CZTS
layers
for
solar
cells
by
an
electrodeposition-annealing
route
Md
Ibrahim
Khalil,
Roberto
Bernasconi,
Luca
Magagnin
*
Dip.Chimica,MaterialieIng.ChimicaGiulioNatta,PolitecnicodiMilano,ViaMancinelli7,20131Milano,Italy
Articlehistory:
Received8April2014
Receivedinrevisedform3September2014
Accepted3September2014
Availableonline6September2014
1.Introduction
Inrecent times abundant,inexpensive andenvironmentally friendlymaterials havegotlotofattention fromresearcherfor thinfilmsolar cells.Cu2ZnSnS4 (CZTS)isa promisingabsorber
material for thin film which has all those qualities unlike its predecessorsCdTe andCu(In,Ga)Se2(CIGS), current commercial
materialforthinfilmphotovoltaicdevices.CZTShasdirectband gapenergy of 1.4–1.5eV and large absorption coefficient over 104cm1[1] aswellas theoreticalconversion efficiencyof 32%,
whichmakesitsuitablefornextgenerationthinfilmsolarcells. Severalmethodshavebeenusedtopreparethisthinfilmnamely sputtering [2–4], photochemical deposition [5], the sol-gel method [6], screen-printing [7], physical vapour deposition
[8–10],asolution-processedapproach[11–13],and electrodepo-sition[14–19].ThehighestconversionefficiencyofCZTSachieved tilldateis12.6%byWangetal.usinghydrazine-basedsolution process [13]. Through electrodeposition, the highest efficient CZTS solar cell was demonstrated by Ahmed et al. with 7.3% conversionefficiency[19].Pointtobenoted,Wangetal.achieved thehighestefficiencywhentheyaddedsulfur(S)andselenium (Se) in the precursor using hydrazine based solutions. Electrodeposition is a potentially suitable preparation method to obtain low-cost precursor films because it is inexpensive, non-vacuum, uses an environment-friendly process, produces
large deposition areas by low temperature growth. Usually electrodeposition base processes can be divided into two categories.One is atwo-stepsprocess wheremetal precursors needsulfurizationafterelectrodepositionofCu-Zn-Sn.Theother oneisasingle-stepelectrodepositionwhereCu,Zn,SnandSare directly deposited from electrolyte to form CZTS compound. However, itis very difficult todeposithomogeneousand good crystalline films from single-step electrodeposition as the reductionpotentialgapofthemetal ionsisconsiderablylarge. For this reasons, up to present, very few reports have been publishedonthisprocess[16,20].Therefore,electrodepositionof metal precursor followed by sulfurization has been preferred. Two-steps process could be accomplished in two approaches eitherbysulfurizationofsequentialelectrodepositedprecursors or sulfurizationof co-electrodeposited Cu-Zn-Sn precursors. In caseofsequentialelectrodepositionofCu-Zn-Sn,therearesome issues like (i) poor adhesion of Sn to the Mo surface [21]
(ii) morphologyofSnprecursorsarequiteporouscomparedto otherelementalprecursors(iii)volatilityofSnduring sulfuriza-tionifSnisdepositedattheuppersurface(iv)non-homogeneous nucleationofZnwhichleadstopoormorphologyofCZTSwith manysecondaryphasesaftersulfurization[22].Ontheotherside, proper complexing agents and optimized parameters are the main issues to deposit Cu-Zn-Sn simultaneously from single electrolyte.Mostofthepublishedworksonco-electrodeposition of Cu-Zn-Sn has been done using trisodium citrate as a complexingagent.Arakietal.fabricated CZTSfrom co-electro-deposited Cu-Zn-Sn precursor with 3.16% power conversion efficiency[14].Gougaudetal. demonstratedrecently theeffect
* Correspondingauthor.Tel.:+390223993124;fax:+390223993180.
oftwo complexingagents(sodiumcitrateandtartaricacid) on co-electrodepositionofCu-Zn-Snwith5.8%conversionefficiency ofCZTS[23].Scraggetal.demonstratedthefabricationofCZTS fromstackedmetalliclayerwith3.2%conversionefficiency[22]. Linet al.recentlyshowedtheoptimizedparameters (tempera-ture,time)topreheatthemetallicstackedprecursorofCu-Zn-Sn before sulfurization with conversion efficiency of 5.6% [24]. Ennaouietal.reportedthefabricationofCZTSfrom co-electro-depositedmetallicprecursorwith3.4%conversionefficiency[15]. Thisgroupusedpyrophosphatealongwithsomeother complex-ing agents to deposit Cu-Zn-Sn simultaneously from single solution. Later precursor was sulfurized by annealing using a gasmixtureofArwith5%H2Sat550Cfor2hours.Thisistheonly
report as faras our concernwhere pyrophosphate along with someothercomplexingagentswasused,evenif co-electrodepo-sitionofCu-Zn-Snfrompyrophosphateelectrolytewasdeveloped longtimeago[25].
Ourresearch introducesa novelelectrodepositionroute for the simultaneous deposition of Cu-Zn-Sn alloy from single electrolyte.Herewe usedonly pyrophosphateasa complexing agent for the co-electrodeposition of Cu-Zn-Sn. Afterwards precursorsareannealedinelementalsulfurenvironmenttoget stoichiometricCZTSfilminsteadofH2Satmospheretoavoidtoxic
environment. 2. Experimental
Electrodeposition was carried out galvanostatically in a conventional electrochemical cell assembly. Mo foil substrate (200
m
m thick from Goodfellows) with an exposed area of 2cm1cmwasusedasaworkingelectrode,agraphiteelectrode asan inertcounter electrode. Beforedeposition, Mosubstrates werecleanedinacetone,dippedin32%HClfor10minutes,distilled waterandfinallydriedunderN2atmosphere.Theelectrolyticbathcontained 0.016M CuSO4
5H2O, 0.072M ZnSO47H2O, 0.037MNa2SnO3
3H2O,0.60MK4P2O7.AllthechemicalswerepurchasedfromSIGMAALDRICHandusedasreceived.ThepHofthesolution bathwas adjusted to11 by using 20% H2SO4 and 30% KOH to
stabilize the electrolyte. Electrodeposition was carried out at 5.5mA/cm2for6minutesinambienttemperaturewithamoderate
stirringforthedesiredcomposition;thehighestcathodiccurrent efficiency CE achieved on Mo was 84%. Current efficiency was calculatedonthebasisofFaraday'slawfromthemetalcontentsof deposits,asdeterminedbyEDSanalysis,asfollows:
CEð%Þ¼100ICuþIZnþISn Itot ¼ 100 2mCu ACu þ 2mZn AZn þ 4mSn ASn Itot F t
whereICu,IZnandISnarepartialcurrentsforcopper,zincandtin,Itot
istotalcurrent;mCu,mZnandmSnareweightsofcopper,zincandtin
deposited;ACu,AZnandASnaretheatomicweightsofmetals;Fis
theFaradayconstantandtisthedepositiontime.
Noprecipitate,e.g.tin(IV)oxides,appearedfromtheelectrolyte after two months storage at room temperature. After electro-plating,theCu-Zn-Snprecursorswereannealedat550C temper-atures for 15minutes or 2hours with different ramping rates (20C/minute or 2C/minute) with 30mg elemental sulfur environment and presence of a very small flow of N2. The
electrochemical characterization of the plating solution was performed using a Solartron analytical Modulab. Graphite was usedascounterandglassycarbon(selectedasinertmaterialfor preliminaryelectrolyteformulation andelectrochemical charac-terization)asstandardworkingelectrode,whileSCEwasusedas reference.The crystallographic phase of as deposited Cu-Zn-Sn precursorsandannealedthinfilmswasanalyzedwiththehelpof X-raydiffractometer(XRD,PhilipsX-pertMPDwithCuK
a
=1.5406Å).Surfacemorphologyandcompositionofthefilmswerestudied by using Scanning Electron Microscopy (Model Zeiss EVO 50) equippedwithanenergydispersiveX-rayanalyzer(EDS)Oxford instrument (Model7060). Ramanspectroscopywas obtainedat roomtemperatureinairbyaLabramHR800WHoribaJobinYvon microscopespectrometer.The514.5nmlineof anAr+laserwas
usedastheexcitationsourcewiththeintensityof10mW.Glow dischargeopticalemissionspectroscopy(GDOES)wasalsocarried outusingSpectrumAnalyticGMBH(ModelGDA750analyzer) 3. ResultsandDiscussion
ThereductionpotentialoftheCu,ZnandSncationscouldbe expressedinthefollowingequations[26]:
Cu2þþ2e!Cu;E¼0:093VþRT 2Fln½Cu 2þðvs:SCEÞ Zn2þþ2e!Zn;E¼1:007VþRT 2Fln½Zn 2þðvs:SCEÞ Sn4þþ2e!Sn2þ;E¼0:096VRT2Fln Sn 2þ Sn4þ ðvs:SCEÞ Sn2þþ2e!Sn;E¼0:384VþRT 2Fln½Sn 2þðvs:SCEÞ
Asthegapamongthereductionpotentialofcopper,zincandtin isconsiderablylarge,itisdifficulttoco-electrodepositthemfrom single electrolyte. Above equations make it clear that Zn has considerablymorenegativepotentialthanCuandSn.Inorderto reducethepotentialgapamongthethreeelements,mostofthe researchersusedtrisodiumcitrateasacomplexingagent.Inour studies we have usedpotassium pyrophosphate as complexing agent.Pyrophosphateisknowntocomplexcopperinanefficient wayoverawidepHrange,giving[CuHq(P2O7)2](6q)species[27].
AlsoZnischelatedbypyrophosphate,givingsimilarcomplexes overagoodpHrange[28].Accordingtothespeciationdiagrams,Sn isnotcomplexedatthepHselected,i.e.11,andispresentinthe solutionasfreestannateionsSn(OH)62[29].Thiswasoneofthe
reasons for selecting pH 11, together with the difficulty in achievingthedesiredchemicalcompositionatvalueslowerthan 10.5.It was found thatpyrophosphate hasnarroweddownthe potential gapmorewithrespecttothetrisodiumcitrate.Thisis visible (from Fig. 1) considering the relative positions of the detectedreductionpeaks:-0.3VfortheCu2+!Cu0,-0.5Vincase
oftheSn2+!Sn0reactionand-1VforZn2+!Zn0.TheSn4+!Sn2+
reactionisnotclearlyvisibleinthegraph,asitisprobablyeither superimposed with the reduction peak of Cu or not present (abehaviorcommonforsomeelectrolytescontainingSn(IV)[30]).
Fig.1showstheCVperformedontheelectrolyteatascanrateof 2mV/sinapotentialrangefrom-2to2Vvs.SCE.Pyrophosphate hasagreateffectinloweringthereductionpotentialofCu,while for theothertwo metals thecomplexation isless efficient.For comparisonitispossibletocitethereductionpotentialsobtained inacitrateelectrolyte:0.2VforCu,-0.6VforSnand-1.1Vinthe caseofZn[31].Thecorrespondingoxidationpotentialswerealso observed intheanodicpartof thecurve.Itis well knownthat Cu-poorandZn-richCu-Zn-SnprecursorisfavorableforgoodCZTS thin films[32].Ourresultshavealsomatchedupwellwiththe literature. Theatomicmetalcomposition ofCu-Zn-Snprecursor was Cu: 39.90% Zn: 34.47% and Sn: 25.63%; Cu/(Zn+Sn) 0.66,Zn/Sn1.34byEDSmeasurementinourexperiment.
Fig.2andFig.3showtheXRDspectraofasdepositedCu-Zn-Sn precursorandsulfurizedthinfilmsafterannealingat550C for 2hours with ramping rate 20C/minute and 2C/minute. XRD
patternofasdepositedCu-Zn-Snprecursorwascomposedofpure
Sn(JCPDScard86–2265)andCu5Zn8(JCPDScard25–1228)which
isnotinagreementwithsomedataoftheliterature[30,33].Our newelectrolytemightbethereasonbehinditastheydeposited fromacidicelectrolyte.FormationsofCu-ZnandCu-Snphaseson asdepositedprecursorhavealsobeenreportedearlier[18,34].As expected,therehasbeennoformationofaZn-Snphaseduringthe depositionofCu-Zn-Snprecursor,asthesetwometalsdonotform any intermetallic compound. XRD pattern of sulfurized film exhibitedmostlythepeaksrelatedtokesteritestructureofCZTS. In case of 20C/minute ramping rate, all the XRD patterns of sulfurizedsamplescorrespondtothekesteritestructureofCZTS (JCPDScard26–0575).Ontheotherhand,incaseof2C/minute rampingrate,kesteritestructureofCZTShasbeenformedalong withsomeothersecondaryphases.Dominantpeak (112)ofthe XRDpatternofthesepolycrystallinefilmsliesatdiffractionangle (2 Theta) 28.53 which was commonly observed by others
[16,35–37].It was reportedearlier thatlow rampingrates used toincreasethegrainsizeofthefilm[15].Themainissuethatwas foundin ourcase is theformation of secondary phases atlow rampingrate, whichcanbeconnectedtotheuseofpuresulfur atmosphereinsteadofH2Spluslongdurationoftheprocessduring
sulfurization.ThepresenceofMoS2(JCPDS06-0097)wasobserved
as well. More work will be necessary on this aspect of the sulfurization process. Fig. 4 shows the as deposited Cu-Zn-Sn precursor(a)onMosubstratewhich transformedinto polycrys-talline thin film (b) after annealing at 550C for 2hours with ramping rate 20C/minute. The thickness of as deposited precursorswasnearabout750nmon200
m
mthickMosubstrate. Earlierwork[19]suggeststhatduringsulfurizationtheconversion ofCuZnSnmetalintotheCu2ZnSnS4thinfilmcausesthreetimesvolumeexpansion withrespecttotheoriginal thicknessof the CuZnSn layer. After sulfurization, we detected a thickness of 2.4
m
mfor theCZTSfilm.InFig.4 (b) someverythinlayersof adsorbedmaterialonsomegrainsofCZTSarevisible.Theseare probablyconnectedtosomesecondaryphasesofCuSnSandSnSas thosezonesshowedadeviationfromstoichiometricratioofCZTS by SEM (EDS) analysis; this is also confirmed by preliminary micro-Ramanevaluations.Theatomiccompositionofsulfurized films was Cu: 20.59% Zn: 15.59% Sn: 11.44% S: 52.59% by EDS measurementwhich isneartothestoichiometricratioof CZTS. Sampleannealedat550Cfor2hourswith2C/minuteramping rate,reportedinFig.4(c)showedalotofsecondaryphasesafter sulfurizationwithnon-stoichiometricratioofCZTS.Annealingat 550Cfor15minuteswith20C/minuterampingrateshoweda nearly stoichiometric ratio for CZTS (Cu: 29.03% Zn: 10.67%Fig.2.XRDpatternofasdepositedprecursorandsulfurizedfilmswith20C/
minuterampingrate.
Fig.3.XRDpatternofasdepositedprecursorandsulfurizedfilmwith2C/minute
rampingrate.
Sn:10.78%S:49.03%),lackofsecondaryphases,butthequalityof thefilm(grainsize)islowerwithrespecttothe2hoursannealed sample (Fig. 5). Fig. 6 shows Raman spectrum of the sample annealed at 550C for 2hours with20C/minute ramping rate with major peaks at 237cm1, 288cm1, 338cm1, 374cm1 whichareingoodagreementwiththeresultofpreviousstudies
[10,38–41]: specifically with commonly acknowledged peak at 338cm1 [44]. No peak was observed corresponding to ZnS (271cm1,352cm1)andCu2SnS3(305cm1,350cm1)indicating
thepresenceofgoodqualitysinglephaseCZTS[41–43].ZnSand Cu2SnS3arethemainsecondaryphaseswhichusedtobepresent
alongwithCZTSaftersulfurization.Thoughupto18phasesofthe ternary Cu-Sn-S system have been described [45], secondary phasespeakswerenotdetectedindicatingagoodqualityofthe electrodepositedfilms.Samplewasfurthercharacterizedbyglow
Fig.4. SEMimagesof(a)asdepositedprecursor,(b)sulfurizedfilm(annealedat550Cfor2hourswithrampingrate20C/minute),and(c)sulfurizedfilm(annealedat550C
for2hourswith2C/minuterampingrates).
Fig.5.XRDresultandSEMimageofthesampleannealedat550Cfor15minutewith20C/minutesrampingrate.
Fig.6.Ramanspectrumofsulfurizedsample(annealedat550Cfor2hourswith
20C/minuterampingrate).
Fig. 7. GDOESresultsonsulfurizedsample(annealedat550Cfor2hourswith
dischargeopticalemissionspectroscopytoseehowthe composi-tionofCZTSchangesalongthefilmthicknessasEDSanalysisonly showstheaveragecompositionofthefilm.FromFig.7,itisevident thatthecompositionofCZTSdoesnotchangealongthethickness ofthefilm.Thecharacterizationofthedeposit/substrateinterface withpossibleformationofaMoS2layerisstillunderinvestigation.
4. Conclusions
CZTS thin filmsweresuccessfully preparedonMosubstrate using co-electrodeposition of Cu-Zn-Sn in a novel electrolyte. Differentcharacterizationresultshavewellmatchedupwiththe literature. It was found that duration of annealing at high temperatureincreases the grain size of thefilm. On theother handitwasalsofoundthatalowrampingratedoesnotincrease thegrainsizeofthefilmratheritcreatessecondaryphasesinthe film.Itwasfoundthatasdepositedprecursorandsulfurizedfilm arequiterough,thusanattempttoreducetheroughness,e.g.using pulseplating,willbesubjectoffuturedevelopments.Atthesame timebyoptimizingthesulfurizationparameters,roughnessofthe film after sulfurization could be decreased. This work demon-strates a novel procedure to fabricate stoichiometricCZTS thin filmsfromco-elctrodepositedmetallicprecursorsand sulfuriza-tion,withhomogeneousdistributionofthe elementsalongthe crosssectionofthefilms.
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