Journal of Alloys and Compounds 622 (2015) 942–947

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Journal of Alloys and Compounds

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Influence of deposition parameters and annealing on Cu2ZnSnS4 thinfilms grown by SILAR

http://dx.doi.org/10.1016/j.jallcom.2014.11.0190925-8388/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author at: Department of Applied Physics, S.V. National Instituteof Technology, Surat 395007, India. Mobile: +91 9904334220.

E-mail address: vipulkheraj@gmail.com (V. Kheraj).

Kinjal Patel a, Dimple V. Shah a, Vipul Kheraj a,b,⇑a Department of Applied Physics, S.V. National Institute of Technology, Surat 395007, Indiab Department of Electrical and Computer Engineering, University of Utah, Salt Lake City, UT 84112, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 19 September 2014Received in revised form 28 October 2014Accepted 3 November 2014Available online 11 November 2014

Keywords:CZTSThin filmsChemical synthesisX-ray diffractionScanning electron microscopy

Cu2ZnSnS4 (CZTS) thin films were deposited on glass substrates using Successive Ionic Layer Adsorptionand Reaction (SILAR) technique at the room-temperature. The deposition parameters such as concentra-tion of precursors and number of cycles were optimised for the deposition of uniform CZTS thin films.Effects of annealing at different temperature under two different ambient, viz. sulphur and tin sulphidehave also been investigated. The structural and optical properties of the films were studied using X-raydiffraction, scanning electron microscopy, Raman spectroscopy and UV–visible spectra in light with thedeposition parameters and annealing conditions. It is observed that a good quality CZTS film can beobtained by SILAR at room temperature followed by annealing at 500 �C in presence of sulphur.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Copper Zinc Tin Sulphide i.e. Cu2ZnSnS4 (CZTS) is a p-typequaternary compound semiconductor having high absorptioncoefficient of the order of 104 cm�1 and a direct band gap of about1.5 eV, which is optimum for a solar photovoltaic (PV) material.Moreover, unlike CuInSe2 (CIS), CuInGa(S,Se)2 (CIGS) and CdTe, allthe constituent elements of CZTS are relatively non-toxic andabundant in the nature [1]. Hence, CZTS has been the material ofinterest for the low cost thin-film solar PV ever since the firstobservation of photovoltaic effects in CZTS based hetero diodewas reported by Ito et al. [2].

Economically viable solar PV technology not only demands lowcost materials but also the low cost synthesis techniques. A varietyof deposition techniques have been employed for synthesis of CZTSthin films like co-evaporation [3–6], ebeam evaporation [7–10],pulsed laser deposition [11,12], sputtering [13–15], spray pyrolysis[16–19], electrodeposition [20–22], and Successive Ion LayerAdsorption and Reaction (SILAR) [23–26]. To date, most CZTS thinfilm depositions involve high vacuum coating techniques and highsubstrate temperatures [3–15], which could be a constraint forfurther cost-reduction for viable solar energy. Therefore, it isimportant to develop low cost processes for deposition of CZTS thin

films. SILAR could be one of the potential techniques to grow CZTSthin films in a cost-effective manner. It is a simple chemical depo-sition method which does not involve vacuum or any other sophis-ticated system. It is easy to implement and can be convenientlyextended to the larger scale for roll-to-roll production.

Film growth mechanism in the SILAR method involves fourimportant steps: (i) Adsorption of cation present in the cationicsolution on the substrate and the formation of Helmholtz ElectricDouble Layer. (ii) Rinsing of loosely adsorbed ions from the diffu-sion layer. (iii) Reaction of anions from anionic precursor solution.(iv) Second rinsing of excess and unreacted species from the diffu-sion layer.

Recently, Zhenghua Su et al. [23] have reported deposition ofCZTS by SILAR using two staked layer, i.e. glass/ZnS/Cu2SnSx fol-lowed by post-growth annealing at 500 �C for 30 min. Shinde et al.[24] reported room temperature deposition of CZTS thin films bythe SILAR using Chloride precursors. The amorphous films obtainedat room temperature were subjected to annealing at 673 K for 4 h toget crystalline films. Mali et al. also used SILAR for the deposition ofCZTS film on Fluorine doped Tin Oxide (FTO) coated glass slide usingsulphate precursors [25]. Later they reported the fabrication of solarcell with SILAR deposited thin films offering 1.85 % efficiency [26].However, the efficiency reported so far using SILAR is still far belowthe Schockley–Quessier limit for a single junction cell based on CZTS(�30%) [27]. The thin film solar cell’s efficiency is mainly limited bythe structural defects and compositional deviations. Being a quater-nary compound, the probability of growing secondary parasitic

Fig. 1. XRD profiles of (a) M3050, (b) M3075, (c) M3100.

K. Patel et al. / Journal of Alloys and Compounds 622 (2015) 942–947 943

phases such as CuS, SnS, and ZnS, during the deposition is very highand these binary and ternary phases lower the efficiency of the cells.Thus, the major challenge in the construction of CZTS based solarcells is to develop impurity free, stoichiometric and phase con-trolled deposition of thin films. A proper understanding of the phaseformations in the material and its dependence on the depositionconditions is essential in order to improve its photovoltaic perfor-mance. Here, we report deposition of CZTS thin film using sulphateprecursors. The concentration of the precursors is varied systemat-ically and the effects of precursors’ concentration on the structuralproperties of CZTS are investigated. To understand the intermediatephases involved during the growth process as well as the final phaseevolution, the films were terminated at different number of cyclesand were subjected to the structural investigations. Moreover,annealing atmosphere and annealing temperature do play animportant role in achieving the dominant CZTS phase in the films.Hence, the grown films were subjected to two different annealingatmospheres. The effects of post growth annealing parameters onthe films’ structural properties have also been studied and reportedhere. Overall, the deposition parameters and post-depositionannealing conditions were optimised for the synthesis of good qual-ity CZTS thin films using SILAR.

2. Experimental

CZTS thin films are grown by SILAR method which is based on successiveimmersion of the substrate into separately placed cation and anion precursor solu-tions. Before the deposition of CZTS, glass slides were cleaned with Methanol, Ace-tone, Trichloroethylene and distilled water respectively. Four beakers containingprecursors were used for deposition of CZTS thin films. The first beaker containedCuSO4, ZnSO4 and SnSO4 dissolved in Millipore water which gives Cu2+, Zn2+, Sn2+

cations respectively. The third beaker contained Na2S dissolved in Millipore waterwhich gives S2� anions. The second and forth beaker contained Millipore water toremove loosely adsorbed ions from the film and avoid precipitation. All beakerswere kept at room temperature. Thin films were deposited with three differentset of concentrations denoted as M1 (0.01 M CuSO4, 0.005 M ZnSO4, 0.01 M SnSO4

and 0.08 M Na2S), M2 (0.02 M CuSO4, 0.01 M ZnSO4, 0.02 M SnSO4 and 0.16 MNa2S) and M3 (0.04 M CuSO4, 0.02 M ZnSO4, 0.04 M SnSO4 and 0.32 M Na2S). Foreach concentration, three samples have been prepared by varying number of dipcycles to 50, 75 and 100. The adsorption and the rinsing time was 10 s for all thesesamples. The samples have been identified by codes with five characters so that firsttwo characters represent the concentration viz. M1, M2 and M3, and the remainingthree characters represent the number of cycles. For example sample M1075 indi-cates that the sample is prepared by precursor concentration set M1 and the num-bers of dip cycles are 75 for that sample.

Further, Sample M2100 was annealed under two different ambient viz. (1) inpresence of elemental sulphur and (2) in presence of tin sulphide (SnS) for10 min. The annealing temperature was chosen to be 400 �C and 500 �C in the caseof elemental sulphur environment. In case of tin sulphide environment, annealingwas carried out at 300 �C and 400 �C temperatures. These annealed samples havebeen indicated by codes starting with letter ‘A’, followed by three digits for theannealing temperature and a letter indicating the annealing ambient. For example,A400S represents sample annealed at 400 �C in presence of sulphur. Sample A300Trepresents sample annealed at 300 �C in presence of tin sulphide.

3. Results and discussion

The structural properties of the CZTS thin film samples werestudied using Bruker’s X-ray diffractometer (XRD) with Cu Ka radi-ation [k = 1.54056 A]. The XRD profiles of as-deposited samplesprepared by varying concentration and number of cycles aredepicted in Fig. 1.

For all the samples with concentrations M1 (M1050, M1075 andM1100) and M2 (M2050, M2075 and M2100) deposited at roomtemperature, no XRD peaks were observed (figure not shown here),which reveal that the films grown at room temperature withprecursor concentration M1 and M2 are completely amorphous innature [28]. Even for concentration M3, the film deposited with50 cycles (M3050) does not show any XRD peak as shown inFig. 1(a). It shows that initially the growth results in the amorphousfilm. However, for the film grown up to 75 cycles (M3075), a peak

starts appearing at about 47.3�. Interestingly, this peak becomesclearer in the sample grown up to 100 cycles (M3100). Thus asthe number of cycles increases with higher concentration of theprecursors, the peak at 47.3� evolves gradually and the film startsshowing the sign of ordering during the growth. However, with asingle minor peak at 47.3� it is not possible to detect the phase asthis peak is common for CZTS, ZnS and CuS. Thus, the phaseidentifications for the amorphous films as well as for the filmsshowing an XRD peak at 47.3� requires further investigation usingRaman spectroscopy [29]. Raman scattering spectroscopy wascarried out on samples deposited by varying concentration andnumber of cycles by JOBIN YVON HR800 through excitation wave-length of 514.5 nm. Raman scattering measurements have beenperformed in the back scattering configuration. Fig. 2 shows Ramanspectra of as-deposited samples deposited by varying concentra-tion and number of cycles.

It is observed from the Raman spectra that the distinct charac-teristic peaks of CZTS are present in all the samples as seen inFig. 2. For samples M1050, M2050 and M3050, peaks observed at143 cm�1, 153 cm�1 and 161 cm�1 can be attributed to the E sym-metry of Transverse Optical Mode of CZTS [30–31]. The peaks at258 cm�1 and 284 cm�1 correspond to E symmetry of LongitudinalMode and A symmetry mode of CZTS respectively [31]. The cationvibrations in the XY plane give rise to the E symmetry modes inCZTS thin films. The intensity of these peaks decrease as the num-ber of cycles is increased for all three concentrations. Further in allthese samples grown up to 50 cycles, a distinct major peak is pres-ent at 324 cm�1 which can be attributed to the B1 symmetry ofCZTS [32]. The B1 modes of CZTS arise due to the displacementof half of the Cu atoms towards the positive Z axis and the rest halfCu atoms towards the negative Z axis. The Zn and Sn atoms remainstationary, where as the anions move in XY plane in this case. Thepeak pertaining to the B1 modes of CZTS is strong and persistenteven in the samples grown up to 75 cycles. Interestingly, this peakshifts to 332 cm�1 when this film is grown up to 100 cycles with

Fig. 2. Raman spectra of (a) M1050, (b) M1075 and (c) M1100 (d) M2050, (e) M2075 and (f) M2100 (h) M3050, (i) M3075 and (j) M3100.

944 K. Patel et al. / Journal of Alloys and Compounds 622 (2015) 942–947

concentration M1 as evident from Fig. 2(c). The peak at 332 cm�1

corresponds to A symmetry mode of CZTS, which includes motionof anions (sulphur) only [32]. The E symmetry of transverse opticalmode and A symmetry mode of CZTS, observed in sample M1050and M1075 are also present in M1100 as detected in form of smallhumps at 153 cm�1, 265 cm�1 and 289 cm�1 [31]. A similar trend isalso observed for the films grown with concentration M2 and M3.However, the shifting of the peak towards 332 cm�1 takes morenumber of cycles as we increase the concentration of the precur-sors. For concentration M2, the peak at 324 cm�1 shift to332 cm�1 after 150 cycles (graph not shown here).

Moreover, the absence of intense peak at 352 cm�1 in all thefilms denies the probability of ZnS in films [34]. The presence oforthogonal SnS and hexagonal SnS2 phases in the film have alsobeen declined by the absence of their characteristic peaks at190 cm�1 and 314 cm�1 respectively [34]. Also, no peak is observedat 318 cm�1 in any film, which confirms the non-existence ofcopper tin sulphide in the films [34]. However, for all samplesdeposited for 50 cycles and 75 cycles, a peak is observed at about473 cm�1, which is the characteristic peak of CuS phase in the film[33]. Thus, during the initial phase of the deposition, the CZTSphase grows along with the growth of CuS phase in the film.However, as the number of cycles increases the intensity of thispeak reduces as compared to the CZTS peaks. Thus, the CZTS phasebecomes dominant at the cost of CuS phase as the deposition con-tinues and the film thickness increases with the number of SILARcycles.

This phase transition could be explained as follow. SILAR is anion by ion deposition process. When glass substrate is immersedin beaker 1, the cation hydroxides make a thin layer on the sub-strate by means of physical adsorption. The unbounded ions arethen rinsed in beaker 2. The third beaker contains anionic solu-tion. When the substrate is immersed into the third beaker, thecationic hydroxides on the substrate react with the anions inthe solution to make a compound thin-film by releasing thehydroxide ions into the solution. In the fourth beaker, againloosely bound and unbound ions are removed from the substrate.By repeating this immersion cycles the film growth takes place onthe substrate.

Now for CZTS, beaker 1 contains mixture of Copper Sulphate,Zinc Sulphate and Tin Sulphate solutions. Hence in beaker 1, thecommon ion SO4

2� suppresses the solubility product of the solution,whereas the ionic product of the solution increases because of thecommon-ion effect. In this condition solution is supersaturated.However, since the chemical reactivity of Zn and Sn is higher thanthat of the Cu, more and more Cu2+ ions are released into thesolution as a result of the displacement reactions. Thus, initiallythe solution becomes rich with the Cu2+. Now when the substrateis immersed in this solution, the nucleation starts on the surface ofthe substrate and the rate of nucleation depends on the degree ofsupersaturation in the solution [35]. Hence, the higher concentra-tion of Cu2+ ions in the solution results in the higher concentrationof Cu-hydroxides on the surface of the substrate compared to theZn and Sn-hydroxides. Consequently, when this substrate isimmersed in the anionic solution, the higher concentration ofCu-hydroxides triggers the growth of CuS which acts as a startingphase for the formation of CZTS. Thus, as observed in the Ramananalysis, the CuS phase is present in all the films grown up to 50cycles. The co-existence of CuS and CZTS phases during the initialstage of the growth is analogous to the formation of CuS phase dur-ing the initial stage of synthesis of CIS [36,37]. However, as thegrowth progresses, the relative concentration of Cu in the cationsolution decreases compared to the Zn and Sn and hence the for-mation of CuS phase is taken over by the formation of CZTS. Thus,the CZTS becomes the dominant phase as the number of cyclesincreases. This is evident from Fig. 2(b) and (c).

Since the as-deposited films show amorphous nature, the filmsgrown up to 100 cycles with precursors’ concentration M2 weresubjected to the post-deposition annealing under two differentambient, viz. sulphur and tin sulphide in order to understand thecrystallization in the films.

XRD profiles of sample A400S, A500S, A300T and A400T areshown in Fig. 3 in the 2h range of 10–60 degree. As shown inFig. 3(a), the XRD profile for samples A400S shows clear diffractionpeaks corresponding to (112), (103), (220) and (312) planes ofCZTS. These peaks indicate that the film becomes polycrystallinedue to annealing at 400 �C under sulphur ambient. Moreover,two sharp peaks are also observed at 2theta value 32� and 33�,

Fig. 3. XRD profiles of samples (a) A400S, (b) A500S, (c) A300T and (d) A400T.Fig. 4. Raman spectra of samples (a) A400S, (b) A500S, (c) A300T and (d) A400T.

K. Patel et al. / Journal of Alloys and Compounds 622 (2015) 942–947 945

which are the characteristic peaks for (103) and (006) planes ofCuS phase respectively [28]. Thus, the CuS phase detected in theas-deposited film also survives in the film annealed at 400 �C.However, the intensity of these peaks shrinks for sample A500Sas shown in Fig. 3(b). Moreover, the peaks pertaining to the CZTSphase become stronger and sharper. These observations indicatethat annealing at 500 �C under sulphur environment results inthe improved CZTS phase with larger grains and the secondaryCuS phase gets suppressed.

On the other hand, in the film annealed under tin-sulphideambient at 300 �C, no sharp peak is observed as shown inFig. 3(c). A minor peak is present at about 48�, which correspondsto the (220) plane of CZTS indicating the beginning of the crystal-lization. As the annealing temperature is raised to 400 �C for sam-ple A400T, the clear strong and sharp peaks resulting by diffractionfrom (112), (220) and (312) planes of CZTS appear as shown inFig. 3(d). The peaks corresponding to the CuS phase, very feeblethough, are present in XRD profile of sample A400T as well. How-ever, as mentioned earlier, the XRD profiles of CZTS and ZnS arevery identical. Hence, Raman spectroscopy is essential to confirmthe formation of CZTS phase in the films.

The Raman spectroscopy on annealed samples was performedusing JOBIN VYON through excitation wavelength of 488 nm.Fig. 4(a)–(d) shows the Raman spectra of sample A400S, A500S,A300T and A400T respectively.

In the Raman Spectra of all annealed samples, no signature peakis observed for any of the secondary phases except the CuS asobserved from Fig. 4. Thus the formation of ZnS or other secondaryphases during the annealing can also be declined and it can beconveniently said that the CZTS phase remains the dominant phaseafter the annealing. A significant peak at 470 cm�1 in Fig. 4(a)reveals the presence of CuS phase in the sample A400S. However,the peak diminishes in sample A500S. Thus, the fact that the CZTSphase becomes dominant at the cost of CuS phase at higher

annealing temperature under sulphur ambient as indicated bythe XRD results also become evident here. Thus, sample A500S,i.e. the film grown by SILAR up to 100 cycles with concentrationM2 and annealed at 500 �C for 10 min under sulphur environmentshows a good quality CZTS phase without any substantial presenceof secondary parasitic phases. The trend is similar in the samplesannealed under tin-sulphide ambient. The major peak at332 cm�1 as observed in Fig. 4(c) shifts toward the higher valuein Fig. 4(d). This indicates the crystallization of the film by anneal-ing at 400 �C under tin-sulphide ambient which is also in agree-ment with the XRD results.

The surface morphology of the films was observed by ScanningElectron Microscopy (SEM) images using ZEISS made Ultra55 FieldEmission gun microscope. Fig. 5(a) shows SEM image of sampleM2100, the film deposited at room temperature with concentra-tion M2 for 100 cycles.

The surface of the film shows some uneven clusters with no dis-tinct grains. This is explicable since the as-grown films are amor-phous in nature as revealed by XRD. However, the annealingunder sulphur atmosphere at 400 �C instigates the grain forma-tions as evident from Fig. 5(b). The film shows a few large clustersin the background of uniformly distributed rice-like grains. Thesegrains grow further at 500 �C. The film annealed at 500 �C undersulphur atmosphere shows polycrystalline nature having uni-formly dispersed grains with narrow size distribution and averagesize of about half a micrometre as shown in Fig. 5(c). A similartrend is observed in the samples annealed under tin sulphideatmosphere. However, size distribution is not uniform for the filmannealed in presence of tin sulphide atmosphere at 300 �C. The filmclearly shows combination of large grains of the order of couple ofmicrometres and small grains of the order of few hundred nano-meters as shown in Fig. 5(d). Here also, as the annealing tempera-ture increases to 400 �C in presence of tin sulphide atmosphere, thegrain growth is observed with the combination of uniformly dis-tributed rice like grains and few large clusters as shown in Fig. 5(e).

Fig. 5. SEM image of sample (a) M2100 (b) A400S (c) A500S (d) A300T and (e) A400T.

946 K. Patel et al. / Journal of Alloys and Compounds 622 (2015) 942–947

The optical properties of the films were studied by UV–Vistransmission measurements. The film annealed at 500 �C undersulphur environment shows the absorption coefficient of the orderof 104 cm�1 in visible range. The energy bandgap is found to beabout 1.5 eV, which is in good agreement with the reported values[2,7,21].

4. Conclusion

We have investigated the deposition CZTS thin films usingSILAR method followed by annealing in presence of sulphur andtin sulphide atmosphere. The films were characterised by XRD,Raman spectroscopy, SEM and UV–visible spectra. As depositedfilms grown at room temperature by SILAR show amorphous nat-ure as revealed by the XRD profiles. For the common anion precur-sor of cationic solution, the CuS and CZTS phase formation startsimultaneously in the initial phase of the reaction, and as the num-ber of cycles increases the CZTS phase becomes dominant which isclearly observed by the Raman Spectra of the as deposited films. Asthe deposited films are annealed in presence of sulphur and tin sul-phide atmosphere, the good grain growth is observed. However,the grain size distribution is found to be better in the film annealedin presence of sulphur at 500 �C. The band gap of this film is 1.5 eV,which is very close to the optimum value of the band gap requiredfor the absorber material in thin film solar cell.

Acknowledgements

Authors are thankful to the UGC-DAE Consortium for ScientificResearch, Indore for the XRD and Raman Characterization of thefilms and Tata Institute of Fundamental Research, Mumbai forthe SEM. VK is also thankful to the Fulbright Scholarship Programand United States-India Educational Foundation.

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