Effect of Cu/Sn Concentration Ratio on the Phase Equilibrium-Related Properties of Cu-Sn-S Sprayed Materials M. Bouaziz, K. Boubaker, M. Amlouk, and S. Belgacem (Submitted April 5, 2010; in revised form August 2, 2010) Copper tin sulfide Cu-Sn-S compounds have been grown by the spray pyrolysis technique at 360 °C using SnCl 2 Æ2H 2 O, CuCl 2 and thiourea (SC(NH 2 ) 2 as precursors. Structural, morpho- logical, and optical properties were studied showing that the obtained films crystallized in tetragonal and orthorhombic structures in terms of the Cu/Sn concentration ratio in the starting solution. Moreover, the deposited films showed a relatively high optical absorption coefficient of around 10 4 cm 21 and exhibited indirect transition gaps which lie within 1.17-1.38 eV domain. Keywords Cu 3 SnS 4 compound, spray pyrolysis, thin films, XPS 1. Introduction The chalcogen family members often form ternary or quaternary compounds that can be represented by the general group formulae: I 2 -IV-VI 3 for Cu 2 SnX 3 or I 3 -II-IV- VI 4 for Cu 3 SnX 4 [1,2] and Cu 2 ZnSnX 4 [3,4] (X = S, Se, Te). The Cu-Sn-S ternary system is a representative system which, with or without additives such as Zn, exhibits such compound-forming behavior with compounds based on this system being the focus of recently growing interest. Such compounds have been the subject of intense study because they are non-linear materials [5] and they may be suitable absorber compounds for photovoltaic cells. [6] These com- pounds exhibit a rather large range of band gap energies being 1 eV for Cu 2 SnS 3 [7,8] and being relatively higher in the range 1.44-1.46 eV for Cu 2 ZnSnS 4 and Cu 2 ZnSnSe 4 . The latter is near optimum value for energy conversion of the solar spectrum and, in principle, could be used in panel devices for either or both electrical or thermal conversion of solar energy. In principle these compounds could be used in photovoltaic and/or thermal devices for either electrical or thermal conversion of solar energy. The international community calls for the use of these new systems as potential alternatives to Cu-In-S (CIS) materials because of the low abundance and production rate of gallium and indium, i.e., 80 and 510 tons, respectively, as recently reported by the U. S. Geological Survey, 2008. These compounds have been prepared by various tech- niques such as conventional reaction, [8-10] solvothermal pro- cesses, [1,2,11,12] evaporation, [4,13] and co-evaporation. [13,14] Dry approaches such as spray pyrolysis techniques have not been extensively applied. [3,15,16] This technique presents two major advantages, namely (1) low cost and (2) large scale of thin film production. The present work reports the structure, the morphology, and the optical properties of Cu 3 SnS 4 sprayed thin films using various molar ratios of r = Cu/Sn in the starting solution [1.8 £ r £ 4.4]. It may be noted that the ideal stoichiometric ratio is r = 3 and it will later be seen that the crystal habit shift from tetragonal symmetry at Cu-deficient ratios to orthorhombic symmetry at Cu-rich ratios. 2. Experimental Details Thin layers of Cu 3 SnS 4 were grown by spraying solutions of tin chloride (SnCl 2 Æ2H 2 O) (10 2 M), copper chloride (CuCl 2 ) and thiourea (SC(NH 2 ) 2 ), precursor of sulfur (20 M) onto glass substrates. The solution was prepared using a mixture of deionized water and methanol. Mixtures with different relative concentration of the pre- cursor solutions r = [Cu]/[Sn] = [1.8-4.4] were sprayed at 360 °C. The as grown films were characterized to evaluate their physical properties and to study the effect of ratio [Cu]/[Sn] on the film behavior. The structure of the films and the different phases present were determined using a Siemens D500 Diffractometer equipped with monochro- matic Co K a (k = 1.7903 A ˚ ). A JEOL 6400 scanning electron microscope was used to determine film’s thick- nesses. The surface morphology was obtained using atomic force microscope (AFM) in tapping mode (VEECO digital instrument 3 A). The optical transmittance and reflectance of the films were recorded using a Schimadzu UV 3100 spectrophotometer, within a wavelength range varying from 400 to 1800 nm. M. Bouaziz, K. Boubaker, M. Amlouk, and S. Belgacem, Unite ´ de Physique des Dispositifs a ` Semi-conducteurs, Faculte ´ des Sciences de Tunis, Campus Universitaire, 2092 Tunis, Tunisia. Contact e-mails: mmbb11112000@yahoo.fr and boubaker_karem@yahoo.com. Section I: Basic and Applied Research JPEDAV (2010) 31:498–503 DOI: 10.1007/s11669-010-9783-8 1547-7037 ÓASM International 498 Journal of Phase Equilibria and Diffusion Vol. 31 No. 6 2010