Growth, structure and optoelectronic characterizations of high quality Cu2ZnSnS4 thin films obtained by close spaced vapor transport

Accepted Manuscript Growth, structure and optoelectronic characterizations of high quality Cu2ZnSnS4 thin films obtained by close spaced vapor transport A. Sagna, K. Djessas, C. Sene, M. Belaqziz, H. Chehouani, O. Briot, M. Moret PII: DOI: Reference:

S0749-6036(15)30048-3 http://dx.doi.org/10.1016/j.spmi.2015.06.019 YSPMI 3828

To appear in:

Superlattices and Microstructures

Received Date: Accepted Date:

11 June 2015 17 June 2015

Please cite this article as: A. Sagna, K. Djessas, C. Sene, M. Belaqziz, H. Chehouani, O. Briot, M. Moret, Growth, structure and optoelectronic characterizations of high quality Cu2ZnSnS4 thin films obtained by close spaced vapor transport, Superlattices and Microstructures (2015), doi: http://dx.doi.org/10.1016/j.spmi.2015.06.019

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Growth, structure and optoelectronic characterizations of high quality Cu2ZnSnS4 thin films obtained by close spaced vapor transport A. Sagna a,b,c, K. Djessasa,b*, C. Senec, M. Belaqzizd, H. Chehouanid, O. Briote, M. Morete a

Laboratoire Procédés Matériaux et Energie Solaire (PROMES)-CNRS, Tecnosud, Rambla de la thermodynamique, 66100 Perpignan, France b Université de Perpignan Via Domitia, 52 avenue Paul Alduy, 68860 Perpignan Cedex 9, France c Laboratoire des Semi-conducteurs et d'Energie Solaire (LASES), Faculté des Sciences et Techniques, Université Cheikh Anta Diop de Dakar, BP 5005 Dakar-Fann, Sénégal d Laboratoire de Procédés Métrologie et Matériaux pour l'Energie et Environnement (LP2M2E), Faculté des Sciences et Techniques de Marrakech, BP 549 avenue Abdelkrim Khattabi, Marrakech, Maroc e CNRS, Université de Montpellier, Laboratoire Charles Coulomb, UMR 5221, Place Eugène Bataillon, F-34095 Montpellier Cedex 5, France * Corresponding author: K. Djessas, E-mail: [email protected] Tel: 0033650264486

ABSTRACT

High quality Cu2ZnSnS4 (CZTS) thin films, as an absorber layer for thin films solar cell, were synthesized successfully using a simple and low cost technique, Close-Space Vapor Transport (CSVT). The films were grown on soda-lime glass substrates using a polycrystalline CZTS ingot as source of evaporation material. Influence of substrate temperature on chemical composition, morphological, structural, electrical and optical properties of the CZTS thin films was investigated using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), Raman spectroscopy, UV-Vis-NIR spectrophotometer, Hall effect and photoluminescence (PL) measurements. The results from XRD and Raman characterization confirmed the formation of kesterite CZTS thin films with a (112) plane preferred orientation and Raman shift of 338 cm-1, respectively. When the substrate temperature was increased from 460 to 540 °C, the composition of the thin films becomes Cu-, Sn-poor and Zn-rich, wherein the optical band gap values increased from 1.34 to 1.52 eV. PL spectra show the presence of broad emission band at 1.28 eV. All CZTS thin films exhibit p-type conductivity. With substrate temperature of 500 °C, the CZTS thin films show the best properties as an absorber layer in thin film solar cell (Eg = 1.48 eV, p = 3.4 x1017 cm-3, ρ= 2.6 Ω/cm, µ = 6.4 cm/V.s).

Keywords: Cu2ZnSnS4; thin films; Absorber; CSVT.

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1. Introduction

Two prominent thin films for photovoltaic (PV) solar technology are Cu(InGa)Se2 (CIGS) and CdTe. However, utilization of these materials could be limited in the future because of scarcity and high cost of In and Te, as well as the toxicity of Cd and Se [1,2]. For these reasons, recently, intensive researches focus on CZTS material. It constitutes abundant elements in the crust of the earth, nontoxic and low cost. Furthermore, the CZTS have an excellent optical absorption of over 104 cm-1, direct optical band gap energy of 1.4-1.5 eV and p-type conductivity [3]. With these advantages, the kesterite CZTS is known to be a potential cost-effective absorber material in solar cells. The highest conversion efficiency of 12.6% have been reported by Wang et al. [4] for CZTS solar cell with mixed S/Se fabricated using a solution-based hydrazine process. Whereas, for the pure sulfide CZTS solar cells, an efficiency of 9.2% has been reported by Kato et al. [5]. These achievements are encouraging but efforts to increase the efficiency and to reduce the cost of production are required. Then to achieve these aims, besides the quality of CZTS thin films, these layers deposition techniques should be low cost. CZTS thin films have been fabricated using a variety of vacuum and non-vacuum techniques including: thermal co-evaporation [6], sputtering [7,8], pulsed laser deposition [9], successive ionic layer adsorption and reaction [10], electrodeposition [11] and sol-gel sulfurization [12]. These techniques usually require expensive equipment, multi-steps processes and high vacuum, which will result in high cost of production. The fabrication of CZTS thin films with good absorber proprieties by a low cost processing is still a challenge. The CSVT is one of the potential techniques to deposit high quality absorber layers with low cost [13]. It is simple process, which does not require high vacuum without sulfurization step. However, to the best of our knowledge, the deposition of CZTS thin films using this low cost approach has not been optimized and reported in the literature. In this work, Cu2ZnSnS4 absorber layers were deposited on soda-lime-glass substrates by CSVT technique. We have investigated to optimize the substrate temperature. Thus, it has been varied from 460 °C to 540 °C. Effects of substrate temperature on the chemical composition, morphological, structural, electric and optical properties of the CZTS thin films were carried out.

2. Experimental details

2.1. Preparation of the CZTS films

Cu2ZnSnS4 thin films were grown on soda lime glass (SLG) substrates using a CSVT system designed by Djessas et al. [14]. We used solid iodine (I2) as a transporting agent. The CZTS ingot used as source material of evaporation is obtained by cooling a molten stoichiometric mixture with controlled composition of Cu (99.9999%), Zn (99.9999%), Sn (99.9999%) and S (99.9995%) elements

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(Alfa Aesar). CZTS alloy ingot was milled using a planetary ball mill system and the milling conditions have been optimized to develop powders grain sizes with few tens of micrometers. After milling, CZTS powder was compressed to form the source using a hydraulic press system with a pressure above 300 bar. The prepared CZTS source was mounted in a reaction zone of graphite holders and then loaded into the quartz reactor. The source of CZTS was evaporated under a controlled I2 atmosphere with pressure of about 2.10-2 atm. The graphite holders were heated by a applying current to a SiC spring resistance. The temperatures were controlled using thermocouples mounted near the substrate and the source. In this work we have controlled the substrate temperature in the range from 460 °C to 540 °C. The source and substrate are separated by a 1 mm thick space. Temperature of the substrate is about 50 °C lower than of the source. Deposition time for all samples was about 15 min.

2.2. Characterization

The morphology and chemical composition of the CZTS thin films were determined by scanning electron microscopy (SEM, JEOL- JMS 5310 LV) and energy dispersive spectroscopy (EDS, JEOL- JMS 5310 LV) at an acceleration voltage of 15 kV. The CZTS thin films structural properties were investigated using a X-ray diffraction (XRD, Empyrean PANalytical) with Cu-Kα radiation ( λ = 0.154056 nm) at 45 kV, 30 mA in Bragg-Brentano geometry ( θ -2 θ ) in the range of 20° to 70°. Complementary structural information was supplied by a Raman setup (HORIBA JOBIN YVON LabRam ARAMIS IR2) equipped with a blue laser diode having an excitation wavelength of 477 nm. The electrical properties of the CZTS films were characterized by Hall measurements at room temperature, using an Ecopia HMS 3000 equipment, with a magnetic field of to 0.56 T. The optical band gap energy was determined by transmittance experiments, measured using an UV-Vis-NIR spectrophotometer (VARIAN cary 5000) in the wavelength domain from 300 nm to 2000 nm. The photoluminescence emission was excited using a 650 nm, 15 mW laser diode, and detected with a liquid nitrogen cooled InGaAs photomultiplier at sample temperatures of 10 K.

3. Results and discussion

3.1. Composition analysis and morphological properties of CZTS thin films The chemical compositions of the CZTS samples determined by the EDS technique, for films deposited at various substrate temperatures, are reported in Table 1. This composition varied significantly for substrate temperatures in a range of 460 °C and 540 °C. In details, the Zn content increased while the Cu and Sn compositions decreased. The significant non-stoichiometry in CZTS samples may be due different reasons: the coexistence of secondary phases, the high concentration of

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intrinsic defects or defect clusters. The CZTS samples deposited at the higher temperatures of 500 °C and 540 °C are in (Cu, Sn) poor and Zn rich states, which is the typical composition domain leading to absorbers with good photovoltaic performances [15]. In fact, Zn rich and (Cu, Sn) poor compositions with (Cu/Zn + Sn) < 1 and Zn/Sn > 1 conditions lead to a decrease of the SnZn, CuSn, CuZn + SnZn and 2CuZn + SnZn defects, which are highly effective recombination centres for electron-hole pairs [16]. For this reason, the highest quality solar cells reported in the literature [15,17] have ratios of Cu/(Zn + Sn) ≈0.80-0.85 and Zn/Sn ≈1.2-1.3. In this work, we observe that the chemical composition of CZTS samples is significantly dependent on the substrate temperature in the CSVT process. The surface and cross section SEM images of CZTS thin films for different substrate temperatures are shown in Fig. 1.The films have uniform thicknesses, around 3 µm and fully cover the substrate, with micrometer-sized crystallites. It can be seen that the substrate temperature has a significant impact on the CZTS morphology. The CZTS thin films deposited at 460 °C (Fig. 1, a1, a2) exhibit a two layers structure that consists of dense small grains at the bottom and dense columnar large grains at the top of the films. This double layer structure is not suitable for an absorber because it introduces additionnal recombination paths for electron-hole pairs and thus decrease the conductivity of the CZTS thin films. The films deposited at relatively higher substrate temperatures (i.e. 500, 540 °C) have a uniform, compact columnar structure (Fig. 1, b1, b2, c1, c2). We see a slight increase in the crystallite sizes compared to CZTS films grown at lower substrate temperatures (460 °C) and it also can be observed that the grain boundaries are better defined. The more compact morphology and increase in grain size for these samples can enhance their electrical properties. From these results, we conclude that the size of the crystallites CZTS thin film is strongly dependent on the substrate temperature.

3.2 Structural properties of CZTS thin films Fig. 2 show XRD patterns of CZTS thin films deposited at different substrate temperatures. The diffraction peaks at 2 θ values of 28.59°, 33.09°, 47.41°, 56.25° and 59.01° that correspond respectively to (112), (200), (220), (312) and (224) planes of the kesterite CZTS structure (JCPDS no. 026-0575) were observed for all samples. Furthermore, it can be observed that small diffractions peaks corresponding to ZnS Wurtzite phase [18,19] appear in the film deposited at a substrate temperature of 540 °C. This presence of ZnS is in good agreement with the significant amount of Zn observed by EDS in Table 1. The lattice parameters calculated from XRD are: a = 5.41 A° and c = 10.78 A°, indicating that the deposited films are tetragonal CZTS (JCPDS no. 026-0575). The XRD characterisation is insufficient to confirm the phase purity of the CZTS films, because the diffraction peaks of CZTS overlap those of Cu2SnS3 (CTS) and cubic ZnS [20]. Raman Spectroscopy was then performed to further investigate the phase purity of our kesterite CZTS.

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Raman spectra of CZTS thin films at room temperature are shown in Fig. 3. A single Raman peak at 338 cm-1 was observed. The peak corresponds to kesterite CZTS, in good agreement with the literature [21]. The strongest peak observed for all the CZTS samples at 338 cm-1 is due to the A1 symmetry which originates from the vibration of S atoms, surrounded by the motionless neighbouring atoms [22]. No characteristics peaks, corresponding to secondary phases such as ZnS (218, 275, 351 cm-1) [23,24], Cu2SnS3 (297,318 cm-1) [24] and Cu2-xS (475 cm-1) [24], were detected. This confirms the existence of phase kesterite CZTS. Nevertheless, ZnS peaks were observed in the XRD pattern. This absence of the ZnS peaks in Raman spectroscopy is due to the wavelength of the laser used (blue laser diode λ = 473 nm). As reported in literature [24,25], in order to detect ZnS, it is necessary to use an ultra violet laser ( λ = 325 nm).

3.3. Electrical and optical properties of CZTS thin films

The electrical properties were determined by Hall effect measurements, using the Van der Pauw method at room temperature. The results are shown in Table 2. All samples exhibited p-type conductivity. The carrier concentration decreases with the increase of the substrate temperature. For the first two samples of Table 2, the Hall mobility increases with substrate temperature. This phenomena can be explained by: (i) the reduction in carrier density from 460 to 500 °C; (ii) the columnar structure with a increase of the CZTS crystallites size described in section 3.1. For the latter factor, an increase in crystallites size reduce the density of grain boundaries, and thus weakens the grain boundaries scattering, increasing the carrier lifetime, consequently increasing the mobility [26]. When the substrate temperature increases from 500 to 540 °C, the mobility decreases along with the increase in resistivity. This may be due to the presence of the ZnS secondary phase observed by RXD. These values of carrier concentrations, resistivities and mobilities correspond well to data for CZTS thin films from the literature [27,28]. Fig. 4 shows the transmittances of the CZTS thin films that decrease to nearly zero, in the visible region. These transmittance measurements have allowed us to obtain the optical band gaps of our CZTS thin films, neglecting the reflectivity (in the visible region, almost zero transmittance). The absorption coefficient α of the CZTS thin films are given as follows [29,30]

1 d

α = ln

1 T

where d and T are the thickness and transmittance of the films, respectively. For all samples, the absorption coefficients calculated are higher than 104 cm-1. After the calculation of the absorption coefficients, the optical band gaps are obtained from the plots of (αhν ) 2 versus photon energy ( hν ), as shown in Fig. 5, by linear extrapolation of the plot and its intersection on the ( hν ) axis.

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The optical band gaps for the CZTS films deposited at substrate temperatures of 460, 500 and 540 °C are estimated to be 1.34, 1.48 and 1.52 eV, respectively. It can be seen that the apparent optical band gap increases with increasing substrate temperature. These results are to be correlated with the EDS data showing that the chemical composition shifts with substrate temperature. Analysis of Fig. 5 reveals that a higher optical band gap is obtained for decreasing Cu/(Zn + Sn) ratio. This phenomenon is in good agreement with the results already reported by Tanaka et al. [12]. For the sample deposited at 460 °C (Tsub), the band gap is much smaller than the expected values (1.4-1.5 eV) reported in the literature. Two possible reasons may be invoked for this result: a) coexistence of a secondary phase such as Cu2SnS3, or b) a high concentration of defects, giving rise to band tailing, effectively lowering the absorption threshold. For this sample, no secondary phases were observed by XRD and Raman. Therefore the narrower band gap for this sample may, most probably, be due, to the presence of a high concentration of defects in the thin film. Theoretical calculations showed that the defects significantly affect the optical absorption of CZTS. In particular defect clusters CuZn + ZnCu and 2CuZn + SnZn decreases the absorption threshold of about 0.11 eV and 0.35 eV respectively [31,32]. As reported in literature [32,33], these defects exist near stoichiometry with (Cu/Zn + Sn = 0.8). This corresponds precisely to the composition in this sample. At higher temperatures (500 and 540 °C), the samples becomes poor in (Cu, Sn) and rich in Zinc and the formation probability of these complexes defects become much smaller, resulting in a larger band gap. On the other hand, ZnCu + Vcu defects clusters may be formed at these higher growth temperatures, which provide an increase of the optical band gap [31]. The band gap value determined in this work are in good agreement with the values reported in the literature [29,34,35]. The low-temperature photoluminescence spectra of CZTS layers deposited at different substrate temperatures are presented in Fig. 6. The maximum of the PL peak is observed around 1.28 eV, and corresponds well with the peak observed previously for CZTS thin films [36]. Two models are often used to discuss the origin of the PL emission in CZTS in the literature [37]: donoracceptor pair transitions and fluctuating potentials. In this quaternary material, compositional fluctuations are most probable. They give rise to a broad range of local band gaps, which are spatially distributed. However, the photoluminescence process happens between lower energy states, after thermalization of the hot carriers. In this case, the lowest value of the band gap fluctuations is the most probable in PL, while the fluctuations strongly broadens the PL line, due to band tailing. This explains that the photoluminescence emission will usually be stockes shifted from the transmission transition. Moreover, the clear asymmetry of our PL peaks strongly support this interpretation. We also observe that the PL peak intensity for the sample grown at 460°C is lower, in agreement with our previous analysis that the density of defects in this sample must be higher. Levanyuk et al. [38] reported that, the luminescence spectrum from a semiconductor with potential fluctuations is usually dominated by three recombination channels: Band-to-Band (BB), Band-to-Impurity (BI), and Band-to-Tail (BT). According to Grossberg et al. [39], at low temperature in CZTS, the recombination channels BI and

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BT usually dominates and the peak position of both bands can be described as ݄ߥ௠௔௫ ൌ ‫ܧ‬௚ െ ‫ ்ܧ‬, where Eg is a band gap energy and ET is a thermal quenching activation energy, or stockes shift amount. The corresponding values of ET deduced from our measures of the band gaps and the PL peaks are then roughly equal to: 60, 200 and 240 meV for the samples grown at 460, 500 and 540 °C respectively. It is seen that the PL line is of deeper origin in samples grown at higher temperatures. Tanaka et al. [40] have measured a separation between the BI (at 1.23 eV) and BT (at 1.35 eV) bands in CZTS of 120 meV. The different ET values measured in our samples suggests that the nature of the photoluminescence in the samples grown at higher temperature should be related to the BI band, while at a growth temperature of 460 °C, the PL would originate from the BT band. 4. Conclusion In summary, using a simple and low cost technique (CSVT), we have successfully deposited high quality CZTS thin films suitable as absorbers for solar cells. The substrate temperature was found to be an important factor influencing the physico-chemical properties of these photovoltaic absorber layers. When the substrate temperature increases, the Cu/(Zn + Sn) ratio decreases and morphological, structural and optoelectronic properties of the thin films improve. However it should be noted that this improvement of the material properties with the Cu/(Zn + Sn) ratio has a limit, beyond which the quality of the layer deteriorates through the appearance of secondary phases. The analysis of XRD and Raman patterns demonstrated the formation of the kesterite phase of CZTS, with a (112) plane preferred orientation and Raman shift of 338 cm-1 indicating the growth of high quality thin films. The photoluminescence analysis confirmed the formation of CZTS. The best optical and electrical properties for our absorbers were obtained at a substrate temperature of 500 °C (Eg = 1.48 eV, p = 3.4 x1017 cm-3, ρ= 2.6 Ω/cm, µ = 6.4 cm/V.s). Acknowledgements This work was financially supported by the Department for Cooperation and Cultural Action of the France in Senegal (No 657/SCAC). The authors also wish to thank Mr. J.L. Gauffier, Mrs. S. Reyjal and Mr. S. Cayez from INSA Toulouse (Laboratory of Physics and Chemistry of Nano-objects) as well as E. Hernandez from PROMES-CNRS laboratory for their technical assistance. References [1] T. Todorov, O. Gunawan, S.J. Chey, T.G. de Monsabert, A. Prabhakar, D.B. Mitzi, Thin Solid Films 519 (2011) 7378-7381. [2] A. Luque, S. Hegedus, Handbook of Photovoltaic Science and Engineering, Wiley, Sussex 2011. [3] K. Ito, T. Nakazawa, Jpn. J. Appl. Phys. 27 (1988) 2094-2097. [4] W. Wang, M.T. Winker, O. Gunawan, T. Gokmen, T.K. Todorov, Y. Zhu, D.B. Mitzi, Adv. Energy Mater. 4 (2014) 1301465. [5] T. Kato, H. Hiroi, N. Sakai, S. Muraoka, H. Sugimoto, Proc. 27th EU-PVSEC (2012) 2236-2239.

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[6] B. Shin, O. Gunawan, Y. Zhu, N.A. Bojarczuk, S.J. Chey, S. Guha, Prog. Photovolt: Res. Appl. 21 (2011) 72-76. [7] N. Song, Y. Wang, Y. Hu, Y. Huang, W. Li, S. Huang, X. Hao, App. Phys. Lett. 104 (2014) 092103. [8] J. Wang, S. Li, J. Cai, B. Shen, Y. Ren, G. Qin, J. Alloy. Compd. 552 ( 2013) 418-422. [9] K. Moriya, K. Tanaka, H. Uchiki, Jpn. J. Appl. Phys. 47 (2008) 602-604. [10] K. Patel, D.V. Shah, V. Kheraj, J. Alloy. Compd. 622 (2015) 942-947. [11] S. Ahmed, K.B. Reuter, O. Gunawan, L. Guo, L.T. Romankiw, H. Deligianni, Adv. Energy Mater. 2 (2012) 253-259. [12] K. Tanaka, Y. Fukui, N. Moritake, H. Uchiki, Sol. Energy Mater. Sol. Cells 95 (2011) 838-842. [13] K. Djessas, S. Yapi, G. Massé, M. Ibannain, J.L. Gauffier, J. Appl. Phys. 95 (2004) 4111. [14] A. Bouloufa, K. Djessas, D. Todorovic, Mater. Sci. Semicond. Process. 12 (2009) 82-87. [15] D.B. Mitzi, O. Gunawan, T.K. Todorov, K. Wang, S. Guha, Sol. Energy Mater. Sol. Cells 95 (2011) 1421-1436. [16] S. Chen, L-W. Wang, A. Walsh, X.G. Gong, S-H. Wei, App. Phys. Lett. 101 (2012) 223901. [17] H. Katagiri, K. Jimbo, M. Tahara, H. Araki, K. Oishi, in MRS Spring Meeting (2009) 125-136. [18] J. Díaz-Reyes, R.S. Castillo-Ojeda, R. Sánchez-Espíndola, M. Galván-Arellano, O. Zaca-Morán, Curr. Appl. Phys. 15 (2015) 103-109. [19] M. Zhou, Y. Gong, J. Xu, G. Fang, Q. Xu, J. Dong, J. Alloy. Compd. 574 (2013) 272-277. [20] A. Walsh, S. Chen, S.H. Wei, X.G. Gong, Adv. Energy Mater.2 (2012) 400-409. [21] K. Wang, B. Shin, K.B. Reuter, T. Todorov, D.B. Mitzi, S. Guha, App. Phys. Lett. 98 (2011) 051912. [22] M. Himmrich, H. Haeuseler, Spectochem. Acta 47A (7) (1991) 933. [23] V.T. Tiong, J. Bell, H. Wang, Beilstein J. Nanotechnol. 5 (2014) 438-446. [24] P.A. Fernandes, P.M.P. Saloméa, A.F. da Cunha, J. Alloy. Compd. 509 (2011) 7600-7606. [25] X. Fontané, L. Calvo-Barrio, V. Izquierdo-Roca, E. Saucedo, A. Pérez-Rodriguez, J.R. Morante, D.M. Berg, P.J. Dale, S. Siebentritt, Appl. Phys. Lett. 98 (2011) 181905. [26] N.M. Shah, J.R. Ray, V.A. Kheraj, M.S. Desai, C.J. Panchal, J. Mater. Sci. 44 (2009) 316. [27] D. Tang, Q. Wang, F. Liu, L. Zhao, Z. Han, K. Sun, Y. Lai, J. Li, Y. Liu, Surf. Coat. Tech. 232 (2013) 53-59. [28] B. Long, S. Cheng, Y. Lai, H. Zhou, J. Yu, Q. Zheng, Thin Solid Films 573 (2014) 117-121. [29] J. Xu, Z. Cao, Y. Yang, Z. Xie, J. Renew. Sust. Energy 6 (2014) 053110. [30] Z. Yan, A. Wei, Y. Zhao, J. Liu, X. Chen, Mater. Lett. 111 (2013) 120-122. [31] D. Huang, C. Persson, Thin Solid Films 535 (2013) 265-269. [32] S. Chen, A. Walsh , X-G. Gong, S-H. Wei, Adv.Mater. 25 (2013) 1522-1539. [33] T. Washio, H. Nozaki, T. Fukano, T. Motohiro, K. Jimbo, H. Katagiri, J. Appl. Phys. 110 (2011) 074511. [34] J. Ge, Y. Wu, C. Zhang, S. Zuo, J. Jiang, J. Ma, P. Yang, J. Chu, Appl. Surf. Sci. 258 (2012) 72507254. [35] J-S. Seol, S-Y. Lee, J-C. Lee, H-D. Nam, K-Y. Kim, Sol. Energy Mater. Sol. Cells 75 (2003) 155-162. [36] M. Grossberg, J. Krustok, J. Raudoja, T. Raadik, Appl. phys. Lett.101 (2012) 102102. [37] J.P. Teixeira, R.A. Sousa, M.G. Sousa, A.F. da Cunha, P.A. Fernandes, P.M.P. Salomé, J.C. Gonzáles, J.P. Leitão, Appl. Phys. Lett. 105 (2014) 163901. [38] A.P. Levanyuk, V.V. Osipov, Sov. Phys. Usp. 24 (1981) 187-212. [39] M. Grossberg, J. Krustok, T. Raadik, M. Kauk-Kuusik, J. Raudoja, Curr. Appl. Phys. 14 (2014) 14241427. [40] K. Tanaka, T. Shinji, H. Uchiki, Sol. Energy Mater. Sol. Cells 126 (2014) 143-148.

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Table caption Table 1. Element compositions and ratio of CZTS films deposited at different substrate temperatures. Table 2. Electrical properties of the CZTS thin films obtained at different substrate temperatures.

Figure caption Fig. 1. Surface and cross section SEM images of CZTS thin films deposited at different substrate temperatures (TSub): Tsub = 460 °C (a1; a2), Tsub = 500 °C (b1; b2) and Tsub = 540 °C (c1; c2). Fig. 2. XRD patterns of CZTS thin films at different substrate temperatures. Fig. 3. Raman spectra of CZTS thin films grown at different substrate temperatures. Fig. 4. Optical transmittance spectra of the CZTS thin films. Fig. 5. (αhν ) 2 versus photon energy ( hν ) graphs of CZTS samples prepared with different substrate temperature. Fig. 6. PL spectrum of CZTS samples, measured at 10 K.

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Table 1. Element compositions and ratio of CZTS films deposited at different substrate temperatures.

Substrate temperature (°C)

Chemical composition

Ratios

S

Cu

Zn

Sn

Cu/(Zn+Sn)

Cu/Sn

Zn/Sn

460

48.76

24.04

15.76

11.44

0.88

2.10

1.37

500

47.14

22.29

20.55

10.02

0.73

2.22

2.04

540

51.50

17.79

23.14

7.57

0.57

2.35

3.06

Table 2. Electrical properties of the CZTS thin films obtained at different substrate temperatures.

Substrate temperature (°C) Carrier concentration (cm-3)

Mobility (cm2/Vs)

Resistivity (Ω.cm)

460

1.2x1018

1.4

3.76

500

3.4 x1017

6.4

2.87

540

2x1017

2.5

8.19

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Highlights

1. CZTS thin films were prepared by Close-spaced vapor transport deposition technique. 2. Effect of substrate temperature on the properties CZTS films have been investigated. 3. XRD and Raman spectra revealed the kesterite phase with a good crystalline quality. 4. The CZTS films deposited at 500 °C showed the best optical and electrical properties.

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