ORIGINAL PAPER A water- and sulfurization-free solution route to Cu 2-x Zn 1+x SnS 4 Renato D’Angelo • Cristy Leonor Azanza Ricardo • Alberto Mittiga • Paolo Scardi • Matteo Leoni Received: 24 April 2014 / Accepted: 28 July 2014 / Published online: 12 August 2014 Ó Springer Science+Business Media New York 2014 Abstract Zinc-rich/copper-poor Cu 2-x Zn 1?x SnS 4 (x = 0.2, CZTS) has been successfully produced in film and powder form using two non-aqueous solutions (of metal salts and thiourea) without the need for sulfurization during the annealing phase. A reaction route is proposed and the choices of the solvents (water, ethyleneglycol, ethanol, methanol) and of the tin source (tin chloride pentahydrate or anhydrous) discussed and justified. A pure and coarse- grained material is obtained with a mix of metal salts in methanol and thiourea in ethylene glycol. The tin penta- hydrate salt seems a better alternative to the commonly used anhydrous chloride. Keywords CZTS Á Non-vacuum Á Dip-coating method Á Sol–gel Á Kesterite 1 Introduction Thin film solar cells aim to become competitive with tra- ditional silicon-based devices, as a low cost per kW of the base materials is coupled to the possibility of building the cell on flexible and non-flat substrates. Energy harvesting in a thin film solar cell is obtained via electron–hole pair formation in a suitable absorber layer. The best absorbers so far contain cadmium (e.g. CdTe, max laboratory effi- ciency of 18.3 % [1]) or indium (CIGS, CuIn x Ga 1-x Se 2 , max efficiency 20 % [2]), the first one being notoriously toxic and the other quite expensive. The research is thus fostered towards finding alternative absorbersnot containing toxic or expensive elements. The most interesting results are currently shown by a synthetic analog of the mineral kesterite with formula Cu 2 ZnSn (S, Se) 4 (CZTS). An efficiency of 11 % has been already achieved in laboratory using a partly selenized CZTS [3]. Many processes have been proposed for the production of CZTS thin films and powders (for solar inks), most of them based on vacuum deposition, complex synthetic routes or requiring a sulfurization step, all too expensive for a mass production. Limiting to cheap production routes, the best stoichiometry, morphology and band gap (1.44–1.51 eV) have been obtained with absorbers synthesized from liquid phase [4]. Conventional aqueous solutions processes entail the possibility of forming M–O–M bonds in the precursor solution or during the annealing step, which can lead to the formation of unwanted extra phases if the wrong solvent is employed. Recent studies [5, 6] have shown the effects of water, ethanol, ethylene glycol and methanol as solvents: near stoichiometric CZTS films were obtained up to 500–550 °C. Park et al. [7] tested nitrogen to replace H 2 S for heat treatment and confirm the optimal stoichiometry of sulfur in the material. Here we report a simple process to obtain CZTS without vacuum and without sulfurization. A Zn-rich/Cu-poor CZTS is obtained, beneficial e.g. for solar energy applications: previous reports [8, 9] suggested that a Cu-poor material leads to the formation of Cu vacancies, which are shallow acceptors in CZTS, while a Zn-rich condition suppresses Cu substitution on the Zn sites, and thus decreases the quantity of deep acceptors. R. D’Angelo Á C. L. Azanza Ricardo Á P. Scardi Á M. Leoni (&) Department of Civil, Environmental and Mechanical Engineering, University of Trento, via Mesiano, 77, 38123 Trento, TN, Italy e-mail: matteo.leoni@unitn.it A. Mittiga ENEA, Casaccia Research Center, Via Anguillarese 301, 00123 Rome, Italy 123 J Sol-Gel Sci Technol (2014) 72:490–495 DOI 10.1007/s10971-014-3462-x