Probing the Local Electronic Structure in Metal Halide Perovskites through Cobalt Substitution.

Single-crystal halide perovskites: Opportunities and challenges

Metal halide perovskite (MHP) materials have experienced intensive attention in the current decade owing to their remarkable properties which includes a tuneable band gap, high photoluminescence quantum yields (PLQY), variable size, morphology, composition and dimensions. The synthesis of MHP nanocrystals (NCs) and quantum dots (QDs) from various methods that have been developed from decades of research. Conventional semiconductors have progressed to achieve controlled morphologies, sizes and compositions. In this context, in the present chapter we review the synthetic methods for MHP NCs highlighting the role of critical factors including the nucleation, growth and ligand passivation to generate defect free and size controlled MHP NCs. Understanding the role of these factors in the mechanism of formation of MHP NCs is crucial to design stable and efficient functional electronic devices. The unique properties of perovskites are related to their distinctive electronic structures which ultimately determine their band gap. These properties make MHP NCs and QDs suitable for use in optoelectronic applications such as light emitting diodes (LEDs), lasers, photodetectors and solar cells. A brief discussion is also given on potential applications of MHPs in some functional devices.

High-throughput experimentation (HTE) seeks to accelerate the exploration of materials space by uniting robotics, combinatorial methods, and parallel processing. HTE is particularly relevant to metal halide perovskites (MHPs), a diverse class of optoelectronic materials with a large chemical space. Here we develop an HTE workflow to synthesize and characterize light-emitting MHP single crystals, allowing us to generate the first reported data set of experimentally derived photoluminescence spectra for low-dimensional MHPs. We leverage the accelerated workflow to optimize the synthesis and emission of a new MHP, methoxy-phenethylammonium lead iodide ((4-MeO-PEAI)2-PbI2). We then synthesize 16 000 MHP single crystals and measure their photoluminescence to study the effects of synthesis parameters and compositional engineering on the emission intensity of 54 distinct MHPs: we achieve an acceleration factor of more than 100 times over previously reported HTE MHP synthesis and characterization methods. Using insights derived from this analysis, we screen an existing database for new, potentially emissive MHPs. On the basis of the Tanimoto similarity of the bright available emitters, we present our top candidates for future exploration. As a proof of concept, we use one of these (3,4-difluorophenylmethanamine) to synthesize an MHP which we find has a photoluminescence quantum yield of 10%.

Mixed cesium‐ and formamidinium‐based metal halide perovskites (MHPs) are emerging as ideal photovoltaic materials due to their promising performance and improved stability. While theoretical predictions suggest that a larger composition ratio of Cs (≈30%) aids the formation of a pure photoactive α‐phase, high photovoltaic performances can only be realized in MHPs with moderate Cs ratios. In fact, elemental mixing in a solution can result in chemical complexities with non‐equilibrium phases, causing chemical inhomogeneities localized in the MHPs that are not traceable with global device‐level measurements. Thus, the chemical origin of the complexities and understanding of their effect on stability and functionality remain elusive. Herein, through spatially resolved analyses, the fate of local chemical structures, particularly the evolution pathway of non‐equilibrium phases and the resulting local inhomogeneities in MHPs is comprehensively explored. It is shown that Cs‐rich MHPs have substantial local inhomogeneities at the initial crystallization step, which do not fully convert to the α‐phase and thereby compromise the optoelectronic performance of the materials. These fundamental observations allow the authors to draw a complete chemical landscape of MHPs including nanoscale chemical mechanisms, providing indispensable insights into the realization of a functional materials platform.

Discovering key factors determining perovskite bandgap under data scarcity inspired by knowledge distillation.

Metal halide perovskites have gained significant interest for use in solar cells and light-emitting devices. Recently, this material has also gained significant interest for its potential in energy storage devices, particularly lithium-ion batteries and photo-batteries, due to their long charge carrier diffusion length, high charge mobility, high light absorption capacity, non-rigid structure, and variable bandgap. This perspective highlights key properties of metal halide perovskites used as electrodes in lithium-ion batteries. The primary discussion is divided into four sections: an explanation of the structure and properties of metal halide perovskites, a very brief description of the operation of a conventional lithium-ion battery, lithium-ion interaction with metal perovskite halides, and the evolution and progress of perovskite halides as electrodes and photo-electrodes. The purpose of this perspective is to build awareness of recent advancements and provide an outlook on this relatively new subfield in order to motivate continued research and development of batteries and photo-batteries containing metal halide perovskites.

Improving the stability of metal halide perovskite solar cells from material to structure

Metal halide perovskites (MHPs) have demonstrated great advances for photovoltaic and optoelectronic applications. However, owing to the presence of the synergy from lattice strain, defects of MHPs, and environment, MHPs suffer from phase transitions and degradation, resulting in the restriction of their practical applications and further commercialization. Multiple metal elements can coexist in MHPs to form alloys due to the high tolerance of lattice and the composition replaceability, which provides a novel strategy for improvement of performance and stability. In this review, the recent advances of alloy engineering of MHPs, focusing on the cation and the metal ion (A‐ and B‐site) alloy strategies, are reviewed. The alloy effects on the crystalline structure, optoelectronic properties, ferroelectricity, carrier dynamics, and stability of perovskites are interpreted. Finally, the prospect of this study is the challenges in the MHPs alloy engineering.

Metal halide perovskites (MHPs) with unique structures and excellent photoelectric properties are deemed promising photocatalytic materials to eliminate the rising environmental and energy issues. However, MHPs often suffer from poor stability, fast charge carrier recombination rates, a deficiency of active sites, and band gap mismatch. The structure and electronic properties of MHPs can be regulated through metal doping strategies and used in various photocatalytic applications. A desired landscape on the relationships of doping strategies-properties-functional applications of MHPs has yet to be presented in a systematic exposition and summary, which is critical for the rational design of metal-doped MHPs. This review first briefly introduces the structural features of MHPs and then discusses the effects generated by different types of metal doping, including main group metal doping, transition metal doping, and rare earth metal doping. Subsequently, various preparation methods for doped MHPs and applications in photocatalytic energy transformation and environmental remediation are summarized. Finally, we present the remaining challenges and future opportunities in the development of metal-doped MHPs for photocatalysis. We believe this review will stimulate deeper research on doping strategies, properties, regulation, and functional application interplays and accelerate more extensive applications of metal-doped MHPs within the thrilling area of photocatalysis.

Metal halide perovskites are an important class of semiconductors now being implemented as photovoltaic absorbers and explored for light emission, among other device applications. The semiconducting properties of halide perovskites are deeply intertwined with their composition and structure. Specifically the symmetry, tilting, and distortions of the metal halide octahedra impact the band structure and other optoelectronic properties. In this review, we examine the various compositions of monovalent A-site cations in three-dimensional (3D) halide perovskites AMX3 (M = divalent metal; X = halide). We focus on how the A-site cation templates the inorganic metal-halide perovskite framework, resulting in changes in the crystal structure symmetry, as well as M-X bonding parameters, summarized in a comprehensive table of AMX3 structures. The A-site cation motion, effects of alloying, and 2D Ruddlesden-Popper perovskite structures with unique A-site cations are further overviewed. Correlations are shown between these A-site cation dominated structural parameters and the resulting optoelectronic properties such as band gap. This review should serve as a reference for the A-site cation structural chemistry of metal halide perovskites and inspire continued research into less explored metal halide perovskite compositions and structures.

Whereas the promise of metal halide perovskite (MHP) photovoltaics (PV) is that they can combine high efficiency with solution-processability, the chemistry occurring in precursor inks is largely unexplored. Herein, we investigate the degradation of MHP solutions based on the most widely used solvents, dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). For the MHP inks studied, which contain formamidinium (FA+), methylammonium (MA+), cesium (Cs+), lead (Pb2+), bromide (Br–), and iodide (I–), dramatic compositional changes are observed following storage of the inks in nitrogen in the dark. We show that hydrolysis of DMF in the precursor solution forms dimethylammonium formate, which subsequently incorporates into the MHP film to compromise the ability of Cs+ and MA+ to stabilize FA+-based MHP. The changes in solution chemistry lead to a modification of the perovskite film stoichiometry, band gap, and structure. The solid precursor salts are stable when ball-milled into a powder, allowing for the storag...

This Ph.D. project was mainly devoted to the study of the connection between magnetocaloric properties and first order phase transitions in ferromagnetic materials based on the La(Fe,Si)13 compound. The magneto caloric effect (MCE) and its application in magnetic cooling cycles rely on the reversible magnetization and demagnetization of a magnetic material by an external magnetic field, resulting in a temperature change that is maximal at temperatures close to a magnetic phase transition. The possibility to improve the performance of the active refrigerator materials, depends on many factors: the need of a Curie temperature close to ambient temperature, a low magnetic and thermal hysteresis and a high magnetic entropy variation for magnetic fields below two Tesla. The latter requisite can be found in first order magnetic phase transitions that, unfortunately, are accompanied by intrinsic thermo-magnetic hysteresis. This drawback for magneto cooling cycles, motivates the present study on the phase transitions dynamics. On the other hand, the investigation of magneto-thermal phenomena in magnetic materials is of great importance also for solving fundamental problems of magnetism and solid state physics, for example, it is recognized that the properties of interest of such functional materials are intimately linked to the detailed micro structure, however, the nature of this link itself is not understood very often. In this Ph.D. project, thermo-magnetic phase transitions in La(Fe,Si)13 compounds were investigated through the comparison of various experimental techniques within a collaboration between the applied superconductivity group of Politecnico of Torino and the electromagnetism division of INRiM (National Institute of Metrological Research). To achieve a proper physical understanding of the connection between thermo-magnetic hysteresis at the microscopic level and the microstructure, a magneto optical method was applied to samples of La-F-Si-13 with cobalt substitutions, so to allow the dynamical visualisation of the phase boundaries motion in a first order phase transition. These type of experiments have been compared with low rate calorimetry data and, from the experimental work, it has been found that the presence of avalanches is a characteristic feature of these alloys and it is related to their thermal hysteresis. The difference between first and second order phase transition dynamics were highlighted thanks to the employment of different techniques, which also favoured the separation of the general aspects of hysteresis, common to all irreversible processes, from features more strictly dependent on specific microstructural properties. For the aim of this Ph.D., other techniques were also used to observe temperature induced magnetic phase transitions in functional magnetic materials. Among them an in-temperature ferromagnetic resonance method was implemented for the study of the magnetization dynamics in canted spin structures. The present research activity has been partially related to the European Project DRREAM [1] (a collaborative research project funded by the EC under the Seventh Framework Program 2013-2015), whose goal is to reduce the use of rare earth elements in the life cycle of technologies that use magnetic phase change materials, in particular magnetic refrigerators

Metal halide perovskites are widely used in optoelectronic devices, including solar cells, photodetectors, and light-emitting diodes. Defects in this class of low-temperature solution-processed semiconductors play significant roles in the optoelectronic properties and performance of devices based on these semiconductors. Investigating the defect properties provides not only insight into the origin of the outstanding performance of perovskite optoelectronic devices but also guidance for further improvement of performance. Defects in perovskites have been intensely studied. Here, we review the progress in defect-related physics and techniques for perovskites. We survey the theoretical and computational results of the origin and properties of defects in perovskites. The underlying mechanisms, functions, advantages, and limitations of trap state characterization techniques are discussed. We introduce the effect of defects on the performance of perovskite optoelectronic devices, followed by a discussion of the mechanism of defect treatment. Finally, we summarize and present key challenges and opportunities of defects and their role in the further development of perovskite optoelectronic devices.

Metal halide perovskite based materials have emerged over the past few decades as remarkable solution-processable optoelectronic materials with many intriguing properties and potential applications. These emerging materials have recently been considered for their promise in low-energy memory and information processing applications. In particular, their large optical cross-sections, high photoconductance contrast, large carrier-diffusion lengths, and mixed electronic/ionic transport mechanisms are attractive for enabling memory elements and neuromorphic devices that are written and/or read in the optical domain. Here, recent progress toward memory and neuromorphic functionality in metal halide perovskite materials and devices where photons are used as a critical degree of freedom for switching, memory, and neuromorphic functionality is reviewed.

Investigations of the electronic structure of d0 transition metal oxides belonging to the perovskite family