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.

有机-无机杂化钙钛矿（organic–inorganic hybrid perovskites，OIHPs）凭借其结构多样性、制备成本低廉且易于合成，在铁电体（ferroelectrics）领域、太阳能电池与电致发光器件领域展现出巨大的科学与应用潜力。然而，窄带隙碘化铅钙钛矿铁电体的相关报道却较为罕见。本工作报道了一种新型二维（two-dimensional，2D）层状碘化铅钙钛矿铁电体(4,4-DFHHA)₂PbI₄（其中4,4-DFHHA即4,4-二氟六氢氮杂卓（4,4-difluorohexahydroazepine）），该材料在室温下的自发极化（spontaneous polarization，Ps）为1.1 μC/cm²，直接带隙为2.32 eV，居里温度（Curie temperature，Tc）高达454 K（高于钛酸钡（BaTiO₃）的393 K）。本研究以非铁电体(HHA)I、(4-FHHA)I与(4,4-DFHHA)I为前驱体（其中HHA即六氢氮杂卓（hexahydroazepine），4-FHHA即4-氟六氢氮杂卓（4-fluorohexahydroazepine）），将其与碘化铅PbI₂复合制备得到碘化铅钙钛矿材料。由于相邻一维（one-dimensional，1D）链之间的间距较大，限域效应并不显著，阳离子仍处于无序状态，因此一维OIHPs (HHA)PbI₃与(4-FHHA)PbI₃在室温下同样为非铁电体。在(4,4-DFHHA)₂PbI₄的二维PbI₄²⁻骨架限域环境中，4,4-DFHHA阳离子发生有序化，其不对称分布催生了自发极化现象。本工作通过限域效应策略丰富了碘化铅钙钛矿铁电体家族，同时为铁电性与光伏效应（photovoltaics）之间的相互作用研究提供了新的思路。