Unusual Electron−Hole Excitation and Separation in Atomically Thin III ₂ −VI ₃ Ferroelectrics for Photocatalysis

Two-dimensional (2D) III2−VI3 ferroelectrics are promising candidates for optoelectronic and photocatalytic applications. Here, using many-body perturbation theory, we investigate the quasiparticle electronic structure and excitonic properties of monolayers Ga2S3, In2S3, Ga2Se3, and In2Se3. We reveal that all systems exhibit indirect band gaps with significant self-energy corrections (>1 eV) due to reduced 2D dielectric screening. Their optical gaps spanning from 1.28 to 2.63 eV enable broad visible-light harvesting dominated by strong excitonic effects. Crucially, the out-of-plane ferroelectric polarization induces an intrinsic spatial asymmetry, promoting the vertical separation of photo-excited electron−hole pairs. This separation exhibits an unusual differential behavior governed by the competition between dielectric screening and polarization: Ga-based systems feature strong, delocalized lowest bright excitons, whereas In-based systems exhibit weakly allowed but exceptionally long-lived excitons localized on opposite surfaces. Benefiting from efficient inter-exciton relaxation from higher-energy states and favorable thermodynamic band-edge alignments for water splitting, this synergistic combination enables highly efficient spatial separation of photogenerated carriers. Our work establishes atomically thin III2−VI3 ferroelectrics as highly versatile platforms for next-generation energy-conversion applications.