First-principles insights into tin-based two-dimensional hybrid halide perovskites for photovoltaics

The two-dimensional (2D) hybrid halide perovskites have recently attracted attention due to their excellent photovoltaic performance. In comparison to their three-dimensional (3D) analogues, they show superior long-term durability and moisture tolerance. Meanwhile, their layered topology offers greater flexibility for electronic structure tuning. To date, most devices containing 2D perovskites have been based on Pb, which presents environment concerns and a possible roadblock to commercialisation due to its toxicity. The development of lead-free alternatives is therefore immensely important to facilitate the uptake of perovskite-based photovoltaics. Herein, we investigate the geometrical, electronic and optical properties of the semiconducting 2D tin perovskites (CH3(CH2)(3)NH3)(2)(CH3NH3)(n-1)SnnI3n+1 (n = 1, 2 and 3), using relativistic hybrid density functional theory calculations. We demonstrate that the band gaps of the series decrease with increasing perovskite-like layer thickness, from 1.85 eV (n = 1) to 1.38 eV (n = 3). We find strong and broad optical absorption across the series, in addition to small effective masses of electrons and holes in the laminar plane. The n = 3 composition displays a high spectroscopic limited maximum efficiency of 24.6%. Our results indicate this series of homologous 2D tin halide perovskites are a promising class of stable and efficient light-absorbing materials for photovoltaics.