Theoretical Dissection of Cation-Deficient Layered Ruddlesden–Popper Oxysulfide Perovskites with a High-Efficiency Carrier Transport Channel

The search for lead-free perovskite materials has triggered intensive interest. Here, we study the electronic structures and optical properties of cation-deficient Ruddlesden-Popper oxysulfide perovskites Ln₂Ti₂O₅S₂ (Ln = Sc, Y, or La), with a tunable band gap of 1.45-2.1 eV and a small exciton binding energy of ∼0.1 eV, among which Y₂Ti₂O₅S₂ has been synthesized experimentally. Sc₂Ti₂O₅S₂ possesses the largest light absorbance in the visible region. We further rationalize the light absorption via the transition dipole moment and suggest potential applications of Sc₂Ti₂O₅S₂ in solar cells and Y₂Ti₂O₅S₂ and La₂Ti₂O₅S₂ in water splitting. In addition, this family exhibits small effective masses within the x-y plane and large ones along the z direction. Most importantly, electron gas-like carrier behaviors are observed within the Ti-O bond region, offering a diffusion channel for electron transport. These findings greatly advance our understanding of lead-free perovskites and offer a novel material platform for future optoelectronic devices.