Halide Double‐Perovskite Light‐Emitting Centers Embedded in Lattice‐Matched and Coherent Crystalline Matrix

Through first‐principles calculations, it is found that two lattice‐matched halide double‐perovskites, Cs₂NaBiBr₆ and Cs₂AgBiBr₆, have a type‐I band alignment and can form highly miscible alloys in which the disordering makes the bandgaps become direct and activates the direct transition from the valence to conduction band edge, leading to a strong optical absorption and high radiative recombination rate. The bandgaps of the alloys are tunable in a wide range of 1.93–3.24 eV, while the lattice constants remain unchanged. This advantage inspires the design of a coherent crystalline matrix based on Cs₂(Na,Ag)BiBr₆ alloys, in which the Ag‐rich and narrower‐bandgap regions are embedded in the Na‐rich and wide‐bandgap region with lattice‐matched and coherent interfaces. The type‐I band alignment drives the photogenerated excitons into the narrower‐bandgap Ag‐rich regions, so the regions become light‐emitting centers with a high photoluminescence quantum yield (PLQY). The bandgaps of the Ag‐rich regions are tunable, so the color of emitted light can be adjusted, making a broadband emission possible. Such kind of coherent crystalline matrix with high‐PLQY and broadband emission can also be fabricated based on the alloys of other lattice‐matched halide double‐perovskites, demonstrating the flexibility of band structure engineering in the coherent heterostructures of various halide double‐perovskites.     

Lead-Free Double Perovskite Cs2AgBiBr6: Fundamentals,Applications, and Perspectives

Cs₂AgBiBr₆, as a benchmark lead-free double perovskite, has emerged as a promising alternative to lead-based perovskites because of its high stability, non-toxicity, exceptional optoelectronic properties, and multifunctionality. To encourage further research on Cs₂AgBiBr₆ and its broad applications, in this review, its fundamental properties including the structure-property relation, synthesis, stability, origin of absorption and photoluminescence, electron-phonon coupling, role of defects, charge carrier dynamics, and bandgap modulation are comprehensively emphasized. The recent progress on the wide applications including solar cells, light/X-ray detectors, and ferroelectric/magnetic devices are highlighted. Moreover, the challenges of Cs₂AgBiBr₆ materials and related applications are discussed and perspectives are provided for guiding the future development of this research area.     

Mixed Halide Formation in Lead-Free Antimony-Based Halide Perovskite for Boosted CO2 Photoreduction: Beyond Band Gap Tuning

Photocatalytic conversion of carbon dioxide (CO₂) into value-added fuels is a vastly promising anthropogenic chemical carbon cycle to combat the greenhouse effect while meeting the ever-increasing energy demand. Recently, lead-based halide perovskites have demonstrated great potential in various applications including photochemical reduction of CO₂. However, in view of lead toxicity, the exploration of a lead-free alternative is crucial for long term application. Herein, a series of lead-free mixed halide perovskites Cs₃Sb₂Cl_xBr_9−x (0 ≤ x ≤ 9) is prepared via a facile antisolvent recrystallization technique, where the incorporation of a secondary halide enhances the charge transfer and separation while allowing precise tuning of bandgap between 2.59 and 2.90 eV. Theoretical calculations further reveal that the formation of mixed Cl/Br halides engenders favorable charge redistribution due to lower octahedral distortion, which in turn strengthens CO₂ adsorption and activation. Under visible light illumination, the optimal dual halide perovskite, Cs₃Sb₂Cl₄Br₅ manifests substantial twofold and fourfold enhancements of CH₄ yield over the single halide perovskite, Cs₃Sb₂Br₉ and Cs₃Sb₂Cl₉, respectively. In brief, this study provides a compelling demonstration of lead-free mixed halide perovskites for photocatalytic CO₂ reduction, and it is anticipated to drive further application of perovskite-based photocatalysts toward a diverse range of artificial photoredox reactions.     

Highly Efficient Blue‐Emitting Bi‐Doped Cs2SnCl6 Perovskite Variant: Photoluminescence Induced by Impurity Doping

Lead halide perovskites show excellent optoelectronic properties but are unsatisfactory in terms of stability and toxicity. Herein, bismuth (Bi)‐doped lead‐free inorganic perovskites Cs₂SnCl₆:Bi are reported as blue emissive phosphors. Upon Bi doping, the originally nonluminous Cs₂SnCl₆ exhibits a highly efficient deep‐blue emission at 455 nm, with a Stokes shift of 106 nm and a high photoluminescence quantum yield (PLQY) close to 80%. Hybrid density functional theory calculations suggest the preferred formation of [Bi_Sn+V_Cl] defect complex, which is believed to be responsible for the optical absorption and the associated blue emission. The Cs₂SnCl₆:Bi also shows impressive thermal and water stability due to its inorganic nature and the formation of protective BiOCl layer. White light‐emitting diodes (LEDs) are constructed using Cs₂SnCl₆:Bi and commercial yellow phosphors combined with commercial UV LED chips, giving the Commission Internationale de I'Eclairage (CIE) color coordinates of (0.36, 0.37). This work represents a significant step toward the realization of highly efficient, stable, and environmentally benign next‐generation solid‐state lighting.     

Exotic Structural and Optoelectronic Properties of Layered Halide Double Perovskite Polymorphs

Discovering new types of layered perovskites has great importance for designing novel optoelectronic devices. In this article, combining first-principle calculations with global structure searching, it is found that Rb₄SnSb₂Br₁₂, a typical halide double perovskite, can unexpectedly possess fertile low formation-energy polymorphs holding van de Walls (vdW) layered structures. Consequently, these polymorphs can be effectively classified into 12 types according to their local octahedral motifs, exhibiting a wide range of bandgap covering the visible spectrum. Interestingly, the structure-dependent bandgap in these polymorphs can be well understood by developing a simple machine learning model. Moreover, as a layered system, the optoelectronic properties of Rb₄SnSb₂Br₁₂ can be effectively tuned by the layer thickness, and both type-I and type-II band alignment can be achieved in single-compound Rb₄SnSb₂Br₁₂ heterojunctions. Finally, it is suggested that the Sn-moderate condition can be considered to grow intrinsic p-type Rb₄SnSb₂Br₁₂ with lower defect density. Those findings not only provide a promising material system for designing the vdW tandem solar cell, but also offer a new opportunity to achieve exotic optoelectronic applications in a single-phase layered perovskite compound.     

Response to Comment on “Prediction of Novel p‐Type Transparent Conductors in Layered Double Perovskites: A First‐Principles Study”

The search for high‐performance p‐type transparent conductors is crucial to many emerging optoelectronic applications. Motivated by the pioneer predictions of the existence of intrinsic p‐type conductivity in several wide‐gap layered halide double perovskites, Hu et al. synthesize the Cs₄CdSb₂Cl₁₂ compound, but fail to observe the p‐type conductivity. They argue that the different conclusion is mostly due to the chemical potential boundary overestimation and bandgap underestimation in the initial calculations. In this study, based on the additional calculations, it is demonstrated that these two factors raised by Hu et al. will not affect the major conclusions, although the pinned Fermi level (E_F‐pin) can slightly be changed. Importantly, it is found that the different conclusion obtains by Xu et al. and Hu et al. is mostly due to the different crystal phases adopted, which can have a big impact on the calculated E_F‐pin positions and the resulting p‐type performance. Finally, besides the careful control of growth conditions for reaching the ideal p‐type chemical potential regions, it is suggested that a moderate strain/pressure or fast quenching may be further adopted to enhance the p‐type conductivity in Cs₄CdSb₂Cl₁₂ and other similar compounds for better experimental observations.     

Understanding the Role of Grain Boundaries on Charge-Carrier and Ion Transport in Cs2AgBiBr6 Thin Films

Halide double perovskites have gained significant attention, owing to their composition of low-toxicity elements, stability in air, and recent demonstrations of long charge-carrier lifetimes that can exceed 1 µs. In particular, Cs₂AgBiBr₆ is the subject of many investigations in photovoltaic devices. However, the efficiencies of solar cells based on this double perovskite are still far from the theoretical efficiency limit of the material. Here, the role of grain size on the optoelectronic properties of Cs₂AgBiBr₆ thin films is investigated. It is shown through cathodoluminescence measurements that grain boundaries are the dominant nonradiative recombination sites. It also demonstrates through field-effect transistor and temperature-dependent transient current measurements that grain boundaries act as the main channels for ion transport. Interestingly, a positive correlation between carrier mobility and temperature is found, which resembles the hopping mechanism often seen in organic semiconductors. These findings explain the discrepancy between the long diffusion lengths >1 µm found in Cs₂AgBiBr₆ single crystals versus the limited performance achieved in their thin film counterparts. This work shows that mitigating the impact of grain boundaries will be critical for these double perovskite thin films to reach the performance achievable based on their intrinsic single-crystal properties.     

In Situ Regulating the Order–Disorder Phase Transition in Cs2AgBiBr6 Single Crystal toward the Application in an X‐Ray Detector

The double perovskite Cs₂AgBiBr₆ single crystal holds great potential for detecting applications because of its low minimum detectable dose rate and toxic‐free merit. Nevertheless, the disordered arrangement of Ag⁺/Bi³⁺ usually gives rise to unexpected structural distortion and thereafter heavily influences the photoelectric properties of the Cs₂AgBiBr₆ single crystal. Herein, phenylethylamine bromide is demonstrated to be capable of in situ regulation of the order–disorder phase transition in the Cs₂AgBiBr₆ single crystal. The improved ordering extent of alternatively arranged [AgX₆]^5? and [BiX₆]^3? octahedra is theoretically and experimentally proven to decrease the defect density and suppress self‐trapped exciton formation, and thereby tune the band gap and enhance the carrier mobility, which consequently promotes its application in an X‐ray detector. The performance of a corresponding detector based on PEA‐Cs₂AgBiBr₆ single crystal displays superior performances, e.g., longer carrier drift distance, higher photoconductive gain, and faster current response (13 vs 3190 µs). Prominently, the as‐fabricated PEA‐Cs₂AgBiBr₆ single‐crystal X‐ray detector has an extremely high sensitivity with a value of 288.8 µC Gy_air^?1 cm^?2 under a bias of 50 V (22.7 V mm^?1), which largely outperforms those of their counterparts with lower ordering structure.     

Oriented Crystallization of Mixed‐Cation Tin Halides for Highly Efficient and Stable Lead‐Free Perovskite Solar Cells

As the most promising lead‐free branch, tin halide perovskites suffer from the severe oxidation from Sn²⁺ to Sn⁴⁺, which results in the unsatisfactory conversion efficiency far from what they deserve. In this work, by facile incorporation of methylammonium bromide in composition engineering, formamidinium and methylammonium mixed cations tin halide perovskite films with ultrahighly oriented crystallization are synthesized with the preferential facet of (001), and that oxidation is suppressed with obviously declined trap density. MA⁺ ions are responsible for that impressive orientation while Br^‐ ions account for their bandgap modulation. Depending on high quality of the optimal MA_0.25FA_0.75SnI_2.75Br_0.25 perovskite films, their device conversion efficiency surges to 9.31% in contrast to 5.02% of the control formamidinium tin triiodide perovskite (FASnI₃) device, along with almost eliminated hysteresis. That also results in the outstanding device stability, maintaining above 80% of the initial efficiency after 300 h of light soaking while the control FASnI₃ device fails within 120 h. This paper definitely paves a facile and effective way to develop high‐efficiency tin halide perovskites solar cells, optoelectronic devices, and beyond.     

All‐Inorganic Bismuth‐Based Perovskite Quantum Dots with Bright Blue Photoluminescence and Excellent Stability

Lead halide perovskite quantum dots (QDs) possess color‐tunable and narrow‐band emissions and are very promising for lighting and display applications, but they suffer from lead toxicity and instability. Although lead‐free Bi‐based and Sn‐based perovskite QDs (CsSnX₃, Cs₂SnX₆, and (CH₃NH₃)₃Bi₂X₉) are reported, they all show low photoluminescence quantum yield (PLQY) and poor stability. Here, the synthesis of Cs₃Bi₂Br₉ perovskite QDs with high PLQY and excellent stability is reported. Via a green and facile process using ethanol as the antisolvent, as‐synthesized Cs₃Bi₂Br₉ QDs show a blue emission at 410 nm with a PLQY up to 19.4%. The whole series of Cs₃Bi₂X₉ (X = Cl, Br, and I) QDs by mixing precursors can cover the photoluminescence emission range from 393 to 545 nm. Furthermore, Cs₃Bi₂Br₉ QDs show excellent photostability and moisture stability due to the all‐inorganic nature and the surface passivation by BiOBr, which enables the one‐pot synthesis of Cs₃Bi₂Br₉ QD/silica composite. A lead‐free perovskite white light‐emitting diode is fabricated by simply combining the composite of Cs₃Bi₂Br₉ QD/silica with Y₃Al₅O₁₂ phosphor. As a new member of lead‐free perovskite QDs, Cs₃Bi₂Br₉ QDs open up a new route for the fabrication of optoelectronic devices due to their excellent stability and photophysical characteristics.     

Silver Ions Assisted Inversion Temperature Crystallization of 2D Cs3Bi2Br9 Nanoflakes for Highly Sensitive X-Ray Detection

2D perovskites have attracted wide attention for optoelectronic applications because of their unique layer structure and tunable outstanding optical/electrical properties. In addition, 2D Cs₃Bi₂Br₉ nanoflakes possess large effective atomic number, high resistivity, high density as well as excellent stability, rendering it a promising material for X-ray detection. Nevertheless, it is full of challenges to synthesize 2D Cs₃Bi₂Br₉ nanoflakes by conventional inversion temperature crystallization (ITC) strategy due to the existence of Br^- vacancies in the Cs₃Bi₂Br₉ crystal nucleus. Herein, an Ag⁺ assisted ITC (SAITC) strategy to grow 2D Cs₃Bi₂Br₉ nanoflakes is proposed. The synthesis mechanism revealed by both experiments and theoretical calculations can be mainly ascribed to the passivated Br⁻ vacancies and enhanced structure stability by adding Ag⁺ which can effectively prevent the oxidation of 2D Cs₃Bi₂Br₉ nanoflakes from growth of hybrid crystals. The synthesized high-crystallinity 2D Cs₃Bi₂Br₉ nanoflakes possess direct bandgap characteristic, and the mobility lifetime can reach 9.8 × 10⁻⁴ cm² V⁻¹. Excitingly, the fabricated device based on 2D Cs₃Bi₂Br₉ nanoflakes demonstrates ultrahigh sensitivity of detecting X-ray (1.9 CGy_air⁻¹cm⁻²) at very low driven voltage (0.5 V) due to the photoconductive gain mechanism. The 2D Cs₃Bi₂Br₉ nanoflakes synthesized by SAITC method have great potential for developing highly sensitive optoelectronic devices.     

Enhanced Incorporation of Guanidinium in Formamidinium‐Based Perovskites for Efficient and Stable Photovoltaics: The Role of Cs and Br

Recently, incorporating guanidium (GA) cations into organolead halide perovskites is shown to effectively improve the stability and performance of the solar cells. However, the underlying mechanisms that govern the GA incorporation have remained unclear. Here, FAPbI₃ is used as a basic framework to investigate experimentally and theoretically the role of cesium (Cs) and bromine (Br) substitutions in GA⁺ incorporation. It is found that simultaneous introduction of the small‐size Cs⁺ and Br^– in the FAPbI₃ lattice is critical to create sufficient space for the large GA⁺ and that the presence of the Cs⁺ prevents the formation of a GA‐contained low‐dimensional phase, which both assist GA⁺ incorporation. Upon entering the perovskite lattice, the GA⁺ can stabilize the lattice structure via forming strong hydrogen bonds with their neighboring halide ions. Such structure modification suppresses halide vacancy formation, thus leading to improved material properties. Compared to the GA‐free perovskite reference samples, the optimal system GA_0.05Cs_0.15FA_0.8Pb(I_0.85Br_0.15)₃ exhibits substantially improved thermal and photothermal stability, as well as increased photocarrier lifetime. Solar cells fabricated with the optimal material system show an excellent photovoltaic performance, with the champion device reaching a power conversion efficiency of 21.3% and an open circuit voltage of 1.229 V.     

An Interdiffusion Method for Highly Performing Cesium/Formamidinium Double Cation Perovskites

The fabrication of high‐quality cesium (Cs)/formamidinium (FA) double‐cation perovskite films through a two‐step interdiffusion method is reported. Cs_x FA_1‐x PbI_{3‐y(1‐x )}Br_{y(1‐x )} films with different compositions are achieved by controlling the amount of CsI and formamidinium bromide (FABr) in the respective precursor solutions. The effects of incorporating Cs⁺ and Br^? on the properties of the resulting perovskite films and on the performance of the corresponding perovskite solar cells are systematically studied. Small area perovskite solar cells with a power conversion efficiency (PCE) of 19.3% and a perovskite module (4 cm²) with an aperture PCE of 16.4%, using the Cs/FA double cation perovskite made with 10 mol% CsI and 15 mol% FABr (Cs_0.1FA_0.9PbI_2.865Br_0.135) are achieved. The Cs/FA double cation perovskites show negligible degradation after annealing at 85 °C for 336 h, outperforming the perovskite materials containing methylammonium (MA).     

Vapor Phase Growth of Centimeter-Sized Band Gap Engineered Cesium Lead Halide Perovskite Single-Crystal Thin Films with Color-tunable Stimulated Emission

Single crystal metal halide perovskites thin films are considered to be a promising optical, optoelectronic materials with extraordinary performance due to their low defect densities. However, it is still difficult to achieve large-scale perovskite single-crystal thin films (SCTFs) with tunable bandgap by vapor-phase deposition method. Herein, the synthesis of CsPbCl_3(1–x)Br_3x SCTFs with centimeter size (1 cm × 1 cm) via vapor-phase deposition is reported. The Br composition of CsPbCl_3(1–x)Br_3x SCTFs can be gradually tuned from x = 0 to x = 1, leading the corresponding bandgap to change from 2.29 to 2.91 eV. Additionally, an low-threshold (≈23.9 µJ cm⁻²) amplified spontaneous emission is achieved based on CsPbCl_3(1–x)Br_3x SCTFs at room temperature, and the wavelength is tuned from 432 to 547 nm by varying the Cl/Br ratio. Importantly, the high-quality CsPbCl_3(1–x)Br_3x SCTFs are ideal optical gain medium with high gain up to 1369.8 ± 101.2 cm⁻¹. This study not only provides a versatile method to fabricate high quality CsPbCl_3(1–x)Br_3x SCTFs with different Cl/Br ratio, but also paves the way for further research of color-tunable perovskite lasing.     

Materials Discovery of Stable and Nontoxic Halide Perovskite Materials for High‐Efficiency Solar Cells

Two critical limitations of organic–inorganic lead halide perovskite materials for solar cells are their poor stability in humid environments and inclusion of toxic lead. In this study, high‐throughput density functional theory (DFT) methods are used to computationally model and screen 1845 halide perovskites in search of new materials without these limitations that are promising for solar cell applications. This study focuses on finding materials that are comprised of nontoxic elements, stable in a humid operating environment, and have an optimal bandgap for one of single junction, tandem Si‐perovskite, or quantum dot–based solar cells. Single junction materials are also screened on predicted single junction photovoltaic (PV) efficiencies exceeding 22.7%, which is the current highest reported PV efficiency for halide perovskites. Generally, these methods qualitatively reproduce the properties of known promising nontoxic halide perovskites that are either experimentally evaluated or predicted from theory. From a set of 1845 materials, 15 materials pass all screening criteria for single junction cell applications, 13 of which are not previously investigated, such as (CH₃NH₃)_0.75Cs_0.25SnI₃, ((NH₂)₂CH)Ag_0.5Sb_0.5Br₃, CsMn_0.875Fe_0.125I₃, ((CH₃)₂NH₂)Ag_0.5Bi_0.5I₃, and ((NH₂)₂CH)_0.5Rb_0.5SnI₃. These materials, together with others predicted in this study, may be promising candidate materials for stable, highly efficient, and nontoxic perovskite‐based solar cells.     

Dual Passivation of Perovskite Defects for Light‐Emitting Diodes with External Quantum Efficiency Exceeding 20%

Solution‐processed metal halide perovskites (MHPs) have attracted much attention for applications in light‐emitting diodes (LEDs) due to their wide color gamut, high color purity, tunable emission wavelength, balanced electron/hole transportation, etc. Although MHPs are very tolerant to defects, the defects in solution‐processed perovskite LEDs (PeLEDs) still cause severe nonradiative recombination and device instability. Here, molecular design of additives for dual passivation of both lead and halide defects in perovskites is reported. A bi‐functional additive, 4‐fluorophenylmethylammonium‐trifluoroacetate (FPMATFA), is synthesized by using a simple solution process. The TFA anions and FPMA cations can bond with undercoordinated lead and halide ions, respectively, resulting in dual passivation of both lead and halide defects. In addition, the bulky FPMA group can constrain the grain growth of 3D perovskite, enhancing electron–hole capture rates and radiative recombination rates. As a result, high‐performance PeLEDs with a peak external quantum efficiency reaching 20.9% and emission wavelength at 694 nm are achieved using formamidinium‐cesium lead iodide‐bromide (FA_0.33Cs_0.67Pb(I_0.7Br_0.3)₃). Furthermore, the operational lifetime of PeLEDs is also greatly improved due to the low trap density in the perovskite film.     

Charge‐Carrier Dynamics,Mobilities, and Diffusion Lengths of 2D–3D Hybrid Butylammonium–Cesium–Formamidinium Lead Halide Perovskites

Perovskite solar cells (PSCs) have improved dramatically over the past decade, increasing in efficiency and gradually overcoming hurdles of temperature‐ and humidity‐induced instability. Materials that combine high charge‐carrier lifetimes and mobilities, strong absorption, and good crystallinity of 3D perovskites with the hydrophobic properties of 2D perovskites have become particularly promising candidates for use in solar cells. In order to fully understand the optoelectronic properties of these 2D–3D hybrid systems, the hybrid perovskite BA_x(FA_0.83Cs_0.17)_1‐xPb(I_0.6Br_0.4)₃ is investigated across the composition range 0 ≤ x ≤ 0.8. Small amounts of butylammonium (BA) are found that help to improve crystallinity and appear to passivate grain boundaries, thus reducing trap‐mediated charge‐carrier recombination and enhancing charge‐carrier mobilities. Excessive amounts of BA lead to poor crystallinity and inhomogeneous film formation, greatly reducing effective charge‐carrier mobility. For low amounts of BA, the benevolent effects of reduced recombination and enhanced mobilities lead to charge‐carrier diffusion lengths up to 7.7 µm for x = 0.167. These measurements pave the way for highly efficient, highly stable PSCs and other optoelectronic devices based on 2D–3D hybrid materials.     

Defect Engineering of Grain Boundaries in Lead‐Free Halide Double Perovskites for Better Optoelectronic Performance

Halide double perovskites (HDPs) are promising lead‐free perovskites for various optoelectronic applications. However, the device performances of HDPs are far below the optimized values, which open a critical question regarding the origin of low performance in these HDPs. In this article, using first‐principles calculations, it is found that some types of grain boundaries (GBs) are easy to form in polycrystalline HDPs. Importantly, the existence of low‐energy Σ5(310) GBs can induce harmful deep‐level defect states within the bandgaps of type‐I (e.g., Cs₂AgInCl₆) and type‐II (e.g., Cs₂AgBiCl₆) HDPs, which may dramatically reduce the device performances. Interestingly, it is found that the formation of some intrinsic defects and defect complexes could effectively eliminate these deep‐levels in type‐II and type‐I HDPs, respectively. Under some exactly predesigned growth conditions identified by utilizing thousands of chemicals through a potential screening process, these defects or defect complexes can spontaneously incorporate into the GB cores, meanwhile the harmful deep‐level defects in the bulk can also be effectively eliminated. In addition, the self‐passivated GBs could generate band bending, which may be beneficial for charge separation. The understanding of GB formation as well as the self‐passivation mechanism in HDPs can provide a new viewpoint and guidance for designing polycrystalline perovskites with improved optoelectronic performance.     

Air‐Stable Cesium Lead Iodide Perovskite for Ultra‐Low Operating Voltage Resistive Switching

CsPbX₃ (X = halide, Cl, Br, or I) all‐inorganic halide perovskites (IHPs) are regarded as promising functional materials because of their tunable optoelectronic characteristics and superior stability to organic–inorganic hybrid halide perovskites. Herein, nonvolatile resistive switching (RS) memory devices based on all‐inorganic CsPbI₃ perovskite are reported. An air‐stable CsPbI₃ perovskite film with a thickness of only 200 nm is successfully synthesized on a platinum‐coated silicon substrate using low temperature all‐solution process. The RS memory devices of Ag/polymethylmethacrylate (PMMA)/CsPbI₃/Pt/Ti/SiO₂/Si structure exhibit reproducible and reliable bipolar switching characteristics with an ultralow operating voltage (<+0.2 V), high on/off ratio (>10⁶), reversible RS by pulse voltage operation (pulse duration < 1 ms), and multilevel data storage. The mechanical flexibility of the CsPbI₃ perovskite RS memory device on a flexible substrate is also successfully confirmed. With analyzing the influence of phase transition in CsPbI₃ on RS characteristics, a mechanism involving conducting filaments formed by metal cation migration is proposed to explain the RS behavior of the memory device. This study will contribute to the understanding of the intrinsic characteristics of IHPs for low‐voltage resistive switching and demonstrate the huge potential of them for use in low‐power consumption nonvolatile memory devices on next‐generation computing systems.     

Revealing the Perovskite Film Formation Using the Gas Quenching Method by In Situ GIWAXS: Morphology,Properties, and Device Performance

The optoelectronic properties, morphology, and consequently the performance of metal halide perovskite solar cells are directly related to the crystalline phases and intermediates formed during film preparation. The gas quenching method is compatible with large-area deposition, but an understanding of how this method influences these properties and performance is scarce in the literature. Here, in situ grazing incidence wide angle X-ray scattering is employed during spin coating deposition to gain insights on the formation of MAPbI₃ and Cs_xFA_1−xPb(I_0.83Br_0.17)₃ perovskites, comparing the use of dimethyl sulfoxide (DMSO) and 2-methyl-n-pyrrolidone (NMP) as coordinative solvents. Intermediates formed using DMSO depend on the perovskite composition (e.g., Cs content), while for NMP the same intermediate [PbI₂(NMP)] is formed independently on the composition. For MAPbI₃ and Cs_xFA_1−xPb(I_0.83Br_0.17)₃ with a small amount of Cs (10% and 20%), the best efficiencies are achieved using NMP, while the use of DMSO is preferred for higher (30% and 40%) amount of Cs. The inhibition of the 2H/4H hexagonal phase when using NMP is crucial for the final performance. These findings provide a deep understanding about the formation mechanism in multication perovskites and assist the community to choose the best solvent for the desired perovskite composition aiming to perovskite-on-silicon tandem solar cells.