Ferroelectricity in a semiconducting all-inorganic halide perovskite

Switchable Bulk Photovoltaic Effect in Intrinsically Ferroelectric 3D All-Inorganic CsPbBr3 Perovskite Nanocrystals

Ferroelectric all-inorganic halide perovskite nanocrystals with both spontaneous polarization and visible light absorption are promising candidates for designing ferroelectric photovoltaic applications. It remains a challenge to realize ferroelectric photovoltaic devices with all-inorganic halide perovskites that can be operated in the absence of an external electric field. Here we report that a popular all-inorganic halide perovskite nanocrystal, CsPbBr3, exhibits a ferroelectricity-driven photovoltaic effect under visible light in the absence of an external electric field. Pristine CsPbBr3 nanocrystals exhibit intrinsic ferroelectric key properties with a notable saturated polarization of ∼0.15 μC/cm2 and a high Curie temperature of 462 K, driven by the stereochemical activity of the Pb(II) lone pair. Furthermore, application of an external electric field allows the photovoltaic effect to be enhanced and the spontaneous polarization to be switched with the direction of the electric field. CsPbBr3 nanocrystals exhibit a robust fatigue performance and a prolonged photoresponse under continuous illumination in the absence of an external electric field. These findings establish all-inorganic halide perovskite nanocrystals as potential candidates for designing photoferroelectric devices by coupling optical functionalities and ferroelectric responses.

Quantifying Structural Polarization by Continuous Regulation of Lone-Pair Electron Expression in Molecular Crystals

Rational design of structural polarization is vital for modern technologies, as it allows the physical properties of functional materials to be tailored. An effective approach for governing polarization involves the utilization of stereochemical lone-pair electrons (LPEs). However, despite the recognized significance of LPEs in controlling structural polarization, there remains a lack of understanding regarding the quantitative relationship between their expression and the extent of structural polarization. Here, by using pressure to continuously tune the LPE expression, we achieve the precise control and quantification of structural polarization, which brings enhanced second harmonic generation (SHG) of the molecular crystal SbI3·3S8. We introduce the I-Sb-I angle (α̅) that describes the degree of LPE expression and establishes a quantitative relationship between α̅ and structural polarization. That is, decreasing α̅ shapes LPE expression from delocalization to localization, which repels the bonding pairs of electrons and thus enhances the structural polarization. In addition, we extend this quantified relationship to a series of molecular crystals and demonstrate its applicability to the design of structural polarization by tailoring LPE expression.

Ferroelectricity in Ultrathin Halide Perovskites

Ferroelectricity has recently been demonstrated in germanium-based halide perovskites. We use first-principles-based simulations to study 4-18 nm CsGeBr3 films and develop a theory for ferroelectric ultrathin films. The theory introduces (i) a local order parameter, which identifies phase transitions into both monodomain and polydomain phases, and (ii) a dipole pattern classifier, which allows efficient and reliable identification of dipole patterns. Application of the theory to both halides CsGeBr3 and CsGeI3 and oxide BiFeO3 ultrathin ferroelectrics reveals two distinct scenarios. First, the films transition into a monodomain phase below the critical value of the residual depolarizing field. Above this critical value, the second scenario occurs, and the film undergoes a transition into a nanodomain phase. The two scenarios exhibit opposite responses of Curie temperature to thickness reduction. Application of a dipole pattern classifier reveals rich nanodomain phases in halide films: nanostripes, labyrinths, zig-zags, pillars, and lego domains.

Controlling the Orientation-Dependent Second Harmonic Generation in Hybrid Germanium Perovskites

Manipulating the crystal orientation plays a crucial role in the conversion efficiency during second harmonic generation (SHG). Here, we provide a new strategy in controlling the surface-dependent anisotropic SHG with the precise design of (101) and (21 ‾ ${\bar 1}$ 0) MAGeI3 facets. Based on the SHG measurement, the (101) MAGeI3 single crystal exhibits larger SHG (1.3×(21 ‾ ${\bar 1}$ 0) MAGeI3). Kelvin probe force microscopy imaging shows a smaller work function for the (101) MAGeI3 compared with the (21 ‾ ${\bar 1}$ 0), which indirectly demonstrates the stronger intrinsic polarization on the (101) surface. X-ray photoelectron spectroscopy confirms the band bending within the (101) facet. Temperature-dependent steady-state and time-resolved photoluminescence spectroscopy show shorter lifetime and wider emission band in the (101) MAGeI3 single crystal, revealing the higher defect states. Additionally, powder X-ray diffraction patterns show the (101) MAGeI3 possesses larger in-plane polar units [GeI3]- density, which could directly enhance the spontaneous polarization in the (101) facet. Density functional theory (DFT) calculation further demonstrates the higher intrinsic polarization in the (101) facet compared with the (21 ‾ ${\bar 1}$ 0) facet, and the larger built-in electric field in the (101) facet facilitates surface vacancy defect accumulation. Our work provides a new angle in tuning and optimizing hybrid perovskite-based nonlinear optical materials.

Near Room Temperature Solvothermal Growth of Ferroelectric CsPbBr3 Nanoplatelets with Ultralow Dark Current

CsPbBr3 exhibits outstanding optoelectronic properties and thermal stability, making it a coveted material for detectors, light-emitting diodes, and solar cells. Despite observations of ferroelectricity in CsPbBr3 quantum dots, synthesizing bulk ferroelectric CsPbBr3 crystals has remained elusive, hindering its potential in next-generation optoelectronic devices like optical switches and ferroelectric photovoltaics. Here, a breakthrough is reported: a novel solvothermal technique enabling the growth of ferroelectric CsPbBr3 nanoplatelets with lateral dimensions in the tens of micrometers. This represents a significant step toward achieving large-area ferroelectric CsPbBr3 crystals. Unlike traditional methods, this approach allows for growth and crystallization of CsPbBr3 in alcohol solutions by enhancing precursor solubility. This study confirms the ferroelectric nature of these nanoplatelets using second harmonic generation, electrical characterizations, and piezoresponse force microscopy. This work paves the way for utilizing ferroelectric CsPbBr3 in novel optoelectronic devices, significantly expanding the potential of this material and opening doors for further exploration in this exciting field.

On-Wire Design of Axial Periodic Halide Perovskite Superlattices for High-Performance Photodetection

Precise synthesis of all-inorganic lead halide perovskite nanowire heterostructures and superlattices with designable modulation of chemical compositions is essential for tailoring their optoelectronic properties. Nevertheless, controllable synthesis of perovskite nanostructure heterostructures remains challenging and underexplored to date. Here, we report a rational strategy for wafer-scale synthesis of one-dimensional periodic CsPbCl3/CsPbI3 superlattices. We show that the highly parallel array of halide perovskite nanowires can be prepared roughly as horizontally guided growth on an M-plane sapphire. A periodic patterning of the sapphire substrate enables position-selective ion exchange to obtain highly periodic CsPbCl3/CsPbI3 nanowire superlattices. This patterning is further confirmed by micro-photoluminescence investigations, which show that two separate band-edge emission peaks appear at the interface of a CsPbCl3/CsPbI3 heterojunction. Additionally, compared with the pure CsPbCl3 nanowires, photodetectors fabricated using these periodic heterostructure nanowires exhibit superior photoelectric performance, namely, high ION/IOFF ratio (104), higher responsivity (49 A/W), and higher detectivity (1.51 × 1013 Jones). Moreover, a spatially resolved visible image sensor based on periodic nanowire superlattices is demonstrated with good imaging capability, suggesting promising application prospects in future photoelectronic imaging systems. All these results based on the periodic CsPbCl3/CsPbI3 nanowire superlattices provides an attractive material platform for integrated perovskite devices and circuits.

A Lead-Free Ferroelectric 2D Dion-Jacobson Tin Iodide Perovskite

2D hybrid organic-inorganic halide perovskites emerge as a new class of 2D semiconductors with the potential to combine excellent optoelectronic properties with symmetry-enabled properties such as ferroelectricity. Although many lead-based ferroelectric 2D halide perovskites are reported, there is yet to be a conclusive report of ferroelectricity in tin-based 2D perovskites. Here, the structures and properties of a new series of 2D Dion-Jacobson (DJ) Sn perovskites: (4AMP)SnI4, (4AMP)(MA)Sn2I7, and (4AMP)(FA)Sn2I7 (4AMP = 4-(aminomethyl)piperidinium, MA = methylammonium, and FA = formamidinium), are reported. Structural characterization reveals that (4AMP)SnI4 is polar with in-plane spontaneous polarization whereas (4AMP)(MA)Sn2I7 and (4AMP)(FA)Sn2I7 are centrosymmetric. Further, (4AMP)SnI4 displays second harmonic generation (SHG) and polarization-electric field hysteresis measurements confirm it is ferroelectric with a spontaneous polarization of 10.0 µC cm-2 at room temperature. (4AMP)SnI4 transitions into a centrosymmetric structure above 367 K. As the first direct experimental observation of the spontaneous ferroelectric polarization of a Sn-based 2D hybrid perovskite, this work opens up environmentally friendly 2D tin halide perovskites for ferroelectricity and other physical property studies.

Direction Dependent Ferroelectricity and Conductivity in a Single Crystal 2D Halide Double Perovskite

Halide ferroelectric materials have garnered a lot of interest because of their distinctive electrical and structural characteristics. In this study, the design and development of a new non-centrosymmetric 2D layered halide double perovskite material, Cl1.14Br2.86PA4AgInBr8 (CPAIn) is reported. This material shows ferroelectric properties above room temperature, with a Curie temperature of 190 °C. This behavior is achieved through the substitution of the halogenated A-site organic linker, 3-chloropropylammonium. CPAIn exhibits anisotropic ferroelectric behavior with higher spontaneous polarization of 6.25 µC cm-2 along the perpendicular direction to the octahedral layers, whereas the value decreases to 0.174 µC cm-2 between sheets. While using bottom contact to study the nature of polarity within a sheet, the P-E loop displays capacitive loop. The nature and value of polarization is highly direction dependent, and to further understand the mechanism of conduction, a combination of temperature-dependent impedance studies and poling dependent conductivity techniques are employed. These directional dependent properties hold immense potential in memory devices, sensors and photovoltaics, piezoelectric devices and energy storage.

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.

Room-Temperature Ferroelectric Epitaxial Nanowire Arrays with Photoluminescence

The development of large-scale, high-quality ferroelectric semiconductor nanowire arrays with interesting light-emitting properties can address limitations in traditional wide-bandgap ferroelectrics, thus serving as building blocks for innovative device architectures and next-generation high-density optoelectronics. Here, we investigate the optical properties of ferroelectric CsGeX3 (X = Br, I) halide perovskite nanowires that are epitaxially grown on muscovite mica substrates by vapor phase deposition. Detailed structural characterizations reveal an incommensurate heteroepitaxial relationship with the mica substrate. Furthermore, photoluminescence that can be tuned from yellow-green to red emissions by varying the halide composition demonstrates that these nanowire networks can serve as platforms for future optoelectronic applications. In addition, the room-temperature ferroelectricity and ferroelectric domain structures of these nanowires are characterized using second harmonic generation (SHG) polarimetry. The combination of room-temperature ferroelectricity with photoluminescence in these nanowire arrays unlocks new avenues for the design of novel multifunctional materials.

From C4H7N2Ge0.4Sn0.6Br3 to C6H11N2Ge0.4Sn0.6Br3: Effective Modulation of the Second Harmonic Generation Effect and Optical Band Gap by Planar π-Conjugated Organic Cation Size

In the search for nonlinear optical (NLO) materials with excellent overall performance, we have devoted ourselves to organic-inorganic hybrids consisting of anionic groups containing stereochemically active lone-pair (SCALP) electron cations and organic planar π-conjugated group cations. Accordingly, in this paper, two novel organic-inorganic hybrid metal halides, C4H7N2Ge0.4Sn0.6Br3 (I) and C6H11N2Ge0.4Sn0.6Br3 (II), have been synthesized. The powder second-harmonic technique shows that both C4H7N2Ge0.4Sn0.6Br3 and C6H11N2Ge0.4Sn0.6Br3 have moderately strong second-order nonlinear optical effects, which are about 2.03 (I) and 1.16 (II) times that of KH2PO4 (KDP), respectively. They also have different optical band gaps of 2.75 (I) and 2.88 eV (II) due to the different sizes of the organic cations, and their photoluminescent and thermal properties were also investigated. This work provides new structural insights for the design and modulation of organic-inorganic hybrid halide materials with multiple excellent optical properties.

Fast and versatile electrostatic disc microprinting for piezoelectric elements

Nanoparticles, films, and patterns are three critical piezoelectric elements with widespread applications in sensing, actuations, catalysis and energy harvesting. High productivity and large-area fabrication of these functional elements is still a significant challenge, let alone the control of their structures and feature sizes on various substrates. Here, we report a fast and versatile electrostatic disc microprinting, enabled by triggering the instability of liquid-air interface of inks. The printing process allows for fabricating lead zirconate titanate free-standing nanoparticles, films, and micro-patterns. The as-fabricated lead zirconate titanate films exhibit a high piezoelectric strain constant of 560 pm V-1, one to two times higher than the state-of-the-art. The multiplexed tip jetting mode and the large layer-by-layer depositing area can translate into depositing speeds up to 109 μm3 s-1, one order of magnitude faster than current techniques. Printing diversified functional materials, ranging from suspensions of dielectric ceramic and metal nanoparticles, to insulating polymers, to solutions of biological molecules, demonstrates the great potential of the electrostatic disc microprinting in electronics, biotechnology and beyond.

Control of Halogen Atom in Inorganic Metal-Halide Perovskites Enables Large Piezoelectricity for Electromechanical Energy Generation

Regulating the strain of inorganic perovskites has emerged as a critical approach to control their electronic and optical properties. Here, an alternative strategy to further control the piezoelectric properties by substituting the halogen atom (I/Br) in the CsPbX3 perovskite (X = Cl, Br) structure is adopted. A series of piezoelectric materials with excellent piezoelectric coefficients (d33 ) are unveiled. Iodine-incorporated CsPbBr2 I demonstrates the record intrinsic piezoelectric response (d33 ≈47 pC N-1 ) among all inorganic metal halide perovskites. This leads to an excellent electrical output power of ≈ 0.375 mW (24.8 µW cm-2 N-1 ) in the piezoelectric energy generator (PEG) which is higher than those of the pristine/mixed perovskite references with CsPbX3 (X = I, Br, Cl). With its structural phase remaining unchanged, the strained CsPbBr2 I retains its superior piezoelectricity in both thin film and nanocrystal powder forms, further demonstrating its repeatability and versatility of applications. The origin of high piezoelectricity is found to be due to halogen-induced anisotropic lattice strain in the unit-cell along the c-axis, and octahedral distortion. This study reveals an avenue to design new piezoelectric materials by modifying their halide constituents and paves the way to design efficient PEGs for improved electromechanical energy conversion.

Superior ferroelectricity and nonlinear optical response in a hybrid germanium iodide hexagonal perovskite

Abundant chemical diversity and structural tunability make organic-inorganic hybrid perovskites (OIHPs) a rich ore for ferroelectrics. However, compared with their inorganic counterparts such as BaTiO₃, their ferroelectric key properties, including large spontaneous polarization (P_s), low coercive field (E_c), and strong second harmonic generation (SHG) response, have long been great challenges, which hinder their commercial applications. Here, a quasi-one-dimensional OIHP DMAGeI₃ (DMA = Dimethylamine) is reported, with notable ferroelectric attributes at room temperature: a large P_s of 24.14 μC/cm² (on a par with BaTiO₃), a low E_c below 2.2 kV/cm, and the strongest SHG intensity in OIHP family (about 12 times of KH₂PO₄ (KDP)). Revealed by the first-principles calculations, its large P_s originates from the synergistic effects of the stereochemically active 4s² lone pair of Ge²⁺ and the ordering of organic cations, and its low kinetic energy barrier of small DMA cations results in a low E_c. Our work brings the comprehensive ferroelectric performances of OIHPs to a comparable level with commercial inorganic ferroelectric perovskites.

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.

Flexo-Ferroelectricity and a Work Cycle of a Two-Dimensional-Monolayer Actuator

Well recognized mechanical flexibility of two-dimensional (2D) materials is shown to bring about unexpected behaviors to the recently discovered monolayer ferroelectrics, especially those displaying normal, off-plane polarization. A "ferro-flexo" coupling term is introduced into the energy expression, to account for the connection of ferroelectricity and bending (strain gradient) of the layer, to predict and quantify its spontaneous curvature and how it affects the phase transitions. With InP as a chemically specific representative example, the first-principles calculations indeed reveal strong coupling ∼P·ϰ between the ferroelectric polarization (P) and the curvature of the layer (ϰ ≡ 1/r), having profound consequences for both mechanics and ferroelectricity of the material. Due to flexural relaxation, the spontaneous polarization and the transition barrier rise significantly, leading to large changes in the Curie temperature, coercive field, and domain wall width and energy, based on Monte Carlo simulations. On the other hand, the polarization switching, characteristic to ferroelectrics, does induce an overall layer bending, enabling a conversion of electrical signal to movement as an actuator; its possible work-cycles and maximum work-efficiency are briefly discussed.

Emergence of Rashba-/Dresselhaus effects in Ruddlesden-Popper halide perovskites with octahedral rotations

Ruddlesden-Popper halide perovskites are highly versatile quasi-two-dimensional energy materials with a wide range of tunable optoelectronic properties. Here we use the all-inorganic Csn+1Pb_nX3n+1Ruddlesden-Popper perovskites with X = I, Br, and Cl to systematically model the effect of octahedral tilting distortions on the energy landscape, band gaps, macroscopic polarization, and the emergence of Rashba-/Dresselhaus splitting in these materials. We construct all uniquen = 1 andn = 2 structures following from octahedral tilts and use first-principles density functional theory to calculate total energies, polarizations and band structures, backed up by band gap calculations using theGWapproach. Our results provide design rules for tailoring structural distortions and band-structure properties in all-inorganic Ruddlesden-Popper perovskites through the interplay of the amplitude, direction, and chemical character of the antiferrodistortive distortion modes contributing to each octahedral tilt pattern. Our work emphasizes that, in contrast to three-dimensional perovskites, polar structures may arise from a combination of octahedral tilts, and Rashba-/Dresselhaus splitting in this class of materials is determined by the direction and Pb-I orbital contribution of the polar distortion mode.

Organolead Halide-Based Coordination Polymers: Intrinsic Stability and Photophysical Applications

ConspectusOrganolead halide-based photovoltaics are one of the state-of-the-art solar cell systems with efficiencies increasing to 25% over the past decade, ascribed to their high light-absorption coefficient, broad wavelength coverage, tunable band structure, and excellent carrier mobility. Indeed, these optical characteristics are highly demanding in photocatalysis and photoluminescence (PL), which also involve the solar energy utilization and charge transport. However, the vast majority of organolead halides are ionically bonded structures and susceptible to degradation upon high-polarity protic molecules (e.g., water (vapor) and alcohol), which are often inevitable in many photochemical applications. Encapsulation is a commonly used stabilization approach by coating protective layers, avoiding the direct contact between organolead halides and polar molecules. However, this may partially hinder the light penetration to the inner hybrid halide materials, and introduce new interface problems that are important in photocatalysis and luminescent sensing. Therefore, developing intrinsically stable organometal halide hybrids is a major target for their applications in optoelectronic applications.In this Account, recent research progress on the synthesis of organolead halide-based coordination polymers for a variety of photoactive applications is described. Herein, we propose a general strategy to advance the intrinsic stability of organometal halide crystalline materials by using coordinating anionic organic linkers, which occupy the excellent photophysical features analogous to those of perovskites. Unlike the organoammonium cations as for ionically bonded structures, the anionic structure-directing agents (e.g., organocarboxylates) render well-defined metal-carboxylate coordination motifs in extended architectures spanning from low-dimensional (0D, 1D) to high-dimensional cationic inorganic Pb-X-Pb (X = F^-/Cl^-/Br^-/I^-) sublattices. This family of organolead halide coordination polymers can endure chemically reactive environments over a wide range of pH and aqueous boiling condition, which have been systematically investigated by experimental studies and theoretical calculations. Many chloride/bromide-based coordination polymers show air-stable, broadband self-trapped emission with large Stokes shift and high color rendition, exhibiting the absolute quantum yields of 35-72%. Among them, the porous frameworks with low-dimensional (0D, 1D) inorganic blocks are recognized as a rare class of porous metal-organic frameworks (MOFs) constructed by lead halides as secondary building units (SBUs). They not only occupy substantially higher light-harvesting and carrier-transport properties than conventional metal oxide-based MOFs, but also allow for isoreticular modification to regulate the PL characteristics by guest molecules. More importantly, combining the high stability with excellent carrier characteristics, a layered organolead iodide coordination polymer shows the overall photocatalytic water splitting without the use of any sacrificial agent under simulated sunlight illumination. Given the wide choice of structurally diverse organocarboxylate linkers, we hope this Account provides deep insights on the importance of coordination chemistry in the discovery of a wide family of intrinsically stable organolead halides to expand their photophysical applications.

Nanowire photochemical diodes for artificial photosynthesis

Artificial photosynthesis can provide a solution to our current energy needs by converting small molecules such as water or carbon dioxide into useful fuels. This can be accomplished using photochemical diodes, which interface two complementary light absorbers with suitable electrocatalysts. Nanowire semiconductors provide unique advantages in terms of light absorption and catalytic activity, yet great control is required to integrate them for overall fuel production. In this review, we journey across the progress in nanowire photoelectrochemistry (PEC) over the past two decades, revealing design principles to build these nanowire photochemical diodes. To this end, we discuss the latest progress in terms of nanowire photoelectrodes, focusing on the interplay between performance, photovoltage, electronic band structure, and catalysis. Emphasis is placed on the overall system integration and semiconductor-catalyst interface, which applies to inorganic, organic, or biologic catalysts. Last, we highlight further directions that may improve the scope of nanowire PEC systems.

Simple Visualization of Universal Ferroelastic Domain Walls in Lead Halide Perovskites

Domain features and domain walls in lead halide perovskites (LHPs) have attracted broad interest due to their potential impact on optoelectronic properties of this unique class of solution-processable semiconductors. Using nonpolarized light and simple imaging configurations, ferroelastic twin domains and their switchings through multiple consecutive phase transitions are directly visualized. This direct optical contrast originates from finite optical reflections at the wall interface between two compositionally identical, orientationally different, optically anisotropic domains inside the material bulk. The findings show these domain walls serve as internal reflectors and steer energy transport inside halide perovskites optically. First-principles calculations show universal low domain-wall energies and modest energy barriers of domain switching, confirming their prevalent appearance, stable presence, and facile moving observed in the experiments. The generality of ferroelasticity in halide perovskites stems from their soft bonding characteristics. This work shows the feasibility of using LHP twin domain walls as optical guides of internal photoexcitations, capable of nonvolatile on-off switching and tunable positioning endowed by their universal ferroelasticity.

Rational Design of Non-Centrosymmetric Hybrid Halide Perovskites

Structural non-centrosymmetry in semiconducting organic-inorganic hybrid halide perovskites can introduce functionalities like anomalous photovoltaics and nonlinear optical properties. Here we introduce a design principle to prepare Pb- and Bi-based two- and one-dimensional hybrid perovskites with polar non-centrosymmetric space groups. The design principle relies on creating dissimilar hydrogen and halogen bonding non-covalent interactions at the organic-inorganic interface. For example, in organic cations like I-(CH₂)₃-NH₂(CH₃)⁺ (MIPA), -CH₃ is substituted by -CH₂I at one end, and -NH₃⁺ is substituted by -NH₂(CH₃)⁺ at the other end. These substitutions of two -H atoms by -I and -CH₃ reduce the rotational symmetry of MIPA at both ends, compared to an unsubstituted cation, for example, H₃C-(CH₂)₃-NH₃⁺. Consequently, the dissimilar hydrogen-iodine and iodine-iodine interactions at the organic-inorganic interface of (MIPA)₂PbI₄ 2D perovskites break the local inversion symmetries of Pb-I octahedra. Owing to this non-centrosymmetry, (MIPA)₂PbI₄ displays visible to infrared tunable nonlinear optical properties with second and third harmonic generation susceptibility values of 5.73 pm V^-1 and 3.45 × 10^-18 m² V^-2, respectively. Also, the single crystal shows photocurrent on shining visible light at no external bias, exhibiting anomalous photovoltaic effect arising from the structural asymmetry. The design strategy was extended to synthesize four new non-centrosymmetric hybrid perovskite compounds. Among them, one-dimensional (H₃N-(CH₂)₃-NH(CH₃)₂)BiI₅ shows a second harmonic generation susceptibility of 7.3 pm V^-1 and a high anomalous photovoltaic open-circuit voltage of 22.6 V.

Insights into the relationship between ferroelectric and photovoltaic properties in CsGeI3 for solar energy conversion

Materials such as oxide and halide perovskites that simultaneously exhibit spontaneous polarization and absorption of visible light are called photoferroelectrics. They hold great promise for the development of applications in optoelectronics, information storage, and energy conversion. Devices based on ferroelectric photovoltaic materials yield an open-circuit voltage that is much higher than the band gap of the corresponding active material owing to a strong internal electric field. Their efficiency has been proposed to exceed the Shockley-Queisser limit for ideal solar cells. In this paper, we present theoretical calculations of the photovoltaic properties of the ferroelectric phase of the inorganic germanium halide perovskite (CsGeI₃). Firstly, the electronic, optical and ferroelectric properties were calculated using the FP-LAPW method based on density functional theory, and the modern theory of polarization based on the Berry phase approach, respectively. The photovoltaic performance was evaluated using the Spectroscopic Limited Maximum Efficiency (SLME) model based on the results of first-principles calculations, in which the power conversion efficiency and the photocurrent density-voltage (J-V) characteristics were estimated. The calculated results show that the valence band maximum (VBM) of CsGeI₃ is mainly contributed by the I-5p and Ge-4s orbitals, whereas the conduction band is predominantly derived from Ge-4p orbitals. It can be seen that CsGeI₃ exhibits a direct bandgap semiconductor at the symmetric point of Z with a value of 1.53 eV, which is in good agreement with previous experimental results. The ferroelectric properties were therefore investigated. With a switching energy barrier of 19.83 meV per atom, CsGeI₃ has a higher theoretical ferroelectric polarization strength of 15.82 μC cm^-2. The SLME calculation also shows that CsGeI₃ has a high photoelectric conversion efficiency of over 28%. In addition to confirming their established favorable band gap and strong absorption, we demonstrate that CsGeI₃ exhibits a large shift current bulk photovoltaic effect of up to 40 μA V^-2 in the visible region. Thus, this material is a potential ferroelectric photovoltaic absorbed layer with high efficiency.

Stereochemically Active Lone Pairs and Nonlinear Optical Properties of Two-Dimensional Multilayered Tin and Germanium Iodide Perovskites

Two-dimensional (2D) metal halide perovskites are promising tunable semiconductors. Previous studies have focused on Pb-based structures, whereas the multilayered Sn- and Ge-based analogues are largely unexplored, even though they potentially exhibit more diverse structural chemistry and properties associated with the more polarizable ns² lone-pair electrons. Herein, we report the synthesis and structures of 2D tin iodide perovskites (BA)₂(A)Sn₂I₇, where BA = n-butylammonium and A = methylammonium, formamidinium, dimethylammonium, guanidinium, or acetamidinium, and those of 2D germanium iodide perovskites (BA)₂(A)Ge₂I₇, where A = methylammonium or formamidinium. By comparing these structures along with their Pb counterparts, we establish correlations between the effect of group IV-cation's lone-pair stereochemical activity on the perovskite crystal structures and the resulting semiconducting properties such as bandgaps and carrier-phonon interactions and nonlinear optical properties. We find that the strength of carrier-phonon interaction increases with increasing lone-pair activity, leading to a more prominent photoluminescence tail on the low-energy side. Moreover, (BA)₂(A)Ge₂I₇ exhibit strong second harmonic generation with second-order nonlinear coefficients of ∼10 pm V^-1 that are at least 10 times those of Sn counterparts and 100 times those of Pb counterparts. We also report the third-order two-photon absorption coefficients of (BA)₂(A)Sn₂I₇ to be ∼10 cm MW^-1, which are one order of magnitude larger than those of the Pb counterparts and traditional inorganic semiconductors. These results not only highlight the role of lone-pair activity in linking the compositions and physical properties of 2D halide perovskites but also demonstrate 2D tin and germanium iodide perovskites as promising lead-free alternatives for nonlinear optoelectronic devices.