Atomically thin two-dimensional hybrid perovskites using hydrophobic superalkali cations with tunable electron...

The direct band gaps of two-dimensional (2D) metal halide perovskites can be tuned via component engineering, but their electron transition type hardly changes. Herein, atomically thin (C₅NH₆)₂MX₄ (M = Ge, Sn, Pb; X = Cl, Br, I) hybrid perovskites with hydrophobic superalkali cations were systematically explored using high-throughput hybrid density functional calculations and ab initio molecular dynamics simulations. We found that the electron transition between the M and X atoms was converted into that between the C₅NH₆ parts and X atoms via X change in the 2D (C₅NH₆)₂MX₄ perovskites. Negative formation energy, stable thermodynamic and kinetic properties, sharp valence bands, and tunable direct band gaps were obtained for the 2D perovskites. A power conversion efficiency (PCE) of 32.54% was obtained in theory for the passivated cubic NH₂CHNH₂PbI₃ (FAPbI₃) perovskite containing the 2D (C₅NH₆)₂PbI₄ perovskite. The hybrid Pb-free (C₅NH₆)₂SnI₄ perovskite with a direct bandgap of 1.56 eV may be viewed as a potential passivation material for perovskite devices. Moreover, the C₅NH₆ cations and X atoms show different hydrogen bonding interactions, which can be extended to other atomically thin organic-inorganic hybrid perovskites.