Lone pair driven anisotropy in antimony chalcogenide semiconductors

Antimony sulfide (Sb₂S₃) and selenide (Sb₂Se₃) have emerged as promising earth-abundant alternatives among thin-film photovoltaic compounds. A distinguishing feature of these materials is their anisotropic crystal structures, which are composed of quasi-one-dimensional (1D) [Sb₄X₆]_n ribbons. The interaction between ribbons has been reported to be van der Waals (vdW) in nature and Sb₂X₃ are thus commonly classified in the literature as 1D semiconductors. However, based on first-principles calculations, here we show that inter-ribbon interactions are present in Sb₂X₃ beyond the vdW regime. The origin of the anisotropic structures is related to the stereochemical activity of the Sb 5s lone pair according to electronic structure analysis. The impacts of structural anisotropy on the electronic, dielectric and optical properties relevant to solar cells are further examined, including the presence of higher dimensional Fermi surfaces for charge carrier transport. Our study provides guidelines for optimising the performance of Sb₂X₃-based photovoltaics via device structuring based on the underlying crystal anisotropy.