Ruddlesden-Popper perovskite sulfides A₃B₂S₇: A new family of ferroelectric photovoltaic materials for the visible spectrum

Perovskite ferroelectric materials exhibit the novel ferroelectric photovoltaic effect, where photon-excited electron-hole pairs can be separated by ferroelectric polarization. Especially, semiconducting ferroelectric materials with small band gaps (Eg) have been extensively studied for applications in solar energy conversion. Traditional route for creating semiconducting ferroelectrics requires cation doping, where E_g of the insulating perovskite ferroelectric oxides are reduced via substitution of certain cations. But cation doping tends to reduce the carrier mobility due to the scattering, and usually lead to poor photovoltaic efficiency. In the present work, based on first-principles calculations, we propose and demonstrate a new strategy for designing stoichiometric semiconducting perovskite ferroelectric materials. Specifically, we choose the parent non-polar semiconducting perovskite sulfides ABS₃ with Pnma symmetry, and turn them into ferroelectric Ruddlesden-Popper A₃B₂S₇ perovskites with spontaneous polarizations. Our predicted Ruddlesden-Popper Ca₃Zr₂S₇ and other derived compounds exhibit the room-temperature stable ferroelectricity, small band gaps (Eg<2.2eV) suitable for the absorption of visible light, and large visible-light absorption exceeding that of Si.