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Applied Surface Science 305 (2014) 506–514

Contents lists available at ScienceDirect

Applied Surface Science

journa l h om epa ge: www.elsev ier .com/ locate /apsusc

PS analysis and structural and morphological characterization ofu2ZnSnS4 thin films grown by sequential evaporation

. Gordilloa, C. Calderóna,∗, P. Bartolo-Pérezb

Departamento de Física, Universidad Nacional de Colombia, Bogotá, ColombiaDepartamento de Física Aplicada, CINVESTAV-IPN, Mérida, Yuc., Mexico

r t i c l e i n f o

rticle history:eceived 27 January 2014eceived in revised form 18 March 2014ccepted 19 March 2014vailable online 27 March 2014

eywords:hin filmsu2ZnSnS4

a b s t r a c t

This work describes a procedure to grow single phase Cu2ZnSnS4 (CZTS) thin films with tetragonal-kesterite type structure, through sequential evaporation of the elemental metallic precursors undersulphur vapor supplied from an effusion cell. X-ray diffraction analysis (XRD) is mostly used for phaseidentification but cannot clearly distinguish the formation of secondary phases such as Cu2SnS3 (CTS)because both compounds have the same diffraction pattern; therefore the use of a complementarytechnique is needed. Raman scattering analysis was used to distinguish these phases.

The influence of the preparation conditions on the morphology and phases present in CZTS thin filmswere investigated through measurements of scanning electron microscopy (SEM) and XRD, respectively.

amanRDPS

From transmittance measurements, the energy band gap of the CZTS films was estimated to be around1.45 eV. The limitation of XRD to identify some of the remaining phases after the growth process areinvestigated and the results of Raman analysis on the phases formed in samples grown by this methodare presented. Further, the influence of the preparation conditions on the homogeneity of the chemicalcomposition in the volume was studied by X-ray photoelectron spectroscopy (XPS) analysis.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

Recent growth of the thin-film PV industry and champion cellfficiencies of 20.3% for Cu(In,Ga)Se2 [1], demonstrate that thin filmhotovoltaic (PV) is becoming a viable option for large-scale powereneration. However, due to the limited availability of In, someoncern exists that high material expenses restrict the capacity toower production costs, especially in view of the desired industrial

ass production. Kesterites used as thin film solar cell absorbersave attracted much attention in the past years [2] due to the

act that this material has several promising attributes for effi-ient, inexpensive solar cells made from abundant and nontoxiclements.

A variety of routes have been undertaken for thin-film depo-ition. These include vacuum and solution based depositionpproaches. For each of these deposition approaches one key bar-ier toward a reliable process is the incompletely understood nature

∗ Corresponding author at: Cra. 30 No. 45-03, Bogotá, Colombia.el.: +57 1 3165000x13017; fax: +57 1 3165135.

E-mail address: clcalderont@unal.edu.co (C. Calderón).

ttp://dx.doi.org/10.1016/j.apsusc.2014.03.124169-4332/© 2014 Elsevier B.V. All rights reserved.

of the Cu–Zn–Sn–S phase diagram and control over the phase pro-gression during film formation [3], which presents a challengefor preparing single phase films. A second common limitationgenerally encountered during process optimization involves thevolatility upon heating of Sn materials [4], which makes com-positional control a challenge during film fabrication. Despitethese challenges, reasonably successful film deposition and devicefabrication has been demonstrated for both vacuum and solution-based deposition approaches. Several groups have reported thefabrication of CZTS thin films using a variety of methods suchas sulphurization of stacked metallic layers [5], rf sputtering [6]and co-evaporation [7]. The highest efficiency of 8.4% has beenachieved using the co-evaporation technique [8], demonstrat-ing the potential of CZTS to be effective photovoltaic absorberlayer.

In this work, we report on studies related to the growthof single phase Cu2ZnSnS4 thin films using a procedure con-sisting in the sequential evaporation of the elemental metallicprecursors in presence of elemental sulphur. The structural and

morphological properties of CZTS thin films have been investi-gated through XRD, Raman spectrometry and SEM measurements.The chemical composition homogeneity was also studied by XPSanalysis.

G. Gordillo et al. / Applied Surface Science 305 (2014) 506–514 507

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ig. 1. Routine to grow single phase Cu2ZnSnS4 thin films, following the Cu/Sn/Znequence.

. Materials and methods

CZTS thin films were prepared in a physical vapour depositionPVD) system equipped with a turbomolecular pump that allowsvaporation at pressures of the order of 2 × 10−5 mbar and twoungsten boats to evaporate Cu and Sn, and two effusion cells usedo evaporate Zn and S, respectively. The temperatures of each effu-ion cell are controlled using PID temperature controllers and theeposition rates of both Cu and Sn were monitored with a Maxtechickness monitor model TM-400 that uses a quartz oscillator asensor.

To grow CZTS films with kesterite type structure a procedureas used, consisting in the sequential evaporation of the metallicrecursors in presence of elemental sulphur evaporated from anffusion cell that provides uncracked sulphur flux. A broad numberf samples were deposited under different sequences (Cu/Zn/Sn,u/Sn/Zn, Sn/Cu/Zn, Sn/Zn/Cu, Zn/Cu/Sn, Zn/Sn/Cu) varying theain deposition parameters in a wide range. XRD measurements

arried out to each one of the prepared samples, allowed us to findhe sequence and deposition parameters leading to the growth ofingle phase Cu2ZnSnS4 thin films. This study revealed that singlehase CZTS films can be obtained evaporating the metallic precur-ors under the Cu/Sn/Zn sequence at temperatures as those shownn Fig. 1. Samples prepared using any other sequence and/or depo-ition routines different to those displayed in Fig. 1 grow in generalith a mixture of the CZTS phase and secondary phases.

The film thickness was determined using a Veeco Dektak 150urface profiler and SEM measurements were carried out with aeol JSM-7600F electron microscope in high vacuum mode. Furtherharacterization involved X-ray diffraction on a Shimadzu-6000iffractometer and Raman spectroscopy on a Horiba Jobin Yvonicro-Raman Spectrometer LabRamHR in backscattering config-

ration with a DPSS laser of 473 nm, 20 mW focused with a 50×bjective. The elemental composition analysis of the CZTS thin filmsas measured using an X-ray photoelectron spectrometer K-Alpha

hermo Scientific.

. Results and discussion

.1. Structural properties

The diffraction patterns displayed in Fig. 2 correspond to sam-les prepared under the sequences Cu/Zn/Sn, Cu/Sn/Zn, Sn/Cu/Zn,n/Zn/Cu, Zn/Cu/Sn and Zn/Sn/Cu, using the same evaporated mass

f Cu, Sn and Zn. It is observed that in general, the diffractionatterns of the studied samples include reflections associated tooth the CZTS phase plus other secondary phases. However, sam-les deposited under the sequences Cu/Sn/Zn and Zn/Sn/Cu present

Fig. 2. XRD pattern of CZTS thin films deposited varying the sequence in whichthe metallic precursors are evaporated (Cu/Sn/Zn, Cu/Zn/Sn, Sn/Cu/Zn, Sn/Zn/Cu,Zn/Cu/Sn, Zn/Sn/Cu).

reflections corresponding only to the kesterite type Cu2ZnSnS4phase oriented preferentially along the (1 1 2) plane (PDF card #00-026-0575). The samples prepared using sequences different to thepreviously mentioned, present additional reflections correspond-ing to secondary phases such as: ZnO, CuS, SnS2, Cu0.999Sn0.501S1.5.

Since some binary and ternary compounds that may be presentin the samples studied show reflections at the same angles thanthose of the reflections corresponding to the Cu2ZnSnS4 phase, isuseful to compare the XRD spectrum of the CZTS film with thoseof thin films of the binary and ternary compounds, to differenti-ate with a greater degree of precision the reflections associatedto Cu2ZnSnS4 from those corresponding to secondary phases thatmay be present. Fig. 3 compares the XRD spectrum of a samplecontaining just the Cu2ZnSnS4 phase with those of thin films of cop-per sulphide, zinc sulphide and tin sulphide prepared separately bycoevaporation of its elemental precursors, as well as with that of aternary compound grown by sequential evaporation of copper sul-phide and tin sulphide followed by annealing at 550 ◦C in sulphurambient.

The following facts can be highlighted from the results of Fig. 3:

(i) Sulphide thin films of Cu, Sn and Zn deposited separately underthe same conditions, used during the growth of the compound

508 G. Gordillo et al. / Applied Surface Science 305 (2014) 506–514

Fig. 3. Comparison of the XRD spectrum of a sample deposited on molybdenumcdo

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C

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do not agree with those obtained by XRD; Raman analysis revealsthat these samples grow with a mixture of the phases Cu3SnS4and Cu2ZnSnS4, being much larger the contribution of the Cu3SnS4phase, whereas the XRD pattern of samples grown under the

ontaining just the Cu2ZnSnS4 phase, with those of thin films of CuS, SnS and ZnSeposited on glass by co evaporation of its elemental precursors, as well as with thatf a CTS film grown by sequential evaporation of CuS and SnS.

Cu2ZnSnS4, grow in the CuS (PDF 00-0006-0464), SnS (PDF 01-079-2193) and ZnS (PDF 00-001-1280) phases, respectively.

ii) Evaporation of Cu at 550 ◦C in presence of elemental sulfur fol-lowed by co-evaporation of Sn and S at 250 ◦C and subsequentannealing at 550 ◦C results in the formation of Cu2SnS3 (PDF01-089-4714). The subsequent co-evaporation of Zn and S at550 ◦C converts the ternary compounds formed in the previ-ous stage in Cu2ZnSnS4 (PDF 00-026-0575), according to thefollowing reactions.

CuS + SnS → Cu2SnS3

u2SnS3 + ZnS → Cu2ZnSnS4

iii) The XRD pattern of the ternary CTS and quaternary CZTScompounds differ very little from each other. This result indi-cates that XRD measurements do not allow to know preciselywhether the observed reflections in the diffractograms cor-respond to Cu2ZnSnS4 or Cu2SnS3 and thus the use of acomplementary technique is needed.

iv) Reflexions corresponding to CuS and SnS phases are easyto identify when are present in a phase mixture with CZTSbecause its diffraction peaks are clearly distinct from the CZTSones. Regarding ZnS, the identification of this phase is very dif-ficult because the most probable ZnS phase to form is the cubicone with a lattice parameter a of 5.410 A that differs little fromthat of CZTS; consequently the XRD peaks of ZnS are very closeto those of the CZTS as is shown in Fig. 3.

A ternary compound can be formed during the growth ofZTS, such as tetragonal Cu2SnS3, cubic Cu2SnS3 and orthorhombicu3SnS4 [9,10]. From these, only the orthorhombic Cu3SnS4 is easyo identify using XRD. The tetragonal and the cubic have the sameroblem as ZnS. The peak proximity for CTZ and CZTS compoundsre also displayed in Fig. 3.

Besides X-ray analysis, Raman spectroscopy study was per-ormed to distinguish the CZTS phase of secondary phases that maye present in these samples. Fig. 4 compares the Raman spectrum

f a sample grown under the sequence Cu/Sn/Zn (sample O6) withhat of a sample grown under the sequence Zn/Sn/Cu (sample E23).he Raman spectrum of a CTS sample grown under the sequenceu/Sn (sample L5) is also plotted in Fig. 4.

Fig. 4. Raman spectra of CTS (sample L5) and CZTS films grown under sequencesCu/Sn/Zn (sample O6) and Zn/Sn/Cu (sample E23).

Raman spectrum of sample O6 shows a single peak at 339 cm−1

which has been attributed to Cu2ZnSnS4 [11]. This peak arises fromthe A1 vibrational mode of the lattice, where the group VI atom (S)vibrates while the rest of atoms remain fixed [12]. Raman spectrumof sample CZTS-E23 shows a strong peak at 317 cm− 1 which hasbeen attributed to orthorhombic Cu3SnS4 [13] and two additionalsmall peaks at 339 cm−1 and 475 cm−1 that have been attributedto Cu2ZnSnS4 and Cu2−xS, respectively [13]. Sample CTS-L5 hasonly a Raman peak at 305 cm−1 which has been attributed to cubicCu2SnS3 [13].

It is observed that the results of the Raman analysis are inagreement with those obtained from XRD measurements only forCZTS samples deposited under the sequence Cu/Sn/Zn. For sam-ples deposited under the sequence Zn/Sn/Cu the Raman results

Fig. 5. Raman spectra of CZTS thin films with different Cu/(Sn + Zn) (M) and Zn/(Sn)(N) ratios.

G. Gordillo et al. / Applied Surface Science 305 (2014) 506–514 509

Fig. 6. (a) Comparison of the transmittance of a typical Cu2ZnSnS4 thin films withtC

sCfittmmrcA(o

Fig. 7. SEM images of surfaces for CZTS thin films deposited under different condi-

resulting in a deficiency of Zn and thus to the preferential formation

hose of thin films of CuS, SnS, ZnS and Cu2SnS3 and (b) Plot of (˛h�)2 vs h� foru2ZnSnS4 thin films prepared under the sequence Cu/Sn/Zn.

equence Zn/Sn/Cu exhibit only reflections corresponding to theu2ZnSnS4 phase. Based on the analysis carried out on CZTS thinlms deposited by sequential evaporation, using Raman spec-roscopy and XRD, it can be concluded that single phase Cu2ZnSnS4hin films can be grown using the sequence Cu/Sn/Zn. It is worth

entioning that in each studied sample (of about 1 cm2) Ramaneasurements were made at four different points. The results

evealed that the obtained spectra differ little from each other, indi-ating that the formation of single phase occurs over a wide area.

s the Raman measurements were made with a laser of 473 nm

20 mW) that only excites the near-surface region, can not be saidf Raman measurements that the formation of single phase occurs

tions: (a) Cu/Sn/Zn sequence, M = 0.36, N = 2, (b) Cu/Sn/Zn sequence, M = 0.24, N = 2and (c) Zn/Sn/Cu sequence, M = 0.36, N = 2.

throughout the volume. However, the atomic concentrations of Zn,Cu, Sn and S determined from high-resolution XPS core level spectraintegrated peak areas at three different depths, indicated that usingan optimal relationship between the mass of evaporated precursorsand optimized deposition conditions allows depositing CZTS filmswith both, atomic composition close to the stoichiometric one andreasonably good homogeneity in chemical composition over theentire volume (see Fig. 10).

Samples deposited under the sequence Zn/Sn/Cu grow predom-inantly in the Cu3SnS4 phase, due to the fact that most of thezinc evaporated onto the glass substrate at 550 ◦C is re-evaporated

of the ternary compound.The influence of the composition ratio on the Raman spectra of

CZTS thin films grown under the sequence Cu/Sn/Zn was studied.

510 G. Gordillo et al. / Applied Surface Science 305 (2014) 506–514

F E23):l e) Zn 2

ForsNmp2pNsdgl

ig. 8. XPS Spectra of CZTS thin films grown under the Zn/Sn/Cu sequence (sampleevel spectra measured at three different depths: (b) S 2p, (c) Sn 3d, (d) Cu 2p and (

or this, CZTS samples were prepared varying the ratio of evap-rated mass of Cu/(Sn + Zn) (=M) between 0.24 and 0.42 and theatio Zn/Sn (=N) between 1.6 and 2.4. In Fig. 5 are compared Ramanpectra of CZTS thin films prepared with different values of M and. This study reveals that all the samples show a single peak whichoves when the composition of the CZTS films is varied. The sam-

le O6 deposited using compositions ratios M and N of 0.36 and, respectively, present a Raman peak at 339 cm−1; however, theosition peak of samples prepared with composition ratios M and

above and below of the composition used for the O6 sample is

hifted to lower frequencies apparently caused by some strain pro-uced by compressive stress, which some authors attribute to aradient in strain, originated by a gradient of crystal quality in theayers [14,15]. The variation of the concentration ratio of Cu and Zn

(a) typical survey spectra of CZTS thin films. Survey scan and high resolution corep.

in CZTS films is frequently used to obtain CZTS films with electricalconductivity suitable for use as absorbent layer in solar cells.

3.2. Optical properties

The optical properties of CZTS thin films were studied by opticaltransmission and reflection measurements at room temperature.Fig. 6 shows the spectral transmittance of a typical Cu2ZnSnS4 thinfilm deposited under the sequence Cu/Sn/Zn as well as the transmit-tance of thin films of its binary precursors (CuS, SnS, ZnS) separately

deposited by co-evaporation of its elemental precursors; the trans-mittance of a Cu2SnS3 film grown by sequential evaporation of CuSand SnS followed by annealing at 550 ◦C in sulphur ambient is alsodisplayed in Fig. 6.

G. Gordillo et al. / Applied Surface Science 305 (2014) 506–514 511

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ig. 9. XPS spectra of CZTS thin films grown under the Cu/Sn/Zn sequences (samplevel spectra measured at three different depths: (b) S 2p, (c) Sn 3d, (d) Cu 2p and (

It is observed that the CuS and CTS films exhibit low transmit-ance values, indicating that these films grow with a high densityf native defects (vacancies, interstitial and antisite). These defectsenerate absorption centers within the energy gap which con-ribute to the photon absorption. In particular, the transmittance ofhe CuS films decreases strongly in the NIR region apparently dueo the formation of a very high density of shallow defects. It is alsobserved that the transmittance curve of the CZTS film has a smalllope: this behavior seems to be caused by absorption of photonsn deep centers generated by structural defects.

The optical band gap of a 800 nm thick CZTS thin film depositednder the sequence Cu/Sn/Zn was obtained using the followingquation [16]:

˛h�) = A(h� − Eg)n (1)

here is the optical absorption coefficient, A is a constant, Eg

s the optical band gap and n = 1/2 for direct transition. The optical

(a) typical survey spectra of CZTS thin films. Survey scan and high resolution corep.

band gap was obtained by extrapolating the linear region of the plot(˛h�)2 vs h�. was determined from the measured transmittance(T) and reflectance (R) using the formula [16]:

= 1d

ln

[(1 − R)2

T

](2)

where d is the thickness of the film. The absorption coefficient of theCZTS film is larger than 104 cm−1 in the visible region. The plot of(˛h�)2 vs h� for CZTS thin films is presented in Fig. 6b. Therefore theestimated optical band gaps of the CZTS thin films is 1.45 eV, which

is in agreement with the previously reported values of 1.4–1.6 eV[17–19]. The study also revealed that there is a decrease in opticalband gap with increasing Cu/(Zn + Sn) ratio of the CZTS thin film.This is in accordance with previous reports [19,20].

512 G. Gordillo et al. / Applied Surface Science 305 (2014) 506–514

s grow

3

Sarmurgi

Fig. 10. Atomic concentration at three different depths of CZTS film

.3. Morphological characterization

The morphology of the CZTS films was characterized throughEM. Fig. 7 shows the surface SEM micrograph of samples O6nd E23 prepared under the sequences Cu/Sn/Zn and Zn/Sn/Cu,espectively, using the same Cu/(Sn + Zn) ratio of 0.36; the SEMicrograph of sample E25 prepared under the sequence Cu/Sn/Zn

sing a Cu/(Sn + Zn) ratio of 0.24 is also displayed in Fig. 7. Theesults show that, in general, the CZTS films consist of compactrain structures, free of voids with sub-micron size, which is signif-cantly affected by the sequence in which these are deposited. The

n under the sequence Cu/Sn/Zn using different composition ratios.

samples prepared under the sequence Cu/Sn/Zn (Fig. 7a and b) tendto grow with grains grouped together in clusters of different size,while the samples prepared under the sequence Zn/Sn/Cu (Fig. 7c)show smooth surfaces and closely packed small grains without anyvoids. The different morphology observed in CZTS films depositedunder the sequence Zn/Sn/Cu is due to the fact that these samplesgrow with a mixture of the CZTS and CTS phases, the latter being

predominant.

It was also found that in the range of composition stud-ied, the grain size becomes larger with increasing Cu/(Zn + Sn)ratio. Samples deposited with a Cu/(Sn + Zn) ratio around 0.36

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re known to have good crystallinity and a single-phase kesteritetructure.

.4. XPS analysis

The elemental composition homogeneity in the volume of CZTSlms grown under different sequences and composition ratios wastudied through XPS measurements carried out at three differentepths (sputter time of 3, 6 and 9 min, respectively). Figs. 8a and 9ahow typical survey spectra of CZTS thin films deposited underoth, Zn/Sn/Cu (sample E23) and Cu/Sn/Zn (sample O6) sequences.

t is observed that besides the C 1s peak visible at about 284.6 eV,eaks corresponding to Cu, Zn, Sn and S are identified in both XPSpectra. High-resolution core level spectra were recorded for the Cup, Zn 2p, Sn 3d and the S 2p regions in order to determine the oxi-ation state and elemental composition (see Figs. 8b–e and 9b–e).

Figs. 8b and 9b show the S 2p core level spectrum whichxhibit two peaks 2p3/2 and 2p1/2 with binding energy at 161.9 and62.8 eV and a peak splitting of 0.9 eV, which are consistent withhe 160–164 eV range expected for S in sulfide phases [21,22]. Then 3d5/2 and Sn 3d3/2 peaks shown in Figs. 8c and 9c are presentt 486.4 and 494.5 eV, respectively, with a separation of 8.1 eV,hich confirms Sn(IV) [21,22]. Figs. 8d and 9d show the Cu 2p core

evel spectra which exhibit binding energy for the Cu 2p3/2 and Cup1/2 peaks at 932.8 and 952.3 eV, respectively, and a peak splittingf 19.5 eV, leading to conclude that copper is in the +1 oxidationtate indicating formation of Cu(I) [21,22]. Fig. 9e shows the Zn 2pore level spectrum of a CZTS film deposited under the sequenceu/Sn/Zn; peaks Zn 2p3/2 and 2p1/2 visible at binding energies of021.8 and 1044.6 eV with a peak separation of 22.8 eV suggesthe presence of zinc(II) [21,22]. Fig. 8e shows the Zn 2p core levelpectrum of a CZTS film deposited under the sequence Zn/Sn/Cu;his types of samples exhibit only the peak Zn 2p3/2 at 1021.9 eVeing its peak area much smaller than that observed in sampleseposited under the sequence Cu/Sn/Zn, indicating that these sam-les grow very Zn deficient; this unusual result is caused by theact that these samples grow predominantly in the Cu2SnS3 phase

ixed with a small amount of CZTS, as was proven through Ramaneasurements (see Fig. 4).Results depicted in Figs. 8 and 9 show that the peak area of the

ore level spectra for elements Cu, Zn, Sn, and S, measured at dif-erent depths differ from each other, indicating that the samplestudied are inhomogeneous in chemical composition, being thenhomogeneity of the sample E23 significantly greater.

The relative atomic concentrations of Zn, Cu, Sn and S were alsoetermined from high-resolution XPS core level spectra integratedeak areas. In Fig. 10 are plotted the atomic concentrations at threeifferent depths (sputter time of 3, 6 and 9 min, respectively) ofZTS films grown under the Cu/Sn/Zn, using different compositionatios M and N. This study shows that in general the CZTS filmsxhibit inhomogeneity in elemental composition throughout theolume and it depends on the mass ratio of precursors evaporated.t was also found that the CZTS films tend to grow with a stoichiom-try influenced by the mass ratio of the precursors evaporated; theest result in both, homogeneity and stoichiometry was obtainedith the sample O6 prepared using composition ratios M and N of

.36 and 2, respectively. The stoichiometry of this sample obtainedy averaging the atomic compositions measured in the three depths

s about S:Sn:Zn:Cu = 4.07:0.97:0.95:2.03.

. Conclusions

CZTS thin films with tetragonal-kesterite type structure wererown using a procedure based on sequential evaporation of thelemental metallic precursors under sulphur vapour supplied from

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cience 305 (2014) 506–514 513

an effusion cell. XRD and Raman spectroscopy studies gave evi-dence of the formation of single phase CZTS films with kesteritestructure using optimized growth parameters. However, the com-position ratio and sequence in which the metallic precursors areevaporated, significantly affects the phase and the structural andmorphological properties of the CZTS films. From Raman mea-surements, evidence was found that, CZTS films prepared withcomposition ratios M and N different than 0.36 and 2, respectively,present some strain associated to compressive stress, which havebeen attributed to a gradient in strain, due to a gradient of crys-tal quality. Study of surface morphology of CZTS films made withSEM revealed that, in general, the CZTS films consist of compactgrain structures free of voids with sub-micron sizes, which are sig-nificantly affected by the sequence in which these are deposited.The samples prepared under the sequence Cu/Sn/Zn tend to growwith grains grouped together in clusters of different sizes, whilethe samples prepared under the sequence Zn/Sn/Cu show smoothsurfaces and closely packed small grains without any voids.

Relative atomic concentrations of Zn, Cu, Sn and S, determinedfrom high-resolution XPS core level spectra integrated peak areas,revealed that, in general, the CZTS films exhibit inhomogeneity inboth stoichiometry and elemental composition; however, it waspossible to find conditions for the preparation of CZTS films withreasonable homogeneity in chemical composition and a stoichiom-etry of S:Sn:Zn:Cu = 4.07:0.97:0.95:2.03 which is close to that ofthe Cu2ZnSnS4 phase. The results also revealed that the Cu2ZnSnS4films are known to get p-type conductivity and an energy bandgap of about 1.45 eV, indicating that this compound is suitable toperform as absorbent layer in thin film solar cells.

Acknowledgements

This paper was supported by Colciencias (Cont. #038-2013),DIB-Universidad Nacional de Colombia, FOMIX-Yucatán 2008-108160 and CONACYT LAB-2009-01 # 123913. The authors wouldlike to thank Eng. Wilian Cauich for his assistance with the XPS andSEM measurements.

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