Ferroelectricity-free lead halide perovskites

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.

Pyroelectric-Accelerated Perovskite Photodetector for Picosecond Light Detection and Ranging

Light detection and ranging (LiDAR) is indispensable in applications such as unmanned aerial vehicles, autonomous driving, and biomimetic robots. However, the precision and available distance of LiDAR are constrained by the speed and sensitivity of the photodetector, necessitating the use of expensive and energy-consuming avalanche diodes. To address these challenges, in this study, a pyroelectricity-based acceleration strategy with 2D-(graded 3D) perovskite heterojunction is proposed to achieve a record high speed (27.7 ns with an active area of 9 mm2, and 176 ps with an active area of 0.2 mm2) and high responsivity (0.65 A W-1) at zero bias. This success is attributed to the unique mechanism where the electrons from the pyroelectric effect at the Cl-rich 2D/3D interface directly recombine with excess holes during light-dark transitions, breaking speed limitations related to carrier mobility and capacitive effect. Furthermore, the introduced pyroelectric effect significantly enhances the photoresponse, resulting in a self-powered external quantum efficiency exceeding 100%. The study also demonstrates precise position detection at the centimeter level. In conclusion, this research presents a pioneering approach for developing high-speed photodiodes with exceptional sensitivity, mitigating energy and cost concerns in LiDAR applications.

Phase Transitions and Dynamics in Mixed Three- and Low-Dimensional Lead Halide Perovskites

Lead halide perovskites are extensively investigated as efficient solution-processable materials for photovoltaic applications. The greatest stability and performance of these compounds are achieved by mixing different ions at all three sites of the APbX3 structure. Despite the extensive use of mixed lead halide perovskites in photovoltaic devices, a detailed and systematic understanding of the mixing-induced effects on the structural and dynamic aspects of these materials is still lacking. The goal of this review is to summarize the current state of knowledge on mixing effects on the structural phase transitions, crystal symmetry, cation and lattice dynamics, and phase diagrams of three- and low-dimensional lead halide perovskites. This review analyzes different mixing recipes and ingredients providing a comprehensive picture of mixing effects and their relation to the attractive properties of these materials.

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.

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.

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.

First-Principles Calculations to Investigate the Effect of Van der Waals Interactions on the Crystal and Electronic Structures of Tin-Based 0D Hybrid Perovskites

The electronic structures of four tin-based 0D hybrid perovskites ((NH3(CH2)2C6H5)2[SnCl6], (C6H10N2)[SnCl6], (C9H14N)2[SnCl6], and (C8H12N)2[SnCl6]) were determined by the DFT method employing the pseudopotential plane wave as implemented in the CASTEP code, and the first transition in each compound has been investigated based on the partial density states and dielectric function. According to the structural properties, incorporating organic cations with the appropriate structure, shape, and strong H-bonding functionality into hybrid perovskite crystals is very beneficial for preventing ion migration and thus enhances the efficiency of hybrid perovskite-based devices. Based on those properties employing the DFT+D method for the dispersion force, the effect of Van der Waals interaction on electronic structure was explained based on the nature of the first electronic transition. The similarity between the experimental and optimized structure was investigated by using a Bilbao crystallographic server. The study of optical properties shows that the Van der Waals interactions have a slight effect on the energy level of the curves. However, the profiles of curves are conserved. The absorption curves of the researched compounds are elaborated.

Comprehensive study of anomalous hysteresis behavior in perovskite-based solar cells

Perovskite solar cells (PSCs) have shown remarkable progress with the rapid increase in power conversion efficiency to reach 25.7% over the last few years. However, it is difficult to precisely determine the energy conversion efficiency for PSC, because of anomalous current density-voltage (J–V) hysteresis. Normal J–V hysteresis has been reported in many papers, where the backward scan performance is higher than the forward scan one. In this work, using Drift–Diffusion Modeling, normal hysteretic behavior associated with ion migration with different scanning rates, pre-bias voltages, and charge-carrier mobility is studied. In addition, the inverted J–V hysteresis by modification of the simulation model, where anions and cations flux towards the transport layers and are accumulated simultaneously on both sides, is achieved. It is also found that the flux parameter values (g_ae and g_ch) play a critical role in the reduction of inverted hysteresis and the efficiency enhancement. It is suggested from the current studies that perovskite interfaces encapsulation, which prevents ions migration, could be of great importance for achieving hysteresis-free PSCs and reliable device characteristics.

The Ferroelectric-Ferroelastic Debate about Metal Halide Perovskites

Metal halide perovskites (MHPs) are solution-processed materials with exceptional photoconversion efficiencies that have brought a paradigm shift in photovoltaics. The nature of the peculiar optoelectronic properties underlying such astounding performance is still controversial. The existence of ferroelectricity in MHPs and its alleged impact on photovoltaic activity have fueled an intense debate, in which unanimous consensus is still far from being reached. Here we critically review recent experimental and theoretical results with a two-fold objective: we argue that the occurrence of ferroelectric domains is incompatible with the A-site cation dynamics in MHPs and propose an alternative interpretation of the experiments based on the concept of ferroelasticity. We further underline that ferroic behavior in MHPs would not be relevant at room temperature or higher for the physics of photogenerated charge carriers, since it would be overshadowed by competing effects like polaron formation and ion migration.

Omni-functional crystal: Advanced methods to characterize the composition and homogeneity of lithium niobate melts and crystals

Lithium niobate (LN) is a type of multifunctional dielectric and ferroelectric crystal that is widely used in acoustic, optical, and optoelectronic devices. The performance of pure and doped LN strongly depends on various factors, including its composition, microstructure, defects, domain, and homogeneity. The structure and composition homogeneity can affect both the chemical and physical properties of LN crystals, including their density, Curie temperature, refractive index, and piezoelectric and mechanical properties. In terms of practical demands, both the composition and microstructure characterizations these crystals must range from the nanometer scale up to the millimeter and wafer scales. Therefore, LN crystals require different characterization technologies when verifying their quality for various device applications. Optical, electrical, and acoustic technologies have been developed, including x-ray diffraction, Raman spectroscopy, electron microscopy, and interferometry. To obtain detailed structural information, advanced sub-nanometer technologies are required. For general industrial demands, fast and non-destructive technologies are preferable. This review outlines the advanced methods used to characterize both the composition and homogeneity of LN melts and crystals from the micro- to wafer scale.

Organic–Inorganic Hybrid Perovskite Materials for Ultrasonic Transducer in Medical Diagnosis

The ultrasonic transducer is considered the most important component of ultrasound medical instruments, and its key active layer is generally fabricated by piezoelectric materials, such as BaTiO3, Pb (Zn, Ti)O3, PVDF, etc. As the star material, perovskite photovoltaic materials (organic and inorganic halide perovskite materials, such as CH3NH3PbI3, CsPbI3, etc.) have great potential to be widely used in solar cells, LEDs, detectors, and photoelectric and piezoelectric detectors due to their outstanding photoelectric and piezoelectric effects. Herein, we firstly discussed the research progress of commonly used piezoelectric materials and the corresponding piezoelectric effects, the current key scientific status, as well as the current application status in the field of ultrasound medicine. Then, we further explored the current progress of perovskite materials used in piezoelectric-effect devices, and their research difficulties. Finally, we designed an ideal ultrasonic transducer fabricated by perovskite photovoltaic materials and considered the future application prospects of organic and inorganic halide perovskite material in the field of ultrasound.

Ferroelasticity in Organic-Inorganic Hybrid Perovskites

Molecular ferroelastics have received particular attention for potential applications in mechanical switches, shape memory, energy conversion, information processing, and solar cells, by taking advantages of their low-cost, light-weight, easy preparation, and mechanical flexibility. The unique structures of organic-inorganic hybrid perovskites have been considered to be a design platform for symmetry-breaking-associated order-disorder in lattice, thereby possessing great potential for ferroelastic phase transition. Herein, we review the research progress of organic-inorganic hybrid perovskite ferroelastics in recent years, focusing on the crystal structures, dimensions, phase transitions and ferroelastic properties. In view of the few reports on molecular-based hybrid ferroelastics, we look forward to the structural design strategies of molecular ferroelastic materials, as well as the opportunities and challenges faced by molecular-based hybrid ferroelastic materials in the future. This review will have positive guiding significance for the synthesis and future exploration of organic-inorganic hybrid molecular ferroelastics.

Stabilization of Metastable Halide Perovskite Lattices in the 2D Limit

Metal halide perovskites constitute a new class of semiconductors that are structurally tailorable, exhibiting rich structural polymorphs. In this perspective, the polymorphism in lead halide perovskites is described-a material system currently used for high-performance photovoltaics and optoelectronics. Strategies for stabilizing the metastable perovskite polymorphs based on crystal size reduction and surface functionalization are critically reviewed. Focus is on an unprecedented stabilization of metastable perovskite lattices in the 2D limit (e.g., with a thickness down to a few unit cells) due to the dominance of surface effects. This stabilization allows the incorporation of various A-cations that deemed oversized for 3D perovskites into the 2D perovskite lattices, which bring new insights on the relationships between the crystal structures and optoelectronic properties and lead to emergent ferroelectricity in halide perovskites. A comprehensive understanding is provided on how the A-cations influence the structural, optoelectronic, and ferroelectric properties, with an emphasis on the second order Jahn-Teller distortion caused by the oversized A-cations. Finally, future perspectives on new structure exploration and studies of fundamental photophysical properties using stabilized perovskite lattices are provided.

Ferroelectricity in a semiconducting all-inorganic halide perovskite

Ferroelectric semiconductors are rare materials with both spontaneous polarizations and visible light absorptions that are promising for designing functional photoferroelectrics, such as optical switches and ferroelectric photovoltaics. The emerging halide perovskites with remarkable semiconducting properties also have the potential of being ferroelectric, yet the evidence of robust ferroelectricity in the typical three-dimensional hybrid halide perovskites has been elusive. Here, we report on the investigation of ferroelectricity in all-inorganic halide perovskites, CsGeX3, with bandgaps of 1.6 to 3.3 eV. Their ferroelectricity originates from the lone pair stereochemical activity in Ge (II) that promotes the ion displacement. This gives rise to their spontaneous polarizations of ~10 to 20 μC/cm2, evidenced by both ab initio calculations and key experiments including atomic-level ionic displacement vector mapping and ferroelectric hysteresis loop measurement. Furthermore, characteristic ferroelectric domain patterns on the well-defined CsGeBr3 nanoplates are imaged with both piezo-response force microscopy and nonlinear optical microscopic method.

The “ferros” of MAPbI3: ferroelectricity, ferroelasticity and its crystallographic foundations in hybrid halide perovskites

Hybrid halide perovskites have been identified as an important novel class of photovoltaic absorbers and have proven their great potential with ever record-breaking efficiencies. Some of the more fundamental properties of halide perovskites, however, still are to be properly understood. The ongoing debate as to whether ferroelectricity and/or ferroelasticity play a role in these materials is just one example for this. Herein, I aim to make these phenomena more approachable to the wider research community and elucidate the foundations and consequences of these phenomena.

A Comparison of Charge Carrier Dynamics in Organic and Perovskite Solar Cells

The charge carrier dynamics in organic solar cells and organic-inorganic hybrid metal halide perovskite solar cells, two leading technologies in thin-film photovoltaics, are compared. The similarities and differences in charge generation, charge separation, charge transport, charge collection, and charge recombination in these two technologies are discussed, linking these back to the intrinsic material properties of organic and perovskite semiconductors, and how these factors impact on photovoltaic device performance is elucidated. In particular, the impact of exciton binding energy, charge transfer states, bimolecular recombination, charge carrier transport, sub-bandgap tail states, and surface recombination is evaluated, and the lessons learned from transient optical and optoelectronic measurements are discussed. This perspective thus highlights the key factors limiting device performance and rationalizes similarities and differences in design requirements between organic and perovskite solar cells.

Stereochemical expression of ns2 electron pairs in metal halide perovskites

Metal halide perovskites (MHPs) are characterized as strongly anharmonic and dynamic lattices. While there is a consensus on the solvation-like polarization effect in these materials, whether static polarization, that is, ferroelectricity, exists or not in 3D MHPs remains controversial. In this Review, we resolve this controversy by analysing the stereochemical expression (SE) of the ns² electron pair (NSEP) on group IV metal cations. The SE-NSEP is key to lattice instability, which governs the breaking of inversion symmetry and induces ferroelectricity. The SE-NSEP is diminishingly small in commonly studied 3D lead iodide or bromide perovskites, indicating an absence of ferroelectricity. In contrast, 2D MHPs promote the SE-NSEP and produce unambiguous ferroelectricity or antiferroelectricity. Irrespective of ferroelectricity, the dynamic manifestation of the SE-NSEP provides the missing link to understanding polar fluctuations and efficient dielectric screening in MHPs, thus, contributing to the long carrier lifetimes and diffusion lengths.

DC bias electric field effects on ac electrical conductivity of MAPbI3suggesting intrinsic changes on structure and charge carrier dynamics

Methylammonium lead iodide (MAPbI₃) emerges as a promising halide perovskite material for the next generation of solar cells due to its high efficiency and flexibility in material growth. Despite intensive studies of their optical and electronic properties in the past ten years, there are no reports on dc bias electric field effects on conductivity in a wide temperature range. In this work, we report the combined effects of frequency, temperature, and dc bias electric field on the ac conductivity of MAPbI₃. We found that the results of dc bias electric fields are very contrasting in the tetragonal and cubic phases. In the tetragonal phase, sufficiently high dc bias electric fields induce a conductivity peak appearance ∼290 K well evidenced at frequencies higher than 100 kHz. Excluding possible degradation and extrinsic factors, we propose that this peak suggests a ferroelectric-like transition. In the absence of a dc bias electric field, the ac conductivity in the tetragonal phase increases with temperature while decreases with temperature in the cubic phase. Also, ac activation energies for tetragonal and cubic phases were found to be inversely and directly proportional to the dc bias electric field, respectively. This behavior was attributed to the ionic conduction, possibly of MA⁺and I^-ions, for the tetragonal phase. As for the cubic phase, the ac conduction dynamics appear to be metallic-like, which seems to change to a polaronic-controlled charge transport to increased dc bias electric fields.

Ferroelastic Domains Enhanced the Photoelectric Response in a CsPbBr3 Single-Crystal Film Detector

The ferroic domain, in metal halide perovskites (MHPs) at a low symmetry phase, was reported to affect optoelectronic properties. Building the relationship between ferroic domains and optoelectronic properties of MHPs will be a non-trivial task for understanding the charge transport mechanism. Here, high-quality CsPbBr₃ single-crystal films (SCFs) were successfully grown by a cast-capping method. Through the phase transition process by heating and cooling the sample, dense domains in CsPbBr₃ SCFs were formed and observed by an in situ polarized optical microscope. These domains were identified as 90° rotation twins by electron backscattered diffraction and transmission electron microscopy. Interestingly, the photocurrent response was dramatically enhanced after introducing ferroelastic domains. The highest responsivity, external quantum efficiency, and detectivity are 380 mA/W, 130%, and 12.9 × 10¹⁰ Jones, respectively, which are surprisingly 25.03, 25, and 7.8 times higher than those of the as-grown CsPbBr₃ SCF, respectively, which may be attributed to the function of the domain wall of separating electrons and holes.

Polar or nonpolar? That is not the question for perovskite solar cells

Perovskite solar cells (PSC) are promising next generation photovoltaic technologies, and there is considerable interest in the role of possible polarization of organic-inorganic halide perovskites (OIHPs) in photovoltaic conversion. The polarity of OIHPs is still hotly debated, however. In this review, we examine recent literature on the polarity of OIHPs from both theoretical and experimental points of view, and argue that they can be both polar and nonpolar, depending on composition, processing and environment. Implications of OIHP polarity to photovoltaic conversion are also discussed, and new insights gained through research efforts. In the future, integration of a local scanning probe with global macroscopic measurements in situ will provide invaluable microscopic insight into the intriguing macroscopic phenomena, while synchrotron diffractions and scanning transmission electron microscopy on more stable samples may ultimately settle the debate.

Crystal engineering and ferroelectricity at the nanoscale in epitaxial 1D manganese oxide on silicon

Ferroelectric oxides have attracted much attention due to their wide range of applications, particularly in electronic devices such as nonvolatile memories and tunnel junctions. As a result, the monolithic integration of these materials into silicon technology and their nanostructuration to develop alternative cost-effective processes are among the central points in the current technology. In this work, we used a chemical route to obtain nanowire thin films of a novel Sr1+δMn8O16 (SMO) hollandite-type manganese oxide on silicon. Scanning transmission electron microscopy combined with crystallographic computing reveals a crystal structure comprising hollandite and pyrolusite units sharing the edges of their MnO6 octahedra, resulting in three types of tunnels arranged along the c axis, where the ordering of the Sr atoms produces natural symmetry breaking. The novel structure gives rise to ferroelectricity and piezoelectricity, as revealed by local direct piezoelectric force microscopy measurements, which confirmed the ferroelectric nature of the SMO nanowire thin films at room temperature and showed a piezoelectric coefficient d33 value of 22 ± 6 pC N-1. Moreover, we proved that flexible vertical SMO nanowires can be harvested providing an electrical output energy through the piezoelectric effect, showing excellent deformability and high interface recombination. This work indicates the possibility of engineering the integration of 1D manganese oxides on silicon, a step which precedes the production of microelectronic devices.

Correlating Crystallographic Orientation and Ferroic Properties of Twin Domains in Metal Halide Perovskites

Metal halide perovskite (MHP) solar cells have attracted worldwide research interest. Although it has been well established that grain, grain boundary, and grain facet affect MHPs optoelectronic properties, less is known about subgrain structures. Recently, MHP twin stripes, a subgrain feature, have stimulated extensive discussion due to the potential for both beneficial and detrimental effects of ferroelectricity on optoelectronic properties. Connecting the ferroic behavior of twin stripes in MHPs with crystal orientation will be a vital step to understand the ferroic nature and the effects of twin stripes. In this work, we studied the crystallographic orientation and ferroic properties of CH₃NH₃PbI₃ twin stripes, using electron backscatter diffraction (EBSD) and advanced piezoresponse force microscopy (PFM), respectively. Using EBSD, we discovered that the orientation relationship across the twin walls in CH₃NH₃PbI₃ is a 90° rotation about ⟨1̅1̅0⟩, with the ⟨030⟩ and ⟨111⟩ directions parallel to the direction normal to the surface. By careful inspection of CH₃NH₃PbI₃ PFM results including in-plane and out-of-plane PFM measurements, we demonstrate some nonferroelectric contributions to the PFM responses of this CH₃NH₃PbI₃ sample, suggesting that the PFM signal in this CH₃NH₃PbI₃ sample is affected by nonferroelectric and nonpiezoelectric forces. If there is piezoelectric response, it is below the detection sensitivity of our interferometric displacement sensor PFM (<0.615 pm/V). Overall, this work offers an integrated picture describing the crystallographic orientations and the origin of PFM signal of MHPs twin stripes, which is critical to understanding the ferroicity in MHPs.

Perovskite random lasers: a tunable coherent light source for emerging applications

Metal halide perovskites have attracted increasing attention due to their superior optical and electrical characteristics, flexible tunability, and easy fabrication processes. Apart from their unprecedented successes in photovoltaic devices, lasing action is the latest exploitation of the optoelectronic performance of perovskites. Among the substantial body of research on the configuration design and light emission quality of perovskite lasers, the random laser is a very interesting stimulated emission phenomenon with unique optical characteristics. In this review article, we first comprehensively overview the development of perovskite-based optoelectronic devices and then focus our discussion on random lasing performance. After an introduction to the historical development of versatile random lasers and perovskite random lasers, we summarize several synthesis methods and discuss their material configurations and stability in synthesized perovskite materials. Following this, a theoretical approach is provided to explain the random lasing mechanism in metal halide perovskites. Finally, we propose future applications of perovskite random lasers, presenting conclusions as well as future challenges, such as quality stability and toxicity reduction, of perovskite materials with regard to practical applications in this promising field.

Nanoscale Ferroelectric Characterization with Heterodyne Megasonic Piezoresponse Force Microscopy

Piezoresponse force microscopy (PFM), as a powerful nanoscale characterization technique, has been extensively utilized to elucidate diverse underlying physics of ferroelectricity. However, intensive studies of conventional PFM have revealed a growing number of concerns and limitations which are largely challenging its validity and applications. In this study, an advanced PFM technique is reported, namely heterodyne megasonic piezoresponse force microscopy (HM-PFM), which uses 106 to 108 Hz high-frequency excitation and heterodyne method to measure the piezoelectric strain at nanoscale. It is found that HM-PFM can unambiguously provide standard ferroelectric domain and hysteresis loop measurements, and an effective domain characterization with excitation frequency up to ≈110 MHz is demonstrated. Most importantly, owing to the high-frequency and heterodyne scheme, the contributions from both electrostatic force and electrochemical strain can be significantly minimized in HM-PFM. Furthermore, a special measurement of difference-frequency piezoresponse frequency spectrum (DFPFS) is developed on HM-PFM and a distinct DFPFS characteristic is observed on the materials with piezoelectricity. By performing DFPFS measurement, a truly existed but very weak electromechanical coupling in CH3NH3PbI3 perovskite is revealed. It is believed that HM-PFM can be an excellent candidate for the ferroelectric or piezoelectric studies where conventional PFM results are highly controversial.

Layer number dependent ferroelasticity in 2D Ruddlesden-Popper organic-inorganic hybrid perovskites

Ferroelasticity represents material domains possessing spontaneous strain that can be switched by external stress. Three-dimensional perovskites like methylammonium lead iodide are determined to be ferroelastic. Layered perovskites have been applied in optoelectronic devices with outstanding performance. However, the understanding of lattice strain and ferroelasticity in layered perovskites is still lacking. Here, using the in-situ observation of switching domains in layered perovskite single crystals under external strain, we discover the evidence of ferroelasticity in layered perovskites with layer number more than one, while the perovskites with single octahedra layer do not show ferroelasticity. Density functional theory calculation shows that ferroelasticity in layered perovskites originates from the distortion of inorganic octahedra resulting from the rotation of aspherical methylammonium cations. The absence of methylammonium cations in single layer perovskite accounts for the lack of ferroelasticity. These ferroelastic domains do not induce non-radiative recombination or reduce the photoluminescence quantum yield.

First-principles identification of the charge-shifting mechanism and ferroelectricity in hybrid halide perovskites

Hybrid halide perovskite solar cells have recently attracted substantial attention, mainly because of their high power conversion efficiency. Among diverse variants, (CH3NH3)PbI3 and HC(NH2)2PbI3 are particularly promising candidates because their bandgap well matches the energy range of visible light. Here, we demonstrate that the large nonlinear photocurrent in β-(CH3NH3)PbI3 and α-HC(NH2)2PbI3 is mostly determined by the intrinsic electronic band properties near the Fermi level, rooted in the inorganic backbone, whereas the ferroelectric polarization of the hybrid halide perovskite is largely dominated by the ionic contribution of the molecular cation. The spatial charge shift upon excitation is attributed to the charge transfer from iodine to lead atoms in the backbone, which is independent of the presence of the cationic molecules. Our findings can serve as a guiding principle for the design of future materials for halide-perovskite solar cells with further enhanced photovoltaic performance.

Intrinsic quantum confinement in formamidinium lead triiodide perovskite

Understanding the electronic energy landscape in metal halide perovskites is essential for further improvements in their promising performance in thin-film photovoltaics. Here, we uncover the presence of above-bandgap oscillatory features in the absorption spectra of formamidinium lead triiodide thin films. We attribute these discrete features to intrinsically occurring quantum confinement effects, for which the related energies change with temperature according to the inverse square of the intrinsic lattice parameter, and with peak index in a quadratic manner. By determining the threshold film thickness at which the amplitude of the peaks is appreciably decreased, and through ab initio simulations of the absorption features, we estimate the length scale of confinement to be 10-20 nm. Such absorption peaks present a new and intriguing quantum electronic phenomenon in a nominally bulk semiconductor, offering intrinsic nanoscale optoelectronic properties without necessitating cumbersome additional processing steps.

Benign ferroelastic twin boundaries in halide perovskites for charge carrier transport and recombination

Grain boundaries have been established to impact charge transport, recombination and thus the power conversion efficiency of metal halide perovskite thin film solar cells. As a special category of grain boundaries, ferroelastic twin boundaries have been recently discovered to exist in both CH3NH3PbI3 thin films and single crystals. However, their impact on the carrier transport and recombination in perovskites remains unexplored. Here, using the scanning photocurrent microscopy, we find that twin boundaries have negligible influence on the carrier transport across them. Photoluminescence (PL) imaging and the spatial-resolved PL intensity and lifetime scanning confirm the electronically benign nature of the twin boundaries, in striking contrast to regular grain boundaries which block the carrier transport and behave as the non-radiative recombination centers. Finally, the twin-boundary areas are found still easier to degrade than grain interior.

First-principles calculations of electronic structure and optical and elastic properties of the novel ABX3-type LaWN3 perovskite structure

The development of ABX₃-type advanced perovskite materials has become a focus for both scientific researchers and the material genome initiative (MGI). In addition to the traditional perovskite ABO₃ and halide perovskite ABX₃, LaWN₃ is discovered as a new ABX₃-type advanced perovskite structure. The elastic and optical properties of this novel LaWN₃ structure are systematically studied via DFT. Based on the calculated elastic constants, the bulk modulus, shear modulus, Young's modulus and Pugh modulus ratio are precisely obtained. Results show that (1) LaWN₃ is an indirect bandgap semiconductor with a hybrid occuring near the Fermi level and the main contributions are La-d, W-d and N-p. (2) LaWN₃ has a certain ductility. The optical constants, such as absorption spectrum, energy-loss spectrum, conductivity, dielectric function, reflectivity and refractive index, are analyzed and the static dielectric constant is 10.98 and the refractivity index is 3.31. (3) The optical constants of LaWN₃ are higher than those of other existing ABX₃-type materials, showing very promising application as a functional perovskite in the future. The existence of this stable LaWN₃ structure might widen the perovskite material's application, such as in photodetectors, light-emitting diodes, perovskite solar cells, fuel cells and so on.

Using first-principles calculation, the stable R3c LaWN₃ as a new ABX₃-type advanced perovskite structure is designed in the plan of the material genome initiative (MGI), which helps to widen the nowadays nitride perovskite material's application.

Properties and growth of large single crystals of one-dimensional organic lead iodine perovskite

Here, we demonstrate for the first time the growth of 2 mm × 4 mm × 8 mm sized single crystal one dimensional organic lead iodine perovskite – DMAPbI₃ ((CH₃)₂NH₂PbI₃).