<p>Ternary antimony-based sulfides generally exhibit lower photovoltaic efficiency compared to binary Sb₂Se₃. This study presents a first-principles comparative analysis of CsSbSe₂ and Sb₂Se₃, with CsSbSe₂ selected for investigation owing to the large ionic radius of Cs, which amplifies structural effects. The analysis indicates that the inferior performance of CsSbSe₂ arises from the high electropositivity of Cs⁺, leading to strong electrostatic attraction toward Se²⁻ and the formation of predominantly ionic Cs–Se bonds. This interaction alters the coordination environment of Sb³⁺, reduces its coordination number, and diminishes Sb–Se orbital hybridization, thereby modifying the electronic band structure and defect characteristics. Moreover, spontaneous intrinsic defects—such as Cs_Sb and Cs_Se—exhibit Fermi-level-dependent formation energies, disrupting the covalent Sb–Se network and promoting the generation of vacancy-related defects and carrier recombination centers. These results underscore the critical importance of cation compatibility and bonding configuration in the design of high-efficiency photovoltaic materials.</p> Graphical abstract <p></p>