Direct Observation of Halide Migration and its Effect on the Photoluminescence of Methylammonium Lead Bromide Perovskite Single Crystals

Resistive switching memories with enhanced durability enabled by mixed-dimensional perfluoroarene perovskite heterostructures

Hybrid halide perovskites are attractive candidates for resistive switching memories in neuromorphic computing applications due to their mixed ionic-electronic conductivity. Moreover, their exceptional optoelectronic characteristics make them effective as semiconductors in photovoltaics, opening perspectives for self-powered memory elements. These devices, however, remain unexploited, which is related to the variability in their switching characteristics, weak endurance, and retention, which limit their performance and practical use. To address this challenge, we applied low-dimensional perovskite capping layers onto 3D mixed halide perovskites using two perfluoroarene organic cations, namely (perfluorobenzyl)ammonium and (perfluoro-1,4-phenylene)dimethylammonium iodide, forming Ruddlesden-Popper and Dion-Jacobson 2D perovskite phases, respectively. The corresponding mixed-dimensional perovskite heterostructures were used to fabricate resistive switching memories based on perovskite solar cell architectures, showing that the devices based on perfluoroarene heterostructures exhibited enhanced performance and stability in inert and ambient air atmosphere. This opens perspectives for multidimensional perovskite materials in durable self-powered memory elements in the future.

Electron Spectroscopy and Microscopy: A Window into the Surface Electronic Properties of Polycrystalline Metal Halide Perovskites

In the past years, an increasing number of experimental techniques have emerged to address the need to unveil the chemical, structural, and electronic properties of perovskite thin films with high vertical and lateral spatial resolutions. One of these is angle-resolved photoemission electron spectroscopy which can provide direct access to the electronic band structure of perovskites, with the aim of overcoming elusive and controversial information due to the complex data interpretation of purely optical spectroscopic techniques. This perspective looks at the information that can be gleaned from the direct measurement of the electronic band structure of single crystal perovskites and the challenges that remain to be overcame to extend this technique to heterogeneous polycrystalline metal halide perovskites.

Aging CsPbBr3 Nanocrystal Wafer for Ultralow Ionic Migration and Environmental Stability for Direct X-ray Detection

The outstanding photoelectric properties of perovskites demonstrate extreme promise for application in X-ray detection. However, the soft lattice of the perovskite results in severe ionic migration for three-dimensional materials, limiting the operation stability of perovskite X-ray detectors. Although ligand-decorated nanocrystals (NCs) exhibit significantly higher stability than three-dimensional perovskites, defects remaining on the interface of NCs could still trigger halide migration under a high bias due to the incomplete ligand decoration. Furthermore, it is still challenging to realize sufficient thickness of absorption layers based on NCs for X-ray detectors through traditional methods. Herein, we develop a centimeter-size and millimeter-thick wafer based on CsPbBr3 NCs through isostatic pressing for X-ray detectors, in which the interfacial defects of NCs are remedied by CsPb2Br5 during aging of wafer in ambient humidity. The wafer shows outstanding sensitivity (200 μC Gyair-1 cm-2) and ultralow dark current drift (1.78 × 10-8 nA cm-1 s-1 V-1 @ 400 V cm-1). Moreover, it shows storage stability with negligible performance degradation for 60 days in ambient humidity. Thus, aging perovskite NC wafers for X-ray detection holds huge potential for next-generation X-ray imaging plates.

Field-Induced Transport Anisotropy in Single-Crystalline All-Inorganic Lead-Halide Perovskite Nanowires

The dynamic crystal lattice of halide perovskites facilitates the coupled transport of ions and electrons, offering innovative concepts in semiconductor iontronic devices that surpass solar cell applications. However, a comprehensive understanding of the intricacies of coupled ionic and electronic transport at the microscale remains ambiguous, owing to the inhomogeneity in ploy-crystalline perovskite thin films. In this work, we employed one-dimensional (1D) single-crystalline CsPbBr3 nanowires (NWs) to investigate the electric field induced ionic transport. Upon poling by an external bias, the previously uniform NW exhibits highly anisotropic ionic transport, which is identified as the origin of the giant switchable photovoltaic effect by spatially resolved scanning photocurrent microscopy. The subsequent ultrafast scanning photoluminescence (PL) microscopy measurements demonstrate significant localization of photocarriers near one terminal of the device, which is attributed to the accumulation of halogen vacancies. In addition, thanks to the enhancement of the local electric field, the poled device shows a 10-fold increase of photoresponse speed. Our findings favor the scale-down of perovskite devices to the submicrometer scale, extending their applications in self-powered iontronic and optoelectronic devices.

Facet-Dependent Photoelectrochemistry on Single Crystal Organic-Inorganic Halide Perovskite Electrodes

Organometallic halide perovskites have garnered significant attention in various fields of material science, particularly solar energy conversion, due to their desirable optoelectronic properties and compatibility with scalable fabrication techniques. It is often unclear, however, how carrier generation and transport within complex polycrystalline films are influenced by variations in local structure. Elucidating how distinct structural motifs within these heterogeneous systems affect behavior could help guide the continued improvement of perovskite-based solar cells. Here, we present studies applying scanning electron microscopy (SECCM) to map solar energy harvesting within well-defined model systems of organometallic halide perovskites. Methylammonium lead bromide (MAPbBr3) single crystals were prepared via a low-temperature solution-based route, and their photoelectrochemical properties were mapped via SECCM using p-benzoquinone (BQ) in dichloromethane as a redox mediator. Correlated SECCM mapping and electron microscopy studies enabled facet-to-facet variations in photoelectrochemical performance to be revealed and carrier transport lengths to be evaluated. The photoelectrochemical behavior observed within individual single crystals was quite heterogeneous, attributable to local variations in crystal structure/orientations, intrafacet junctions, and the presence of other structural defects. These observations underscore the significance of controlling the microstructure of single perovskite crystals, presenting a promising avenue for further enhancement of perovskite-based solar cells.

What Can Glow Discharge Optical Emission Spectroscopy (GD-OES) Technique Tell Us about Perovskite Solar Cells?

The emerging broad range of applications of the glow discharge optical emission spectroscopy (GD-OES) technique in the field of perovskite solar cells (PSCs) research is reviewed. It can provide a large palette of information by easily and quickly tracking the depth distribution of light to heavy elements. After a discussion of the advantages and the limitations of the technique and a comparison with other analytical techniques, how GD-OES is employed to give structural information on perovskite solar cells is shown. GD-OES has allowed the full perovskite film formation process investigation, from the initial precursor layers containing soaking and complexed solvent to the final crystallized 3D perovskite layers. The A-site elemental cations distribution is followed-up during the film formation. In addition, this technique gives a deep insight into the action mechanism of additives and their effects on the film formation. It provides fruitful information on optimized light absorbing layers and on the selective contact layers which ensure the charge transport in PSCs. It allows to directly visualize halide ions migration and their blocking by ad-hoc chemical engineering and to study the films and PSCs ageing. GD-OES opens new perspectives to explain the final performances of the devices.

Extreme-Ultraviolet Excited Scintillation of Methylammonium Lead Bromide Perovskites

Inorganic-Organic lead halide materials have been recognized as potential high-energy X-ray detectors because of their high quantum efficiencies and radiation hardness. Surprisingly little is known about whether the same is true for extreme-ultraviolet (XUV) radiation, despite applications in nuclear fusion research and astrophysics. We used a table-top high-harmonic generation setup in the XUV range between 20 and 45 eV to photoexcite methylammonium lead bromide (MAPbBr₃) and measure its scintillation properties. The strong absorbance combined with multiple carriers being excited per photon yield a very high carrier density at the surface, triggering photobleaching reactions that rapidly reduce the emission intensity. Concurrent to and in spite of this photobleaching, a recovery of the emission intensity as a function of dose was observed. X-ray photoelectron spectroscopy and X-ray diffraction measurements of XUV-exposed and unexposed areas show that this recovery is caused by XUV-induced oxidation of MAPbBr₃, which removes trap states that normally quench emission, thus counteracting the rapid photobleaching caused by the extremely high carrier densities. Furthermore, it was found that preoxidizing the sample with ozone was able to prolong and improve this intensity recovery, highlighting the impact of surface passivation on the scintillation properties of perovskite materials in the XUV range.

Ion Migration in Perovskite Light-Emitting Diodes: Mechanism, Characterizations, and Material and Device Engineering

In recent years, perovskite light-emitting diodes (PeLEDs) have emerged as a promising new lighting technology with high external quantum efficiency, color purity, and wavelength tunability, as well as, low-temperature processability. However, the operational stability of PeLEDs is still insufficient for their commercialization. The generation and migration of ionic species in metal halide perovskites has been widely acknowledged as the primary factor causing the performance degradation of PeLEDs. Herein, this topic is systematically discussed by considering the fundamental and engineering aspects of ion-related issues in PeLEDs, including the material and processing origins of ion generation, the mechanisms driving ion migration, characterization approaches for probing ion distributions, the effects of ion migration on device performance and stability, and strategies for ion management in PeLEDs. Finally, perspectives on remaining challenges and future opportunities are highlighted.

Quasi-2D Perovskite Crystalline Layers for Printable Direct Conversion X-Ray Imaging

Polycrystalline perovskite film-based X-ray detector is an appealing technology for assembling large scale imager by printing methods. However, thick crystalline layer without trap and solvent residual is challenging to fabricate. Here, the authors report a solution method to produce high quality quasi-2D perovskite crystalline layers and detectors that are suitable for X-ray imaging. By introducing n-butylamine iodide into methylammonium lead iodide precursor and coating at elevated temperatures, compact and crystalline layers with exceptional uniformity are obtained on both rigid and flexible substrates. Photodiodes built with the quasi-2D layers exhibit a low dark current and stable operation under constant electrical field over 96 h in dark, and over 15 h under X-ray irradiation. The detector responds sensitively under X-ray, delivering a high sensitivity of 1214 µC Gy_air ^-1  cm^-2 and a sensitivity gain is observed when operated under higher fields. Finally, high resolution images are demonstrated using a single pixel device that can resolve 80-200 µm features. This work paves the path for printable direct conversion X-ray imager development.

Instability Issues and Stabilization Strategies of Lead Halide Perovskites for Photo(electro)catalytic Solar Fuel Production

Photo(electro)catalysis is a promising route to utilizing solar energy to produce valuable chemical fuels. In recent years, lead halide perovskites (LHPs) as a class of high-performance semiconductor materials have been extensively used in photo(electro)catalytic solar fuel production because of their excellent photophysical properties. However, instability issues make it arduous for LHPs to achieve their full potential in photo(electro)catalysis. This Perspective discusses the instability issues and summarizes the stabilization strategies employed for prolonging the stability or durability of LHPs in photo(electro)catalytic solar fuel production. The strategies for particulate photocatalytic systems (including composition engineering, surface passivation, core-shell structures construction, and solvent selection) and for thin-film PEC systems (including physical protective coating, A site cation additive, and surface/interface passivation) are introduced. Finally, some challenges and opportunities regarding the development of stable and efficient LHPs for photo(electro)catalysis are proposed.

Reduced Barrier for Ion Migration in Mixed-Halide Perovskites

Halide alloying in metal halide perovskites is a useful tool for optoelectronic applications requiring a specific bandgap. However, mixed-halide perovskites show ion migration in the perovskite layer, leading to phase segregation and reducing the long-term stability of the devices. Here, we study the ion migration process in methylammonium-based mixed-halide perovskites with varying ratios of bromide to iodide. We find that the mixed-halide perovskites show two separate halide migration processes, in contrast to pure-phase perovskites, which show only a unique halide migration component. Compared to pure-halide perovskites, these processes have lower activation energies, facilitating ion migration in mixed versus pure-phase perovskites, and have a higher density of mobile ions. Under illumination, we find that the concentration of mobile halide ions is further increased and notice the emergence of a migration process involving methylammonium cations. Quantifying the ion migration processes in mixed-halide perovskites shines light on the key parameters allowing the design of bandgap-tunable perovskite solar cells with long-term stability.

Co-evaporation of CH3NH3PbI3: How Growth Conditions Impact Phase Purity, Photostriction, and Intrinsic Stability

Hybrid organic-inorganic perovskites are highly promising candidates for the upcoming generation of single- and multijunction solar cells. Despite their extraordinarily good semiconducting properties, there is a need to increase the intrinsic material stability against heat, moisture, and light exposure. Understanding how variations in synthesis affect the bulk and surface stability is therefore of paramount importance to achieve a rapid commercialization on large scales. In this work, we show for the case of methylammonium lead iodide that a thorough control of the methylammonium iodide (MAI) partial pressure during co-evaporation is essential to limit photostriction and reach phase purity, which dictate the absorber stability. Kelvin probe force microscopy measurements in ultrahigh vacuum corroborate that off-stoichiometric absorbers prepared with an excess of MAI partial pressure exhibit traces of low-dimensional (two-dimensional, 2D) perovskites and stacking faults that have adverse effects on the intrinsic material stability. Under optimized growth conditions, time-resolved photoluminescence and work functions mapping corroborate that the perovskite films are less prone to heat and light degradation.

Revealing Dynamic Effects of Mobile Ions in Halide Perovskite Solar Cells Using Time-Resolved Microspectroscopy

Halide perovskites are promising candidate materials for the next generation high-efficiency optoelectronic devices. Since perovskites are electronic-ionic mixed conductors, ion dynamics have a critical impact on the performance and stability of perovskite-based applications. However, comprehensively understanding ionic dynamics is challenging, particularly on nanoscale imaging of ionic dynamics in perovskites. In this review, mobile ion dynamics in halide perovskites investigated via luminescence spectroscopy combined with confocal microscopy are discussed, including mobile ion induced fluorescence quenching, phase segregation in mixed halide hybrid perovskite, and mobile ion accumulation at the interface in perovskite devices. Steady-state and time-resolved luminescence imaging techniques, combined with confocal microscopy, are unique tools for probing ionic dynamics in perovskites, providing invaluable insights on ionic dynamics in nanoscale resolution, along with a wide temporal range from picoseconds to hours. The works in this review are not only for understanding mobile ions to improve the design of perovskite-based devices but also foster the development of microspectroscopic methodologies in a broader solid-state physics context of investigating ionic transports in polycrystalline materials.

External Field-Tunable Internal Orbit-Orbit Interaction in Flexible Perovskites

In hybrid metal halide perovskites, electrons carry both orbital and spin momenta through s-p wave function hybridization. This leads to a hypothesis that the orbit-orbit interaction between excitons can occur through orbital magnetic dipoles forming short-range interaction or through orbital polarizations forming long-range interaction to influence optoelectronic properties. This Letter reports an interesting phenomenon: the orbit-orbit interaction can be electrically switched between orbital magnetic dipoles and orbital polarizations in a flexible perovskite (MAPbI_3-xCl_x) solar cell by scanning an external voltage between forward and reverse biases (0.2 and -0.2 V). Essentially, this phenomenon presents an external mechanism for electrically controlling the internal orbit-orbit interaction in hybrid perovskites. It was further observed that this bias-switchable orbit-orbit interaction is sensitive to temperature, becoming negligible when the temperature is decreased from 300 to 250 K. This observation indicates that the mobile ions driven by an external electrical field provide an intrinsic mechanism for electrically switching the orbit-orbit interaction through polarization and spin parameters while applying an external voltage between forward and reverse biases. These results provide a comprehensive understanding of tuning the orbit-orbit interaction in flexible perovskites toward developing orbitronic actions.

Understanding the Stability of MAPbBr3 versus MAPbI3: Suppression of Methylammonium Migration and Reduction of Halide Migration

Solar cells based on metal halide perovskites often show excellent efficiency but poor stability. This degradation of perovskite devices has been associated with the migration of mobile ions. MAPbBr₃ perovskite materials are significantly more stable under ambient conditions than MAPbI₃ perovskite materials. In this work, we use transient ion drift to quantify the key characteristics of ion migration in MAPbBr₃ perovskite solar cells. We then proceed to compare them with those of MAPbI₃ perovskite solar cells. We find that in MAPbBr₃, bromide migration is the main process at play and that contrary to the case of MAPbI₃, there is no evidence for methylammonium migration. Quantitatively, we find a reduced activation energy, a reduced diffusion coefficient, and a reduced concentration for halide ions in MAPbBr₃ compared to MAPbI₃. Understanding this difference in mobile ion migration is a crucial step in understanding the enhanced stability of MAPbBr₃ versus MAPbI₃.

Controlled Size Growth of Thermally Stable Organometallic Halide Perovskite Microrods: Synergistic Effect of Dual-Doping, Lattice Strain Engineering, Antisolvent Crystallization, and Band Gap Tuning Properties

Organometallic halide perovskites, as the light-harvesting material, have been extensively used for cost-effective energy production in high-performance perovskite solar cells, despite their poor stability in the ambient atmosphere. In this work, methylammonium lead iodide, CH₃NH₃PbI₃, perovskite was successfully doped with KMnO₄ using antisolvent crystallization to develop micrometer-length perovskite microrods. Thus, the obtained KMnO₄-doped perovskite microrods have exhibited sharp, narrow, and red-shifted photoluminescence band, as well as high lattice strain with improved thermal stability compared to undoped CH₃NH₃PbI₃. During the synthesis of the KMnO₄-doped perovskite microrods, a low boiling point solvent, anhydrous chloroform, was employed as an antisolvent to facilitate the emergence of controlled-size perovskite microrods. The as-synthesized KMnO₄-doped perovskite microrods retained the pristine perovskite crystalline phases and lowered energy band gap (∼1.57 eV) because of improved light absorption and narrow fluorescence emission bands (fwhm < 10 nm) with improved lattice strain (∼4.42 × 10^-5), Goldsmith tolerance factor (∼0.89), and high dislocation density (∼5.82 × 10^-4), as estimated by Williamson-Hall plots. Thus, the obtained results might enhance the optical properties with reduced energy band gap and high thermal stability of doped-perovskite nanomaterials in ambient air for diverse optoelectronic applications. This study paves the way for new insights into chemical doping and interaction possibilities in methylamine-based perovskite materials with various metal dopants for further applications.

Development of Halide Perovskite Single Crystal for Radiation Detection Applications

Preface: Recently, low-cost perovskite single crystals have attracted intensive attention due to their excellent optoelectronic properties and improved stability when compared to polycrystalline films for various applications, such as solar cells (Kojima et al., 2009; Lee et al., 2012; Tsai et al., 2016; Sahli et al., 2018), lasers (Gu et al., 2016; Veldhuis et al., 2016), radiation detection (Kim et al., 2017), and so on. The unique optoelectronic properties and low-cost growing processes for large-sized single crystals also make them greatly suitable for radiation detection. In this review, we summarize various synthesis methods of perovskite single crystals and introduced the high radiation detection performance of the perovskite single crystal. The advantages and limitations of halide perovskite single crystals as radiation detector candidates will be discussed in detail, and corresponding future development trends can be expected by overcoming current obstacles (Leijtens et al., 2018; Boyd et al., 2019), such as ion migration (Eames et al., 2015), stability, etc.

Origin of Open-Circuit Voltage Losses in Perovskite Solar Cells Investigated by Surface Photovoltage Measurement

Increasing the open-circuit voltage (V_oc) is one of the key strategies for further improvement of the efficiency of perovskite solar cells. It requires fundamental understanding of the complex optoelectronic processes related to charge carrier generation, transport, extraction, and their loss mechanisms inside a device upon illumination. Herein, we report the important origin of V_oc losses in methylammonium lead iodide perovskite (MAPI)-based solar cells, which results from undesirable positive charge (hole) accumulation at the interface between the perovskite photoactive layer and the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole-transport layer. We show strong correlation between the thickness-dependent surface photovoltage and device performance, unraveling that the interfacial charge accumulation leads to charge carrier recombination and results in a large decrease in V_oc for the PEDOT:PSS/MAPI inverted devices (180 mV reduction in 50 nm thick device compared to 230 nm thick one). In contrast, accumulated positive charges at the TiO₂/MAPI interface modify interfacial energy band bending, which leads to an increase in V_oc for the TiO₂/MAPI conventional devices (70 mV increase in 50 nm thick device compared to 230 nm thick one). Our results provide an important guideline for better control of interfaces in perovskite solar cells to improve device performance further.

CH3NH3Br solution as a novel platform for the selective fluorescence detection of Pb2+ ions

The development of a simple fluorescent sensor for detecting the Pb²⁺ heavy metal is fundamentally important. The CH₃NH₃PbBr₃ perovskite material exhibits excellent photoluminescence properties that are related to Pb²⁺. Based on the effects of Pb²⁺ on the luminescent properties of CH₃NH₃PbBr₃, we design a novel platform for the selective fluorescence detection of Pb²⁺ ions. Herein, we use a CH₃NH₃Br solution at a high concentration as the fluorescent probe. Incorporation of PbBr₂ into the CH₃NH₃Br solution results in a rapid chemical reaction to form CH₃NH₃PbBr₃. Hence, the nonfluorescent CH₃NH₃Br material displays a sensitive and selective luminescent response to Pb²⁺ under UV light illumination. Moreover, the reaction between CH₃NH₃Br and PbBr₂ could transform Pb²⁺ into CH₃NH₃PbBr₃, and therefore, CH₃NH₃Br may also be used to extract Pb²⁺ from liquid waste in recycling applications.

Ferroelectricity-free lead halide perovskites

Direct piezoelectric force microscopy (DPFM) is employed to examine whether or not lead halide perovskites exhibit ferroelectricity. Compared to conventional piezoelectric force microscopy, DPFM is a novel technique capable of measuring piezoelectricity directly. This fact is fundamental to be able to examine the existence of ferroelectricity in lead halide perovskites, an issue that has been under debate for several years. DPFM is used to detect the current signals, i.e. changes in the charge distribution under the influence of the scan direction and applied force of the atomic force microscope (AFM) tip in contact mode. For comparison, (i) we use DPFM on lead halide perovskites and well-known ferroelectric materials (i.e. periodically poled lithium niobate and lead zirconate titanate); and (ii) we conduct parallel experiments on MAPbI₃ films of different grain sizes, film thicknesses, substrates, and textures using DPFM as well as piezoelectric force microscopy (PFM) and electrostatic force microscopy (EFM). In contrast to previous work that claimed there were ferroelectric domains in MAPbI₃ perovskite films, our work shows that the studied perovskite films Cs_0.05(FA_0.83MA_0.17)_0.95Pb(I_0.83Br_0.17)₃ and MAPbI₃ are ferroelectricity-free. The observed current profiles of lead halide perovskites possibly originate from ion migration that happens under an applied electrical bias and in strained samples under mechanical stress. This work provides a deeper understanding of the fundamental physical properties of the organic-inorganic lead halide perovskites and solves a longstanding dispute about their non-ferroelectric character: an issue of high relevance for optoelectronic and photovoltaic applications.

Enabling Self-passivation by Attaching Small Grains on Surfaces of Large Grains toward High-Performance Perovskite LEDs

This paper reports a new method to generate stable and high-brightness electroluminescence (EL) by subsequently growing large/small grains at micro/nano scales with the configuration of attaching small grains on the surfaces of large grains in perovskite (MAPbBr₃) films by mixing two precursor solutions (PbBr₂ + MABr and Pb(Ac)₂·3H₂O + MABr). Consequently, the small and large grains serve, respectively, as passivation agents and light-emitting centers, enabling self-passivation on the defects located on the surfaces of light-emitting large grains. Furthermore, the light-emitting states become linearly polarized with maximal polarization of 30.8%, demonstrating a very stable light emission (49,119 cd/m² with EQE = 11.31%) and a lower turn-on bias (1.9 V) than the bandgap (2.25V) in the perovskite LEDs (ITO/PEDOT:PSS/MAPbBr₃/TPBi[50 nm]/LiF[0.7 nm]/Ag). Therefore, mixing large/small grains with the configuration of attaching small grains on the surfaces of large grains by mixing two precursor solutions presents a new strategy to develop high-performance perovskite LEDs.

Emergence of multiple fluorophores in individual cesium lead bromide nanocrystals

Cesium-based perovskite nanocrystals (PNCs) possess alluring optical and electronic properties via compositional and structural versatility, tunable bandgap, high photoluminescence quantum yield and facile chemical synthesis. Despite the recent progress, origins of the photoluminescence emission in various types of PNCs remains unclear. Here, we study the photon emission from individual three-dimensional and zero-dimensional cesium lead bromide PNCs. Using photon antibunching and lifetime measurements, we demonstrate that emission statistics of both type of PNCs are akin to individual molecular fluorophores, rather than traditional semiconductor quantum dots. Aided by density functional modelling, we provide compelling evidence that green emission in zero-dimensional PNCs stems from exciton recombination at bromide vacancy centres within lead-halide octahedra, unrelated to external confinement. These findings provide key information about the nature of defect formation and the origin of emission in cesium lead halide perovskite materials, which foster their utilization in the emerging optoelectronic applications.

Inorganic perovskite nanocrystals attract lots of research attention but the origin of their photoluminescence remains debatable. Here Zhang et al. show that behavior of both CsPbBr₃ and Cs₄PbBr₆ nanocrystals is like individual molecular fluorophores and independent of the structural dimensionalities.

High-Performance Solution-Processed Organo-Metal Halide Perovskite Unipolar Resistive Memory Devices in a Cross-Bar Array Structure

Resistive random access memories can potentially open a niche area in memory technology applications by combining the advantages of the long endurance of dynamic random-access memory and the long retention time of flash memories. Recently, resistive memory devices based on organo-metal halide perovskite materials have demonstrated outstanding memory properties, such as a low-voltage operation and a high ON/OFF ratio; such properties are essential requirements for low power consumption in developing practical memory devices. In this study, a nonhalide lead source is employed to deposit perovskite films via a simple single-step spin-coating method for fabricating unipolar resistive memory devices in a cross-bar array architecture. These unipolar perovskite memory devices achieve a high ON/OFF ratio up to 10⁸ with a relatively low operation voltage, a large endurance, and long retention times. The high-yield device fabrication based on the solution-process demonstrated here will be a step toward achieving low-cost and high-density practical perovskite memory devices.

Halide lead perovskites for ionizing radiation detection

Halide lead perovskites have attracted increasing attention in recent years for ionizing radiation detection due to their strong stopping power, defect-tolerance, large mobility-lifetime (μτ) product, tunable bandgap and simple single crystal growth from low-cost solution processes. In this review, we start with the requirement of material properties for high performance ionizing radiation detection based on direct detection mechanisms for applications in X-ray imaging and γ-ray energy spectroscopy. By comparing the performances of halide perovskites radiation detectors with current state-of-the-art ionizing radiation detectors, we show the promising features and challenges of halide perovskites as promising radiation detectors.

Halide lead perovskites have emerged recently as possible candidates for high performance radiation detectors besides efficient solar cells. Here Wei et al. review the recent progress on perovskite based radiation detectors and suggest that they may compete with the conventional counterparts.

Effect of Crystal Grain Orientation on the Rate of Ionic Transport in Perovskite Polycrystalline Thin Films

In this work, we examine the effect of microstructure on ion-migration-induced photoluminescence (PL) quenching in methylammonium lead iodide perovskite films. Thin films were fabricated by two methods: spin-coating, which results in randomly oriented perovskite grains, and zone-casting, which results in aligned grains. As an external bias is applied to these films, migration of ions causes a quenching of the PL signal in the vicinity of the anode. The evolution of this PL-quenched zone is less uniform in the spin-coated devices than in the zone-cast ones, suggesting that the relative orientation of the crystal grains plays a significant role in the migration of ions within polycrystalline perovskite. We simulate this effect via a simple Ising model of ionic motion across grains in the perovskite thin film. The results of this simulation align closely with the observed experimental results, further solidifying the correlation between crystal grain orientation and the rate of ionic transport.

Spin-Coated CH₃NH₃PbBr₃ Film Consisting of Micron-Scale Single Crystals Assisted with a Benzophenone Crystallizing Agent and Its Application in Perovskite Light-Emitting Diodes

Owing to the superior properties of optical and electronic properties, perovskite single crystals have been in high demand recently. However, the growth of large-sized single crystals requires several processing steps and a long growth time, which engenders great difficulties in device integration. Herein, benzophenone (BP) was firstly introduced as a crystallizing agent to facilitate the construction of a high-quality CH₃NH₃PbBr₃ (MAPbBr₃) film consisting of micron-scale single crystals in a one-step spin-coating method. We studied the influence of the BP concentration upon the size and shape of the micron-scale single crystals. Moreover, due to the enhanced morphology of the MAPbBr₃ film with low-defect micron-scale single crystals, perovskite light-emitting diodes (PeLEDs) have been demonstrated with a maximum luminance of 1057.6 cd/m² and a turn-on voltage as low as 2.25 V. This approach not only proposes a concise and highly repeatable method for the formation of micron-scale perovskite single crystals, but also paves a way for the realization of efficient PeLEDs.

Zero-Dimensional Cesium Lead Halides: History, Properties, and Challenges

Over the past decade, lead halide perovskites (LHPs) have emerged as new promising materials in the fields of photovoltaics and light emission due to their facile syntheses and exciting optical properties. The enthusiasm generated by LHPs has inspired research in perovskite-related materials, including the so-called "zero-dimensional cesium lead halides", which will be the focus of this Perspective. The structure of these materials is formed of disconnected lead halide octahedra that are stabilized by cesium ions. Their optical properties are dominated by optical transitions that are localized within the individual octahedra, hence the title "'zero-dimensional perovskites". Controversial results on their physical properties have recently been reported, and the true nature of their photoluminescence is still unclear. In this Perspective, we will take a close look at these materials, both as nanocrystals and as bulk crystals/thin films, discuss the contrasting opinions on their properties, propose potential applications, and provide an outlook on future experiments.

Perovskite Nanowire Extrusion

The defect tolerance of halide perovskite materials has led to efficient optoelectronic devices based on thin-film geometries with unprecedented speed. Moreover, it has motivated research on perovskite nanowires because surface recombination continues to be a major obstacle in realizing efficient nanowire devices. Recently, ordered vertical arrays of perovskite nanowires have been realized, which can benefit from nanophotonic design strategies allowing precise control over light propagation, absorption, and emission. An anodized aluminum oxide template is used to confine the crystallization process, either in the solution or in the vapor phase. This approach, however, results in an unavoidable drawback: only nanowires embedded inside the AAO are obtainable, since the AAO cannot be etched selectively. The requirement for a support matrix originates from the intrinsic difficulty of controlling precise placement, sizes, and shapes of free-standing nanostructures during crystallization, especially in solution. Here we introduce a method to fabricate free-standing solution-based vertical nanowires with arbitrary dimensions. Our scheme also utilizes AAO; however, in contrast to embedding the perovskite inside the matrix, we apply a pressure gradient to extrude the solution from the free-standing templates. The exit profile of the template is subsequently translated into the final semiconductor geometry. The free-standing nanowires are single crystalline and show a PLQY up to ∼29%. In principle, this rapid method is not limited to nanowires but can be extended to uniform and ordered high PLQY single crystalline perovskite nanostructures of different shapes and sizes by fabricating additional masking layers or using specifically shaped nanopore endings.