2017·8.1 MB·English

crystals
Article
Reactive Mechanism of Cu ZnSnSe Thin Films
2 4
Prepared by Reactive Annealing of the Cu/Zn Metal
Layer in a SnSe + Se Atmosphere
x
LiyongYao1,JianpingAo2,*,Ming-JerJeng3,4,*,JinlianBi2,ShoushuaiGao2,GuozhongSun2,
QingHe2,ZhiqiangZhou2,YiZhang2,YunSun2andLiann-BeChang3,4
1 TianjinInstituteofPowerSource,Tianjin300384,China;[email protected]
2 InstituteofPhotoelectronicThinFilmDevicesandTechnologyofNankaiUniversity,
KeyLaboratoryofPhotoelectronicThinFilmDevicesandTechnologyofTianjin,KeyLaboratory
ofOpticalInformationScienceandTechnologyofMinistryofEducation,CollaborativeInnovation
CenterofChemicalScienceandEngineering,NankaiUniversity,Tianjin300071,China;
[email protected](J.B.);[email protected](S.G.);[email protected](G.S.);
[email protected](Q.H.);[email protected](Z.Z.);
[email protected](Y.Z.);[email protected](Y.S.)
3 DepartmentofElectronicEngineering,ChangGungUniversity,
KueiShan333,Taiwan;[email protected]
4 DepartmentofOtolaryngology-HeadandNeckSurgery,ChangGungMemorialHospital,
Linkou244,Taiwan
* Correspondence:[email protected](J.A.);[email protected](M.-J.J.);
Tel.:[phone](ext.8011)(J.A.);[phone](ext.3507)(M.-J.J.);
Fax:[phone](J.A.);[phone](M.-J.J.)
(cid:1)(cid:2)(cid:3)(cid:1)(cid:4)(cid:5)(cid:6)(cid:7)(cid:8)(cid:1)
Received:13November2018;Accepted:20December2018;Published:23December2018 (cid:1)(cid:2)(cid:3)(cid:4)(cid:5)(cid:6)(cid:7)
Abstract: Cu ZnSnSe (CZTSe) thin films were prepared by a two-step process with the
2 4
electrodeposition of a Cu/Zn metallic stack precursor followed by a reactive anneal under a Se
+Sncontainingatmosphere. WeinvestigatetheeffectoftheSe andSnSe (x=1,2)partialpressures
x x
andannealingtemperatureonthemorphological,structural,andelementaldistributionoftheCZTSe
thinfilms. Linescanningenergydispersivespectroscopy(EDS)measurementsshowthepresence
of a Zn-rich secondary phase at the back-absorber region of the CZTSe thin films processed with
higherSnSe partialpressureandlowerannealingtemperatures. TheZn-richphasecanbereduced
x
byloweringtheSnSe partialpressureandbyincreasingtheannealingtemperature. Averythin
x
MoSe filmbetweentheCZTSeandMointerfaceisconfirmedbyX-raydiffraction(XRD)andgrazing
2
incidenceX-raydiffraction(GIXRD)measurements.Thesemeasurementsindicateastrongdependence
of these process variations in secondary phase formation and accumulation. A possible reaction
mechanismofCZTSethinfilmswaspresented.InapreliminaryoptimizationofboththeSnSe partial
x
pressureandthereactiveannealingprocess,asolarcellwith7.26%efficiencyhasbeenfabricated.
Keywords: Cu ZnSnSe (CZTSe);electrodeposition;metallicstackprecursor;Se andSnSe (x=1,2)
2 4 x x
partialpressures;annealingtemperature
1. Introduction
Earth-abundantandnon-toxiclightabsorbermaterials,suchasCu2ZnSn(SxSe1−x)4 (CZTSSe),
are an attractive candidate for thin film solar cells to meet terawatt scale production as they have
an adjustable bandgap between 1.0 eV and 1.5 eV and a theoretical efficiency of above 30% [1].
Nevertheless, these materials are still at an incipient state of development, with a maximum
Crystals2019,9,10;doi:10.3390/cryst9010010 www.mdpi.com/journal/crystals
Crystals2019,9,10 2of14
device efficiency of 11.6% [2] and 12.6% [3] for Cu ZnSnSe (CZTSe) and CZTSSe, respectively,
2 4
at the laboratory scale. Their values are still far from the record cell efficiency of above 21% for
Cu(In,Ga)Se [4]andCdTe[5]. BecauseofthethermodynamicstabilityofCZTS(e)thinfilms[6]and
2
the volatility of tin selenide (SnSe ) [7], a two-step process was generally used to prepare CZTSe
x
thinfilms[2,3,7–13]. AprecursorwithallorpartoftheCZTS(e)componentswaspreparedfirstby
eitheravacuumornon-vacuummethodandwasthentreatedbysubsequentthermalannealingto
improvethecrystallinity. HighpressureinertgasandSepartialpressurehasbeenappliedtoensure
the surface stability at the expense of excessive Se diffuse through CZTSe thin films, which react
withMotogeneratethickMoSe films[14]. ThickMoSe filmsseemthermodynamicallyinevitable
2 2
whenconsideringthehighSepartialpressureorhightemperature[15,16]. Thecompactnessofthe
precursor[8]andtheannealingprocessmustbecarefullydesignedandcontrolled[10,11]. Infact,
thestabilityofCZTSethinfilmsismainlydecidedbythechemicalequilibrium[6,17]:
Cu ZnSnS(e) ←→Cu Se+ZnSe+SnS(e)(g)+1/2S(e) (g) (1)
2 4 2 2
Therefore,thedecompositionreactioncanbereducedtosomeextentwithhighSepartialpressure,
especially when accompanied by high inert gas pressure. However, CZTS(e) thin films were also
stable when SnS(e) and S(e) coexisted in the annealing atmosphere which has been confirmed by
experiments[6]. LowSepressureannealingprocesseswerefairlyeffectivetoavoidthickMoSe films.
2
However,mostpublishedworkswhichadoptlowSepressure,mainlyinvestigatethefeasibilityof
thismethod[12,17]. Therefore,thesolarcellefficiencyturnsouttobelowerthanthatpreparedwith
highSepartialpressure. SnSe andSeareoftenadoptedinlowpressureprocesses,andthentheSn
x
isintroducedthroughthegasphase. Moreover,thesurfacemorphologyofSnmetalintheCuZnSn
precursorpreparedbyelectrodeposition[18]orphysicalvapordeposition[12,19]hasbeenstudied
andwasshowntohaveasignificanteffectontheabsorberproperties. Wewillinvestigatewhethera
precursorofonlyCuZnmetalcanbeanalternativechoiceforpreparinghighqualityCu ZnSnSe thin
2 4
films[12,17,19].
Inthiswork,wepresentasystematicstudyofCu ZnSnSe thinfilmspreparedbyatwo-stage
2 4
process as a function of SnSe (x = 1, 2) partial pressure and reactive annealing temperature.
x
Wedemonstratetheimportanceofaccuratecontrolofbothpartialpressureandreactiveannealing
temperature, which are significant determining factors for elemental diffusion and distribution.
Apreliminaryreactionanalysiswasalsoproposedbasedontheexperimentalresultsandliterature
data. WebelievetheformationofCZTSethinfilmsinvolvesseveralsequentialstepsandthecontrol
stepisasolid-statereaction:
Cu SnSe +ZnSe←→Cu ZnSnSe (2)
2 3 2 4
Theoptimizationofexperimentalconditionswasobtainedfromtheexperimentalresults.Wehave
achievedamaximumcellefficiencyof7.26%byusingarelativelylowpressureatabout580◦Cand
identifiedtheformationmechanismsofCu ZnSnSe thinfilms.
2 4
2. MaterialsandMethods
ThemetalstacksofcopperandzincwiththeratioofCu/Znof1.6wereelectrodepositedona
molybdenumlayerbytheconstantcurrentmethodusingatwo-electrodesystematroomtemperature.
A4cm2 ×4cm2pieceofsoda-limeglassonwhicha1200nm-thickdoubleMolayerwasdeposited
byDC–magnetronsputteringwasusedasthecathodeintheelectrodepositionprocess. Copperwas
electrodepositedfromasolutionthatwasfreshlypreparedandcontained187.5g/LofCuSO ·5H O
4 2
and 75 g/L of H SO . The solution of zinc was prepared in-house using 0.2 M zinc vitriol that
2 4
was dissolved in 0.5 M methane sulfuric acid [11]. The pH of the solution was adjusted to 2.2 by
addingsodiumhydroxide. Thecopperandzinclayerswereelectrodepositedusingdirectcurrent
with current densities of 50 mA/cm2 and 20 mA/cm2, respectively. The typical deposition time
was about 15 seconds. Samples of the metallic precursors (4 cm2 × 4 cm2 in area) were reactively
Crystals2019,9,10 3of14
annealed in a laboratory-made furnace capable of working in vacuum (1 × 10−4 Pa) or an inert
gas (Ar) atmosphere [13]. The sample temperature, Sn temperature and the Se temperature are
controlledindividually. TheelementsSnandSeareplacedintheselenizationfurnace,underheating.
Se evaporates into the selenization furnace and reacts with Sn to form Sn–Se binary phases,
whichevaporateeasilyduetoahighsaturatedvaporpressure. TheSeandSnSe partialpressuresare
x
controlledbythreeparameters: theSnandSetemperature,andthetotalpressure.
The electrodeposited metal stacks were annealed at 300 ◦C in inert gas at 1000 Pa for 30 min.
Subsequently,theSeandSnwereheatedto270◦Cand300◦Crespectively,thenthetubewaspurged
withinertgasto1×10−3 Paandthevalvewasclosed. Thesesampleswerefollowedbydifferent
selenizationprocesses. TwosetsofexperimentsarelistedinTable1. Theparametersofthefirstsetof
experimentsare: totalpressureof15PawithSntemperatureof550◦C,namedasA1;totalpressureof
15PawithSntemperatureof520◦C,namedA2;andtotalpressureof25PawithSntemperatureof
520◦C,namedA3. Thesubstratetemperaturewaskeptat570◦CandtheSetemperaturewaskeptat
270◦C.Thetotalpressurewasadjustedbythevalveofthevacuumpumpmanuallywithaccuracyof
±1Pa. Theparametersofthesecondsetofexperimentsare: sampletemperaturesof570◦C,600◦C
and630◦C,namedasa1,a2anda3,respectively;tinsourcetemperatureof550◦Cinasealedvacuum
withmaximumtotalpressureofabout40Pa.
Table1.Parametersoftheexperiments.
SampleNumber SampleTemperature(◦C) SnTemperature(◦C) SeTemperature(◦C) TotalPressure(Pa)
A1 570 550 270 15
A2 570 520 270 15
A3 570 520 270 25
a1 570 550 270 ~40
a2 600 550 270 ~40
a3 630 550 270 ~40
AstandardprocedurewasappliedtofabricatesolarcellsbasedontheCZTSethinfilmwithoutan
etchingprocess. First,a50nmthickCdSwasdepositedusingchemicalbathdeposition. Next,50nm
i-ZnOand450nmAl-ZnOweredepositedbyRFsputtering. Finally,a50nmNi/2µmAlmetalgrid
wasdepositedontopofthedeviceviaelectron-beamevaporationthroughametalmasktocreatea
metallicgridpattern. TheactiveareaoftheCZTSesolarcellwas0.358cm2.
The structures of the selenized samples were characterized using a Philips X-pert Pro
diffractometer(PANalyticalLtd.,EAAlmelo,theNetherlands)withCuradiationandaRenishawInvia
Ramanspectroscope(RenishawLtd.,GloucestershireUK)TheexcitationwavelengthoftheRaman
was514nm. Thepenetrationdepthofthislaserisaround100nmintotheabsorber. Thecomposition
oftheCZTSethinfilmwasmeasuredusingaMagixPW2403X-rayfluorescentspectrometer(XRF,
(PW2403(PANalyticalLtd., EAAlmelo, theNetherlands))withaRh-anode, whichwascalibrated
byinductivelycoupledplasmaspectroscopy(ICP,PANalyticalLtd.,EAAlmelo,theNetherlands).
Surfaceandcross-sectionalimagesweretakenusingascanningelectronmicroscope(SEM)(JEOL
JSM-6700,(JEOLLtd.,Akishima-shi,Japan)). Thedepthprofilesoftheelementswereobtainedby
linescanningenergydispersivespectroscopy(EDS).Phaseidentityanddeterminationweremadeby
Ramanspectroscopy(Renishaw+Nanonics).Current–voltage(J–V)measurementsofCZTSesolarcells
weremadeunderilluminationbyastandardAM1.5spectrumof1000W/m2atroomtemperaturewith
aconstantlightsolarsimulator,whichwascalibratedusingastandardmono-crystallineSisolarcell.
3. Results
AsshowninFigure1,depthdistributionofelementsinCuZnprecursorswasmeasuredbyline
scanning energy dispersive spectroscopy (EDS) along the depth direction to estimate the effect of
thermaltreatmentontheelementaldistribution(Figure1a,b),andtheXRDpatternwasusedtoanalyze
thephasetransition(Figure1c).ThreepeakswereobservedinFigure1a,correspondingtotheelements
Crystals2019,9,10 4of14
Mo,CuandZnfromthebottomtothesurfaceintheMo/Cu/Znstackedstructure,fairlyconsistent
with the electrodeposition sequence. The overlap of Cu and Zn signals in Figure 1b, obviously in
Crycsotanlst r2a0s1t8t, o8, Fxi gFOuRre P1EaE,Rr eRvEeVaIlEsWa m oreuniformdistributionofCuZnafterannealing. TheCuZnalloyw4 aosf 14
themainphaseoftheelectrodepositedlayerinFigure1c(blackline),correspondingtotheCuZn/Mo
to the CuZn/Mo stacked structure. After thermal treatment, the Cu Zn brass phase is the main
stackedstructure. Afterthermaltreatment,theCu Zn brassphas0.e64isth0.3e6mainphasealongwith
0.64 0.36
phsaosme ealmoningo wrCituha snodmZen mpihnaosre sC.uTw aondu nZknn opwhanspeesa. kTswaot 3u5n.8k◦naonwdn4 3p.9e7a◦ksa raet l3a5b.e8l°e danasd“ 4*3”.,9w7°h iacrhe cloaubledled
as “*”, which could be copper oxide phases. These oxides may come from the electrodeposition
be copper oxide phases. These oxides may come from the electrodeposition process. The copper
process. The copper has an obvious volume variation in the process of selenization, owing to the
hasanobviousvolumevariationintheprocessofselenization,owingtothegreatdisparitiesinthe
grevaotl udmisepoafrCitiue2sS iena nthdeC vuoSleu2mpee rocfo Cppue2Srea taonmd[ C20u].STe2h peecro pcpoeprpdeirs tartiboumti o[2n0a]t. tThheeb coottpopmerw dililsitnreibvuittaibolny at
thec rbeoattteopmin whiolll einseavtitthaeblbya ccrke-caoten tpaicnt hinotleersf aacte thdeu briancgk-thcoenrteaacctt iivnetearnfanceea ldinugri,nwgh tihceh riesadcetitvriem aennntaelaltiong,
whthiceha idsh deseitornimbeentwtael etno tthheea abdsohrebseiornla ybeertwaneednt htheem aoblysobrdbeenru lmayseur basntrda tteh.eF umrothlyerbmdeonreu,mel esmubensttraalte.
Fudrtishterribmuotiroen, ehlaesmgerenattali ndfliusternibceutoinonth heasse lgerneiazta tiinofnlureenacctei oonnp trhoec essesleansiwzaetliloans trheeacqtuioanli tpyroofcaebsss oarsb ewrell
as ltahyee rq,uwahliitcyh olfe aadbstoordbievre lrasyeedr,e vwicheicphe rlefoardm taon dceiv[e9r]s.eW deevhiacvee pdeorfnoermmaanncyee [x9p].e rWime ehnatvseto dsoenleen mizaeny
exptheeriMmoe/nCtsu t/oZ snelmeneitzael pthreec Muroso/Crud/irZenc tmlye(twali tphoreuctuarnsonre adliinregcttolyf o(wrmithaomute taanlnaellaolyin)gin toan foSrem+ aS nmSeetal
allaotym) oisnp haenr eSfeo r+p SrenpSaer iantgmCoZsTpSheerthe infofirl mprse.pWaerifnogu nCdZthTaStet htheiand hfielsmiosn. bWetew feoeunntdhe tChZatT Stheet haindhfielmsion
beatwndeetnh ethMe oClZayTeSrew thaisna fiblmig apnrodb tlheme M. Toh learyeefor rwe,atsh ae bfoigrm partoibolnemof. aTCheurZenfoarlelo, tyhwe faosremssaetniotina loffo ar CthueZn
allfooyll owwa-su epssseenletniaizl aftoior ntheex pfoelrliomwe-nutps. selenization experiments.

FigFuigrue re1. 1L.iLnein escsacnanninnign geneneregrgyy ddisisppeerrssiivvee ssppeeccttrroossccooppyy ((EEDDSS)) pprroofiflielesso offt htehee leecletrcotrpolaptleadteCd uCZunZn
prpecreucrusorsrosr (sa()a b)ebfeofroer eanandd (b(b) )aaftfeterr ththeerrmmaall ttrreeaattmmeenntt ffrroomm EEDDSSm meeaassuureremmenent.tX. X-r-aryayd idffirfafrcaticotnio(nX (RXDR)D)
papttaetrtnersn osfo tfhteh eeleelcetcrtoroddeeppoossititeedd CCuuZZnn pprreeccuurrssoorrss ((cc)) bbeeffoorreea anndda fatfetrerth tehremrmalatlr etaretmatemnet.nt.
XRDpatternsoftheelectrodepositedCZTSeabsorberlayersareshowninFigure2. TheCZTSe
XRD patterns of the electrodeposited CZTSe absorber layers are shown in Figure 2. The CZTSe
phases are identified for all samples [21]. All samples have peaks at 17.44◦, 22.05◦, 42.8◦, 63.83◦,
phases are identified for all samples [21]. All samples have peaks at 17.44°, 22.05°, 42.8°, 63.83°, and
and68◦ andthoselabeledas“*”. ThestrongestCZTSepeakis(112),inagreementwithotherliterature
68° and those labeled as “*”. The strongest CZTSe peak is (112), in agreement with other literature
data[21]. AllsamplesexceptA3havepeaksat24.5◦,29.2◦,and43.8◦,whichcorrespondtoCu Se
data [21]. All samples except A3 have peaks at 24.5°, 29.2°, and 43.8°, which correspond to Cu2Se x(or
2 x
(or Cu Se) phases according to (ICCD#00-053-0523, ICCD#00-026-0557 and ICCD#00-027-1131).
2–x
Cu Se) phases according to (ICCD#00-053-0523, ICCD#00-026-0557 and ICCD#00-027-1131). This
2–x
ThisindicatesthatsampleA3wasmoreconsistentwithpureCZTSephases. Binarysecondaryphases
indicates that sample A3 was more consistent with pure CZTSe phases. Binary secondary phases of
of Cu Se and SnSe were not easily observable because of the similarity of their crystal structures.
x
Cu Se and SnSe were not easily observable because of the similarity of their crystal structures. No
x
MoSe related diffraction peaks are observed as expected because of the low Se partial pressure
x
employed in this work.
To further confirm the minor MoSe phase at the CZTSe and Mo interface, grazing incidence X-
2
ray diffraction (GIXRD) was carried out on one of the samples with most of the CZTSe film
mechanically stripped off from the sample, as shown in Figure 3. Only CZTSe and Mo related
diffraction peaks was present and no MoSe related phase was observed (MoSe with a main
2 2
Crystals2019,9,10 5of14
NoMoSe relateddiffractionpeaksareobservedasexpectedbecauseofthelowSepartialpressure
x
employedinthiswork.
TofurtherconfirmtheminorMoSe phaseattheCZTSeandMointerface,grazingincidenceX-ray
2
Crystals 2018, 8, x FOR PEER REVIEW 5 of 14
diffraction(GIXRD)wascarriedoutononeofthesampleswithmostoftheCZTSefilmmechanically
Crystals 2018, 8, x FOR PEER REVIEW 5 of 14
strippedofffromthesample,asshowninFigure3. OnlyCZTSeandMorelateddiffractionpeakswas
diffraction peak at 2θ = 31.7° and 55.8° according to ICCD#00-029-0914), which is consistent with the
XdRifDfr ampcrteeiasoesnnu tpreaenmadke nnaott 2iMnθ o F=Si eg321u.rr7ee°l a 2at.en dd p5h5a.8se° awcacsoordbsinergv teod I(CMCoDSe#20w0-i0th29a-0m9a1i4n),d wiffhriacchti oisn cpoenaskisatte2nθt w=i3t1h.7 t◦he
and55.8◦ accordingtoICCD#00-029-0914),whichisconsistentwiththeXRDmeasurementinFigure2.
XRD measurement in Figure 2.

Figure 2. XRD patterns of the A1–A3 and a1–a3 samples selenized in different selenium conditions.
FigurFei g2u. rXeR2D. X pRaDttepranttse ronf sthofe tAhe1–AA1–3A a3ndan ad1–a1a–3a s3asmamplpelse ssesleelnenizizeedd inin ddiiffffeerreenntt sseelleenniiuumm ccoonndditiitoionns.s.
MMoo ppeMeaakoksps aeaarreke s llaaabrbeeellleaedbde alaessd ““a●●s””“,, CC”u,uC22SSuee2xxS ppexeeaapkkessa kaasrreea rlleaablbaeeblleeeldde d aasas s“““▲▲(cid:78)””” aaannnddd CCCuuu222ZZZnnnSSSnnnSSSeee444 ( (C(CCZZZTTTSSeSe)e)p )p epeaeakaksksas ra earere
lalabbeeleleldad ba aesls e“ “d**”a”.s . “*”.

FFigiguurreFe i3 g3.u .G rGerra3az.ziGinnrgga z iininnccgiiddineencnicdceee nXXc--erraaXyy- r ddayiiffffdrriaaffccrttaiiooctnnio ((nGG(IIGXXIRRXDDRD)) pp)apatatttteetrernrnn oooff f tththheee sssaaammmpppllleee w wwiitiththh mm moosoststo to foft fht hteheCe C ZCZTZSTTeSSee
fiflimlm h hfiaalvmvininhgga vb bieneeegnnb memeeencchmhaaenncihiccaaanlllilcyya slslttyrriispptrppipeeddp e oodffffo.. ff.
BBeeccaauussee ooff tthhee ssiimmiillaarriittyy iinn ddiiffffrraaccttiioonn ppaatttteerrnnss,, XXRRDD ccaannnnoott uunnaammbbiigguuoouusslyly idideenntitfiyfy ZZnnSSee, ,
CCuu2S2SnnSSee33 a anndd C CZZTTSSee,, aanndd tthheerreeffoorree iitt iiss nneecceessssaarryy ttoo uussee ootthheerr cchhaarraacctteerriizzaattiioonn t teecchhnniqiquueess t oto i dideenntitfiyfy
CCZZTTSSee s suucchh a ass R Raammaann ssppeeccttrroossccooppyy.. AAss sshhoowwnn iinn FFiigguurree 44,, tthhee RRaammaann ppeeaakk p poossiittioionnss a at t1 19955 c cmm−1−,11,17733
ccmm−1− 1a anndd 2 23344 ccmm−−11 wweerree iiddeennttiiffiieedd aass tthhee mmaaiinn ppeeaakkss ooff tthhee CCZZTTSSee pphhaassee.. TThhee wweeaakk p peeaakk a at t2 24499 c cmm−1− 1
ccoorrrreessppoonnddss t too Z ZnnSSee pphhaasseess [[1122]],, CCuu22SSnnSSee33 [[2222]] oorr ddiissoorrddeerreedd CCZZTTSSee pphhaasseess [ [2233]]. .T Thhisis i sis c coonnfifrirmmeedd t oto
bbee C Cuu2S2SnnSSee33 a accccoorrddiinngg ttoo tthhee ZZnn--ppoooorr ssuurrffaaccee ccoonnffiirrmmeedd bbyy tthhee ffoollllooww--uupp l liinnee s sccaannnniningg E EDDSS a annaalylysissi s
oof fC CZZTTSSee t thhiinn f fiillmmss.. FFoorr ssaammpplleess AA11−−AA33,, tthhee iinntteennssiittyy aatt 224499 ccmm−−11 ddeeccrreeaasseedd w wiitthh l olowweerr S SnnSSeex xp paartritaial l
pprreessssuurree, ,r reevveeaalliinngg tthhaatt CCuu22SSnnSSee33 aatt tthhee ssuurrffaaccee rreeggiioonn iiss rreedduucceedd.. FFoorr ssaammpplleess a a11--aa33, ,s saammpplele a a11 a anndd
aa22 h haavvee a ann o obbvviioouuss ppeeaakk iinntteennssiittyy aatt 224499 ccmm−−11,, aanndd aa22 hhaass aa wweeaakk ppeeaakk iinntteennssiittyy h heerree, ,r reevveeaalilningg t hthaat t
Crystals2019,9,10 6of14
Because of the similarity in diffraction patterns, XRD cannot unambiguously identify ZnSe,
Cu SnSe and CZTSe, and therefore it is necessary to use other characterization techniques to
2 3
identify CZTSe such as Raman spectroscopy. As shown in Figure 4, the Raman peak positions at
195cm−1,173cm−1and234cm−1wereidentifiedasthemainpeaksoftheCZTSephase.Theweakpeak
at249cm−1correspondstoZnSephases[12],Cu SnSe [22]ordisorderedCZTSephases[23]. Thisis
2 3
confirmedtobeCu SnSe accordingtotheZn-poorsurfaceconfirmedbythefollow-uplinescanning
2 3
CrysEtDalsS 2a0n18a,l 8y,s xi sFoOfRC PZETERSe RtEhVinIEfWil m s. ForsamplesA1−A3,theintensityat249cm−1decreasedwithlow6 eorf 14
SnSe partialpressure,revealingthatCu SnSe atthesurfaceregionisreduced. Forsamplesa1-a3,
x 2 3
annsaemalipnleg aa1t a6n0d0 °aC2 hina vae saenaloinbvgi oautmspoespakheinrete rnessituyltast i2n4 9lecsms C−1u,2aSnndSea32 phhaassaesw. eTahkisp ewaiklli nbtee ndsisitcyuhsseered, in
ther efovlelaolwinigntgh astecatninoenasl.i ngat600◦CinasealingatmosphereresultsinlessCu SnSe phases. Thiswillbe
2 3
disTchues seRdaminatnhe sfpolelcotwruinmg swecittiho nLs.orenzian fitting was carried out and the peak position, peak
intensitTyh aenRda mfualnl swpiedcttrhu mhawlfi tmhaLxoirmenuzmian (fiFtWtinHgMw)a sdcaatrar iewdeoreu taannadlythzeedp.e aAksp loissitteiodn ,inp eTakabinlet e2n,s iptyeak
posaintidonfu lalnwdi dFthWhHalMfm cahxaimnguemd (fFrWomH M19)4d.2a9ta cwme−r1e aton a1ly9z5e.1d1. Acmsl−i1s taenddin fTroabmle 62.,9p6e ackmp−1o stioti o6n.8a5ncdm−1,
resFpWecHtivMelcyh, afonrg seadmfrpolmes 1A914 .a2n9dc mA−2 1wtioth1 9lo5w.11erc Smn−S1ea pnadrtfiraolm pr6e.s9s6ucrme a−n1dto hi6g.8h5ecrm S−e 1p,arretsipale cptriveseslyu,re.
x
forsamplesA1andA2withlowerSnSe partialpressureandhigherSepartialpressure. However,
However, it changed little with further adxjustment to the Se and SnSe partial pressure for A2 and A3.
it changed little with further adjustment to the Se and SnSe partial pressure for A2 and A3.
This indicates that higher Se and lower SnSe partial pressure were preferred for the formation of pure
This indicates that higher Se and lower SnSe partial pressure were preferred for the formation of
CZTSe phases. However, excessive SnSe would cause secondary phases. Annealing temperature has
x
pureCZTSephases. However,excessiveSnSe wouldcausesecondaryphases. Annealingtemperature
an obvious influence on the Raman spectruxm parameters. The peak position decreases and the
hasanobviousinfluenceontheRamanspectrumparameters. Thepeakpositiondecreasesandthe
FWHM increases with increasing annealing temperature from 570 °C to 630 °C. The broad CZTSe
FWHMincreaseswithincreasingannealingtemperaturefrom570◦Cto630◦C.ThebroadCZTSepeaks
peaks are observed and Cu SnSe was confirmed to be present at the surface layer. More information
2 3
areobservedandCu SnSe wasconfirmedtobepresentatthesurfacelayer. Moreinformationcould
2 3
could be obtained from the Raman spectra, like the order–disorder character and the degree of
beobtainedfromtheRamanspectra,liketheorder–disordercharacterandthedegreeofcrystallization.
crystallization. The broadening peak may arise from a high defect density and local compositional
The broadening peak may arise from a high defect density and local compositional fluctuations.
fluctuations. The local disorder in the cation sub-lattice could also be a reason for the peak shoulder
The local disorder in the cation sub-lattice could also be a reason for the peak shoulder [23,24].
[23,24]. A higher peak position accompanied with lower FWHM values corresponds to higher
AhigherpeakpositionaccompaniedwithlowerFWHMvaluescorrespondstohighercrystallization
cryqsutaallliitzya[t1io2]n. quality [12].

FigFuirgeu r4e. R4a.maRna smpaenctrsopseccotproys ocof pthye oAf1–thAe3 Aan1d–A a31–aan3 dsama1p–ale3s ssealmenpilzeesds welietnhi zdeidffewreintht sdelieffneirzeanttion
consdelietinoinzast. ionconditions.
Table 2. Raman peak position, full width half maximum (FWHM) and peak intensity calculated by
Raman spectra with Lorenzian fitting.
Sample Peak Position(cm−1) FWHM (cm−1) Peak Intensity (a.u.)
A1 194.29 6.96 2658.24
A2 195.11 6.85 2438.50
A3 195.13 6.89 2609.58
a1 195.34 6.46 3676.71
a2 195.05 6.94 2734.85
a3 194.78 7.37 2357.60
Crystals2019,9,10 7of14
Table2. Ramanpeakposition,fullwidthhalfmaximum(FWHM)andpeakintensitycalculatedby
RamanspectrawithLorenzianfitting.
Sample PeakPosition(cm−1) FWHM(cm−1) PeakIntensity(a.u.)
A1 194.29 6.96 2658.24
A2 195.11 6.85 2438.50
A3 195.13 6.89 2609.58
a1 195.34 6.46 3676.71
a2 195.05 6.94 2734.85
a3 194.78 7.37 2357.60
Tofurtheranalyzetheeffectofannealingconditiononthemorphology,secondaryphasesand
elemental distribution, scanning electron microscopy (SEM) with line scanning EDS was applied.
Inordertofacilitatethediscussion,gasphasepartialpressureanalysisintheseleniumatmosphere
willbediscussedfirst. ItiswellknownthatonlySnSe andSnSephaseswereverifiedexperimentally
2
intheSn–Sesystem,andnoSn Se wasfound[7]. Thesaturatedvaporpressure(unitsinPa)ofSe,
2 3
SnSe and SnSe at the temperature adopted in this work are listed in Table 3. The statured vapor
2
pressureofSnSe istwoorderofmagnitudeshigherthanthatofSnSe;therefore,SnSe wasconsidered
2 2
asthemainSn-containingvaporgasinthisworkandtheSnSephasewasneglected.
Atthebeginningoftheselenizationprocess, theannealingfurnacewasquicklyinjectedwith
Se vapor and the Sn-containing vapor phase was negligible because the saturated vapor pressure
of Sn was low [25]. Subsequently, Sn reacts with Se to generate SnSe , and SnSe evaporates into
2 2
theannealingfurnaceandreactswiththeprecursor. TheSnSe partialpressurewasaffectedbythe
2
reactionrateofSewithSn,i.e.,SntemperatureandtheSepartialpressure. Theanalysisofpartial
pressureislistedinTable4basedonthedataintableandtheexperimentalconditions. Figure5shows
thecross-sectionalandsurfaceSEMmicrographsofsamplesA1–A3. LargeCZTSegrainwithasmooth
andcompactsurfacewasobservedinFigure5a,d,accompaniedbylargeholesandZn-richsecondary
phasesattheCZTSe/Mointerface(whichwillbeconfirmedbyEDSandshownhereinlater). Theholes
andZn-richsecondaryphasesbetweenCZTSeandModiminishedwithincreasingSepartialpressure
and decreasing SnSe partial pressure, as shown in Figure 5b. The surface roughness increased,
x
andgrainsizedecreasedinFigure5b,e. WithfurtherlowerSnSe partialpressuresandhigherSe
x x
partialpressures,isolatedhill-shapedgrainswereobtainedandcoveredthewholefilm. Almostno
holesandsecondaryphasesexistedattheinterfaceinFigure5c,f. Unsurprisingly,thebiggestsurface
roughnesswasobservedinthetrendofA1andA2.ItwasobviousthatwithhigherSe partialpressure
x
andlowerSnSe partialpressure,thegraintendsgrowlongitudinallythroughthefilm. Incontrast,
x
withlowSepartialpressureandhigherSnSepartialpressure,thegraintendstogrowthlaterallyat
thesurface.
Table3.Thesaturatedvaporpressure(unitsinPa)ofSnSeandSnSe [7,13,25].
2
Temperature(◦C)
SaturatedVaporPressure 520 550 570
SnSe 0.13(0.10–0.19) 0.42(0.31–0.56) 0.85(0.63–1.11)
SnSe 6.89(0.8–18.54) 19.16(2.33–51.58) 36.4(4.53–97.97)
2
Table4.Thepartialpressure(unitsinPa)ofSeandSnSe .
2
Sample TotalPressure Sex SnSe2
A1 A1(15Pa) MuchLower(<<16.3Pa) Lower(<19.16Pa)
A2 A2(15Pa) Lower(<16.3Pa) muchlower(<6.89Pa)
A3 A3(25Pa) Low(≈16.3Pa) Lower(≈6.89Pa)
a1 a1(~40Pa) Low(≈16.3Pa) Low(≈19.16Pa)
a2 a2(~40Pa) Low(≈16.3Pa) Low(≈19.16Pa)
a3 a3(~40Pa) Low(≈16.3Pa) Low(≈19.16Pa)
Crystals2019,9,10 8of14
Crystals 2018, 8, x FOR PEER REVIEW 8 of 14
Crystals 2018, 8, x FOR PEER REVIEW 8 of 14

FigFFuigirgueu r5ree. 5 C5..rC oCrsroso-ssssse--scseteicocttiniooannla aalln aadnn dsdu ssruufrarfcfaaecc esec ssaccnaannninnniignn gge leeellceetccrttorronon nm mmiciirccorroosscscocoopppyyy ( S((SSEEEMMM)) )m mmiciiccrrrooogggrrraaappphhhsss ooofff ttthhheee AAA111–––AAA333
anadan nadd1a –a1a1–3–a as33as msaampmlpepslle epssrp eprpreeappraeardree dudnu udnneddree rdr didfififefffreeerrneentn tcto cconondndidtiiitotiiononsns.s .T. TThhhee es scscacaannnnnniniinnggg l illniinneee i siiss i iinnndddiiicccaaattteeeddd bbbyyy aaa bbbllluuueee llliiinnneee
in i(nian–( a(ca–)–.c c).).
FiFgiugruer 6e s6hoshwosw thset hlieneli nsceasncnainnngi nEgDSE DprSopfirleosfi olefs thoef sthame spalmesp Ale1s–AA31 –(tAh3e (stchaennscinangn liinneg (lbinluee( lbinluee)
Figure 6 shows the line scanning EDS profiles of the samples A1–A3 (the scanning line (blue line)
is liiinns dei)nicidasiticenaddt eiidcna tiFneid gFuiingrueF ri5gea u5,rbae,,cb5),a.c ,)Tb. ,hcTe)h. eST nhSSeneSSxe npxS aperxatripatiala rlpt ipraelrsepssruseursersseu srf erfsoromfmro h mhigihghih g thtoo t loloowlwow ffoofror rtthhthee e ppprrreeepppaaarrraaatttiiiooonnn ooofff
sasmsaampmlpep lAele1A A, 1A1,,2A A a22na adnn dAdA 3A w33 wewreeer resea ssmaammplppel leAe AA1,11 A,, AA3 33a naandnd dA AA2,22 r,, errseepssppeecectcittviivveeelyllyy.. .S SSaaammmpppllelee A AA111 i iisss ZZZnnn---pppoooooorrr aaattt ttthhheee sssuuurrrfffaaaccceee
regrreieoggnioi onanna dann dSdnS S-npn-o-ppoooro oarrta attht tethh ebeo bbtoottotttmoomm re rrgeegigoiionon.n H.. HHowoowweveevevree,rr ,s, sasamammppplellee A AA222 i siiss S SSnnn--p-ppoooooorrr a aannnddd ZZZnnn---rrriiiccchhh aaattt ttthhheee sssuuurrrfffaaaccceee
regrreiegoginoi,on inn,,id ninidcdaicitcaiantitgnin gtght athhta altotl wolowewre erSrnS SnSneSSxe expx appraatrirattiilaa pll rpperrseesssussuruerrese ssf offoor rrs assaammmppplellee A AA222 c ccaaannn r rreeesssuuulllttt i iinnn bbbeeetttttteeerrr SSSnnn iiinnn---dddiiiffffffuuusssiiiooonnn
thtahtnha anhnih ghihgigehrhe eSrrnS SSnneSxSe epxxa ppratairrattlii aapll rppersreesssussruuerrsee sfso ffroo rsr assmaammpplpell eeA AA11.1 .L. LLooowwweere rrS SSnnnSSSeeex xxp ppaaarrtrittaiiaal ll ppprrreeessssssuuurrreee iiisss bbbeeennneeefffiiiccciiiaaalll tttooo ttthhheee
intineinrtetderirdfdfiufiffsufiuossinoio nonfo oSffnS S nanna adnn dZdZ nZn fnof froo rar a raer reaeacatccitvtivieve eaa nannnneneaealailnliingng gCC Cuuu//Z/ZZnnn m mmeeetattaallllillciicc s sstattaaccckkk p pprrreeecccuuurrrsssooorrr... IIIttt iiisss wwweeellllll kkknnnooowwwnnn
thtahtth aaat tha aihg hihgigehrhe etrort ttoaotlta apllrp persresesusssruuer receac cnaa npn rpperrveevevneentn ttth ttheh eeS SnSn nlo llososss ssfr fforromomm t htthheee s susuurrfrfafaaccceee o oofff C CCZZZTTTSSSeee ttthhhiiinnn fffiiilllmmmsss ooowwwiiinnnggg tttooo ttthhheee
highhhigi ghshas tsauatrutaurtraeatdtee dvda vvpaaoppro orpr rppersresesusssruuerr eoe ofo fSfS nSnSnSeSexe.x x.T. ThTehhreeerrfeeoffrooerr,ee ,,p pprorroopppeerelrryllyy a aadddjujjuussstittniinnggg ttthhheee aaannnnnneeeaaallliiinnnggg pppaaarrraaammmeeettteeerrr bbbyyy
slisgslhilgitghlhytt liylnyi cnirncecrareseaianssiginn gSgnS SSnneSSxe epxxa ppraatirrattilia apll rpperrseesssussruuerr eea naadnnd dth ttheh eeto ttotoattlaa llp pprerresessssusuurerree o oof ffs ssaaammmpppllleee A AA333 llleeeaaadddsss tttooo mmmooorrreee uuunnniiifffooorrrmmm
eleeemlelememneten ndttid sdtirsisitbtrriubibtuiuottinoio ntnht athhnaa ntnht tohhsooess esehs shohowowwnn nfo fforo rsra ssamammpplpelleses sA AA11 1a anannddd A AA222,, ,a aasss s sshhhooowwwnnn i iinnn F FFiiiggguuurrreee 666ccc...
(a)(a) A1A1(b()b) CuCu AA22((cc)) AA33
SeSe
MoMo SeSe ZZnn
sity(a.u.) nsity(a.u.) ZnZn SnSCnuCu SnSn ZnZn MMoo CCSuSunn SSee MMoo
n e
nte Int
I
0 0 1 1DeDpteh2p(tμh2m(μ)m) 3 3 4040 1 1 DeDpetph2t(h2μ(mμm)) 33 4040 11 DDeepptth2h2((μμmm)) 33 44

FFiigguurree 66.. LLiinnee ssccaannnniinngg EEDDSS pprroofifilleess ooff ssaammpplleess AA11––AA33..
Figure 6. Line scanning EDS profiles of samples A1–A3.
FFigiguurree7 7s shhoowwsst thheec crroossss--sseeccttiioonnaall aanndd ssuurrffaaccee SSEEMM mmiiccrrooggrraapphhss ffoorr ssaammpplleess aa11––aa33.. AAss sshhoowwnn
Figure 7 shows the cross-sectional and surface SEM micrographs for samples a1–a3. As shown
inin FFiigguurree 77aa,, tthhee 550000 nmnm-t-htihcikc ksesceocnodnadryar pyhpashea fsoer fsoarmspalme pa1le exai1stesx aits ttshea Ct ZthTeSCe/ZMToS ien/tMerfoacien,t ewrhfaicche,
in Figure 7a, the 500 nm-thick secondary phase for sample a1 exists at the CZTSe/Mo interface, which
wish iccohnifsircmonedfi rtmo ebde toa bZena-rZicnh- rpichhaspeh absye tbhye tlhineeli nsceasncnainnngi nEgDESD pSrporfiolfiesle osfo sfasmamplpel eaa11. .ItI trreevveeaallss aann
is confirmed to be a Zn-rich phase by the line scanning EDS profiles of sample a1. It reveals an
ininssuuffiffcicieiennttr reeaacctitoionno offt thheep phhaasseessa attt thhee iinntteerrffaaccee rreeggiioonn.. NNoo sseeccoonnddaarryy pphhaassee,, eexxcceepptt aa ffeeww ppiinn hhoolleess
insaauttft fhtiheceiCe CnZZtT TrSeSeae/c/MtMiooon ii nnotftee trrhffeaa ccpeeh,, iiasss ooesbb ssaeetrr tvvheeedd i naattt eaarnnf aaacnnenn reeeaaglliiinongng .t teNemmop pseeerrcaaottnuudrreea rooyff 6p600h00a °s◦CeC,, , eaaxss cssehhpootw wan nf eiinwn F Fpiigignuu rhreeo 7l7ebbs. .
at the CZTSe/Mo interface, is observed at an annealing temperature of 600 °C, as shown in Figure 7b.
ItIt aalslsoo rreevveeaalsls ththaatt aa ssuuffiffcicieienntt rreeaaccttioionn ooff ththee pphhaasseess ooccccuurrrreedd aatt tthhee inintteerrffaaccee rreeggiioonn.. SSeeccoonnddaarryy
It paphlhsaoas seresesav apeppaplesea arterhedadtw wai tihstuhf fuffurictrhitehenretri n ricneracecraetsiaoessne soo foftf h ttheheae n pannhenaaseleianslig notgce cmtuemprreeprdaert uaatrtu etrhteoe t 6oi3n 06t3e◦0rCf °a,Ccae,s asrshe ogshwioonnw.i nnS eFincigo Funirdgeua7rryec,
phases appeared with further increases of the annealing temperature to 630 °C, as shown in Figure
w7hc,i cwhhwicihll wbeildl bisec udsissceudslsaetde rl.aCteorh. eCroehnetraenndt acnomd pcoamctpgarcati ngroafinth oef uthpep eurplpaeyre rlawyeitrh waitflha ta sfulartf ascuerfwacaes
7c, which will be discussed later. Coherent and compact grain of the upper layer with a flat surface
owbsaesr voebdsefrovreadl lftohrr eaells tahmrepel essa.mTphelegs.r aTihneb gouranidna broyubnedcoamrye bsemcoomreecso mmporaec tcaonmdptahcet garnadin tshiez egirnacinre asiszees
was observed for all three samples. The grain boundary becomes more compact and the grain size
increases with increasing temperature, as shown in Figure 7d,e,f. The grain size exceeding 10 μm was
increases with increasing temperature, as shown in Figure 7d,e,f. The grain size exceeding 10 μm was
obtained for sample a3.
obtained for sample a3.
Crystals2019,9,10 9of14
withincreasingtemperature,asshowninFigure7d,e,f. Thegrainsizeexceeding10µmwasobtained
Crystals f2o0r18s,a 8m, xp lFeOaR3 .PEER REVIEW 9 of 14
Crystals 2018, 8, x FOR PEER REVIEW 9 of 14

Figure 7. Cross-sectional and surface SEM micrographs of samples a1–a3.
FigurFeig 7u.r Ce7ro.Cssr-ossesc-steioctnioanla alnandd ssuurrffaaccee SSEEMMm micircorgorgaprahpshofs soafm spalmespal1e–sa 3a.1–a3.
Figure 8 shows the line scanning EDS profiles of samples a1–a3. The elemental signal intensity
Figure8showsthelinescanningEDSprofilesofsamplesa1–a3. Theelementalsignalintensity
Figure 8 shows the line scanning EDS profiles of samples a1–a3. The elemental signal intensity
variation of the surface regions of samples a1–a3 was sharper than that of samples A2 and A3. They
variation of the surface regions of samples a1–a3 was sharper than that of samples A2 and A3.
variation of the surface regions of samples a1–a3 was sharper than that of samples A2 and A3. They
reveal a smooth surface in samples a1–a3. Sample a1 has a Sn-rich and Zn-poor composition at the
Theyrevealasmoothsurfaceinsamplesa1–a3. Samplea1hasaSn-richandZn-poorcomposition
reveal as usrmfaoceo rtehg isounr afancde a iZnn -sraicmh pcolemsp ao1si–taio3n. aSta thmep bloet taom1 hreagsi oan .S Tnh-irsi cinhd iacnatde sZ thna-tp Sono arn dco Zmn pdioffsuitsiioonn at the
atthesurfaceregionandaZn-richcompositionatthebottomregion. ThisindicatesthatSnandZn
surfaced iirsfe fiugnsiaiodonne qaiusnaidnte aa df oZerqn su-aarmitcephflo ecrso samanmnpepoalseleistdia onantn 5ea7at0l et h°dCea. btH5o7ot0two◦emCve. rrHe, ogfowiro etnvh.ee Tr ,shafoimsr pitnhleed asincaanmtepeasllee tdah naantt e6Sa0nl0e ad°nCad,t t 6Zh0en0 Z ◦dnCi-f,fusion
rich composition at the bottom region disappears which indicates that Zn diffusion is sufficient. A
is inadtehqeuZant-eri cfhorc osmampopsilteiosn aantntheealbeodtt oamt 5r7eg0i o°nCd. iHsaopwpeeavrsewr,h fiochr itnhdei csaatemspthlaet aZnnndeifafulesdio nati s6s0u0ffi °cCie,n tt.he Zn-
Sn-rich surface region still exists, which may originate from the sealed selenization conditions with
rich coAmSpno-rsiicthiosnur afatc ethreeg bioonttsotimlle rxeisgtsio,wn hdicihsampapyeoarrisg iwnahteicfhro imndthiecasteeasle tdhsaetl eZninz adtiiofnfucsoinodni tiiosn ssuwffiitchient. A
Sn-richr ersleaultaritvfiavelceyley h rhiegigghhieoerrnS S nsnStSeilexlx p epaxarritstiiatalsl,p pwrreeshsssiuucrhree c mcoomamypp aoarrreeidgdt itonoat ththaeat tf oorfof ssmaamm tphplleee sss AeAa11–l–eAAd33 ..s WWelehhneenniz ffauutrrittohhneerr ciinnoccnrrdeeaaistsiiinonggn s with
ththeea annnneeaalilninggt etemmppeerraatutureret oto6 63300◦ °CC( (ssaammppllee aa33)),, aa tthhiicckk ZZnn--ppoooorr ssuurrffaaccee rreeggiioonn aanndd iinntteerrffaaccee rreeggiioonn
relatively higher SnSe partial pressure compared to that of samples A1–A3. When further increasing
rerveveaealslst hthaatti titi sisu unxnfafavvoorarabblelet otof oformrmC CZZTTSSeea att6 63300◦ °CC..
the annealing temperature to 630 °C (sample a3), a thick Zn-poor surface region and interface region
reveals th(aat )it is unfavSonrable to foram1 C(bZ)TSe at 630 °C. a2 (c) Se a3
Zn Zn Cu
u.) Cu Mo Cu Sn Mo Zn Mo
(a) sity(a. Se Sn Zn Mo a1 (b)Cu SeZn Mo a2 (c)Cu Sn Se Zn a3
) Cun Sn Mo
a.u. SeInte Sn
( Se
y
t
si
n
e 0 1 2 3 40 1 2 3 40 1 2 3 4
nt Depth(μm) Depth(μm) Depth(μm)
I
Figure8.LinescanningEDSprofilesofsamplesa1–a3.
Figure 8. Line scanning EDS profiles of samples a1–a3.
0 1The com2position o3f the sam40ples is li1sted in T2able 5. T3he ratio o40f Cu/Zn1was abo2ut 1.6 for3all 4
TheD ecpotmhp(μomsi)tion of the samples is listedD einp thT(aμbmle) 5. The ratio of Cu/Zn wasD aebpothu(tμ 1m.6) for all
samples. For samples A1–A3, the Zn/Sn ratio is lower and the Sn content is higher at low SnSe
x
psaarmtiapllepsr.e Fsosru sraem, ipnleasg Are1e–mAe3n, tthwe iZthn/tShne raotbiose irsv laotwioenr aonfdS tnhed Sifnfu csoinotnenint isF ihgiughreer6 a.t Floowr SthneSesxa pmaprtlieasl
pressure, in agreemeFnit gwuirteh 8t.h Le ionbes secravnatnioinng o Ef DSnS dpirfofufisleiosn o ifn s aFmigpulrees 6 a. 1F–oar3 t.h e samples a1–a3, the
a1–a3,theZn/SnratiodecreasesandtheSncontentincreaseswithincreasingannealingtemperature.
Zn/Sn ratio decreases and the Sn content increases with increasing annealing temperature. The Zn
The Zn diffusionis morerapid for sample a2 incomparison with a1, andthe higher Sn content is
diffusion is more rapid for sample a2 in comparison with a1, and the higher Sn content is accessible.
Thacec ecsosmiblpeo. Hsiotiwonev eorf, tthheeZ sna/mSnplreasti oisf olrisstaemdp ilne aT3awbilteh 5p.o oTrhZen rdaitfifou soiofn Cisua/Zbnno rwmaasl. aSbamouptl e1A.61 for all
However, the Zn/Sn ratio for sample a3 with poor Zn diffusion is abnormal. Sample A1 and a3 have
sampleasn.d Fao3r hsaavmepdlieffse Aren1t–Aco3m, pthoesi tZionn/sS,na lrtahtoiuog ihs lboowtheorf atnhdem thhea vSen pcooonrteelnetm ise nhtidgihffeurs aiotn l.oHwo SwneSveer ,partial
x
different compositions, although both of them have poor element diffusion. However, minor
pressumrei,n oinr daigffreereenmceesncta nwbietho btsheerv oedbsbeertwvaeteinoFni goufr eSsn6 adaifnfdu8scio.Sna minp FleigAu1rhea s6p. oFoorrS tnhaen dsaZmnpdlifefsu saio1n–,a3, the
differences can be observed between Figures 6a and 8c. Sample A1 has poor Sn and Zn diffusion,
whereassamplea1hasgoodSndiffusionandpoorZndiffusion.
Zn/Sn rwathieor edaes csraemapseles a a1n hdas t ghoeo dS nS nc doinfftuesniot ni nacnrde paoseosr Zwni tdhif fiunsciorena. sing annealing temperature. The Zn
As shown above, the formation process of CZTSe thin film from a CuZn precursor under
diffusion is Ams osrheo wranp aidbo fvoer, tshaem foprlme aat2io nin p croocmesps aorfi sCoZnT Swei tthhi na 1fi,l man fdro tmh ea hCiugZhne rp rSencu crosonrt eunntd eisr aSncc +e ssible.
Sn+Seatmosphereisaffectedbymanyfactors,suchasannealingtemperature,Separtialpressure,
HowevSeer ,a tthmeo Zspnh/eSrne irsa atfifoe cftoedr sbaym mpalney afa3c wtoristh, s upcoho ars Z annn deiaflfinugs itoenm pise raabtunroer, mSea pla. rStaiaml pprlees sAur1e ,a SnndS eax 3 have
partial pressure and total pressure. To verify the diffusion process, another set of experiments were
different compositions, although both of them have poor element diffusion. However, minor
carried out with annealing times of 1 min, 2 mins, 4 mins, 8 mins and 16 min, named as s1,s2, s4, s8
differences can be observed between Figures 6a and 8c. Sample A1 has poor Sn and Zn diffusion,
and s16, respectively. The cross-section SEM images and line scanning EDS profiles are shown in
whereas sample a1 has good Sn diffusion and poor Zn diffusion.
Figure 9. Secondary phases at the CZTSe/Mo interface and the size of voids in the film both decrease
As shown above, the formation process of CZTSe thin film from a CuZn precursor under Sn +
and finally disappear with an increase in annealing time. The line scanning EDS profiles of samples
Se atmosphere is affected by many factors, such as annealing temperature, Se partial pressure, SnSe
x

partial pressure and total pressure. To verify the diffusion process, another set of experiments were
carried out with annealing times of 1 min, 2 mins, 4 mins, 8 mins and 16 min, named as s1,s2, s4, s8
and s16, respectively. The cross-section SEM images and line scanning EDS profiles are shown in
Figure 9. Secondary phases at the CZTSe/Mo interface and the size of voids in the film both decrease
and finally disappear with an increase in annealing time. The line scanning EDS profiles of samples
Crystals2019,9,10 10of14
Crystals 2018, 8, x FOR PEER REVIEW 10 of 14
SnSe partialpressureandtotalpressure. Toverifythediffusionprocess,anothersetofexperiments
x
werecarriedoutwithannealingtimesof1min,2mins,4mins,8minsand16min,namedass1,s2,s4,
s1 through s16 reveal the process of elemental diffusion. At the initial stage, as shown in Figure 9f,
s8ands16,respectively. Thecross-sectionSEMimagesandlinescanningEDSprofilesareshownin
Zn and Cu were the main elements and little Sn existed in the middle region. When increasing the
Figure9. SecondaryphasesattheCZTSe/Mointerfaceandthesizeofvoidsinthefilmbothdecrease
annealing time, Sn diffuses to the bottom region and Zn diffuses to the surface, as shown in Figure
andfinallydisappearwithanincreaseinannealingtime. ThelinescanningEDSprofilesofsamples
9f,j. For all samples, a Cu-rich surface region is observed, owing to the surface tension of liquid phase
s1throughs16revealtheprocessofelementaldiffusion. Attheinitialstage,asshowninFigure9f,
Cu2–x SZen aat nadboCvuew 5e7r0e °thCe. mainelementsandlittleSnexistedinthemiddleregion. Whenincreasingthe
annealingtime,SndiffusestothebottomregionandZndiffusestothesurface,asshowninFigure9f,j.
ForallsampleTsa,balCe u5-. rCicohmspuorfsaitcieonre ogfi osnamispoleb saenrnveeadl,inogw uinngdetor dthifefesruernfta cceontednitsiioonns.o fliquidphase
Cu Seatabove570◦C.
2–x
Sample Cu Zn Sn Se Zn/Sn Cu/Zn
Table5.Compositionofsampleannealingunderdifferentconditions.
A1 23.05 14.23 12.49 50.23 1.14 1.62
Sample Cu Zn Sn Se Zn/Sn Cu/Zn
A2 22.48 13.79 13.13 50.60 1.05 1.63
A1 23.05 14.23 12.49 50.23 1.14 1.62
A2 A3 22.48 22.161 3.7193.84 1143..0133 50.775 0.600.99 11..0650 1.63
A3 22.16 13.84 14.03 50.77 0.99 1.60
a1 22.75 14.15 12.91 50.39 1.1 1.61
a1 22.75 14.15 12.91 50.39 1.1 1.61
a2 22.49 13.88 13.11 50.52 1.06 1.62
a2 22.49 13.88 13.11 50.52 1.06 1.62
a3 22.15 13.67 13.31 50.87 1.03 1.62
a3 22.15 13.67 13.31 50.87 1.03 1.62

FigureF i9g.u Crer9o.sCs-rsoescst-sioecnt iSoEnMSE Mimiamgaegse sanandd lilninee ssccaannnniinnggE EDDSSp rporfiolefisleosf soafm spalmespsl1e–ss 1s61.–s16.
It was reported that the formation of CZTSe thin films involves reaction (1) and (2)[19,26]. The

sample temperature of the Cu Zn precursor is heated to a higher temperature than 570 °C for 3 min.
x y
The main selenization reactions of the Cu Zn precursor are [19,26]:
x y
CuxZny+Sex(g)→Cu2Se+ZnSe (3)
Cu2Se+SnSex(g)→Cu2SnSe3 (4)
Cu2SnSe3+ZnSe→Cu2ZnSnSe4 (5)