Tunable crystal structure of Cu2SnS3 deposited by spray pyrolysis and its impact on the chemistry and electronic structure

Abstract

Cu2SnS3 (CTS) thin films were grown by spray pyrolysis using two synthetic routes: stoichiometric and substoichiometric in an aprotic media. The use of dimethyl sulfoxide (DMSO) as a solvent improved the solubility of metal chlorides at room temperature, which was crucial to avoid metal-hydroxylation. In this sense, a mechanism for the solvation of metal chloride precursors in an aprotic media is proposed. Different sulfurization temperatures allowed the presence of diverse crystalline structures of Cu2SnS3, corroborated by Raman Spectroscopy and X-ray diffraction. For the substoichiometric composition Cu/Sn = 1.76 and a temperature annealing interval of 350–600 °C, the films showed the monoclinic-Cu2SnS3 phase. While for the stoichiometric composition Cu/Sn = 2, the tetragonal-Cu2SnS3 phase was present between 300 and 400 °C. At 500 °C, the Cu content increase allows the orthorhombic-Cu3SnS4 phase formation. The presence of Sn4+ and Cu+ species was confirmed by X-ray photoelectron spectroscopy (XPS) measurements. Simultaneously, electronic structures of the surface of the film were obtained by XPS combined with inverse photoemission spectroscopy (IPES). Bandgap values were 1.33 eV, 1.25 eV, and 1.15 eV for those films with tetragonal, orthorhombic, and monoclinic phases. The proximity of the valence band maximum (VBM) to the Fermi level causes that these compounds have a p-type nature, producing a small cliff in the conduction band alignment at the CdS/CTS interface. Photoresponse measurements revealed that all the samples are photoconductive to be considered as an option for solar applications.