(NH₄)₄Sn₂S₆·3H₂O: Crystal Structure, Thermal Decomposition, and Precursor for Textured Thin Film

Understanding the condensation of the dimeric thiostannate(IV) [Sn 2 S 6 ] 4– to SnS 2 is of key importance for the development of solution processing of advanced tin(IV) sulfide based electronic devices such as photovoltaics (e.g., Cu 2 ZnSnS 4, CZTSSe) and thin-film transistors. Here, we report the crystal structure of tetraammonium thiostannate(IV) trihydrate ((NH 4 ) 4 Sn 2 S 6 ·3H 2 O), which can be used as a more environmentally friendly alternative to the hydrazinium analogue in solution processed advanced tin(IV) sulfide based electronic devices, e.g., CZTSSe. Hirshfeld surface analysis shows that crystal bound water molecules play a significant role in the structure and interact strongly with the sulfur atoms in the dimeric thiostannate(IV) complex [Sn 2 S 6 ] 4– . The thermal decomposition and corresponding condensation of ((NH 4 ) 4 Sn 2 S 6 ·3H 2 O) to SnS 2 have been studied by TG/DSC-MS and solid-state 119 Sn MAS NMR. It involves the formation of the relatively more condensed thiostannate(IV) complex [Sn 4 S 10 ] 4– at 90 °C via evaporation of ammonia, hydrogen sulfide, and water from the structure. With increasing temperature, more tin is transformed from tetrahedral to octahedral coordination, and at 220 °C, crystalline SnS 2 is formed. In an aqueous ammonium sulfide based solution, the structure of dimeric [Sn 2 S 6 ] 4– is retained, and aqueous solutions of (NH 4 ) 4 Sn 2 S 6 ·3H 2 O can be spin coated and thermally decomposed to form crystalline SnS 2 thin films. X-ray scattering techniques show that the solution processed SnS 2 thin film is highly textured with the ab plane parallel to the substrate. Furthermore, AFM and TEM reveal that the thin film is continuous and with an inherent porous surface structure from the gaseous formation byproducts.