Halide Double Perovskite Ferroelectrics

Achieving High Operating-Temperature Self-powered X-Ray Detection in Multilayered Hybrid Perovskites through Arylamine Intercalation

Polar metal halide hybrid perovskites (PHPs) that exhibit outstanding bulk photovoltaic effect (BPVE), excellent semiconductor features, and strong radiation absorption ability, have shown prominent advantages in highly sensitive direct X-ray detection. However, it is still a challenge to explore PHPs with high BPVE temperature ranges, answering the demand of developing thermally stable passive X-ray detection. Herein, by intercalating arylamine into lead tribromide and inducing order-disorder phase transition, a 2D multilayered PHPs (BZA)2(MA)Pb2Br7 (BZPB, BZA = benzylamine, MA = methylamine) is synthesized. BZPB crystallizes in a polar space group Aea2 at a low-temperature phase and demonstrates a significant open-circuit of 0.3 V deriving from BPVE under X-ray irradiation. Meanwhile, the strong X-ray absorption coefficient and outstanding carrier transport capability of the bilayered lead halide framework associated with the polar BPVE give BZPB excellent X-ray detection abilities. At 0 V bias, the impressive sensitivity of BZPB is 98 µC Gy-1 cm-2. Importantly, the introduction of the rigid BZA ring increases the energy barrier of phase transition and thus dramatically enhances the X-ray detection operating temperature of BZPB up to 409 K without significant performance degradation. This work strongly reveals the great potential of rational design of metal halide hybrid perovskites for X-ray detection applications.

The past 10 years of molecular ferroelectrics: structures, design, and properties

Ferroelectricity, which has diverse important applications such as memory elements, capacitors, and sensors, was first discovered in a molecular compound, Rochelle salt, in 1920 by Valasek. Owing to their superiorities of lightweight, biocompatibility, structural tunability, mechanical flexibility, etc., the past decade has witnessed the renaissance of molecular ferroelectrics as promising complementary materials to commercial inorganic ferroelectrics. Thus, on the 100th anniversary of ferroelectricity, it is an opportune time to look into the future, specifically into how to push the boundaries of material design in molecular ferroelectric systems and finally overcome the hurdles to their commercialization. Herein, we present a comprehensive and accessible review of the appealing development of molecular ferroelectrics over the past 10 years, with an emphasis on their structural diversity, chemical design, exceptional properties, and potential applications. We believe that it will inspire intense, combined research efforts to enrich the family of high-performance molecular ferroelectrics and attract widespread interest from physicists and chemists to better understand the structure-function relationships governing improved applied functional device engineering.

Precise Design of Molecular Ferroelectrics with High TC and Tunable Band Gap by Molecular Modification

Molecular ferroelectric materials are widely applied in piezoelectric converters, non-volatile memorizers, and photovoltaic devices due to their advantages of adjustable structure, lightweight, easy processing, and environmental friendliness. However, designing multifunctional molecular ferroelectrics with excellent properties has always been a great challenge. Herein, a multiaxial molecular ferroelectric is successfully designed by modifying the quasi-spherical cation dabco with CuBr₂ to obtain halogenated [Bretdabco]CuBr₄ (Bretdabco = N-bromoethyl-N'-diazabicyclo [2.2.2]octane), which crystallizes in polar point groups (C₆). Typical ferroelectric behaviors featured by the P-E hysteresis loop and switched ferroelectric domain are exhibited. Notably, the molecular ferroelectric shows a high T_C of 460 K, which is rare in the field and could greatly expand the application range of this material. In addition, the band gap is adjustable through the regulation of halogen. Both the UV absorption spectra and theoretical calculations indicate that the molecular ferroelectrics belong to a direct band gap (2.14 eV) semiconductor. This tunable and narrow band gap semiconductor molecular ferroelectric material with high T_C can be utilized more effectively in the study of optoelectronics and sensors, including piezoelectric energy harvesters. This research may provide a promising approach for the development of multiaxial molecular ferroelectrics with a tiny band gap and high T_C.

Emerging Halide Perovskite Ferroelectrics

Halide perovskites have gained tremendous attention in the past decade owing to their excellent properties in optoelectronics. Recently, a fascinating property, ferroelectricity, has been discovered in halide perovskites and quickly attracted widespread interest. Compared with traditional perovskite oxide ferroelectrics, halide perovskites display natural advantages such as structural softness, low weight, and easy processing, which are highly desirable in applications pursuing miniaturization and flexibility. This review focuses on the current research progress in halide perovskite ferroelectrics, encompassing the emerging materials systems and their potential applications in ferroelectric photovoltaics, self-powered photodetection, and X-ray detection. The main challenges and possible solutions in the future development of halide perovskite ferroelectric materials are also attempted to be pointed out.

Rational Design of Ferroelectric 2D Perovskite for Improving the Efficiency of Flexible Perovskite Solar Cells Over 23

Despite the great progress of flexible perovskite solar cells (f-PSCs), it still faces several challenges during the homogeneous fabrication of high-quality perovskite thin films, and overcoming the insufficient exciton dissociation. To the ends, we rationally design the ferroelectric two-dimensional (2D) perovskite based on pyridine heterocyclic ring as the organic interlayer. We uncover that incorporation of the ferroelectric 2D material into 3D perovskite induces an increased built-in electric field (BEF), which enhances the exciton dissociation efficiency in the device. Moreover, the 2D seeds could assist the 3D crystallization by forming more homogeneous and highly-oriented perovskite crystals. As a result, an impressive power conversion efficiency (PCE) over 23 % has been achieved by the f-PSCs with outstanding ambient stability. Moreover, the piezo/ferroelectric 2D perovskite intrigues a decreased hole transport barriers at the ITO/perovskite interface under tensile stress, which opens new possibilities for developing highly-efficient f-PSCs.

First-principles study on photoelectric properties of all-inorganic two-dimensional double perovskite Cs3AgBiBr7

Two-dimensional (2D) all-inorganic double perovskite materials have attracted great interest owing to their unique photoelectric characteristics, such as high quantum efficiency and relative stability. However, few studies have been conducted on the 2D all-inorganic double perovskite Cs₃AgBiBr₇, and its photoelectric properties are unclear. In this study, we present a detailed investigation of the band structure, optical absorption spectrum, carrier mobility and exciton binding energy of the double perovskite Cs₃AgBiBr₇ based on the first-principles. The results show that this system has an indirect band gap and low carrier mobility, high exciton binding energy (2041.38 meV) and significant light absorption in the UV region. We also find that the material may be a potential exciton insulation candidate owing to the exciton binding energy beyond the band gap. Our calculated results also show that low dimensional perovskite Cs₃AgBiBr₇ is more suitable for luminescence than a photovoltaic device. We hope our theoretical results will inspire and promote the experimental exploration of 2D all-inorganic double perovskite materials for photoelectric applications.

Ferroelectric Ordering in Nanosized PbTiO3

The insight into the three-dimensional configuration of ferroelectric ordering in ferroelectric nanomaterials is motivated by the application of the development of functional nanodevices and the structural designing. However, the atomic deciphering of the spatial distribution of ordered structure remains challenging for the limitation of dimension and probing techniques. In this paper, a neutron pair distribution function (nPDF) was utilized to analyze the spontaneous polarization distribution of zero-dimensional PbTiO₃ nanoparticles in three dimensions, via the application of reverse Monte Carlo (RMC) modeling. The comprehensive identification with transmission electron microscopy verified the linear characteristics of polarization along the c-axis in the main body, while electric polarization distribution on the surface was enhanced abnormally. In addition, the correlation of dipole vectors extending to three unit cells below the surface is retained. This work shows an application of the micro/macroscale information to effectively decode the polarization structure of nanoferroelectrics, providing new views of designing nanoferroelectric devices.

Ferroelectric hybrid organic-inorganic perovskites and their structural and functional diversity

Molecular ferroelectrics have gradually aroused great interest in both fundamental scientific research and technological applications because of their easy processing, light weight and mechanical flexibility. Hybrid organic-inorganic perovskite ferroelectrics (HOIPFs), as a class of molecule-based ferroelectrics, have diverse functionalities owing to their unique structure and have become a hot spot in molecular ferroelectrics research. Therefore, they are extremely attractive in the field of ferroelectrics. However, there seems to be a lack of systematic review of their design, performance and potential applications. Herein, we review the recent development of HOIPFs from lead-based, lead-free and metal-free perovskites, and outline the versatility of these ferroelectrics, including piezoelectricity for mechanical energy-harvesting and optoelectronic properties for photovoltaics and light detection. Furthermore, a perspective view of the challenges and future directions of HOIPFs is also highlighted.

Two-Step Dielectric Responsive Organic-Inorganic Hybrid Material with Mid-Band Light Emission

Hybrid organic-inorganic perovskite (HOIP) have received tremendous scientific attention because of the phase transition and photovoltaic properties. However, achieving the special perovskite structure with both two-step dielectric response and luminescence characteristics is rarely reported. Herein, we report an organic-inorganic hybrid perovskite, [(BA)₂  ⋅ PbI₄ ] (Compound 1, BA=n-butylamine) by introducing flexible organic cations (HBA⁺ ), with direct mid-band gap as 2.28 eV. Interestingly, this material exhibits two-step reversible dielectric response at 350 K and 460 K (in heating process), respectively. Besides, the photoluminescence was found: it emits charming green light under 365 nm lamp (Photoluminescence quantum yield is 9.52 %). The outstanding two-step dielectric response and luminescence characteristics of this compound might pave the way for the application of dielectric and ferroelectric functional materials in temperature sensors and mechanical switches.

Two Manganese(II)-Based Hybrid Multifunctional Phase Transition Materials with Strong Photoluminescence, High Quantum Yield, and Switchable Dielectric Properties: (C6NH16)2MnBr4 and (C7NH18)2MnBr4

Multifunctional materials have always been an attractive research area, but how to combine multiple excellent properties in one compound remains a considerable challenge. Organic-inorganic hybrid compounds are widely used in the design of such materials due to their rich properties and flexible assembly. Herein, two new manganese(II)-based organic-inorganic hybrid compounds, (C₆NH₁₆)₂MnBr₄ (1) and (C₇NH₁₈)₂MnBr₄ (2), are prepared by the solution method. Compounds 1 and 2 both emit extremely strong green light under UV excitation, with high quantum yields of 45.93 and 50.98%, respectively. In addition, reversible solid-state phase transitions and obvious switchable dielectric properties are shown at 378/366 and 361/352 K, respectively. The coexistence of the dual stimulus-response characteristics of temperature and light in compounds 1 and 2 opens a new path for exploring more multifunctional phase transition materials.

Physical Fields Manipulation for High-Performance Perovskite Photovoltaics

With the efforts of researchers from all over the world, metal halide perovskite solar cells (PSCs) have been booming rapidly in recent years. Generally, perovskite films are sensitive to surrounding conditions and will be changed under the action of physical fields, resulting in lattice distortion, degradation, ion migration, and so on. In this review, the progress of physical fields manipulation in PSCs, including the electric field, magnetic field, light field, stress field, and thermal field are reviewed. On this basis, the influences of these fields on PSCs are summarized and prospected. Finally, challenges and prospective research directions on how to make better use of external-fields while minimizing the unnecessary and disruptive impacts on commercial PSCs with high-efficiency and steady output are proposed.

The construction of a two-dimensional organic–inorganic hybrid double perovskite ferroelastic with a high Tc and narrow band gap

Two-dimensional (2D) hybrid double perovskites have attracted extensive research interest for their fascinating physical properties, such as ferroelectricity, X-ray detection, light response and so on. In addition, ferroelastics, as an important branch of ferroic materials, exhibits wide prospects in mechanical switches, shape memory and templating electronic nanostructures. Here, we designed a 2D phase-transition double perovskite ferroelastic through a structurally progressive strategy. This evolution is core to our construction process from 0D to 1D and AgBi-based 2D. In this way, we successfully synthesized 2D lead-free ferroelastic (DPA)₄AgBiBr₈ (DPA = 2,2-dimethylpropan-1-aminium) with a high Curie temperature (T_c), which shows a narrower band gap than 0D (DPA)₄Bi₂Br₁₀ and 1D (DPA)₅Pb₂Br₉. Moreover, the mechanism of structural phase transition and molecular motion are fully characterized by temperature dependent solid-state NMR and single crystal XRD. (DPA)₄AgBiBr₈ injects power into the discovery of new ferroelastics or the construction and dimensional adjustment in new hybrid double perovskites.

By using a lead-free AgBi-based scheme, we successfully synthesized a two-dimensional double perovskite ferroelastic (DPA)₄AgBiBr₈ with high T_c of 375 K and a narrow band gap of 2.44 eV, where DPA is 2,2-dimethylpropan-1-aminium.

Centimeter-Sized Single Crystal of a One-Dimensional Lead-Free Mixed-Cation Perovskite Ferroelectric for Highly Polarization Sensitive Photodetection

Linear dichroic anisotropic photonic materials are highly attractive due to their great potentials in many applications, which in combination with the ferroelectric properties could broaden their research and applications. However, to date, the linear dichroism conversion phenomenon has not been observed in one-dimensional (1D) large-size single-crystal materials: in particular, lead-free perovskite ferroelectric crystals. Here, we propose a new ferroelectric design strategy: namely, partial organic cation substitution for precisely designing 1D polarization-sensitive perovskite ferroelectrics. As an example, the 1D mixed-cation perovskite ferroelectric (n-propylammonium)(methylammonium)SbBr₅ was synthesized, which exhibits a fascinating ferroelectricity with a notable reversible polarization of 2.9 μC/cm² and a large ferroelectricity-driven polarization ratio of 6.9. Importantly, the single-crystalline photodetectors also exhibit superior optoelectronic anisotropic performances at the paraelectric phase, having a large photoelectric anisotropy ratio (∼35), an excellent polarization-sensitive dichroism ratio (∼1.31), highly sensitive detectivity up to ∼10⁹ Jones, and a fast response rate (∼45/68 μs). This finding provides a significant and effective pathway for the targeted design of new functional lead-free linear dichroic anisotropic photonic ferroelectrics.

High-Curie Temperature Multilayered Hybrid Double Perovskite Photoferroelectrics Induced by Aromatic Cation Alloying

Due to the breakthrough development of layered hybrid perovskites, the multilayered hybrid double perovskites have emerged as outstanding semiconducting materials owing to their environmental friendliness and superior stability. Despite recent booming advances, the realization of above-room temperature ferroelectricity in this fascinating family remains a huge challenge. Herein, when the molecular design strategy of aromatic cation alloying is applied, an above-room temperature "green" bilayered hybrid double perovskite photoferroelectric, (C₆H₅CH₂NH₃)₂CsAgBiBr₇ (BCAB), is successfully developed with a notable saturation polarization of 10.5 μC·cm^-2 and high-Curie temperature (T_c ∼ 483 K). Strikingly, such a T_c achieves a new record in multilayered hybrid perovskite ferroelectrics, which extends the ferroelectric working temperature to a high level. Further computational investigation reveals that the high-T_c originated from the high phase-transition energy barrier switched by the rotation of the aromatic cation in the confined environment of the inorganic layers. In addition, benefiting from the attractive polarization and remarkable photoelectric properties, a bulk photovoltaic effect (BPVE) with a prominent zero-bias photocurrent (2.5 μA·cm^-2) is achieved. As far as we know, such a high-T_c multilayered hybrid double perovskite ferroelectric is unprecedented, which sheds light on the rational design of an environmental photoferroelectric for high performance photoelectric devices.

Structure and Optical Properties of Hybrid-Layered-Double Perovskites (C8H20N2)2AgMBr8 (M = In, Sb, and Bi)

The Pb-free hybrid-layered-double perovskites (HLDPs) have been proposed as green and stable semiconducting materials for optoelectronic devices, but the synthesized members are still limited. Here, we report the synthesis of three new HLDPs, (C₈H₂₀N₂)₂AgMBr₈ (M = In, Sb, and Bi), by a solution method using 1,4-bis(ammoniomethyl)cyclohexane (C₈H₂₀N₂²⁺) as the organic spacing cation. All three of these HLDPs show ⟨100⟩-oriented layered structures with Ag and In/Sb/Bi order arranged in corner-sharing octahedral layers. The first-principle calculations indicate the indirect-gap nature of (C₈H₂₀N₂)₂AgInBr₈ and (C₈H₂₀N₂)₂AgSbBr₈, while their Bi counterpart shows a direct band gap after considering spin-orbit coupling. The band gaps obtained by diffuse-reflectance spectroscopy are 3.11, 2.60, and 2.70 eV for M = In, Sb, and Bi, respectively. (C₈H₂₀N₂)₂AgInBr₈ shows a broadband red emission centered at 690 nm, and it is mainly attributed to the self-trapped-excitons mechanism. Our results not only provide a series of new "Pb-free" HLDPs with chemical diversity but also help us to further understand the structure-property relationships of HLDP materials.

High-Temperature and Large-Polarization Ferroelectric with Second Harmonic Generation Response in a Novel Crown Ether Clathrate

Molecular ferroelectrics of high-temperature reversible phase transitions are very rare and have attracted increasing attention in recent years. In this paper is described the successful synthesis of a novel high-temperature host-guest inclusion ferroelectric: [(C₆ H₅ NF₃ )(18-crown-6)][BF₄ ] (1) that shows a pair of reversible peaks at 348 K (heating) and 331 K (cooling) with a heat hysteresis about 17 K by differential scanning calorimetry measurements, thus indicating that 1 undergoes a reversible structural phase transition. Variable-temperature PXRD and temperature-dependent dielectric measurements further prove the phase-transition behavior of 1. The second harmonic response demonstrates that 1 belongs to a non-centrosymmetric space group at room temperature and is a good nonlinear optical material. In its semiconducting properties, 1 shows a wide optical band gap of about 4.43 eV that comes chiefly from the C, H and O atoms of the crystals. In particular, the ferroelectric measurements of 1 exhibit a typical polarization-electric hysteresis loop with a large spontaneous polarization (P_s ) of about 4.06 μC/cm² . This finding offers an alternative pathway for designing new ferroelectric-dielectric and nonlinear optical materials and related physical properties in organic-inorganic and other hybrid crystals.

All-Inorganic Lead-Free Perovskite(-Like) Single Crystals: Synthesis, Properties, and Applications

Due to their nontoxicity, stability, and unique optoelectronic properties, all-inorganic lead-free halide semiconductors with perovskite and perovskite-like structures have successfully emerged as promising optoelectronic materials for various applications, such as solar cells, light-emitting diodes (LEDs), photodetectors, and X-ray detectors. To further explore their practical potentials, researchers have paid more attention in all-inorganic lead-free perovskite (-like) (ILFP) single crystals. For these single crystals, the advantages of large sizes, uniform surface morphology, and few defects can facilitate their excellent performances and practical applications. Besides, compared with the low dimensional and polycrystalline ILFP materials, the ILFP single crystals feature enhanced performances, including a longer carrier diffusion length and a larger light absorption coefficient, which attract a great deal of attention. Therefore, focus is on the researching progress of ILFP single crystals and the development of their preparation methods, as well as the novel properties of ILFP single crystals. In addition, the reported applications of ILFP single crystals are proposed to highlight their practical importance. With the perspective of the evolution and challenges, the current limitations of the materials and devices are discussed, followed by an inspirational outlook on their future development directions.

Phase Transitions in Low-Dimensional Layered Double Perovskites: The Role of the Organic Moieties

Halide double perovskites are an interesting alternative to Pb-containing counterparts as active materials in optoelectronic devices. Low-dimensional double perovskites are fabricated by introducing large organic cations, resulting in organic/inorganic architectures with one or more inorganic octahedra layers separated by organic cations. Here, we synthesized layered double perovskites based on 3D Cs2AgBiBr6, consisting of double (2L) or single (1L) inorganic octahedra layers, using ammonium cations of different sizes and chemical structures. Temperature-dependent Raman spectroscopy revealed phase transition signatures in both inorganic lattice and organic moieties by detecting variations in their vibrational modes. Changes in the conformational arrangement of the organic cations to an ordered state coincided with a phase transition in the 1L systems with the shortest ammonium moieties. Significant changes of photoluminescence intensity observed around the transition temperature suggest that optical properties may be affected by the octahedral tilts emerging at the phase transition.

Chiral Switchable Low-Dimensional Perovskite Ferroelectrics

Low-dimensional hybrid organic-inorganic perovskites (HOIPs) possess more localized electronic states and narrower conduction and valence bands to promote self-trapping of excitons and stronger exciton emission; therefore, they are widely used as building blocks for various applications in the fields of optoelectronics, photovoltaics, light-emitting diodes, luminescence, fluorescence, and so forth. Despite the past decades of intensive study, the discovered low-dimensional chiral HOIPs are rare as of the 1D chiral HOIP single crystals reported in 2003, as well as the low-dimensional chiral HOIP ferroelectrics are particularly scarce since the first chiral two-dimensional (2D) and/or one-dimensional (1D) HOIP ferroelectrics reported. Herein, two new low-dimensional HOIPs with the same conformational formula [R-MPA]₂CdCl₄ (R-MPA⁺ = (R)-(-)-1-methyl-3-phenylpropylamine) were successfully synthetized by means of regulating the stoichiometric proportion of R-MPA and CdCl₂ in two ways of 1:1 (1) and 2:1 (2). By combining single-crystal X-ray diffraction, circular dichroism (CD) spectroscopy, differential scanning calorimetry, temperature-dependent dielectric constant, temperature-dependent second-harmonic generation (SHG) effect, polarization-dependent SHG response, and P-E hysteresis loop, we reveal that 1 is a 1D nonchiral molecular ferroelectric and 2 is the first zero-dimensional (0D) chiral ferroelectric with distinct CD signals; meanwhile, 2 exhibits increased properties of high-T_c, large dielectric constant, SHG isotropy, and ferroelectricity than that of 1. These results not only shed light on the high tunability of the low-dimensional HOIP ferroelectrics but also open up an avenue to explore multifunctional chiral ferroelectrics.

A lead-free layered Dion–Jacobson hybrid double perovskite constructed by an aromatic diammonium cation

A two-dimensional Dion–Jacobson type lead-free hybrid double perovskite [(3AMPY)₂AgBiI₈·H₂O] (3AMPY = 3-(aminomethyl) pyridinium) constructed by an aromatic diammonium cation exhibits an attractive narrow band gap (E_g = 1.86 eV).

An unprecedented hexagonal double perovskite organic-inorganic hybrid ferroelastic material: (piperidinium)2[KBiCl6]

An unprecedented A2MIMIIIX6-type double perovskite adopting a fully hexagonal BaNiO3-type structure, (piperidinium)2[KBiCl6], undergoes a 2/mF1[combining macron] ferroelastic phase transition at 285 K with a spontaneous strain of 0.0615, arising from the order-disorder transition of organic cations together with the synchronous displacement of inorganic chains.

Effects of ITO Substrate Hydrophobicity on Crystallization and Properties of MAPbBr3 Single-Crystal Thin Films

Fabricating perovskite single-crystal thin films (SCTFs) in controllable manner is the major challenge for the promising potential applications in optoelectronic devices. Although modifying the substrate surface is frequently used to realize the controlled growth of perovskite SCTFs, it is still unclear how the substrate condition affects the crystallization process. In this work, we systemically investigated the effects of the surface hydrophobicity of indium tin oxide substrates on the crystallization process of MAPbBr3 SCTFs prepared by the space-confined method. Comprehensive characterizations show that the surface morphology and crystallinity of SCTFs are improved, and the defect density is reduced when increasing the substrate hydrophobicity. The best MAPbBr3 thin film obtained has a full width at half-height of the rocking curve of the (001) crystal plane of 0.044°. The mechanism of the substrate hydrophobicity on the crystal growth is also discussed. These results will provide guidance to the controllable growth of high-quality SCTFs for perovskite SCTF devices.

Chiral tetranuclear copper(ii) complexes: synthesis, optical and magnetic properties

The chiral tetranuclear Cu(ii) cubane complexes with the general molecular formula [Cu₄(R-L₁)₄] (R-1) and [Cu₄(S-L₁)₄] (S-1) exhibit ferromagnetic exchange coupling, which is in contrast to the literature reports. This is corroborated by theoretical calculations.