Cu 2 ZnSnS 4 thin films deposition by ultrasonic spray pyrolysis W. Daranfed a , M.S. Aida a,⇑ , N. Attaf a , J. Bougdira b , H. Rinnert b a Université de Constantine, Laboratoire de Couches Minces et Interfaces, Faculté des Sciences, Algeria b Université de Lorraine, Institut Jean Lamour, UMR 7198, Vandoeuvre 54506, France article info Article history: Received 24 May 2012 Received in revised form 29 June 2012 Accepted 15 July 2012 Available online 23 July 2012 Keywords: CZTS Thin film Spray technique Solar cells abstract Cu 2 ZnSnS 4 (CZTS) thin films were deposited by ultrasonic spray pyrolysis technique. The substrate tem- perature was varied from 280 to 360 °C in order to investigate its influence on CZTS films properties. The deposition rate shows two activation energies 0.16 and 0.53 eV, respectively at low and high substrate temperatures. This indicates that CZTS deposition by spray pyrolysis passes by two different processes with increasing temperature substrate. The temperature 320 °C corresponds to the transition between these two processes. The X rays diffraction (XRD) analysis indicated that the deposited films have a keste- rite hexagonal structure with (1 1 2) preferential orientation and a crystalline size, ranged from 30 to 52 nm with increasing substrate temperature. Stannate ZnSnO 3 is present as a secondary phase. The pres- ence of this secondary phase causes films optical band broadening. Broad emissions at around 1.27 eV was observed in the photoluminescence spectrum measured at 77 K, it is accompanied with a small peak located at 1.75 eV due the presence of zinc stannate phase ZnSnO 3 . Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Solar cells based on CuIn(Ga)Se 2 (CIGS) and CdTe have attained efficiency of 19.8% and 16.5%, respectively [1]. Nevertheless, issues are encountered with these absorbing materials such as scares of In and Ga and toxicity of Cd. This may limit the mass production of solar cells based on these materials. These problems have stimu- lated the research for an alternative absorber layer that can be pre- pared with abundant and environmental friendly materials. Recently quaternary Cu 2 ZnSnS 4 (CZTS), which is composed of abundant and safety materials, has emerged as CIGS and CdTe alternative. It becomes a promising material for thin films solar cell absorber layer. CZTS is derived from CuInSe material by replacing scarce In(III) by abundant elements Zn(II) and Sn(IV) in the ratio 50:50, and toxic selenium by sulfur. CZTS has a band gap ranged from 1.4 to 1.6 eV, which is close to the optimal value required as solar cell absorber layer [2–4], a high absorption coefficient in the visible solar spectrum wavelengths [1] and a p-type conductivity. Therefore, these properties promote CZTS as potential candidate for photovoltaic materials. The theo- retical predicted efficiency of CZTS based solar cell is larger than 30% [5–7]. Efficiency up to 6.77% has been already reached in solar cells produced with CZTS as absorber layer [8]. Several techniques have been employed for preparing CZTS thin films namely: sputtering [2,9,10], thermal evaporation [11,12], electron-beam-evaporated precursors [13,14], electrodeposition [15]. Generally these techniques are multi steps processes, they are based on sequential or co-deposition of precursor metallic stacked layers followed by a subsequent sulfurization or annealing in H 2 S atmosphere. However for a large scale solar cell production a single step deposition process is more economic and well suitable. This can be achieved by co-sputtering technique [16] co-evapora- tion [17] or by vacuum free chemistry methods such as spray-pyro- lysis [18,19], photochemical deposition [20,21] and sol–gel [22]. Among these techniques, spray pyrolysis is a versatile and low- cost one. This technique is very attractive; it has been largely used to produce oxide and sulfide II–VI semiconductors. However, few studies have been devoted to CZTS deposition by spray pyrolysis [19,23,24]. To our knowledge ultrasonic spray pyrolysis has not been tested in CZTS deposition. In the present work, CZTS thin films have been deposited by ultrasonic spray pyrolysis. The effect of substrate temperature on structural, chemical composition, optical and electrical properties of CZTS films has been investigated. It is well known that aqueous precursor synthesis route leads to oxide contamination [25]. In order to minimize contamination with oxide and hydroxide spe- cies a non-aqueous precursor material is used. 2. Experimental details CZTS Thin films were deposited by spray ultrasonic in air. The initial solution is prepared from copper chloride CuCl 2 (0.01 M), zinc acetate (0.005 M), tin chloride SnCl 2 (0.005 M) and thiourea SC(NH 2 ) 2 (0.04 M). These salts, used as sources of different elements (Cu, Zn, Sn and S), are diluted in methanol. In order to optimize the temperature deposition, the substrate temperature was changed from T = 280 to 360 °C with a step of 20 °C and the spraying duration was fixed at 45 min. The struc- 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.07.063 ⇑ Corresponding author. Tel.: +21 3663146426. E-mail address: aida_salah2@yahoo.fr (M.S. Aida). Journal of Alloys and Compounds 542 (2012) 22–27 Contents lists available at SciVerse ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom