Piezoelectric materials produce electricity when strained, making them ideal for different types of sensing applications. The most effective piezoelectric materials are ceramic solid solutions in which the piezoelectric effect is optimized at what are termed morphotropic phase boundaries (MPBs). Ceramics are not ideal for a variety of applications owing to some of their mechanical properties. We synthesized piezoelectric materials from a molecular perovskite (TMFM)x(TMCM)1–xCdCl3 solid solution (TMFM, trimethylfluoromethyl ammonium; TMCM, trimethylchloromethyl ammonium, 0 ≤ x ≤ 1), in which the MPB exists between monoclinic and hexagonal phases. We found a composition for which the piezoelectric coefficient d33 is ~1540 picocoulombs per newton, comparable to high-performance piezoelectric ceramics. The material has potential applications for wearable piezoelectric devices.

Piezoelectric sensors that can work under various conditions with superior performance are highly desirable with the arrival of the Internet of Things. For practical applications, a large piezoelectric voltage coefficient g and a high Curie temperature T c are critical to the performance of piezoelectric sensors. Here, we report a two-dimensional perovskite ferroelectric (4-aminotetrahydropyran)2PbBr4 [(ATHP)2PbBr4] with a saturated polarization of 5.6 μC cm–2, high T c of 503 K [above that of BaTiO3 (BTO, 393 K)], and extremely large g 33 of 660.3 × 10–3 V m N–1 [much beyond that of Pb­(Zr,Ti)­O3 (PZT) ceramics (20 to 40 × 10–3 V m N–1), more than 2 times higher than that of poly­(vinylidene fluoride) (PVDF, about 286.7 × 10–3 V m N–1)]. Combined with the advantages of molecular ferroelectrics, such as light weight, easy and environmentally friendly processing, and mechanical flexibility, (ATHP)2PbBr4 would be a competitive candidate for next-generation smart piezoelectric sensors in flexible devices, soft robotics, and biomedical devices.

The past decade has witnessed much progress in designing molecular ferroelectrics, whose intrinsic mechanical flexibility, structural tunability, and easy processability are desirable for next-generation flexible and wearable electronic devices. However, an obstacle in expanding their promising applications in nonvolatile memory elements, capacitors, and sensors is effectively modulating the Curie temperature (T c ).Here, taking advantage of fluorine substitution on the reported molecular ferroelectric, (pyrrolidinium)MnCl 3 , we present enantiomeric perovskite ferroelectrics, namely, (R)and (S)-3-(fluoropyrrolidinium)MnCl 3 . The close van der Waal's radii and the similar steric parameters between H and F atoms ensure the minimum disruption of the crystal structure, while their different electronegativity and polarizability can trigger significant changes in the physical and chemical properties. As expected, the T c gets successfully increased from 295 K in (pyrrolidinium)MnCl 3 to 333 K in these two homochiral compounds. Such a dramatic enhancement of 38 K signifies an important step toward designing high-T c molecular ferroelectrics. In the light of the conceptually new idea of fluorine substitution, one could look forward to a continuous succession of new molecular ferroelectric materials and technology developments.

Two-dimensional (2D) organic–inorganic perovskites (OIPs), with improved material stability over their 3D counterparts, are highly desirable for device applications. It is their considerable structural diversity that offers an unprecedented opportunity to engineer materials with fine-tuning functionalities. The isosteric substitution of hydrogen by an electronegative fluorine atom has been proposed as a useful route to improve the photovoltaic performance of 2D OIPs, whereas its valuable role in developing ferroelectricity is still waiting for further exploration. Herein, for the first time we applied fluorinated aromatic cations in extending the family of 2D OIP ferroelectrics, and successfully obtained [2-fluorobenzylammonium]2PbCl4 as a high-performance ferroelectric semiconductor. The failures in the nonferroelectric [4-fluorobenzylammonium]2PbCl4 and [3-fluorobenzylammonium]2PbCl4 demonstrate that the selective introduction of fluorine in correct structural positions is particularly essential. This work represents an unprecedented proof-of-concept in the use of fluorinated aromatic cations for the targeted design of excellent 2D OIP ferroelectrics, and is believed to inspire the future development of low-cost, high-efficiency, and stable device applications.

electrical transport performance, and long carrier lifetime. Besides these factors, the ferroelectric photovoltaic effect (FEPV) was also believed to contribute to the good performance in PSC. [6] Conventional photovoltaic devices utilize heterojunctions to create asymmetric electric potential to separate the photoinduced charge carriers. While, for FEPV, the photogenerated electron-hole pair is separated by the spontaneous polarization or domain walls in homogeneous ferroelectric materials. [7] Since FEPV is independent to the bending of energy band, it can generate extremely large open-circuit voltage comparable to the bandgap and is expected to enhanced efficiency in PSC. Although FEPV draws a very exciting picture, the ferroelectricity of MAPbI 3 is still controversial. [8] To study the detailed contribution of FEPV in PSC, obtaining a lead-iodide-based polar perovskite materials with settled ferroelectricity is very necessary as a useful complement to the current PSC materials and model system to investigate the role of ferroelectric polarization in photovoltaics.Besides the intensive competition on PCE, PSC are facing a more severe problem of stability. During operation, the applied electric field or optoelectrical field may induce migration of organic molecules and halogen ions. Even without light and electric field, in ambient condition, the moisture and oxygen may also decompose the perovskite. [9] Those factors limit the lifetime of PSC for only ≈1000 h, which is far away from that of conventional silicon-based solar cell panel (20-25 years). One possible solution to this problem is reducing the dimensionality from 3D to quasi-2D, which has large formation energy, high moisture stability, and long lifetime, however, such low-dimensional perovskite will affect transport properties and reduce PCE. [5,10] Until very recently, fluorination on 2D lead iodide perovskite was reported to enhance charge transport and PCE. [11] Thus, in order to achieve a good balance between PCE and stability, a fluorinated 2D lead iodide perovskite ferroelectric material is highly demanded, which is also a good platform to study the fundamental mechanism behind high PCE.Although we have reported several lead-based perovskite ferroelectrics with 2D layered structure, the ferroelectricity can only coexist with chlorine or bromine, even slight doping of iodine would alter the crystal structure and vanish the precious ferroelectric property. [12] Until now, rational design and synthesis of HOIP ferroelectrics is still very challenging and Hybrid perovskite materials are famous for their great application potential in photovoltaics and optoelectronics. Among them, lead-iodide-based perovskites receive great attention because of their good optical absorption ability and excellent electrical transport properties. Although many believe the ferroelectric photovoltaic effect (FEPV) plays a crucial role for the high conversion efficiency, the ferroelectricity in CH 3 NH 3 PbI 3 is still under debate, and obtaining ferroelectric lead i...