Fabrication and sulfurization of Cu ₂ SnS ₃ thin films with tuning the concentration of Cu-Sn-S precursor ink

In this study, Cu-Sn-S nanoinks were synthesized by combining chelating polyetheramine to Cu, Sn, S powders of various concentrations. X-ray diffraction patterns indicate that nanoinks synthesized at low concentrations are composed almost entirely of binary phases SnS and Cu ₂ S. Synthesizing nanoinks at higher concentrations decreased the quantity of binary phase and led to the appearance of ternary phase Cu ₄ SnS ₄ . Following sulfurization, single phase Cu ₂ SnS ₃ (CTS) thin film was obtained from nanoinks of low concentration; however, impurities, such as Cu ₂ S were detected in the thin film obtained from nanoinks of high concentration. This can be attributed to the fact that lower concentrations reduce the reactivity of all the elements. As a result, the SnS phase reacted more readily and more rapidly, resulting in the early formation of a stoichiometric CTS thin film during sulfurization. Under these reaction conditions, Cu ₂ S and SnS transform into CTS and thereby prevent the formation of unwanted phases of Cu ₂ S and Cu ₄ SnS ₄ . Raman spectra revealed that second phase Cu ₂ S phase remained in the high-concentration samples, due to an increase in reactivity due to the participation of a greater proportion of the copper in the reaction. The surface microstructure of low-concentration samples display closely packed Cu ₂ SnS ₃ grains with a flat morphology and an atomic composition ratio of Cu:Sn:S = 34.69:15.90:49.41, which is close to stoichiometric. Hall measurement revealed that low-concentration sample has superior electrical properties; i.e., a hole concentration of 5.23 × 10 ¹⁷  cm ⁻³ , mobility of 14.2 cm ² /V-s, and optical band-gap energy of 1.346 eV, which are suitable for thin-film solar cells.