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 (Ps) of
1.1 μC/cm2 at room temperature, a direct bandgap
of 2.32 eV, and a high Curie temperature Tc 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 PbI42– 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.