High-temperature decomposition of Cu2BaSnS4 with Sn loss reveals newly identified compound Cu2Ba3Sn2S8

Abstract

The earth-abundant quaternary compound Cu₂BaSnS₄ is being currently studied as a candidate for photovoltaics and as a photocathode for water splitting. However, the chemical stability of this phase during synthesis is unclear. The synthesis of other quaternary tin–sulphur-based absorbers (e.g., Cu₂ZnSnS₄) involves an annealing step at high temperature under sulphur gas atmosphere, which can lead to decomposition into secondary phases involving Sn loss from the sample. As the presence of secondary phases can be detrimental for device performance, it is crucial to identify secondary phase chemical, structural and optoelectronic properties. Here we used a combination of in situ EDXRD/XRF and TEM to identify a decomposition pathway for Cu₂BaSnS₄. Our study reveals that, while Cu₂BaSnS₄ remains stable at high sulphur partial pressure, the material decomposes at high temperatures into Cu₄BaS₃ and the hitherto unknown compound Cu₂Ba₃Sn₂S₈ if the synthesis is performed under low partial pressure of sulphur. The presence of Cu₄BaS₃ in devices could be harmful due to its high conductivity and relatively lower band gap compared to Cu₂BaSnS₄. The analysis of powder diffraction data reveals that the newly identified compound Cu₂Ba₃Sn₂S₈ crystallizes in the cubic system (space group I3d) with a lattice parameter of a = 14.53(1) Å. A yellow powder of Cu₂Ba₃Sn₂S₈ has been synthesized, exhibiting an absorption onset at 2.19 eV.