2D Hybrid Perovskites: From Static and Dynamic Structures to Potential Applications.

2D perovskites have received great attention recently due to their structural tunability and environmental stability, making them highly promising candidates for various applications by breaking property bottlenecks that affect established materials. However, in 2D perovskites, the complicated interplay between organic spacers and inorganic slabs makes structural analysis challenging to interpret. A deeper understanding of the structure-property relationship in these systems is urgently needed to enable high-performance tunable optoelectronic devices. Herein, this study examines how structural changes, from constant lattice distortion and variable structural evolution, modeled with both static and dynamic structural descriptors, affect macroscopic properties and ultimately device performance. The effect of chemical composition, crystallographic inhomogeneity, and mechanical-stress-induced static structural changes and corresponding electronic band variations is reported. In addition, the structure dynamics are described from the viewpoint of anharmonic vibrations, which impact electron-phonon coupling and the carriers' dynamic processes. Correlated carrier-matter interactions, known as polarons and acting on fine electronic structures, are then discussed. Finally, reliable guidelines to facilitate design to exploit structural features and rationally achieve breakthroughs in 2D perovskite applications are proposed. This review provides a global structural landscape of 2D perovskites, expected to promote the prosperity of these materials in emerging device applications.

Kinetics Controlled Perovskite Crystallization for High Performance Solar Cells.

The power conversion efficiencies (PCEs) of perovskite solar cells have recently developed rapidly compared to crystalline silicon solar cells. To have an effective way to control the crystallization of perovskite thin films is the key for achieving good device performance. However, a paradox in perovskite crystallization is from the mismatch between nucleation and Oswald ripening. Usually, the large numbers of nucleation sites tend to weak Oswald ripening. Here, we proposed a new mechanism to promote the formation of nucleation sites by reducing surface energy from 44.9 mN/m to 36.1 mN/m, to spontaneously accelerate the later Oswald ripening process by improving the grain solubility through the elastic modulus regulation. The ripening rate is increased from 2.37 Åm ⋅ s-1 to 4.61 Åm ⋅ s-1 during annealing. Finally, the solar cells derived from the optimized films showed significantly improved PCE from 23.14 % to 25.32 %. The long-term stability tests show excellent thermal stability (the optimized device without encapsulation maintaining 82 % of its initial PCE after 800 h aging at 85 °C) and an improved light stability under illumination. This work provides a new method, the elastic modulus regulation, to enhance the ripening process.

Au Nanocluster Assisted Microstructural Reconstruction for Buried Interface Healing for Enhanced Perovskite Solar Cell Performance.

The heterogeneity of perovskite film crystallization along the vertical direction leads to voids and traps at the buried interfaces, hampering both efficiency and stability of perovskite solar cells. Here, bovine serum albumin-functionalized Au nanoclusters (ABSA), combined with heavy gravity, high surface charge density, and strong interactions with the electron transport layer, are designed to reconstruct the buried interfaces for not only high-quality crystallization, but also improved carrier transfer. The ABSA macromolecules with amine function groups and larger surface charge density interact with the perovskite to improve the crystallinity, and gradually migrate towards the buried interface, healing the defective voids, hence suppressing surface recombination velocity from 3075 to 452 cm s-1 . The healed buried interface and the higher surface potential of ABSA-modified TiO2 lead to improved carrier extraction at the interface. The resulting solar cell attains a power conversion efficiency of 25.0% with negligible hysteresis and remarkable stability, maintaining 92.9% of their initial efficiency after 3200 h of exposure to the ambient atmosphere, they also exhibit better continuous irradiation stability compared to control devices. These findings provide a new metal-protein complex to eliminate the deleterious voids and defects at the buried interface for improved photovoltaic performance and stability.

Understanding Microstructural Development of Perovskite Crystallization for High Performance Solar Cells.

Solution crystallization in film devices has attracted broad interest from various fields such as perovskite solar cells. However, the detailed perovskite crystallization kinetics remain unclear due to the difficulty of in situ observation of grain cluster growth during annealing. This article presents the development of an in situ laser scanning confocal polarized microscopy with a temperature-controlled stage to observe nucleation and growth of perovskite crystal clusters. It is found that enhanced interactions by a liquid crystal with perovskite form a new intermediate complex that induces diffusion-controlled growth according to Avrami equation. The retarded cluster growth (63 nm s-1 ) originates from enlarged diffusion activation energy 40 kJ mol-1 compared with 152 nm s-1 and 37 kJ mol-1 for the Control film during annealing. Finally, the optimized perovskite films with enhanced crystallographic and optical characteristics are applied in solar cells to achieve a champion efficiency of 24.53% with open circuit voltage of 1.172 V and fill factor of 82.78%. The bare device without any protection maintains 89% of its initial efficiency after 6600 h of aging in ambient environment. This work implies that the in situ observation using fluorescence microscopy is a critical for understanding of crystallization kinetics in film devices.

On the Mechanism of Solvents Catalyzed Structural Transformation in Metal Halide Perovskites.

Metal halide perovskites show the capability of performing structural transformation, allowing the formation of functional heterostructures. Unfortunately, the elusive mechanism governing these transformations limits their technological application. Herein, the mechanism of 2D-3D structural transformation is unraveled as catalyzed by solvents. By combining a spatial-temporal cation interdiffusivity simulation with experimental findings, it is validated that, protic solvents foster the dissociation degree of formadinium iodide (FAI) via dynamic hydrogen bond, then the stronger hydrogen bond of phenylethylamine (PEA) cation with selected solvents compared to dissociated FA cation facilitates 2D-3D transformation from (PEA)2 PbI4 to FAPbI3 . It is discovered that, the energy barrier of PEA out-diffusion and the lateral transition barrier of inorganic slab are diminished. For 2D films the protic solvents catalyze grain centers (GCs) and grain boundaries (GBs) transforme into 3D phases and quasi-2D phases, respectively. While in the solvent-free case, GCs transform into 3D-2D heterostructures along the direction perpendicular to the substrate, and most GBs evolve into 3D phases. Finally, memristor devices fabricated using the transformed films uncover that, GBs composed of 3D phases are more prone to ion migration. This work elucidates the fundamental mechanism of structural transformation in metal halide perovskites, allowing their use to fabricate complex heterostructures.

Electron-Phonon Coupling Suppression by Enhanced Lattice Rigidity in 2D Perovskite Single Crystals for High-Performance X-Ray Detection.

2D Dion-Jacobson (DJ) perovskite single crystals (PSCs) usually demonstrate better X-ray detection performance than Ruddlesden-Popper (RP) PSCs. However, the mechanism of the improved performance is still elusive. Here, by the aid of strong interactions between dimethylbiguanide (DGA) and PbI2 , a novel DJ-perovskitoid (DGA)PbI4 is designed. From the comparison of (DGA)PbI4 to other 2D PSCs, it is discovered that the tiniest lattice distortion and increased hydrogen bonds in the atom-scaled analysis strengthen lattice rigidity and weaken electron-phonon coupling to suppress disordered scattering of carriers, resulting in significantly improved carrier transport and stability. Therefore, high carrier mobility (78.1 cm2 V-1 s-1 ) and a pronounced sensitivity of 4869.0 µC Gyair -1 cm-2 are achieved using (DGA)PbI4 , which are the best in 2D Pb-based PSC devices to date. Finally, the (DGA)PbI4 devices exhibit good spatial resolution in X-ray imaging and excellent long-term stability to work as a promising candidate for medical diagnostics and nondestructive determination.

Spontaneous Interface Healing by a Dynamic Liquid-Crystal Transition for High-Performance Perovskite Solar Cells.

Low-temperature solution processing of thin-film semiconductors is more cost-effective than traditional vacuum processing; however, it leads to more defects during fast bulk crystallization and residual tensile stress. Herein, a new strategy of dynamic liquid-crystal transition (DLCT) is developed to solve these problems in one step. The design principle is used to suggest that the DLCT molecule should firstly interact with the perovskite grains in the bulk and meanwhile go through a dynamic transition to spontaneously heal the interface. A thermotropic LC molecule (CBO6SS6OCB) is then designed to demonstrate the strategy. The LC interacting with perovskite colloid forms an intermediate adduct to retard the crystallization. The annealing processes stimulate the concentrated LC solid, causing it to flow to the electron transport layer to release the residual stress to attain improved electron extraction. Consequently, the device efficiency is increased to 24.38%, where its VOC of 1.184 V is among the best for the formamidine-based perovskite solar cells. Furthermore, the ambient stability (93.0% of initial efficiency after 2000 h of aging) and light stability (96.3% of initial efficiency after 500 h of aging) are much improved. This work conceives a new engineering of additive phase transition for high-performance perovskite solar cells.

Deep-Level Transient Spectroscopy for Effective Passivator Selection in Perovskite Solar Cells to Attain High Efficiency over 23.

Most studies choose passivators essentially in a trial-and-error fashion in an attempt to attain high efficiency in perovskite solar cells (PSCs). Using deep-level transient spectroscopy (DLTS) measurements, the type of defects in perovskite films was determined to guide the passivator selection for PSCs. Three kinds of positively charged defects were found in the target PSC system. Fluorinated phenylethylamine hydroiodide (FPEAI) was chosen to passivate the surface defects due to the electronegativity and hydrophobicity of fluorine. Due to the decreased surface roughness, increased hydrophobicity, lowered defect density, and improved carrier dynamics as observed by ultrafast transient absorption spectroscopy (TAS), a PSC with meta-F-PEAI had the best efficiency over 23 % with open-circuit voltage of 1.155 V and fill factor of 80.15 %. In addition, the long-term stability of the PSC was significantly improved. The present work provides a new means to select the best passivator for different types of defects.

Thermal regelation of single particles and particle clusters in ice.

We investigate the migration by thermal regelation of single particles and clusters of particles surrounded by ice subjected to a temperature gradient. This phenomenon is relevant to the casting of porous materials, to cryopreservation of biological tissue, and to the degradation of paleoclimatic signals held in ice sheets, for example. Using carefully controlled laboratory experiments, we measure the migration rates of single particles and clusters as they approach the freezing front. We find that clusters migrate at a constant rate, while single particles accelerate towards the freezing front. This fundamental difference is attributed to the fact that, during regelation, melt water passes through the interstices of a cluster, limited by its constant permeability, but for a single particle must flow through a thin layer of pre-melted ice whose thickness diverges as the freezing temperature is approached, reducing the viscous resistance to migration. We extend existing theories of particle and cluster migration to include the influences of different thermal conductivities and of latent heat on the local temperature field in and around the particle or cluster. We find that if the specific latent heat is large or the viscous resistance to flow is sufficiently small then the migration rate is determined solely by heat transport.

Publisher Correction: Nucleation-controlled growth of superior lead-free perovskite Cs3Bi2I9 single-crystals for high-performance X-ray detection.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Nucleation-controlled growth of superior lead-free perovskite Cs3Bi2I9 single-crystals for high-performance X-ray detection.

The organic-inorganic hybrid lead halide perovskites have emerged as a series of star materials for solar cells, lasers and detectors. However, the issues raised by the toxic lead element and marginal stability due to the volatile organic components have severely limited their potential applications. In this work, we develop a nucleation-controlled solution method to grow large size high-quality Cs3Bi2I9 perovskite single crystals (PSCs). Using the technique, we harvest some centimeter-sized single crystals and achieved high device performance. We find that X-ray detectors based on PSCs exhibit high sensitivity of 1652.3 µC Gyair-1 cm-2 and very low detectable dose rate of 130 nGyair s-1, both desired in medical diagnostics. In addition, its outstanding thermal stability inspires us to develop a high temperature X-ray detector with stable response at up to 100 °C. Furthermore, PSCs exhibit high X-ray imaging capability thanks to its negligible signal drifting and extremely high stability.

Robust Spin-Gapless Behavior in the Newly Discovered Half Heusler Compound MnPK.

Spin gapless semiconductors have aroused high research interest since their discovery and a lot of effort has been exerted on their exploration, in terms of both theoretical calculation and experimental verification. Among different spin gapless materials, Heusler compounds stand out thanks to their high Curie temperature and highly diverse compositions. Especially, both theoretical and experimental studies have reported the presence of spin gapless properties in this kind of material. Recently, a new class of d0 - d Dirac half Heusler compound was introduced by Davatolhagh et al. and Dirac, and spin gapless semiconductivity has been successfully predicted in MnPK. To further expand the research in this direction, we conducted a systematical investigation on the spin gapless behavior of MnPK with both generalized gradient approximation (GGA) and GGA + Hubbard U methods under both uniform and tetragonal strain conditions by first principles calculation. Results show the spin gapless behavior in this material as revealed previously. Different Hubbard U values have been considered and they mainly affect the band structure in the spin-down channel while the spin gapless feature in the spin-up direction is maintained. The obtained lattice constant is very well consistent with a previous study. More importantly, it is found that the spin gapless property of MnPK shows good resistance for both uniform and tetragonal strains, and this robustness is very rare in the reported studies and can be extremely interesting and practical for the final end application. This study elaborates the electronic and magnetic properties of the half Heusler compound MnPK under uniform and tetragonal strain conditions, and the obtained results can give a very valuable reference for related research works, or even further motivate the experimental synthesis of the relative material.

Interactions between Nanoparticles and Polymers in the Diffusion Boundary Layer during Freezing Colloidal Suspensions.

The present work investigated diffusion interactions between nanoparticles and polymers during freezing colloidal suspensions. The size effects of nanoporous media formed by packed nanoparticles on the diffusion instability of the polymer solution were explored. It is found that small particles under low pulling speeds will obstruct the diffusion of polymers and the corresponding morphology will be banded structures. The intrinsic reason is the inhibited tube-like motion of polymer chains in the nanoporous particle layer. The increased particle size or the decreased solute size will solve the diffusion problem. On the other hand, the small pulling speed constructs an increased length of the particle layer in front of the freezing interface, which presents a longer diffusion path to impede the polymer diffusion. Instead, an increased pulling speed shortens the length of the particle layer so that it is easy for polymers to go through a short porous media. Hence, the diffusion of polymers will control the freezing morphology of the suspension and create dendrites. These results imply that a relatively larger particle size and a moderately higher pulling speed are beneficial for well-developed microstructures in the production of porous ceramics with the freeze-casting method.

Interfacial undercooling in solidification of colloidal suspensions: analyses with quantitative measurements.

Interfacial undercooling in the complex solidification of colloidal suspensions is of significance and remains a puzzling problem. Two types of interfacial undercooling are supposed to be involved in the freezing of colloidal suspensions, i.e., solute constitutional supercooling (SCS) caused by additives in the solvent and particulate constitutional supercooling (PCS) caused by particles. However, quantitative identification of the interfacial undercooling in the solidification of colloidal suspensions, is still absent; thus, the question of which type of undercooling is dominant in this complex system remains unanswered. Here, we quantitatively measured the static and dynamic interface undercoolings of SCS and PCS in ideal and practical colloidal systems. We show that the interfacial undercooling primarily comes from SCS caused by the additives in the solvent, while PCS is minor. This finding implies that the thermodynamic effect of particles from the PCS is not the fundamental physical mechanism for pattern formation of cellular growth and lamellar structure in the solidification of colloidal suspensions, a general case of ice-templating method. Instead, the patterns in the ice-templating method can be controlled effectively by adjusting the additives.

Interface instability modes in freezing colloidal suspensions: revealed from onset of planar instability.

Freezing colloidal suspensions widely exists in nature and industry. Interface instability has attracted much attention for the understandings of the pattern formation in freezing colloidal suspensions. However, the interface instability modes, the origin of the ice banding or ice lamellae, are still unclear. In-situ experimental observation of the onset of interface instability remains absent up to now. Here, by directly imaging the initial transient stage of planar interface instability in directional freezing colloidal suspensions, we proposed three interface instability modes, Mullins-Sekerka instability, global split instability and local split instability. The intrinsic mechanism of the instability modes comes from the competition of the solute boundary layer and the particle boundary layer, which only can be revealed from the initial transient stage of planar instability in directional freezing.

In situ observation the interface undercooling of freezing colloidal suspensions with differential visualization method.

Interface undercooling is one of the most significant parameters in the solidification of colloidal suspensions. However, quantitative measurement of interface undercooling of colloidal suspensions is still a challenge. Here, a new experimental facility and gauging method are designed to directly reveal the interface undercooling on both static and dynamic cases. The interface undercooling is visualized through the discrepancy of solid/liquid interface positions between the suspensions and its solvent in a thermal gradient apparatus. The resolutions of the experimental facility and gauging method are proved to be 0.01 K. The high precision of the method comes from the principle of converting temperature measurement into distance measurement in the thermal gradient platform. Moreover, both static and dynamic interface undercoolings can be quantitatively measured.