Confinement-Driven Ferroelectricity in a Two-Dimensional Hybrid Lead Iodide Perovskite

Organic-inorganic hybrid perovskites (OIHPs) hold a great potential for scientific and technological endeavors in the field of ferroelectrics, solar cells, and electroluminescent devices, because of their structural diversity, low cost of manufacture, and ease of fabrication. However, lead iodide perovskite ferroelectrics with narrow band gap have rarely been reported. Here, we present a new two-dimensional (2D) layered lead iodide perovskite ferroelectric, (4,4-DFHHA)₂PbI₄ (4,4-DFHHA = 4,4-difluorohexahydroazepine), with a spontaneous polarization (P_s) of 1.1 μC/cm² at room temperature, a direct bandgap of 2.32 eV, and a high Curie temperature T_c of 454 K (beyond that of BaTiO₃, 393 K). On the basis of the nonferroelectrics (HHA)I, (4-FHHA)I, and (4,4-DFHHA)I (HHA = hexahydroazepine, 4-FHHA = 4-fluorohexahydroazepine), we assembled them with PbI₂ to form lead iodide perovskites. Because the space between adjacent one-dimensional (1D) chains is relatively large and the confinement effect is not obvious, the cations are still in a disordered state, and 1D OIHPs (HHA)PbI₃ and (4-FHHA)PbI₃ are also nonferroelectrics at room temperature. In the confined environment of the 2D PbI₄^2- framework for (4,4-DFHHA)₂PbI₄, the 4,4-DFHHA cations become ordered, and their asymmetric distribution leads to the spontaneous polarization. This work offers an efficient strategy for enriching the family of lead iodide perovskite ferroelectrics through the confinement effect and should inspire further exploration of the interplay between ferroelectricity and photovoltaics.