  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 and optical properties 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 solar cells via device structuring based on the
underlying crystal anisotropy.
