Kesterite Cu 2 ZnSn(S 1 À x ,Se x ) 4 film synthesis through ethanol-thermal route with sulfurization/selenization treatments Lei Cao a , Shuai Ma a , Jing Sui a , Jincheng Bai a , Hongzhou Dong a,b , Qian Zhang a,b , Lifeng Dong a,b,c,n a College of Materials Science and Engineering, Qingdao University of Science and Technology, Qingdao, Shandong 266042, PR China b State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi 030001, PR China c Department of Physics, Astronomy, and Materials Science, Missouri State University, Springfield, MO 65897, USA article info Article history: Received 13 September 2014 Accepted 10 October 2014 Available online 18 October 2014 Keywords: Cu 2 ZnSn(S 1Àx ,Se x ) 4 Ethanol-thermal method Sulfurization Selenization Films abstract This contribution reports, for the first time, the successful synthesis of kesterite Cu 2 ZnSn(S 1 Àx ,Se x ) 4 films by a simple ethanol-thermal method, followed by sulfurization/selenization annealing treatments for stoichiometric control of the products. The annealing processes under various experimental conditions have a significant impact on microscopic properties of the films, such as morphology and crystallinity. In particular, the energy band gap of obtained nanocrystal films was tuned from 1.43 eV to 1.1 eV after the implementation of different annealing treatments. Correspondingly, a preliminary evaluation of photovoltaic response was conducted, and the measurements indicated that the Se/S ratio, controlled by sulfurization/selenization processes, can be a decisive factor in the optoelectronic performance of Cu 2 ZnSn(S 1Àx ,Se x ) 4 films. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Functioning as light absorbers, Cu 2 ZnSn(S 1Àx ,Se x ) 4 (0 rx r1, CZTS/Se) films have been regarded as one of the most promising alternatives to toxic and costly CuInGa (1Àx) Se 2 (CIGS) for the devel- opment of thin film solar cells. CZTS/Se contains only earth-abundant and relatively nontoxic elements that barely generate environmental contamination; moreover, it demonstrates a direct energy band possessing a considerable absorption coefficient ( 410 4 cm À1 ) [1] and has a crystal structure analogous to CIGS, indicating that attaining a power conversion efficiency over 20%, achieved by CIGS-based solar cells currently [2], might be possible. Previously, Jiang et al. fabricated CZTS films by a sol–gel method [3]; Saucedo et al. synthesized CZTS films via a spray pyrolysis route [4]; Guh et al. developed a co-evaporation method for CZTS film preparation [5]; Scragg et al. reported sputter-synthesized CZTSe films [6]; Deligianni et al. achieved electrodeposited CZTSe films [7], and Li et al. reported CZTS/Se synthesis by an aqueous solution process [8]. The current problem is that most experimental methods focus only on either CZTS or CZTSe deposition, yet hardly address the influence of Se/S ratio, although adjusting this ratio could assist in tuning the photovoltaic properties of CZTS/Se films [9]. The ethanol-thermal technique is similar to the familiar hydro- thermal method, except that it substitutes ethanol for deionized water as a reactant and generates relatively higher pressure during the reaction. This convenient method has been utilized for synthesiz- ing entities such as Cu 3 BiS 3 nanorods [10] and Cd 1Àx Zn x S nanopar- ticles [11], but never for the quaternary compound Cu 2 ZnSn(S 1Àx , Se x ) 4 . In this study, we report for the first time CZTS/Se synthesis by a simple ethanol-thermal approach. Phase homogeneity and morphol- ogy of the products are desirably controlled. The impact of sulfuriza- tion and selenization annealing on the photoelectrical properties of obtained films was also investigated. We confirm that the proposed synthesis method can produce quality CZTS/Se thin films for photo- voltaic applications. 2. Experiments Material preparation: The precursor solution was prepared by dissolving CuCl 2 Á 2H 2 O (0.8 mmol), ZnCl 2 (0.6 mmol), SnCl 2 Á 2H 2 O (0.6 mmol), and H 2 NCSNH 2 (4.0 mmol) in an ethanol solution (20 mL) under magnetic stirring. Subsequently, the solution was loaded into a Teflon-lined stainless steel autoclave of 60 mL capacity. The FTO glasses used as substrates were cleaned ultra- sonically in sequence with acetone, ethanol, and deionized water Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/matlet Materials Letters http://dx.doi.org/10.1016/j.matlet.2014.10.051 0167-577X/& 2014 Elsevier B.V. All rights reserved. n Corresponding author at: College of Materials Science and Engineering, Qing- dao University of Science and Technology, Qingdao, Shandong, 266042, PR China. Tel.: þ86 532 84022869. E-mail address: DongLifeng@qust.edu.cn (L. Dong). Materials Letters 139 (2015) 101–103