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...

Molecular ferroelectrics (MOFEs) with excellent piezoelectricity are highly desirable for their easy and environmentally friendly processing, light weight, low processing temperature, and mechanical flexibility. However, although 100 years have passed...

The X-site ion in organic-inorganic hybrid ABX 3 perovskites (OHPs) varies from halide ion to bridging linkers like HCOO À ,N 3 À ,NO 2 À ,and CN À .However,nonitrite-based OHP ferroelectrics have been reported so far.N ow,b ased on non-ferroelectric [(CH 3 ) 4 N][Ni(NO 2 ) 3 ], through the combined methodologies of quasi-spherical shape,h ydrogen bonding functionality,a nd H/F substitution, we have successfully synthesized an OHP ferroelectric, [FMeTP][Ni(NO 2 ) 3 ] (FMeTP = N-fluoromethyl tropine). As an unprecedented nitrite-based OHP ferroelectric,t he well-designed [FMeTP]-[Ni(NO 2 ) 3 ]u ndergoes the ferroelectric phase transition at 400 Kwith an Aizu notation of 6/mmmFm, showing multiaxial ferroelectric characteristics.T his work is ag reat step towards not only enriching the molecular ferroelectric families but also accelerating the potential practical applications.Molecular ferroelectrics with cheap,l ightweight, flexible, wearable,a nd environmentally benign characteristics have been amajor research topic in the past decades owing to the potential applications in data storage,p iezoelectric sensors, actuators,a nd nonlinear optical devices. [1] It is their intrinsic structure-property flexibility that brings forth particular interest for molecular design towards new materials with satisfactory ferroelectric performance near or even above room temperature. [2] Nevertheless,d iscovering ar eliable method to control the ordered organization of molecules into apolar lattice and thereby modulate ferroelectricity is an imminent ongoing challenge.R ecently,s everal design strategies involving the selection of molecular building blocks have been developed. [1b,c] Fore xample,t he Zn 2+ /Dy 3+ Schiff base antenna complex and the lower-symmetric quasi-spherical cations modified from the spherical [(CH 3 ) 4 N] + ,1 ,4diazabicyclo[2.2.2]octonium (dabco), or quinuclidinium were found to be promising for constructing molecular ferroelectrics with high Curie point (T c ). [3] Accordingly,w eh ave obtained several outstanding organic-inorganic hybrid perovskite (OHP) ferroelectrics with ageneral formula ABX 3 (A is an organic cation, Bi sametal, and Xi susually halides), such as [(CH 3 ) 3 NCH 2 Cl]MCl 3 (M = Mn, Cd) and [MeHdab-co]RbI 3 ,where the structural transitions arise mainly from the dynamic motions of the A-site moieties. [3b, 4] Remarkably,b y extending the X-site from monatomic anions to bridging linkers like HCOO À , [5] N 3 À , [6] NO 2 À , [7] and CN À , [8] aw ide range of larger and longer organic cations can be accommodated for enriching the OHP families and inducing abundant properties such as ferromagnetism and antiferromagnetism. Fort he classical pseudohalide NO 2 À ,o nly two kinds of inorganic ferroelectrics have been reported, NaNO 2 and MCd(NO 2 ) 3 (M = NH 4 ,K,Rb, Cs,Tl), [9] whereas there are no nitrite-based ferroelectrics in the OHPs.I nt his way,t he earlier work of non-ferroelectric [(CH 3 ) 4 N][Ni(NO 2 ) 3 ], adopting at rigonal centrosymmetric space group P3 m1a t room temper...

Ferroelectrics are attractive due to their great application potential in information storage, optoelectronics, spintronics and sensing. As an important characteristic affecting semiconducting applications, the energy band structure is important for the development of light-emitting devices. Although it is a tremendous challenge to tune the bandgap in inorganic ferroelectric materials due to the strict requirement of structural symmetry for the fragile ferroelectricity, hybrid organic–inorganic perovskite (HOIP) ferroelectrics, which have a flexible structure, provide a new method to optimize the ferroelectric performance and bandgap. Based on the overview of methods for designing ferroelectrics, this Perspective systematically provides in-depth insight into the relationships between the structure–property and bandgap of HOIP ferroelectrics. In addition, we discussed the challenges and directions of HOIP ferroelectrics in semiconducting applications for the future.