Properties Of Metal Halide Perovskites Research Articles

Metal halide perovskites for solar‐to‐chemical energy conversion in aqueous media

AbstractSolar‐driven energy conversion is a promising technology for a sustainable energy future and environmental remediation, and an efficient catalyst is a key factor. Recently, metal halide perovskites (MHPs) have emerged as promising photocatalysts due to their exceptional photoelectronic properties and low‐cost solution processing, enabling successful applications in H2 evolution, CO2 reduction, organic synthesis, and pollutant degradation. Despite these successes, the practical applications of MHPs are limited by their water instability. In this review, the recently developed strategies driving MHP‐catalyzed reactions in aqueous media are outlined. We first articulate the structures and properties of MHPs, followed by elaborating on the origin of instability in MHPs. Then, we highlight the advances in solar‐driven MHP‐based catalytic systems in aqueous solutions, focusing on developing external protection strategies and intrinsically water‐stable MHP materials. With each approach offering peculiar sets of advantages and challenges, we conclude by outlining potentially promising opportunities and directions for MHP‐based photocatalysis research in aqueous conditions moving forward. We anticipate that this timely review will provide some inspiration for the design of MHP‐based photocatalysts, manifestly stimulating their applications in aqueous environments for solar‐to‐chemical energy conversion.

Eliminating Non-Corner-Sharing Octahedral for Efficient and Stable Perovskite Solar Cells.

The metal halide (BX6)4- octahedron, where B represents a metal cation and X represents a halide anion, is regarded as the fundamental structural and functional unit of metal halide perovskites. However, the influence of the way the (BX6)4- octahedra connect to each other has on the structural stability and optoelectronic properties of metal halide perovskite is still unclear. Here, the octahedral connectivity, including corner-, edge-, and face-sharing, of various CsxFA1-xPbI3 (0≤x≤0.3) perovskite films is tuned and reliably characterized through compositional and additive engineering, and with ultralow-dose transmission electron microscopy. It is found that the overall solar cell device performance, the charge carrier lifetime, the open-circuit voltage, and the current density-voltage hysteresis are all improved when the films consist of corner-sharing octahedra, and non-corner sharing phases are suppressed, even in films with the same chemical composition. Additionally, it is found that the structural, optoelectronic, and device performance stabilities are similarly enhanced when non-corner-sharing connectivities are suppressed. This approach, combining macroscopic device tests and microscopic material characterization, provides a powerful tool enabling a thorough understanding of the impact of octahedral connectivity on device performance, and opens a new parameter space for designing high-performance photovoltaic metal halide perovskite devices.

Low-cost, stable all-inorganic charge transporting layer based FAPI perovskite solar cells achieving ≈ 26% efficiency using SCAPS-1D

The superior opto-electronic properties of metal halide perovskites have uncovered great potential as light harvesters for thin-film photovoltaics, capturing the dominant research trend for over a decade now. While Perovskite Solar Cells (PSCs) depict promising results of a successful photovoltaic technology, further replacement of conventional charge transporting layers (CTLs) with inorganic counterparts can impart sustainable device stability and cost-effectiveness. Here, we represent a theoretical study aimed to analyse the performance of Formamidinium Lead Iodide (FAPI) based PSCs using SCAPS-1D simulation tool (version 3.3.09). The solar cell is designed into a novel, and cost-effective inverted p-i-n architecture comprising of inorganic transporting layers. The introduction of a thin CuI buffer layer accompanied with the hole transporting layer (HTL) was found to boost the device performance greatly. Furthermore, a systematic analysis of the device parameters was carried out based on crucial factors like thickness of the absorber, CTLs, and buffer layer, interfacial defect densities (Nit), series resistances (RS) and working temperature (T). In addition to this, we also conducted a study on the combined effect of absorber defect density (Nt) and thickness on our simulated PSC. Our optimized device configuration ITO/CuI/Cu2O/FAPI/ZnO/Al, resulted in an ideal device efficiency of ≈ 26% and a reasonably high efficiency of 21.58% adopting a more realistic approach. This study therefore focuses on an economical architectural design strategy with an emphasis on device efficiency, and its long-term sustainability.

Superior optical limiting performance of lead-free metal halide perovskites nano-crystals embedded chalcogenide glass

The nonlinear optical properties of metal halide perovskites (MHP) are highly attractive, making them an emerging hotspot in the field of perovskite photonics. In this study, the third-order optical nonlinear properties of lead-free MHP (CsSnCl3) nano-crystals (NCs) embedded in a chalcogenide glass (GeS2-Sb2S3) substrate were characterized using the Z-scan technique. The spectral range examined was from 750 to 900 nm. Experimental results revealed that the incorporation of CsSnCl3 NCs enhanced the nonlinear refraction and nonlinear absorption behavior of the GeS2-Sb2S3 glass. Furthermore, the magnitude of these effects increased with the growth size of the CsSnCl3 NCs. Importantly, the MHP glasses exhibited strong two-photon absorption due to the large multiple-absorption cross-section of MHP crystals and the intrinsic high optical nonlinearity of the chalcogenide glass. The superior optical limiting performance induced by this process was successfully demonstrated.

Interfacial alloying between lead halide perovskite crystals and hybrid glasses

The stellar optoelectronic properties of metal halide perovskites provide enormous promise for next-generation optical devices with excellent conversion efficiencies and lower manufacturing costs. However, there is a long-standing ambiguity as to whether the perovskite surface/interface (e.g. structure, charge transfer or source of off-target recombination) or bulk properties are the more determining factor in device performance. Here we fabricate an array of CsPbI3 crystal and hybrid glass composites by sintering and globally visualise the property-performance landscape. Our findings reveal that the interface is the primary determinant of the crystal phases, optoelectronic quality, and stability of CsPbI3. In particular, the presence of a diffusion “alloying” layer is discovered to be critical for passivating surface traps, and beneficially altering the energy landscape of crystal phases. However, high-temperature sintering results in the promotion of a non-stoichiometric perovskite and excess traps at the interface, despite the short-range structure of halide is retained within the alloying layer. By shedding light on functional hetero-interfaces, our research offers the key factors for engineering high-performance perovskite devices.

Mitigating Environmental Effects in Halide Perovskites through Hybrid Perovskite-Polymer Nanocomposites: A Short Review

Metal halide perovskites are currently making notable strides in the development of next-generation optoelectronic and photovoltaic devices. Prototype lab-based devices have exhibited high efficiencies for photovoltaic applications. However, the high performance cannot be maintained for practically useful lengths of time due to degradation of the perovskite layer on exposure to environmental factors such as moisture, oxygen, and heat. This limits the widespread use of these materials in commercial devices. This review of hybrid perovskite-polymer nanocomposites begins by highlighting the significant structural and optoelectrical properties of metal halide perovskites. The discourse moves on to the elucidation of the deleterious effects of environmental stressors on the movement of charge carriers in photovoltaic devices based on these materials. Various mitigatory measures are then considered, with the narrative eventually narrowing down to perovskite-polymer nanocomposites, where the synergistic combination of the two materials is critically analyzed. The conclusion looks at the challenges and opportunities presented by polymer-metal halide perovskite nanocomposites and the potential role they may play in the development of low-cost photovoltaic and optoelectronic devices.

Charge Selective Contacts to Metal Halide Perovskites Studied with Photoelectron Spectroscopy: X‐Ray, Ultraviolet, and Visible Light Induced Energy Level Realignment

AbstractThe electronic properties of metal halide perovskites (MHPs) are crucial for achieving the full potential of MHPs‐based optoelectronic devices, and they are extensively studied by photoelectron spectroscopy (PES). However, there are numerous complexities during PES measurements, which can result in inconsistent experimental data or even misinterpretation of results. Here, it is demonstrated that in the presence of charge selective junctions with MHPs, significant electronic energy level realignment can occur during PES measurements due to the sample excitation with high‐energy photons used in PES, amounting up to over 0.8 eV in the present case. This is caused by unbalanced charge carrier accumulation within the perovskite due to the charge‐selective interface. X‐ray photoelectron spectroscopy further reveals that photoexcitation due to bremsstrahlung, as produced in commonly employed twin‐anode lab‐sources, can readily produce sizable photoinduced shifts of core levels of MHPs films, whereas monochromatized X‐ray lab‐sources (irradiation flux reduced by >50 times by eliminating the bremsstrahlung) induce negligible shifts within the range of presently applied anode powers. The data and measurement conditions presented here are intended to enable others to obtain reliable MHP electronic property information from PES measurements.

Using pressure to unravel the structure–dynamic-disorder relationship in metal halide perovskites

The exceptional optoelectronic properties of metal halide perovskites (MHPs) are presumed to arise, at least in part, from the peculiar interplay between the inorganic metal-halide sublattice and the atomic or molecular cations enclosed in the cage voids. The latter can exhibit a roto-translative dynamics, which is shown here to be at the origin of the structural behavior of MHPs as a function of temperature, pressure and composition. The application of high hydrostatic pressure allows for unraveling the nature of the interaction between both sublattices, characterized by the simultaneous action of hydrogen bonding and steric hindrance. In particular, we find that under the conditions of unleashed cation dynamics, the key factor that determines the structural stability of MHPs is the repulsive steric interaction rather than hydrogen bonding. Taking as example the results from pressure and temperature-dependent photoluminescence and Raman experiments on MAPbBr_3 but also considering the pertinent MHP literature, we provide a general picture about the relationship between the crystal structure and the presence or absence of cationic dynamic disorder. The reason for the structural sequences observed in MHPs with increasing temperature, pressure, A-site cation size or decreasing halide ionic radius is found principally in the strengthening of the dynamic steric interaction with the increase of the dynamic disorder. In this way, we have deepened our fundamental understanding of MHPs; knowledge that could be coined to improve performance in future optoelectronic devices based on this promising class of semiconductors.

Atomic Model for Alkali Metal-Doped Tin–Lead Mixed Perovskites: Insight from Quantum Dynamics

Defects such as metal vacancies act as nonradiative recombination centers to deteriorate the photoelectric properties of metal halide perovskites. Nonadiabatic molecular dynamics has demonstrated that alkali metal dopants markedly improve the performance of mixed tin-lead perovskites. Alkali dopants increase the formation energy of tin vacancies to 1 eV, so that the defect concentration is decreased. When tin vacancies exist, alkali metals are easily doped into perovskites. Tin vacancies produce iodine trimers that create midgap states and cause rapid electron-hole recombination. Alkali metal additives eliminate the trap state, weaken nonadiabatic coupling, and decelerate charge recombination with a coefficient of ≤5.5 compared with the performance of the defective tin-lead mixed perovskite. Our research has constructed a theoretical model at the atomic level for alkali metal passivation that enhances defect tolerance of tin-lead mixed perovskites, generating valuable inspiration for optimizing high-performance perovskites.

Disentangling Dual Emission Dynamics in Lead Bromide Perovskite

AbstractMetal halide perovskites exhibit intriguing properties for optoelectronic and electronic applications. Understanding their intrinsic photoexcitation and band‐tail dynamics is essential for exploring novel functionality and improving device performance. This report addresses the band‐tail luminescence of lead bromide perovskite (APbBr3) single crystals, where A is methylammonium, formamidinium, and cesium. Using time‐resolved photoluminescence (PL) spectroscopy, the dynamics of well‐defined dual emission peaks are unambiguously resolved from the picosecond to the microsecond scale. Whereas the long‐lived states exhibit similar recombination dynamics at high temperatures, a clear, fast transition occurs between the high and low energy states at low temperatures, indicating direct communication between these two states. These time‐resolved PL spectra shed light on the intrinsic optical properties of metal halide perovskites.

Lattice Dynamics and Electron-Phonon Coupling in Double Perovskite Cs2NaFeCl6.

Phonon-phonon and electron/exciton-phonon coupling play a vitally important role in thermal, electronic, as well as optical properties of metal halide perovskites. In this work, we evaluate phonon anharmonicity and coupling between electronic and vibrational excitations in novel double perovskite Cs2NaFeCl6 single crystals. By employing comprehensive Raman measurements combined with first-principles theoretical calculations, we identify four Raman-active vibrational modes. Polarization properties of these modes imply Fm3̅m symmetry of the lattice, indicative for on average an ordered distribution of Fe and Na atoms in the lattice. We further show that temperature dependence of the Raman modes, such as changes in the phonon line width and their energies, suggests high phonon anharmonicity, typical for double perovskite materials. Resonant multiphonon Raman scattering reveals the presence of high-lying band states that mediate strong electron-phonon coupling and give rise to intense nA 1g overtones up to the fifth order. Strong electron-phonon coupling in Cs2NaFeCl6 is also concluded based on the Urbach tail analysis of the absorption coefficient and the calculated Fröhlich coupling constant. Our results, therefore, suggest significant impacts of phonon-phonon and electron-phonon interactions on electronic properties of Cs2NaFeCl6, important for potential applications of this novel material.

Cyclodextrin-induced phase transformation of cesium copper bromide perovskite.

Phase transformation represents a fascinating way to tune the structural and optical properties of metal halide perovskites. Macrocyclic cyclodextrin could trigger transformation of cesium copper bromide, driven by strong interactions of the macrocyclic hydroxyl groups with the perovskite cesium and bromide ions.

Recent Developments of Optoelectronic Synaptic Devices Based on Metal Halide Perovskites

AbstractMassive data processing with high computing efficiency and low operating power is required owing to the rapid development of artificial intelligence and information technology. However, the von Neumann structure computing system with the separated memory and processor can cause large energy consumption and a low running speed during massive data processing. Therefore, the brain‐inspired neuromorphic computing system is developed, that can provide hardware support for emulating biological synaptic functions and realizing highly intensive data processing with low power consumption. As a neuromorphic device, the optoelectronic synaptic device (OSD) is regarded as an ideal device to replace the von Neumann‐based computer because of its ultrafast signal transmission, large bandwidth, low energy consumption, and wireless communication. Owing to their unique optoelectronic property, metal halide perovskites (MHPs) have received growing attention as effective photosensitive materials in OSDs. Therefore, the review introduces the recent progress on OSDs based on MHPs (MHPs‐OSDs) including the structures and properties of MHPs, and the architectures and performance characteristics of MHPs‐OSDs. Furthermore, applications of MHPs‐OSDs are presented. Finally, the outlook and opportunity of MHPs‐OSDs are discussed.

Nonequilibrium Lattice Dynamics in Photoexcited 2D Perovskites.

Interplay between structural and photophysical properties of metal halide perovskites is critical to their utility in optoelectronics, but there is limited understanding of lattice response upon photoexcitation. Here, 2D perovskites butylammonium lead iodide, (BA)2 PbI4 , and phenethylammonium lead iodide, (PEA)2 PbI4 , are investigated using ultrafast transient X-ray diffraction as a function of optical excitation fluence to discern structural dynamics. Both powder X-ray diffraction and time-resolved photoluminescence linewidths narrow over 1 ns following optical excitation for the fluence range studied, concurrent with slight redshifting of the optical bandgaps. These observations are attributed to transient relaxation and ordering of distorted lead iodide octahedra stimulated mainly by electron-hole pair creation. The c axis expands up to 0.37% over hundreds of picoseconds; reflections sampling the a and b axes undergo one tenth of this expansion with the same timescale. Post-photoexcitation appearance of the (110) reflection in (BA)2 PbI4 would suggest a transient phase transition,however, through new single-crystal XRD, reflections are found that violate glide plane conditions in the reported Pbca structure. The static structure space group is reassigned as P21 21 21 . With this, a nonequilibrium phase transition is ruled out. These findings offer increased understanding of remarkable lattice response in 2D perovskites upon excitation.

Photoelectron spectroscopic studies on metal halide perovskite materials

Modifications of the surface and interfacial properties of metal halide perovskites (MHPs) have become pivotal strategies in designing high-performance photovoltaics and light-emitting diodes. Particularly, photoelectron spectroscopy (PES) plays a vital role in the advancement of MHP-based optoelectronic devices. Herein, we review the recent studies of PESs for perovskite devices. We first give an overview of the principles of PES and related techniques. Different properties of halide perovskite material unveiled by PES are then discussed, including elemental compositions, chemical states, electronic band structures, interfaces of MHPs and functional layers, ions distribution, and thermal- and moisture-induced degradation mechanism. We also revisit some of the issues when carrying out PES measurements on MHPs, such as irradiation and ambient effects and the uncertainty in determining band edge. Based on these findings, we foresee that PES techniques combined with advanced optical and electron microscopies will become increasingly important to correlate the molecular dynamics with the interfacial and chemical properties of MHP materials and devices.

Great Influence of Organic Cation Motion on Charge Carrier Dynamics in Metal Halide Perovskite Unraveled by Unsupervised Machine Learning.

Unsupervised machine learning combined with time-dependent density functional theory reveals the significant influence of organic cation on the charge carrier lifetime of FAPbI3 (FA = HC(NH2)2+) by analyzing their mutual information (MI) between the geometric features and the nonadiabatic coupling (NAC) and bandgap. Analysis of MI values demonstrates that the NAC and bandgap are dominated by the orientation and shape of the inorganic octahedron because iodine and lead atoms are composed of the band edge states. Counterintuitively, the correlated motion promotes the contribution of the FA cation to the NAC; in particular, one type of FA rotation even supersedes the influence of the velocities of the lead and iodine atoms due to the enhanced hydrogen bond interaction. Our study demonstrates the importance of the correlated motion on the excited-state lifetimes of FAPbI3, which provides a guidance for optimizing the optoelectronic properties of metal halide perovskites.

Unveiling the Valence State of Interstitial Bromine on Charge Carrier Lifetime in CH3NH3PbBr3 by Quantum Dynamics Simulation.

Interstitial halogens are detrimental to the optoelectronic properties of metal halide perovskites. Using nonadiabatic (NA) molecular dynamics, we demonstrate that the valence state of interstitial bromine strongly changes the carrier lifetimes of MAPbBr3 (MA = CH3NH3+). Both neutral and negatively charged interstitial bromine create no midgap states, and they decrease the bandgap, weaken the NA coupling, and accelerate decoherence in a different extent with respect to pristine MAPbBr3, making free charge recombination either slow down about a 3-fold or remain largely unchanged. In contrast, a positively charged interstitial bromine forms a Br trimer and introduces a deep electron trap state, causing a 1.4-fold increase of charge recombination followed by a rapid electron trapping or across the bandgap because of an enhanced NA coupling. The simulations uncover the influence of different charged interstitial bromine defects on MAPbBr3 carrier lifetimes and provide rational guidelines for optimizing perovskite solar cells.

Nacre inspired robust self-encapsulating flexible perovskite photodetector

The unique properties of metal halide perovskites give them great potential for applications in portable and wearable optoelectronics. However, the intrinsic brittleness of perovskites favors easy crack propagation that deteriorates the optoelectronic performance of the devices. Inspired by nacre, we propose flexible perovskite thin films fabricated by antisolvent-assisted self-encapsulation of polymer and imprinting techniques during crystallization. Such an approach results in synchronous formation of polymer-glued perovskite grains exhibiting nacre-like “brick-and-mortar” structure. The deformable composite architecture bonding hemisphere-shaped grating (HG) and porous photonic crystal (PC) (HG-PC) improves its crystalline quality and light-harvesting capability. An HG-PC photodetector (PD) with high responsivity of 17.31 A/W and detectivity of 5.02 × 1013 Jones is achieved. Our flexible HG-PC PD retains 95% of the initial photocurrent after 1000 bending cycles at 2 mm curvature radius. This solvent-driven self-encapsulation strategy offers an innovative and universal approach for highly efficient flexible optoelectronics.

Recent advancements in batteries and photo-batteries using metal halide perovskites

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.

Low Exciton Binding Energies and Localized Exciton–Polaron States in 2D Tin Halide Perovskites

AbstractAside from band gap reduction, little is understood about the effect of the tin‐for‐lead substitution on the fundamental optical and optoelectronic properties of metal halide perovskites (MHPs), especially when transitioning from 3D to lower dimensional structures. Herein, we take advantage of the spectroscopic isolation of excitons in 2D MHPs to study the intrinsic differences between lead and tin MHPs. The exciton's spectral fine structure indicates a larger polaron binding energy in tin MHPs. Additionally, the electroabsorption responses of the 2D MHPs demonstrates that tin MHPs have exciton binding energies 1.5–2× lower than that of their lead counterparts. Despite the lower binding energy, the excitons in tin MHPs are more Frenkel‐like with small radii, small polarizabilities, and large dipole moments. These results are interpreted as consequences of small polaron formation and disorder‐induced dipole moments. This work highlights the wide range of intrinsic differences between lead and tin MHPs as well as the complexity of excited states in these systems.